# Some LME49600 implementations



## wakibaki

Here's the first:-
   

   
  It's no more than the headphone amp reference design from the LME49600 datasheet with the addition of dual regulated power supplies. There is a slight departure in that the servos don't use amps from a second package, but rather each channel comprises one LME49600 + one LME49720. This simplifies and compacts the layout somewhat:-
   

   
   
  This is a step on the way to a more comprehensive integrated desktop amp (including volume control, PSU and offset protection on a single PCB), but I think it makes an interesting stand-alone module. It'll probably fit in 50 * 50 mm.
   
  Here's ground:-
   

   
  And +/- rails shown for L/R channels for clarity:-
   

   
  I'd obviously prefer to route the power rails superimposed, but the tracks are hardly extensive and no PCB is without compromises.
   
  Unlike a lot of stuff I do, it's not really suited to homebrew PCBs due to the PTHs and via-in-pad construction employed, despite the fact that all but one of the second layer links are straight. I still have to get the LME49600s in my hand to verify the footprint.
   
  I'm toying with the idea of doing one with only LM317s aboard but since I'm currently aiming at using a single AC wallwart I don't have the luxury of a dual-secondary-type supply.
   
  w


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## Avro_Arrow

Personally, I wouldn't run the LME49600 (or BUF634) in wide band mode in a headphone amp.
  You are more interested in the slew rate and that does not change with the bandwidth.


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## wakibaki

I took this from the datasheet A_A, I'm firmly of the opinion that the manufacturer's recommendations are there to be followed, particularly in the light of no previous experience of the part, which is what I've got.
   
  w


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## Avro_Arrow

If you make a space for a PTH resistor, you can leave it open, jumper it or put whatever
  resistance you like to try...just a thought...
   
  Edit: It doesn't need to be a PTH resistor, an SMD will serve the same purpose.


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## Avro_Arrow

If you want to do an all 317 power supply, consider 338 instead...it has lower output impedance.


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## Avro_Arrow

If you do a small run of boards I might be interested in one depending on the final price and shipping.


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## wakibaki

Quote: 





avro_arrow said:


> If you make a space for a PTH resistor, you can leave it open, jumper it or put whatever
> resistance you like to try...just a thought...
> 
> Edit: It doesn't need to be a PTH resistor, an SMD will serve the same purpose.


 


  Thanks for the input. That's easy to accommodate, R4 & R7:-
   

   
  I might get some made. I can get 100x100mm boards at a very reasonable price. I have to cut them up myself, I haven't decided how to fill the other 3 25x25mm sections or whether to go with a single integrated PCB. I think I could get everything into the 100x80 that fits into an O2 enclosure.
   
  Here's the protection circuit I'm planning to use. I haven't verified it in simulation yet, but I'm pretty sure it will work, it's simple scheme and straightforward to understand.
   

   
  Presettable +ve and -ve voltage references are presented to an LM2902 configured as a comparator. They are compared with LP filtered voltages derived from the amplifier outputs. The comparison is configured so that the outputs are low in normal operation. Resistors and diodes constrain the voltages input to the bases of the 4 transistors to a maximum of -0.6V to avoid exceeding the permitted transistor base-emitter voltage. The transistors are normally cut-off. This results in the voltage at the base of the 5th. transistor rising at switch on, albeit slowly due to the capacitor there. Eventually the transistor turns on and the relay closes.
   
  If the DC element of the LP filtered output voltage exceeds the reference in either direction, the comparator output changes polarity, turning on one of the 4 transistors, turning off the 5th and causing the relay to open. The LP filter values are not set in stone.
   
  This is where I'm at with the layout:-
   

   
  The existing amp has a gain of three. A 3rd. module might comprise a volume control and preceding additional gain stage, and the 4th. would be the PSU.
   
  w


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## Avro_Arrow

I drew up my own version to muck around with:
   

   
  Here's the "I'm crazy" part.
  I want to use either a BUF634 TO-220 or the LME49600 TO-263
  Here's what I'm going to do...glue the TO-263 package to
  a heat sink and mount it like the TO-220. With a little reforming
  of the leads it just might fit.
   
  The other "crazy" thing I was thinking about was making a non-symmetrical
  op amp from an OPA132 and LME49990 on a Brown Dog adapter.
  OPA132 for the servo and LME49990 for the signal.


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## wakibaki

Quote: 





avro_arrow said:


> I want to use either a BUF634 TO-220 or the LME49600 TO-263
> Here's what I'm going to do...glue the TO-263 package to
> a heat sink and mount it like the TO-220. With a little reforming
> of the leads it just might fit.
> ...


 

 I think you can do that with the heat sink, and the asymmetrical opamp is an interesting idea, but I'd rather steer clear of the adapter, it's bound to introduce some strays. You can see I've removed the ground plane around the amplifier pins as per the layout advice for the TPA6120.
   
  I kind of fancied the OPA2277 for the servo role, but in the end I went with the layout resulting from using the second amp from the LME49720 package. When all is said and done, and without figuring the offsets in the circuit given, the '49720 is still pretty hot in a DC precision application. 
   
  w


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## Avro_Arrow

I would solder the Brown Dog in just like a DIP to minimize strays.
   
  Of course the "proper" thing to do would be to just use single SOICs
  right in the layout...


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## wakibaki

Here's the general topology for the integrated amp:-
   

   
  The component values are a bit small for comfortable reading, but you get to see the general idea.
   
  A half-wave rectified supply provides +/-15V regulated. CRCRC filters mean that ripple to the regulators is already fairly well controlled. Ferrites should help in keeping any diode switching noise off the wallwart cable.
   
  The amp itself is provided with switchable gain, a modest selection between 3x and 5x. A conventional volume pot of 10k is employed. I have an existing design using a TI PGA23xx digital volume control, but this provides a massive 35dB of gain which is unnecessary and would result in clearly audible hiss (undriven) at the top of the range in sensitive IEMs and a damaging output level with even a comparatively quiet source.
   
  The protection circuit will certainly obviate any thump at switch-on and hopefully at switch-off too.
   
  The layout so far:-
   

   
  w


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## Avro_Arrow

I know this digital pot would take up more space but it is easy
  to implement with no uController.
   
  Looking good so far...


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## wakibaki

Well, I've got room, I haven't even started on the other side of the board.
   
  Here's the one with the regular pot:-
   

   
  Gotta get some sleep...
   
  w


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## Avro_Arrow

This link shows a simple way to power the DS1802.
  You can take from it what you will. You will probably
  have a much better idea than me for it.
  DS1802 is also supposed to work with a rotary encoder though
  I have not done it myself.


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## Avro_Arrow

The board looks really good.
  It's nice to see a professional layout.


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## wakibaki

Thanks. The design doesn't seem to be attracting much interest though. I have a couple of other things I'm working on, a USB DDS signal generator based on an ebay AD9850 module + PIC 18F4550 and another compact headphone amp based on the AD3897. I might get on with writing them up and leave this on the back burner for a while.
   
  The digipot is interesting, THD is not as good as the high-end TI and Cirrus chips though, but still probably below the threshold of hearing.  The problem, as I see it though, is that you have to go with the uproc if you want a readout of the setting, in which case you might as well go with the high-end chips. If I could get another decimal place in the THD over the existing chips I'd go for it, because people like the margin of performance, unless they're tube enthusiasts.
   
  I had another idea for a simplified protection circuit, I'll write it up in the AD3897 thread I'm going to start.
   
  w


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## wakibaki

Went back and reworked these to give some copper fill to the 49600's and regs for cooling. Should have thought about this earlier. 
   
   
  I fiddled with the protection scheme too.
   

   
   
  The input voltage needs to be controlled so as not to drop too much voltage across the regs, because they won't dissipate a lot. I could maybe get them a bit more copper area by connecting the 49600's to the bottom with a big hole like I used for the TPA6120's and stealing some of the top from them.
   
  The protection circuit:-
   

   
  The original layout with modified fills:-
   

   
  I did look at the output offsets with the LME49720. They can be pretty horrific. 65mV worst case. Which is higher than I was going to set the offset protection.
   
  I think maybe I'll try fitting in single opamps instead of the duals. This would permit the use of say, LME49990 and OPA123 as Avro_Arrow has suggested here and elsewhere. This is probably the thing to do, why spoil the ship for a ha'porth of tar? I can get a bit more room for the amplifier proper if I move the pot and make the front panel layout assymmetrical.
   
  Of course there's room on the bottom, but it's pretty much unbroken ground plane for the moment, I don't want to complicate the ground returns. I could go to 100 * 100 mm for the same price board, but it wouldn't fit the O2 enclosure then.
   
  Nobody got any input?
   
  w


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## kevin gilmore

The reference design with the  lme49720 with an lme49600 where one section of the 49720
  is used as a servo. It doesn't work, typical outputs are 10+mv. Sounds really good however.
  Much better solution is to use lme49710 with the lme49600 and then a very low offset opamp
  like tl071 or op27 as the servo.  You can easily get to 200 microvolts offset this way.
  Also the gain of the servo is way too high, the 1k resistor should be at least 10k.
  And to reduce offsets further replace the 1M resistors with 100k, and the 1uf caps
  with 10uf mlcc caps.


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## wakibaki

Thanks, Kevin.
   
  My calculations of offset due to input bias current were leading me to suspect as much, but in the absence of a first prototype or an example of the National reference board, you can probably understand why I was taking the design from the datasheet at face value.
   
  The move to replacing the 1M's in the servo is fairly obvious measure, fiddling with these values and the servo gain are amongst the range of options available subsequent to completing the layout.
   
  I have in fact re-laid the board to accommodate 2 single opamps to substitute for the dual 49720, making it possible to build it with a variety of opamps. I'll look at the footprints of 10u MLCCs and make sure that they can be accommodated. Changing the servo authority will have an impact on the gain, what are the symptoms of leaving it as is?
   
  Thanks again.
   
  The board as it currently stands:-
   

   

   
  I'll stew on this now for a few days, there are a couple of things I will probably change, I find that I like to get a board routed even if it's a bit dubious in places, and then come back and improve it. I need to get in some 49600's to verify the footprint, maybe go to 35V electrolytics instead of these 10mm square jobs.
   
  w


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## kevin gilmore

The servo resistor should be at least a factor of 10 greater than the gain resistor
  to ground. Then you can adjust the feedback resistor for the overall gain
  you require.


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## Avro_Arrow

Here is the board I came up with.
  I'm no master but I think it looks OK.
   
  The dual op amp position will have a brown bog adapter
  with an OPA132 on one side and LME49990 on the other.
   
  Power supply is off board.
   

   
  And the schematic it was generated from:
   

   
  Component value are not final.
  Let me know if you see any flaws in my layout.
  I tried to make it home etching friendly.
   
  Edit:
   
  I forgot to mention I was using a BUF634 as I have several in TO-220.
  I also have LME49600 so I will do a version for that too.
  It will be fun to compare the two.


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## wakibaki

Looks good to me, it's certainly suited to a homebrew PCB. You'll need some isolation between the tab and heatsink, or between the heatsink and ground plane.
   
  I'd be tempted to put an exclusion zone for the groundplane around the IC pins and tracking to keep stray capacitance to an absolute minimum, but this is arguably unnecessary, unknown in exact effect and subject to discovery since this is a prototype. It's certainly less risky than what I've done, the simplicity minimises the likelihood of any unwanted interactions or feedback. I think you're in love with the Brown Dog idea though, why not make the PCB accept the chips direct?
   
  IMO it makes more sense than the one that was being worked up on diyaudio, my approach is to either make an amplifier module to work with some external modules or a complete integrated amplifier with an enclosure in mind.
   
  I ordered a couple of these chips, so I guess I'm committed to building some kind of amp using them. 
	

	
	
		
		

		
			




   
  w


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## sek@

Hi,
   
  I hope nobody minds if I chime in over here too, as I'm also currently fiddling with the BUF634/LME496x0 buffers, but am not particularly happy with how things go over on diyaudio at the moment. 
   
  Quote: 





avro_arrow said:


> Here is the board I came up with.


 

 Looks good so far. I have a couple questions/remarks, though.
   
  Your circuit of course reminds me of the datasheet application. I assume R5 and R10 differ from the datasheet servo configuration on purpose (i.e. to increase stability)?
   
  I don't quite get your grounding yet. You mention the PSU being off board, but some things aren't obvious from the board layout.
   
  First off, the circuit differentiates between signal ground and power ground. Why? If it's on purpose, where do they join? How? There's no connector. The solder pad only features signal ground.
   
  Secondly, how do you handle signal ground input? You have a single solder pad (per channel) for signal input, but no place to connect the return wire (i.e. cable shield). That would mean you'd have to essentially run a couple of inches of unshielded signal wire from the chassis connectors to the PCB.
   
  Have you made your choice regarding chassis input/output connectors and how to handle their insulation/grounding yet?
   
  What's with the heat sinks? They look power referenced in the schematic (why actually?), but they're floating on the board. 
   
  What's your reason to mix SMD and through-hole components? You've got through-hole capacitors on the board already? I may suggest SMD components throughout.
   
  Finally, where's the headphone return wire (ground) connected? Also at the PSU? 
   
   


> I forgot to mention I was using a BUF634 as I have several in TO-220.


 
   
  The TO-220 cased variant of the BUF634 has straight legs. There is a library part in Eagle that has the 5-pin device with all holes in a row. That's slightly more difficult to etch at home, but way more gentle on the pins.
   
  Cheers,
  Sebastian.


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## Avro_Arrow

Those are all very good observations and questions. Let's see if I can be as good at answering them.
   
  It doesn't show up well in the picture, but the heat sink mounting pins and ground plane are isolated.
  I tied the pins on the heat sink to V- because the tab on BUF634 is also at V-. I could have just left
  them tied to nothing. I may run a jumper from V- to the heat sink pin.
   
  The reason I wanted to try the brown bog idea is easier routing. One dip is easier to route than two soic
  in this case. There will not be a socket. If I want to "roll", it will involve soldering.
   
  The grounding scheme is an off board star ground. The grounding point will probably be at the headphone
  jack. The ground connection on the board is to serve the boards ground plane. The ground plane is
  split to help isolate left and right.
   
  I know lot's of people love PTH components and others love SMD, but I tend to use both whenever
  it suits me. I used PTH components because this is a single sided board and they provide convenient
  jumpers. 
	

	
	
		
		

		
		
	


	



   
  I'll increase the ground exclusion around the input pins on the op amps.
   
   
  I have done a board in the past with TO-220 BUF634 using the stock footprint. I found the pad spacing
  too close. Even when I zoomed right in, it still looked like the pads were actually touched. When I printed the
  transparency my printer could not resolve the fine spacing and I had to use a needle to scratch out
  between the pads. I have done TSSOP with my set up but the BUF634 footprint just did not work.
  After that I changed the BUF634 footprint from straight legs footprint to staggered. I guess I should
  rework the BUF634 straight footprint to have better spacing.
   
  I chose this modular approach to give the builder freedom in choosing the case, power supply, etc.
  Thats why I also chose not to include a volume control.


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## Avro_Arrow

Here is the power supply if you need one:
   

   
  And it's schematic:


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## sek@

Quote: 





avro_arrow said:


> It doesn't show up well in the picture, but the heat sink mounting pins and ground plane are isolated.


 

 It looked like that in the picture, and it's certainly easier to solder that way. 
   
   


> The grounding scheme is an off board star ground. The grounding point will probably be at the headphone
> jack. The ground connection on the board is to serve the boards ground plane. The ground plane is
> split to help isolate left and right.


 
   
   
  Upon closer look I can see that you use the ground plane for joint power and signal grounding. Good.
   
  Wouldn't using the output connector as a star point (only) make it difficult to connect the input ground/shield wire? Even if you use a separate connection between input socket and ground star, you could still only incorporate a shielded input wiring by leaving the shield open at least at one end, if at all. What am I missing?
   


> I guess I should rework the BUF634 straight footprint to have better spacing.


 
   
  Yes, the original case drawing is unusable, but CadSoft's libraries contain other TO-220-5 cases (i.e. for voltage regulators) that can be borrowed. If all else fails, the pad size and shape can be reduced, i.e. simply made into regular round pads.
   

   
  Cheers,
  Sebastian.


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## Avro_Arrow

I will add a connection at the board for input grounding.
   
  I used one of CADSofts "other" TO-220 footprints for my BUF634.
  I was thinking of just splaying the legs apart slightly to increase
  the spacing. Easier than having to bend the pins for the current
  footprint I am using.


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## Avro_Arrow

Revised amp board:
   

   
  I made the heat sinks a little smaller...I think they were a little too big.
  Added mounting holes...duh.
  Cut out a little more around op amps.
  Heat sink pins now floating, not connected to V-


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## sek@

> Cut out a little more around op amps.


 
   
  With regard to this I find the concept of guard rings around pins and traces very interesting. Here's an introduction to some ground related issues (from TI).
   
  Do you think you can handle a two-sided PCB manufacturing process, with the upper layer being ground plane only? All it takes for this to be effective are a reasonable number of vias (i.e. hand soldered on both sides using resistor wire cuttoffs).
   


> Heat sink pins now floating, not connected to V-


 
   
  There's no reason not to ground the heat sink (granted the buffers are mounted in an insulated way), especially if EMI is taken into consideration. OTOH it's not absolutely necessary either.
   
  Cheers,
  Sebastian.


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## sek@

Quote: 





avro_arrow said:


> Here is the power supply if you need one:


 

 Just some quick remarks here for now.
   
  The heat sinks can be made the same size as on the amplifier boards.
   
  Large capacitance is needed more on the input side than on the output side of the regulator. Additionally such large capacitors might benefit from some film or ceramic decoupling capacitors or even snubbers.
   
  Seeing that you have the board space available, consider increasing the size of C1 and C2. Up to at least 470uF is a good idea to improve ripple rejection.
   
  The very large V+ and V- copper areas form an effective antenna loop, together with the wire between PSUs and amp. A top layer ground plane would solve this issue almost completely (just like I described in my post above).
   
  Cheers,
  Sebastian.
   
  PS: By joining the negative side of the positive supply to the positive side of the negative supply you effectively form a (local) ground star. That's fine, but you could leverage on this by having the screw terminal offer multiple ground point connections. The board space is there.


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## Avro_Arrow

I actually use this supply for a wide variety of projects and experiments...that's why the heat sinks are oversized.
  You could indeed use smaller heat sinks.
   
  It would be very easy to add a ground plane.
   
  I will look at rearranging the filter caps.
   
  I already have a similar power supply I built last year but
  I lost the files in a Hard Drive crash. I decided to design
  this one to replace it so I am wide open to suggestions.
   
  Quote: 





sek@ said:


> Just some quick remarks here for now.
> 
> The heat sinks can be made the same size as on the amplifier boards.
> 
> ...


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## Avro_Arrow

Yes, I have done double sided boards before, exactly as you described. I even used cut off resistor wire... 
  The heat sink can easily be tied to any potential you wish with a jumper to the mounting pin.
   
  One of the goals was to try and keep it as simple as possible for people with only beginner level home etching skill.
   
  If I were to do a board for a board house, it would be much tighter and complex.
   
  Quote: 





sek@ said:


> With regard to this I find the concept of guard rings around pins and traces very interesting. Here's an introduction to some ground related issues (from TI).
> 
> Do you think you can handle a two-sided PCB manufacturing process, with the upper layer being ground plane only? All it takes for this to be effective are a reasonable number of vias (i.e. hand soldered on both sides using resistor wire cuttoffs).
> 
> ...


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## Avro_Arrow

I should add that Wakibaki is doing the board house quality board.
  I'm just doing an alternate (simpler) version.
  I don't want to seem like I am hijacking his thread...
  
  Quote: 





avro_arrow said:


> I actually use this supply for a wide variety of projects and experiments...that's why the heat sinks are oversized.
> You could indeed use smaller heat sinks.
> 
> It would be very easy to add a ground plane.
> ...


 


   


  Quote: 





sek@ said:


> With regard to this I find the concept of guard rings around pins and traces very interesting. Here's an introduction to some ground related issues (from TI).
> 
> Do you think you can handle a two-sided PCB manufacturing process, with the upper layer being ground plane only? All it takes for this to be effective are a reasonable number of vias (i.e. hand soldered on both sides using resistor wire cuttoffs).
> 
> ...


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## wakibaki

Quote: 





avro_arrow said:


> I should add that Wakibaki is doing the board house quality board.
> I'm just doing an alternate (simpler) version.
> I don't want to seem like I am hijacking his thread...


 

 No problem at all Avro_Arrow, I'm happy to see anybody posting layouts they're working on, in fact I was thinking of suggesting sek@ post one of his own since he obviously has software and skills, it's much easier to see the advantages of a particular strategy with an example in front of you, rather than add in people's suggestions, design by remote control, as it were.
   
  I think the EMC considerations are, to a degree, secondary here, neither board has a clock. Crosstalk between tracks is a concern though. Certainly you want to keep RFI out of the circuit, but the reference design, while doublesided, has no extensive ground plane, and we are concerned to keep stray capacitance in the area of the active device pins at a minimum as oscillation is a real concern with such high bandwidth devices.
   
  I've made some changes, the regs have gone to TO220 instead of SOT223-3. This gives me a bit more than 3 sq. in. of board area to cool the 49600's, about the same as on the reference board. I was aiming at using some 16V wallwarts I've got here, if the regs are set to 18V out they will be dissipating about 0.4W for a 250mA peak sinewave output, which I think means they will cope without heatsinking, music power should be 10dB down or more re. sinewave.
   

   
  w


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## Avro_Arrow

I'm trying to get my head around what Kevin said...
  In my schematic, the gain resistor is R1, the feedback resistor is R3.
  Is the servo resistor R5?
  
  Quote: 





kevin gilmore said:


> The servo resistor should be at least a factor of 10 greater than the gain resistor
> to ground. Then you can adjust the feedback resistor for the overall gain
> you require.


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## kevin gilmore

in your schematic...
   
  R1 is the resistor to ground, in the schematic 1k.
  R5 is the servo resistor, currently 10k, but can go higher depending
  on the offset of the audio opamp.
  R3, currently 1k is the gain setting resistor, so calculate the
  R1,R5 pair, and set R3 for the voltage gain required.
  i'm using 4.7k for R3.


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## Avro_Arrow

Thanks Kevin
  
  Quote: 





kevin gilmore said:


> in your schematic...
> 
> R1 is the resistor to ground, in the schematic 1k.
> R5 is the servo resistor, currently 10k, but can go higher depending
> ...


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## sek@

Hi wakibaki,
  
  Quote: 





wakibaki said:


> I was thinking of suggesting sek@ post one of his own since he obviously has software and skills


 
   
  I'd be glad to, but unfortunately most of it is work related (robotics, microcontrollers). That stuff can't be released here.
  I wish I'd have the time to get to my DIY projects as often as I seem to come across like. And please don't make me look more experienced than I am. 
   
  My current headphone related project is a balanced amplifier with integrated analog volume control. 
   
  Here's part of the schematic. It's prototyped (one iteration) and almost set, give or take a couple of component values:

   
  And here's part of the board. It's tow-sided, currently unrouted, but mostly figured out. Power supply and some of the bypassing are omitted in this detail:

   
  Other than that, the things I mentioned are specifically those that caused me problems in the past, both with DIY projects and with actual products. That does of course not imply that builders of the projects in this thread will, though. YMMV.
   
   


> I think the EMC considerations are, to a degree, secondary here, neither board has a clock.


 
   
  I tend to think that - after everything has been wired correctly and a good parts choice has been made - EMI is the only concern left! 
   
  At the same time, it's a phenomenon of perpetually increasing relevance. The number of wireless devices (and clocks, as you narrow it) around us (and on us) rises considerably and this rise will most likely even become steeper in the future. 
   
  Thus, a design that worked well in the past may have GSM or ISM related RFI problems today. A design that works well today... 
   
  Cheers,
  Sebastian.


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## sek@

One more thing!
  
  Quote: 





avro_arrow said:


> The heat sink can easily be tied to any potential you wish with a jumper to the mounting pin.


 
   
  Thinking about this approach, this could help to solve one of my remaining issues: heat dissipation.
   
  If I were to mount the voltage regulators upright (on heat sinks) and then were to tightly solder the heat sink to the negative supply copper, the sinks could help the dissipation via the copper plane.
   
  I wonder how strong the effect would be...
   
  But enough with the threadjacking, back to wakibaki's design!


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## Avro_Arrow

There are solderable board level heat sinks for SMD devices such as this Wakefield Heatsink.


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## sek@

Quote: 





avro_arrow said:


> There are solderable board level heat sinks for SMD devices


 

 Sure, I'm using those with THE WIRE. But they take a lot of horizontal space, i.e. they don't allow for close spacing of the buffers (because the fins would overlap).


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## kevin gilmore

here is what the default servo filter does
http://gilmore.chem.northwestern.edu/defaultfilter.jpg
  amplitude in green, phase in red


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## wakibaki

Is this a Proteus plot Kevin? Certainly looks like one. Where are the probe and stimulus placed?
   
  Changing the resistor changes the range of servo authority as I understand it, i.e. the default resistor permits the servo to influence the DC level from rail-to-rail, being the same value as the feedback resistor (the one from output to inverting input), and because it is in parallel with the resistor to ground (being the same value as that one too) and because the servo opamp output is a virtual ground it increases the circuit gain to 3.
   
  1 + Rf/(Rg||Rs) = 1 + 1000/(1000||1000) = 1 + 1000/500 = 1 + 2 = 3.
   
  Increasing it to 10k would decrease the servo authority to (+/-)1/10 * rail voltage, and reduce the circuit gain to 2.1
   
  1 + Rf (Rg||Rs)= 1 + 1000/(1000||10,0000) = 1 + 1000/909.09 = 1 + 1,1 = 2.1.
   
  What other _negative effects_ arise from using the default values? I can't visualise what these are purely on the basis of the gain/phase plot.
   
  w


----------



## kevin gilmore

more clear

http://gilmore.chem.northwestern.edu/defaultfilter2.jpg


----------



## wakibaki

Both circuits have gain virtually flat @ just over 1Hz, phase ~0 degrees by 10 Hz, I don't think the differences are anything to get excited about. It'll be a rare CD player having output (and a rare CD having content) @10Hz. There's no real inconsistency with the datasheet's claim that the servo cutoff is over 2 decades below 20Hz, which should be entirely adequate to avoid any audible perturbations.
   
  I was surprised when I first saw the values for the servo in the datasheet. I doubt that there's any necessity to give the servo that much authority, but the main concern, as I see it, is that a fault might result in the servo pulling the circuit to a large DC offset. Setting the authority to 1/10th. however, merely restricts the offset in a fault condition to 1.5V (for, say, 15V rails), which is probably still catastrophic for most headphones, and is why the circuit I have drawn includes output offset protection, which might not have been considered necessary in an AC coupled circuit, depending on the exact implementation.
   
  I will, however, use a value of 10k to set the servo authority to 1/10 in the light of your input (I see no real necessity for any greater) and modify the gain setting resistors accordingly if experience of the first prototype shows it to be desirable. Thanks for taking the time and putting in the effort to do the simulations and post them. Please get back to us if I have misunderstood the significance of the plots or if you have any further observations
   
  w


----------



## sek@

A very interesting discussion about the servo circuit. 
   
  I read that as follows:
   
  Under normal conditions the servo acts to minimize the output DC voltage occuring from both an offset inherent to the opamp/buffer circuit and the input signal. Depending on the choice of feedback and servo/gain resistors, the influence ("authority") can be adjusted, but is of course defined as a proportion of the signal, limited by the supply voltages. So far, so good.
   
  Thus, if either a full scale DC voltage is applied to the input or any active device fails, introducing either rail voltage into the circuit's signal chain, the servo is _incapable_ to effectively double as a _safety feature_ in order to protect the headphones.
   
  In order to account for this, among other things, wakibaki incorporates a relais based protection circuit into his design.
   
  The question for me is:
   
  Is the servo worth the trouble under normal conditions, i.e. is the nulling effect that significant to the sound out of the headphone (compared to, say, a fully DC coupled signal chain)?
  Seeing that protection is covered by the relais circuit, isn't the servo expendable in all but the most delicate cases?
   
  As of my understanding, the circuit is rather stable, using high quality/low drift components. All causes of output offset voltage that stem from the circuit itself should essentially be constant, making it easy to deal with them without a servo (i.e. selection, trimming) or to just live with it and leave well enough alone.
   
  And in order to put things into perspective, I can report a maximum of approx. 50 mV DC out of THE WIRE, which is basically the circuit at hand minus the servo. That's about 80 uW into a 32 Ohm headphone or 5 uW into 600 Ohm. 
   
  Thanks,
  Sebastian.


----------



## Avro_Arrow

Here is how I take it.
   
  Since this is a DC coupled design from end to end, any DC offset
  in the source will be amplified. The servo is able to null out any
  reasonable DC offset at the input to keep the output near zero.
   
  If the above fails, then Wakibaki's fail safe will kill the output.
   
  The SE-SE Wire can not correct for DC offset at the input.
   
  edit: spelling


----------



## sek@

Hi again,
  
  Quote: 





> Originally Posted by *Avro_Arrow* /img/forum/go_quote.gif
> 
> The servo is able to null out any reasonable DC offset at the input to keep the output near zero.


 
   
  Granted, but my question was actually wether this brings any advantage over taking care how you build and set up your components. Let's ignore sources with insensible DC offset for the moment.
   
   


> The SE-SE Wire can not correct for DC offset at the input.


 
   
  I was referring to wakibaki's circuit _without_ the servo, which is essentially a SE-SE _with_ protection circuit.


----------



## Avro_Arrow

If you remove source offset from the discussion,
  I don't think the servo adds anything over and above what a properly designed circuit will do.
  The only other thing I can think of is if your signal is AC offset (more swing on one side than the other)
  the servo will push it back to the center. I'm not sure under what circumstances the above condition
  happens, but I have seen it when editing audio files in Audacity.


----------



## sek@

Quote: 





avro_arrow said:


> If you remove source offset from the discussion,
> I don't think the servo adds anything over and above what a properly designed circuit will do.


 
   
  The question that remains is wether it adds something you don't want.
   
   


> The only other thing I can think of is if your signal is AC offset (more swing on one side than the other)


 
   
   
  Different swing on either side by definition is a DC component.
   
  Different wave form shape on either side, however, is part of the music.


----------



## kevin gilmore

there are some otherwise nice dac's out there with unusual i/v converters that put out more than .1 volts dc.
  The resulting offset after amplification can damage some sensitive headphones.
  My opinion is that the servo is absolutely required.


----------



## Avro_Arrow

The servo only works at low frequency, just a few Hz.
  At audible frequencies, it should be invisible.
   
  Probably the way to truly answer your question is to build the
  amp and listen to it with the servo enabled and disabled to see if you can
  hear the difference.
   
  I don't want to seem like I am arguing for or against servos...I think
  like any other circuit element, it has it's uses.


----------



## wakibaki

If we take a 16 ohm headphone with 100dB/mW and we want to drive it to 100dB then the voltage across it is sqrt(0.001 * 16) = 126mV.
   
  Then 1 LSB would be 0.126/(2^16) = ~2uV. (rms), 2.73uVpk.
   
  So I think ideally we would like to see 1/2 LSB offset, and 50mV is unacceptable. It's about ~18,000 times 1 LSB and  I was thinking of setting the protection to 50mV and AMB's E12 protection circuit operates @ 60mV IIRC.
   
  If we use OPA132 with the 1M resistors and a gain of 2 we get 1050uV offset and with OPA277 we get 1020uV.and with LTC1050CS8, 160uV, all worst case. With LTC1050CS8 & 100k this comes down to ~25uV worst case (10 LSBs) and 2.5uV typical. Farnell stock LTC1050CS8
   
  Feel free to disagree with my arithmetic.
   
  These numbers look a lot better if we are using more of the amplifier's output range, of course, and it's capable of nearly 100 times this voltage swing.
   
  w
   
  LTC1050CS8 is really only good for +/- 16V


----------



## sek@

Thanks for replying to my questions everyone!
  
  Quote: 





kevin gilmore said:


> there are some otherwise nice dac's out there with unusual i/v converters that put out more than .1 volts dc.


 
   
  As I mentioned earlier, I'm leaving source DC offset out of the equation with my question.
  But you're of course right in that this can't be a recommendation for everyone and any source.
   
  If, though, the known source (like a DAC that is known to be DC free after it's reconstruction filter) is no issue... 
  Also, we don't quite cover the presumably relevant case of balanced operation.
   
  I agree with you, Kevin, an amp like this better has the servo in it!
   
  Quote: 





avro_arrow said:


> listen to it with the servo enabled and disabled to see if you can hear the difference.


 
   
  The more I think about it, the more I see this point and also find myself starting to think about incorporating it. 
  
  Quote: 





wakibaki said:


> So I think ideally we would like to see 1/2 LSB offset, and 50mV is unacceptable. It's about ~18,000 times 1 LSB


 

 Well, we'd still have to distinguish between AC and DC components at the amplifier output.
   
  The output offset will contribute to heating the voice coils, reducing the headphone's dynamic range through phenomenons like reduced dissipation, power compression or even biasing (depending on construction). It also poses the danger of overheating the phones even without a music signal present at all. And it causes nasty clicks at turn-on and turn-off...
   
  A DC offset will not be noise or distortion in itself (depending on whether you measure for audible distortion), though. Nor will it alter any AC component (the music). If it did, it never were to be considered _offset_ to begin with and a low frequency servo wouldn't help much anyway. The servo circuit discussed here is not intended to fight motorboating or unstable supplies, although it would do to that to a degree.
   


> Then 1 LSB would be 0.126/(2^16) = ~2uV. (rms), 2.73uVpk.


 
   
  Does it really make sense to refer output offset level to DAC resolution? What did I miss?
   
  What if you increase your desired resolution to, say, 20 bit (or maybe even 24 bit)?
   
  Then 1 LSB would be 0.126/(2^20) ≈ 120 nV. 
   
  The worst case you mentioned above would have a totally different meaning now, perceiving this as a proportion of LSB gets difficult.
    
  I gave an example calculation referring to (thermal) power levels above.
   
   


> ~25uV worst case (10 LSBs)


 
   
   
  25 uV represented by 2.72 uV steps is a factor of ~9.2, which is inbetween bits 3 (2^3=8) and 4 (2^4=16) referred to LSB, so 25 uV would correspond to 4 bit (as 2^3=8 < 9.2 < 2^4=16).
   

  Anyway, the 50mV I mentioned are a worst case scenario in itself, the real world offset without a servo can still be made much lower (approximately by a decade).
   
  I'd like to add that a servo circuit should be a measure to make AC coupling (using series capacitors) unnecessary, not as an excuse to ignore inherent offset that could be minimized by design or adjustment.
   
  Cheers,
  Sebastian.


----------



## wakibaki

Quote: 





sek@ said:


> 50 mV DC out of THE WIRE, which is basically the circuit at hand minus the servo. That's about 80 nW into a 32 Ohm headphone or 5 nW into 600 Ohm.


 

  50mV in 32R is 78uW, not 78 nW.
   
  25uV is between the 3rd. and 4th significant bits, but it is still 10 _times_ the 1st. or lowest significant bit. Try to keep your eye on the ball instead of indulging in mindless nitpicking.
   
  w


----------



## sek@

> 50mV in 32R is 78uW, not 78 nW.


 
   
  Thanks for checking, I corrected my mistake in the original post above.
   
   


> 25uV is between the 3rd. and 4th significant bits, but it is still 10 times the 1st. or lowest significant bit.


 
   
  Great, so you do agree with me on the math, I also corrected a typo in my last post, thanks. The reason I explicitly presented the numbers is because I still don't see how your attempt at producing as low as 2.5 uV DC offset brings us to your self-imposed goal of getting below 1/2 LSB. We're still off, even with your benevolent choice of 16 bit resolution and your best case calculation.
   
  I'd rather like to discuss the relevance of relative DC offset level, assuming the absolute level is low enough not to cause damage or dynamic compression or bias, than get lost in the details of a certain example.
   


> Try to keep your eye on the ball instead of indulging in mindless nitpicking.


 
   
  No reason to get grumpy... 
   
   
  Speaking of getting the eye back on the ball, here's the question at hand again:
   


> Does it really make sense to refer output offset level to DAC resolution?


 
   
  Cheers,
  Sebastian.


----------



## wakibaki

OK, I've been looking at this a lot, the first thing is LTC1050 is only good for 16V total, it'll have to be LTC1150. Which is expensive. It also means some modifications to the circuit due to the reduced range of output swing available, and the fact that it's a chopper-stabilized amplifier.
   
  I started looking at the output filter I was going to use to feed the comparators. It has a rollof of 0.16Hz, same as the integrator. This is 2 decades down on 16Hz so a rail-rail output of 16Hz will only be suppressed by 40dB or 100 times, meaning that there will be a residual of 150mV pk. This means that the threshold of the comparator controlling the relay can be set no lower than 150mV or it will false trigger for a 16Hz full-scale input, This means that the DC level detectable can be no less than 150mV. The residual @ 20Hz will be somewhat less than the one @ 16Hz, but a little margin is required. Setting the threshold to 150mV simply means that no input at 20Hz could cause false triggering of the relay. 
   
  It is important to understand what would happen in the circuit using the filter to trigger the relay. If the servo authority is set to 1/10th. of the full-scale voltage then a DC offset in the source of >1.5V will defeat the servo and the phones can be exposed to a continuous offset of up to 150mV before the relay operates. The DC servo nulls out any offset voltage in the source up to the range of its authority (up to 1.5V in this case). If the source offset exceeds 1.5V then the excess (the offset - 1.5V) appears at the amplifier output up to the point where the servo comparator sees 150mV (total offset = 1.65V), when it causes the relay to drop out.
   
  If, instead of using the filter at the output, the relay comparator looks at the servo output, then the relay can be forced to drop out before the servo runs out of adjustment. Now what we have is an arrangement where the servo compensates for any DC offset in the source (or any arising anywhere) until it can no longer compensate, whereupon the relay drops out.
   
  The LTC1150 has an output swing of +10.5V into 10k. It really requires a filter at the output to eliminate switching spikes.
   
  If we take the 10k resistor that is supplying the servo feedback and split it into 2 * 4k7 resistors we can create a filter with a rolloff @ ~1.6Hz using a 20uF cap at the junction of the 2 resistors and also insulate (to a degree) the filter from the 1k resistors in the amplifier proper which will otherwise load the filter. This is an order of magnitude higher than the loop unity gain frequency and should not contribute to any instability. It has the added benefit that the LTC1150 now sees >10k, keeping its output voltage up.  If we set the relay comparator threshold to 10V (at the servo output) then the relay will operate for input offsets of just over +/-1V and the 20Hz residual if sampling at the junction of the 2 * 4k7s will be only 4V (settling to 2.5Vpk), so a 20Hz full-scale output will only cause triggering when an offset of >0.5V is already present. The servo authority is reduced by a factor of 9.4 instead of 10.
   

   
  The last question to be resolved is how quickly the circuit responds to an offset. The time constant of the integrator is 1 second, but the comparator sampling point is now the junction of the 2 4k7 resistors, which is being pumped by the amplifier output via the feedback resistor. The sim says 566mS to reach 10V feedback. This is not ideal, but the integrator tau is the same as as that of the filter I was going to use and the response is better than the DC servo response was in the original circuit. 
   

   
  As regards using 1 * 16-bit LSB as a standard for assessing offset performance; (taking +/-15V rails)
   
  1) 16/44k1 is the limit of audibility. There's no point in using 24 bits, it's unrepresentative of what can be heard. Less than 16 bits is throwing away resolution that arguably _can_ be heard  
   
  2)There's no point working to get the offset down below 1/2 LSB, so it sets a limit on how hard we need to work. Once we get it down to 1/2 LSB we can stop trying to get it down any further.
   
  3)What else are you going to use? Different phones have different sensitivities, but not many are more sensitive than 100dB/mW. If you get the offset down to 1/2 LSB of the most sensitive phones working at a 'normal' level then the offset will be insignificant for all other phones. It's a ballpark thing.
   
  You can pick an arbitrary number out of the air, such as the 50mV that has been discussed and say, that'll be OK on the basis of experience, and quite honestly I doubt that even the most sensitive phones suffer any ill effects from a 50mV offset. I'm less sure that a 50mV offset has no audible effect in such phones, 1 mW in 16R is only 126 mV rms. I'd be a lot happier arguing that 2.5uV is not significant as opposed to 50 mV. Yes, 2.5uV is still not 1/2 LSB but it's only just over 1 LSB, which has to be a lot better than the ~1000uV that the OPA132 was going to get us. It is less than 1 LSB for a DAC having a 2V output.
   
  I also decided to substitute the PIC for the AND gates, it's just better all round. It has 5 inputs, less pins, no cap, lower parts count. I've added a high-speed switch off so thumps at turn-on and turn-off should both be eliminated now..
   

   
  OK, I'm going to run some more sims. If anybody sees any holes in my thinking, please point them out, there are few subjects with as many lurking traps as electronic design.
   
  w


----------



## kevin gilmore

Lets start with C2 in the wrong place, needs to be after the resistor.
  Correctly shown in the 3rd diagram. Need to run analysis again.
   
  This seems to be an even more simple protection circuit with a pair of lm339.
http://gilmore.chem.northwestern.edu/protector.pdf


----------



## wakibaki

Thanks, I changed the sim. It makes little practical difference to the result, the 20 Hz residual is reduced to less than a volt, which is all to the good, since the threshold is 5V at the sampling point. Similarly the rise time at the sampling point for a step input is reduced. This means that the relay will drop out @ 220mS with a 15V step at the amp input. Because of the amplifier gain the output reaches 15V for a DC input of ~6.5V and the time to dropout is not greatly increased. 
   

   

   
  Your protection circuit is a considerable improvement in component count and complexity, and while looking at it I realised that it could be modified to provide fast turn-off in the case of loss of AC supply.
   
  For the reasons I have already pointed out, as drawn it would anyway only operate after the servo had been defeated, and it cannot detect an offset much below 150mV, so I have redrawn my circuit to incorporate yours, but sampling at the junction of the 2 * 4k7's in the servo loop.
   

   
  I'm wondering now if I can get it into 100 * 50 mm.
   
  w
   
  Oh, anything else?


----------



## G.Trenchev

You'll get better results if running the buffers in high bias WB mode(with a jumper instead of resistor).
  Otherwise,I'm pretty sure that offset will be lower that 1 mV even without the servo(I've got same topology amp,just with BUF634).


----------



## wakibaki

Quote: 





g.trenchev said:


> You'll get better results if running the buffers in high bias WB mode(with a jumper instead of resistor).
> Otherwise,I'm pretty sure that offset will be lower that 1 mV even without the servo(I've got same topology amp,just with BUF634).


 

 The option to run in NB is something I added at the suggestion of Avro_Arrow. Evidently another member felt they had better results that way. Obviously the link can be made with a 0R resistor.
   
  sek@ reports 50mV offset from his build of THE WIRE.
   
  The purpose of the servo is not only to control any internally generated offsets, but (in conjunction with the relay) to ensure that no offsets in the source are passed on (amplified) to the phones.
   
  Thanks for the input though, let us know if anything else occurs to you...
   
  w
   
  I bookmarked your blog.


----------



## Avro_Arrow

I asked Wakibaki to add a place for a jumper or resistor.
  It gives you the option of running NB, WB or something in between.
  It just gives the builder/listener an option to run it the way they want.
   
  Perhaps it will turn out that WB works best in all situations and we
  can get rid of the jumper...remember this is a prototype...
   
  Whether it is in Wide Band or Narrow Band, the slew rate does not change
  and band width is well above what is needed for audio.
  
  Quote: 





g.trenchev said:


> You'll get better results if running the buffers in high bias WB mode(with a jumper instead of resistor).
> Otherwise,I'm pretty sure that offset will be lower that 1 mV even without the servo(I've got same topology amp,just with BUF634).


----------



## Avro_Arrow

So I sat on it, revised it a little, but I'm pretty sure this is the design I'm going to use.
  Wakibaki's board is highly integrated, I'm doing a more simple, modular design.
   

   
  Other than cleaning up the fonts a little, I think it's ready for the board house.
  What do you think..?


----------



## sek@

Hi,
  
  Quote: 





wakibaki said:


> sek@ reports 50mV offset from his build of THE WIRE.


 
   
  Just to clarify, as I'm afraid this has been mistaken for a normal or expected value.
   
  Please keep in mind that the BAL-BAL employs an OPA1632, which has an output common-mode adjustment pin referenced to ground using a capacitor. Thus, at turn-on (and turn-off) the conditions haven't settled yet and the output level is approaching it's final value with a certain time constant. The way this happens depends on which supply settles first and wether there is a high level input signal at this time (as this would then make for a transient). It's also worth mentioning that the time constant is rather short, so the condition settles so quickly that it's probable not even considered an offset voltage at all (but just a turn-on thump, if anything).
   
  The stable output offset after settling is usually not dominantly a differential-mode voltage (appearing across the voice coil), at least the differential-mode offset is way lower than 50mV. It's a fully differential circuit, after all. 
   
  I made another measurement of the SE-SE (incorporating LME49990 and BUF634) just for the sake of clarifying this issue in this very thread. It showed 0.5mV DC output voltage right after turn on and over the course of a couple of minutes, so it's probably safe to say that my SE-SE simply has half a millivolt output offset.
   
  Cheers,
  Sebastian.
   
  PS: I'll make a more detailed measurement of my BAL-BAL as soon as I have the chance.


----------



## sek@

Quote: 





> Other than cleaning up the fonts a little, I think it's ready for the board house.
> What do you think..?


 
   
  I like the improvements you made.
   
  I recommend a design rule check (DRC function in Eagle) before sending to a board house. From over here it looks as if your clearance around pads and wires is rather small. This way you might have to rely on the solder mask resist for easy soldering. Some board houses even supply Eagle design rules as a .drc file to be loaded into the project.
   
  Particularly the distance/clearance around the SMD pads looks inconveniently small to me.

 The electrolytic caps (C13 through C16) look like they can accomodate up to 220uF (assuming 8mm devices and 25V types). If you want larger capacitance you could go for 10mm devices.
   
  Finally, seeing that all connectors except the output pad have the same, larger diameter, I recommend switching to a larger pad type at the output, too. More precisely I'd recommend a larger drill diameter for all the connectors.


----------



## Avro_Arrow

Thanks for the review sek@
   
  The clearances were better than it shows in the image I posted...however,
  I took your advice and increased the clearance on the ground plane to 16 mils.
  Pads were all made the same size and drills were also increased.
  I also nudged a few of the components around for better clearance and
  added the option for SMD caps in the servo. C13 to C16 were going to be Tantalums.
  The power supply is co-located and has 1500uF on the output.
   
  Here is the new version:


----------



## Avro_Arrow

Here is the power supply board...just a standard 317/337 dual rail.
   
  Top:
   

   
  Bottom:


----------



## qusp

Quote: 





wakibaki said:


> The option to run in NB is something I added at the suggestion of Avro_Arrow. Evidently another member felt they had better results that way. Obviously the link can be made with a 0R resistor.
> 
> sek@ reports 50mV offset from his build of THE WIRE.
> 
> ...


 


  there is no 50mv offset *except* referenced to ground, which goes nowhere near the headphones, as mentioned above its simply a start up behavior and only on the BAL-BAL;  common mode offset is still for all intents and purposes nil and there is no offset to ground after the VCOM pin wakes up
   
  I see no reason for WB mode in audio, just makes the layout more critical for no benefit IMO but some people seem to hear an improvement so best to leave the option I guess.
   
  no comment on shaky legs there =D
   
  havent had much time for diy lately, got catching up to do at work, keen to get the LPUHP done once thats taken care of, massively parallel LME49600 FTW!
   
_edited one word above to make sense_


----------



## sek@

Quote: 





qusp said:


> there is no 50mv offset *except *referenced to ground, which goes nowhere near the headphones, as mentioned above its simply a start up behavior and only on the BAL-BAL;  common mode offset is still for all intents and purposes nil


 
   
  I measured one channel of a BAL-BAL (featuring OPA1632 and LME49610) again using an oscilloscope and +/-12V supplies. I took readings between the non-inverting and the inverting outputs (i.e. the normal headphone output) and from both non-inverting output to ground and inverting output to ground, respectively.
   
  The (differential-mode) output offset voltage turned out to be exactly +1mV, the (common-mode) offset voltages to ground were -1.2mV and -0.8mV, respectively.
   
   


> I see no reason for WB mode in audio, just makes the layout more critical for no benefit IMO but some people seem to hear an improvement so best to leave the option I guess.


 
   
  I experienced instability with the wrong type of output load (cheap coaxial measurement lead, almost clean sinusoidal oscillation at 13 MHz).
   
  I wish THE WIRE had a resistor for tuning the BW pin current. But if the head amp I'm putting it into isn't going to be stable under all loads, I might as well just cut the copper trace and convert the amp to low BW mode. 
   
   
  Quote: 





avro_arrow said:


> I also nudged a few of the components around for better clearance and
> added the option for SMD caps in the servo. C13 to C16 were going to be Tantalums.
> The power supply is co-located and has 1500uF on the output.


 

 Just squeezing the SMD caps onto the round, drilled pads is totally cool. Having actual component pads on the top layer in a single sided job might not be, though, at least not to the board house. 
   
  Cheers,
  Sebastian.
   
  Edit: typos corrected and changes in quote incorporated.


----------



## Avro_Arrow

I dropped the SMD back to the bottom layer.
  At the board house, all the boards start out double sided...If I don't
  put anything in the top layer, they just etch it all off. It's easier
  on the home etcher if I keep it all single sided though.
   
  Here is the revision and a close up of the servo area to better
  see the clearances.


----------



## G.Trenchev

According to the datasheet,WB mode increases small-signal response and PSRR...and if you're after maximum performance...It's just 15mA anyway
	

	
	
		
		

		
			




  Make sure you put tantalum caps close to the buffers.


----------



## wakibaki

Buffers arrived today, the footprints fit fine. I'm expecting to get some 4-pole slide switches tomorrow which will simplify the fast switch-off of the relay.
   
  w


----------



## Avro_Arrow

I think I might build a "Lightspeed Attenuator" for use with this project.
   
  Anyone have an opinion on the design?
  The threads I have read either love it or think
  the idea of using LDR for audio is daft.


----------



## wakibaki

I seem to recall that LDR attenuators have quite high distortion and are problematic to calibrate in terms of gain step and channel-channel matching.
   
  I thought about building a self-calibrating one with a PIC controller with A/D, but I was put off by the distortion data, which I think was developed by Nelson Pass.
   
  Given that part of the rationale for the design is ultralow distortion, I think it'd be a curious pairing.
   
  Curious is not an overriding reason not to do it though, I'd be happy if people only thought I was weird 
	

	
	
		
		

		
		
	


	




.
   
  If you want some code to run a PGA volume control using a PIC...
   
  w


----------



## Avro_Arrow

Thats what I like about the modular approach...if it sux, I can toss it.
   
  Here is the device the design is based on:
  http://www.silonex.com/audiohm/index.html
   
  This is the exact device:
  http://www.silonex.com/datasheets/specs/images/pdf/104539.pdf
   
  I'm sure people think I am weird too...


----------



## wakibaki

Intended for audio, sorted. That's the same one _*I*_ picked from the list.
   
  It says low distortion. Ask them how much. Probably hard to quantify, application dependent.
   
  2 per channel, I guess. 
   
  Have you got a source for them? How are you going to indicate the set level?
   
   I was looking for a 1k log pot, best I could find was 5k. What max R are you aiming at? Any ideas about a mute?
   
  w


----------



## Avro_Arrow

For a "mute", just throw a fixed shunt resistor on a switch after the LDR.
   
  To indicate volume level, I have a knob that has numbers to "11".
   
  In the schematic, the author uses a dual 100K pot.
   
  Yes, you use two LDR per channel...shunt attenuator fashion.
   
  I found two sources for them with a short search
  One only has the unsorted version.
  The other has the sorted version.
   
  I might as well post the schematic so we are all on the same page...


----------



## wakibaki

Some distortion info & alternate configurations here:- http://silonex.com/audiohm/levelcontrol.html
   
  They also recommend controlling them with current sources rather than voltage. I saw somebody doing that on diyaudio.
   
  Maybe you should start a thread...
   
  w


----------



## Avro_Arrow

Lightspeed is already a product/kit and I don't want to step on any toes...
   
  There is already an extensive thread here.


----------



## wakibaki

I found a few redundant components, made the output relay run off the on/off switch. This should mean that the relay opens before the capacitance in the rest of the circuit discharges, hopefully preventing any switch-off transient.
   

   
  Output offset should be 50 microvolts worst case, under 25 typical. Parts should perform close to their datasheet specs, I've been careful with the layout of both signal and power. The servo and protection traces carry very little current and what there is is largely DC levels.
   
  I'm close to sending off for 10 boards, just waiting to check the dimensions of some 5k pots I ordered.
   
  w


----------



## sek@

I think it looks great. 
   
  Besides the (certainly great) sound, I'm particularly interested in how the volume control will influence noise and offset performance with this setup.
   
  What pots are you going to try?


----------



## wakibaki

The pots came, but in the end I decided to go with this 10k Alps one from RS http://uk.rs-online.com/web/p/trimmers-potentiometer-rheostat/2499159/
   
  I can get it in 1k in the same footprint from Farnell, but it's out of stock at the moment. 5k I haven't seen, but they're probably available, it's just a case of locating them.
   
  I hope the shaft length is OK, the datasheet was useless... I've got a diamond cut-off disk if the worst comes to the worst.
   
  I'm ordering Panasonic 0.1% resistors throughout the signal path and servo since that's what Farnell have in metal film. I have some 47u tants and I intend to piggyback 1u and 0u1 MLCCs on the bypass pads. With the 49990s and LTC1150s the whole thing will be built to the very highest standards, I've also ordered a nice red anodized case.
   
  Boards (x10) should take about 2 weeks to arrive. I stewed over them for a few days as I get a better deal on the shipping if I order 2 lots and I'm modifying my TPA6120 amp to get rid of the on'off transients. I like to have a couple of days go by without any changes occurring to me before I commit. I've got some 12 thou copper sheet and some UV sensitive polymer so I'm going to try to make a solder paste stencil.
   
  w


----------



## wakibaki

Home etched solder stencil for this build:-
   

   
  My first try at toner transfer, couldn't get the UV polymer to work. Obviously there have been a few pinholes, but the copper is thick enough that they don't go thru and thru. There are a couple of other problems, missing dividers between a couple of holes, but it's still useable for limited numbers with a bit of manual manipulation. I had to let it stew for a while to get all the holes to etch to close to their intended size, some cleared quicker than others. I can get 2 that size (on one stencil) professionally made at Itead for just over $50 shipped. As it happens I have another SMT board that size coming, when I've proved both I may get the stencil.
   
  Can't wait to see the boards...
   
  w


----------



## wakibaki

Nearly 3 weeks, boards arrived from China this morning.
   

   
  Looking good, I can probably get one built in the next 24 hours. First try of my homemade solder stencil, it matches the boards pretty good.
   
  A box for this design...
   

   
  w


----------



## wakibaki

Here's the board with the solderpaste stencilled on.
   

   
  The stencil is a bit thick at 12 thou, so the solder paste is a bit thick, but I'll get thinner copper sheet for the next.
   
  My dedicated reflow oven.
   

   
  This cost less than my comparatively cheap soldering iron. I'd recommend anyone to get one of these and use SMT where possible.
   
  The board after reflow.
   

   
  Surface tension has pulled a couple of components askew and a couple have shorted, but fortunately they were joined by tracks anyway. I removed a solder string between the LME49600 pins, the only one. All the SO chips worked out fine, the surface tension has pulled the excess solder onto the pads and pins.
   
  w


----------



## wakibaki

I tested the PSU side of the board before I put the regulators in, that all worked OK.
   
  Then I stuffed and soldered the TH components...
   

   
  Finally, after some work with the diamond needle files...
   

   
  The only problem with reflowed prototypes is that you have to place all the components in one go. Otherwise I'd have built it from the PSU up, testing as I went. All that remains is to plug it in and throw the switch. I've got a variac, a lab variable AC supply that is, but it's a lot of trouble to get it down, I guess I'll just have to bite the bullet.
   
  I may be gone some time.
   
  w


----------



## wakibaki

OK, it works, the amplifier section anyway. The relay transistor's not turning on, I had to short across it, but I'll get to the bottom of that in a minute. Otherwise it could do with a bit more gain, at least for these old Koss phones I'm using to test it. Chris Isaacs, Baja Sessions. 
   
  I'd describe the sound as 'transparent'. It's flawless as far as I can tell.
   





   
  A bit of Chet Atkins & Mark Knopfler next.
   
  w


----------



## sek@

Congratulations wakibaki! 
   
  I'm sure you'll sort out the relais driver issue. Reworking boards in the non-audio section shouldn't be an issue in the end.
   
  As for the gain, same thing with the wire. It's obviously a matter of personal preference and choice of headphones, so there likely is no such thing as a one-setting-fits-all preset, I guess. Reworking two (or four) resistors is no biggy, I suppose.
   
  Cheers,
  Sebastian.


----------



## wakibaki

Thanks Sebastian, I was beginning to feel that I was talking to myself. Odd that there's so little apparent interest, this is potentially a very high-performance design at a very reasonable cost.
   
  The first thing I've done is measure the DC offset. I say measure, both my digital meters have a lower limit of 1mV and both read zero. I can't find any unshrouded probes to plug into my old analog meter.
   
  <1mV is good, it means the servo is working OK, and the rest of the amplifier obviously sounds OK
   
  Anyway that meant I could plug in my sensitive IEMs, Shure e2c's. Now I can hear a very quiet 100Hz hum from the fullwave voltage doubler with nothing playing, it's totally inaudible even in the quietest passages with music playing, but I may be able to do something about it anyway. All I had in SMT diodes to build it with was ultra-fast Shottkys which are more likely to cause radiation than slower ones and if conventional rectifiers don't do the trick I can try strapping a cap across them.
   
  Now there's more volume than you can really use. I have a couple of jumpers in there which double the gain, they can be removed. I'm mostly driving it using a Sony NW-E003 Walkman anyway, they are notorious for their weedy output. I may wire the jumpers to a panel switch on the rear panel, there's room.
   
  OK, take it all out of the box again and try to figure out the relay transistor...
   
  w
   
  Edit: found the relay transistor fault, problem with the footprint. I had to skew the transistor to make it work, but no big deal. About 4 seconds delay before switch-on, no pops on switch-on or switch-off.


----------



## Avro_Arrow

I was beginning to feel like a judge on "American Idol", just telling everyone their stuff was looking
  good and well designed...so I took a break.
   
  Your Amp does look good and I am also surprised at the lack of interest.
  Maybe if you had started several fights and made lots of outlandish claims, there
  would be more interest (in reference to "he who is to remain nameless").
   
  That's one of the reasons I put so little effort into showing off any of my designs, that
  and the fact I have a "MOT" banner and would get banned...
   
  I'm still haven't decided on the fate of my design. I bought a PPA and that basically
  blew my budget for getting my boards made. My supplies for etching the board myself
  have been depleted...my printer ran out of toner and I don't have an transparencies left.
  I do still have some double sided photo board left.


----------



## sek@

Quote: 





wakibaki said:


> Thanks Sebastian, I was beginning to feel that I was talking to myself.


 
   
  I took your intermediate status updates as just that. 
   


> Odd that there's so little apparent interest, this is potentially a very high-performance design at a very reasonable cost.


 
   
  Yeah, you should at least give it a uniquely fancy name, hire someone who advocates your design and then spread it virally across all platforms... 
   



> <1mV is good


 
   
  <1mV is very good. 
   
  We had this little argument earlier about DC of servo-controlled vs. servo-less amps. My offset is no biggy, but below 1 mV is certainly zip even in a headphone amp.
   


> I can hear a very quiet 100Hz hum from the fullwave voltage doubler with nothing playing [...] and if conventional rectifiers don't do the trick I can try strapping a cap across them.


 
   
  Is that normal for the type of regulators you chose? I hadn't noticed any audible mains buzz with my [you know what] amp. Could be increased by the supply you use (i.e. variac). I found that in my house for toroids I need a mains DC filter in order to get a PSU buzz free... 
   


> I have a couple of jumpers in there which double the gain, they can be removed.


 
   
  Ah, okay, I hadn't paid attention, sorry. All the better.


----------



## wakibaki

I thought I might call it Wakibaki's Smokin'.
   
  The hum is a little puzzling since I have a very similar design in my TPA6120 desktop/portable, I've built 2 of those and there's no hum audible, although it has so much gain (>30dB) that you can turn it up to the point where the noise floor is audible.
   
  Maybe these chips don't have as good PSRR, or maybe it's a quirk of the layout, despite the fact that the layout is more relaxed in this one (more room). I'll fiddle around with it over the next couple of days, try some different wallwarts, see if I can pick it up on another amp, things like that. It's only running 14V rails at the moment, I can turn that up a bit as well and fiddle with the CRCRC values.
   
  Avro_Arrow, sek@, Kevin Gilmore, thanks for your support, if you want a board PM me your mailing address, I'll send you one FOC.  
   
  Test results follow...
   
  w


----------



## wakibaki

Initial RMAA test results using M-Audio Delta AP 2496
   
  Showing representative levels (soundcard straight thru):-
   

   
  1st. column shows amplifier-soundcard without additional loading, 2nd. column amplifier loaded with 16 ohms resistive, 3rd. column, soundcard-soundcard straight thru.
   

   
  The 3rd. column actually shows the soundcard alone with FR flatness 0.01dB worse than with the amplifier, but I think that just shows the limits of RMAA testing under these conditions.
   
  w


----------



## Avro_Arrow

Why thank you, that is very generous.
  PM sent.
   
  Quote:


wakibaki said:


> I thought I might call it Wakibaki's Smokin'.
> 
> The hum is a little puzzling since I have a very similar design in my TPA6120 desktop/portable, I've built 2 of those and there's no hum audible, although it has so much gain (>30dB) that you can turn it up to the point where the noise floor is audible.
> 
> ...


----------



## kevin gilmore

Thank you, PM sent.


----------



## wakibaki

2x boards sent.
   
  w


----------



## sek@

Wait, that was serious? Hmmm... 
   
  PM sent.


----------



## G.Trenchev

wakibaki,could you post results at 16/44.1 or 24/44.1 ?I want to compare my amp to yours.


----------



## wakibaki

Quote: 





g.trenchev said:


> wakibaki,could you post results at 16/44.1 or 24/44.1 ?I want to compare my amp to yours.


 
   
  No problem, just give me some time to get to it.
   
  In the meantime, 1kHz square wave loaded with 100nF:-
   

   
  w


----------



## wakibaki

Sebastian, board in the post. I'll post a detailed final schematic when I figure out how, the editor says 'You don't have permission to create attachments'. Maybe I'll just put a .zip file on my website for download.
   
  w


----------



## sek@

Great, and greatly appreciated!


----------



## wakibaki

OK, if you right-click on the image and choose 'save image as...', you should be able to save an image with sufficient detail to read.
   

   
  Here's the layout:-
   

   
  Q3 has to be mounted upside down and rotated to make B,C & E line up correctly, or perhaps I can find an alternate txistor with the right footprint. Anyway, I'm sure you guys can make it work, I did.
   
  w


----------



## wakibaki

OK, I found the cure for the hum.
   
  It was necessary to attack the ground planes with a Dremel with cutting disk.
   
  Here's what had to be done, top:-
   

   
  You can see the 2 cuts near the input jack, J3.
   
  And the bottom:-
   

   
  3 cuts here, with the long vertical one interrupted by tracks. The top 2 parts of the cut are probably unnecessary, but I really wanted to force the current up into the area above the regulators.
   
  And that's got rid of the hum. Whoopee! It's a nice amplifier. Shame about the cuts, but that's what prototypes are for... I should have worked out this issue earlier, but as usual I was impatient to get some results.
   
  w


----------



## Avro_Arrow

At least we are brave enough to try even if we make a few mistakes along the way.
  Good job finding the source of the hum.


----------



## kevin gilmore

my precision mill should have no trouble with that once i get the board.


----------



## sek@

Precision mill, my ‰¶¿
  I usually just cut a V-groove with a blade. 
   
  So it was a loop of return currents, presumably between top and bottom layer (via plated through holes)?


----------



## wakibaki

I think it's most useful to think of it in terms of how you would ground it if it were point-to-point wiring.
   
  You might bring the quiet grounds such as the feedback to the input or output jack (which would be joined) and return the decoupling caps up to the regulators which would form a star for the PSU.
   
  As it was, current from the caps down the LHS of the board could migrate diagonally from close to the input jack up to the regulators, polluting the signal areas in the process. Cutting the 2 ground planes forces the PSU return current to flow in a clockwise direction into the area above the regulators and separates the noisy PSU area from the signal area.
   
  I think.
   
  I've always found grounding a difficult issue, particularly in PCBs with a ground plane, but although I've generally taken care I think I've been lucky with the results, which led me to be a bit casual in this case.
   
  Anyway, whatever the reasons, the cuts make a huge difference, I can't tell by listening whether the amp is turned on or off regardless of volume setting.
   
  Please let me know when the boards arrive, they're all marked 'no intrinsic value' so there should be no holdup at customs.
   


> Originally Posted by *G.Trenchev* /img/forum/go_quote.gif
> 
> wakibaki,could you post results at 16/44.1 or 24/44.1 ?I want to compare my amp to yours.


 
   
  Here are the results @ 24/44k1 showing the soundcard alone and soundcard + amp. Edited to include results for 16 ohm load.
   

   
  w


----------



## shadow419

Quote: 





wakibaki said:


> Thanks Sebastian, I was beginning to feel that I was talking to myself. Odd that there's so little apparent interest, this is potentially a very high-performance design at a very reasonable cost.
> 
> .....snip


 
   
  I'm sure they're a lot more people lurking, like myself.


----------



## Avro_Arrow

If there are people lurking that are interested in the project, they should make themselves known, like you did.
  The more people that show interest, the more effort a designer is likely to put into a project.
   
  Quote: 





shadow419 said:


> I'm sure they're a lot more people lurking, like myself.


----------



## wakibaki

Quote: 





shadow419 said:


> I'm sure they're a lot more people lurking, like myself.


 
   
  Good. Come out, come out, wherever you are...
   
  I just took delivery of a new USB oscilloscope, so I've been playing with that...
   
  Output RMS voltage 1kHz test tone (Hantec DSO2150 USB scope):-
   
  Unloaded 317mV 
   
  Loaded 309mV
   
  Load 16 ohms
   
  Output impedance (calculated) 0.41 ohms
   
  w
   
  I see a newer version of RMAA (6.2.4) has been available since 02 April 2012. Good, there may be some improvements coming.


----------



## J-Pak

I'm interested as well 
   
  Does it accept balanced input only?


----------



## Avro_Arrow

The amp is single ended, not balanced
   
  Quote: 





j-pak said:


> I'm interested as well
> 
> Does it accept balanced input only?


----------



## J-Pak

Quote: 





avro_arrow said:


> The amp is single ended, not balanced


 
   
  Even better, my source is S/E


----------



## shadow419

Quote: 





avro_arrow said:


> If there are people lurking that are interested in the project, they should make themselves known, like you did.
> The more people that show interest, the more effort a designer is likely to put into a project.


 
  Probably should have chimed in sooner, but my knowledge of circuits is very limited.  Didn't want to post just for the sake of posting without any contribution to the discussion.  I do enjoy tackling diy designs though if  for nothing else than the satisfaction of completing something.


----------



## Nebby

Count me as interested


----------



## 2000impreza

I'm very interested as well. Missed out on "the wire" awhile back.


----------



## sek@

Just as a quick update: board arrived fine. Thanks very much.


----------



## tyre

I'm also interested, but quietly lurking as the discussion is largely over my head.


----------



## Avro_Arrow

@ wakibaki
   
  You officially have a following...


----------



## Avro_Arrow

Got my board today...Thank you!


----------



## wakibaki

Quote: 





sek@ said:


> Just as a quick update: board arrived fine. Thanks very much.


 
  Quote: 





avro_arrow said:


> Got my board today...Thank you!


 
   
  Good. Let me know any problems such as identifying parts. You have some options, you don't have to build it exactly as the schematic. For example, if a regular (say OPA277) precision opamp were used for the DC servo instead of the chopper-stabilized one then the RC filter at the servo output could go.
   
   


avro_arrow said:


> @ wakibaki
> 
> You officially have a following...


 
   
  Let's hope those turn out to be participants rather than followers. 
	

	
	
		
		

		
			




   
  w


----------



## sek@

Hi,
   
  Quote: 





wakibaki said:


> You have some options, you don't have to build it exactly as the schematic. For example, if a regular (say OPA277) precision opamp were used for the DC servo instead of the chopper-stabilized one then the RC filter at the servo output could go.


 
   
  Have you tried this option? Do you recommend it over the chopper amp?


----------



## wakibaki

Quote: 





sek@ said:


> Hi,
> 
> Have you tried this option? Do you recommend it over the chopper amp?


 
   
  No and no. The chopper amp will give the lowest output offset. I'm about to test a build (TPA6120 amp) that uses a dual OPA2277, however. It should come close to the chopper. The chopper-stabilized opamps are the single most expensive component apart from the case.
   
  I just mentioned it to point out that you still have some freedoms as to exactly how you build it, how much gain, how much servo authority, how much input offset will cause the relay to drop out, what rail voltage you pick, values for the CRCRC PSU filter, wallwart voltage. The last 3 will affect how much the regulators will dissipate, you might want to bolt them to the case and connect them with wandering leads to 3-way headers on the PCB.
   
  Here's a BOM. Prices are approximate in UKP.
   

   
  This is the DC power jack http://www.ebay.co.uk/itm/DC-Power-PCB-Socket-Connector-2-5mm-Pack-5-/290637034182?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item43ab5132c6#ht_1799wt_906
   
  These are the 3.5 stereo jacks http://www.ebay.co.uk/itm/4-PCB-3-5mm-mount-Stereo-Jack-Socket-Connector-3-5-mm-/220488902014?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item335629a57e#ht_1930wt_955
   
  I think that's most of the awkward components (from a fiting POV) dealt with.
   
  If you're having trouble getting parts to fit, I may be able to help. 
   
  w


----------



## J-Pak

Quote: 





wakibaki said:


> Good. Let me know any problems such as identifying parts. You have some options, you don't have to build it exactly as the schematic. For example, if a regular (say OPA277) precision opamp were used for the DC servo instead of the chopper-stabilized one then the RC filter at the servo output could go.
> 
> 
> 
> ...


 
   
  Hoping to be a participant, will there be another run of boards? Cheers
   
  Maybe this goes against the philosophies of the design, but I'll mention it if you are open to suggestions- IMO the option to use whatever pot/attenuator the user wanted would be a nice feature (given the high transparency of the amp). And perhaps the headphone connector off the board, I personally prefer 4 pin XLR over TRS even for single ended amps.


----------



## wakibaki

I have loaned my working amp to a friend to get his listening impressions.
   
  There were 10 boards. I built 1, I gave away 3.
   
  Nebby has asked me for a board, I told him he can have one as long as he really intends to build it. Not that I have any reason to think that he won't, but obviously, at this stage, the more amps working out there in the wild, the better from the point of view of getting some feedback.
   
  I want to keep 3 boards.
   
  I will give away another 2 to the first responders _*to this thread*_ who will undertake to build them in the next couple of weeks.
   
  In about 3 weeks (hopefully) I will respin the board on the basis of what has been learned, (about the ground plane, if nothing else). Realistically, most people will take a couple of weeks to get the components.
   
  I can't continue to give away boards _ad infinitum_, at some point I will be obliged to set up a facility to take payment. If people like the amp I may supply a semi-kit or even complete amplifiers.
   
  @ J-Pak
   
  The intent with this board is to produce an integrated amp in a small, off-the-shelf case and which will run off a wallwart. There's no reason, however, why the board can't be installed in a larger case and flying leads used to permit the connection of any attenuator or connector the builder wishes.
   
  Both Avro_Arrow and I have shown some designs for PCBs using the LME49600 in a simpler stand-alone amp earlier in the thread. Perhaps you would find one of them more suitable, but you'll have to push to get something done, there's a lot of demand on our attention ATM.
   
  w


----------



## J-Pak

Quote: 





wakibaki said:


> I have loaned my working amp to a friend to get his listening impressions.
> 
> There were 10 boards. I built 1, I gave away 3.
> 
> ...


 
   
  I'm in a GB at Diyaudio for some 49600, so I will take a board. Thank you for the kind offer, pm coming.


----------



## shadow419

If I didn't have so many unfinished projects on my plate, I'd take your offer.  With 3 O2 builds for friends and 2 "The Wire" builds all in different states of completion, I have too much to finish one of your boards in the near future.  When/if you do a re-spin, I'd buy a pcb or perhaps a kit from you for sure though.


----------



## wakibaki

OK, the 3 spare boards are going to Nebby, J-Pak and tcpoint. I'll post them tomorrow (Monday).
   
  Remember that the boards have 2 known problems, detailed earlier in the thread. They need the relay transistor mounted upside-down and rotated to make B, C & E line up correctly and they need the ground planes cut top and bottom as shown in the pictures. Cutting the planes shouldn't be a problem, but remember that the intent is to prevent current flow, so the cuts need to separate the sections completely.
   
  I still haven't checked that the relay drops out when the input offset gets too great.
   
  w
   
  Edit: Post offices are closed here for the Diamond Jubilee until Wed. 6 June. Sorry.


----------



## J-Pak

Thanks! Working on a Mouser BOM now, will share once it's done.


----------



## J-Pak

Looking through Mouser and some of the SMD thin films are obscenely priced, ie Panasonic 1206 package resistors for $1.73 each or the 10 ohm 2512 package- Vishay 2.5w at $6.57 each 
	

	
	
		
		

		
		
	


	



   
  I haven't checked Digikey yet, but I might as well go with Farnell if they will ship to the US.


----------



## ujamerstand

Newark is farnell's US branch. Get a raspberry pi while you're at it.


----------



## Avro_Arrow

I thought they were all sold out of Raspberry Pi...
   
  Quote: 





ujamerstand said:


> Newark is farnell's US branch. Get a raspberry pi while you're at it.


----------



## J-Pak

Quote: 





avro_arrow said:


> I thought they were all sold out of Raspberry Pi...


 
   
  Yeah none in stock at the moment. Newark is also out of stock on several resistors that Farnell still has.
   
  If the other 2 people that are getting boards want me to buy most of the parts and distribute at cost I am happy to do this. As Newark has minimum purchases of 5 or 10 on SMD resistors.


----------



## tcpoint

J-Pak,
   
  I probably have 35% of the parts, already.  I'll have to check.  Some of the resistors can be thick film (esp. in power supply section).  I'll see what parts I have in the next day or two and I'd be happy to go in on an order.


----------



## wakibaki

Quote: 





j-pak said:


> Looking through Mouser and some of the SMD thin films are obscenely priced, ie Panasonic 1206 package resistors for $1.73 each or the 10 ohm 2512 package- Vishay 2.5w at $6.57 each
> 
> 
> 
> ...


 
   
  OK, there's an error there (mine), the 10R resistors are 1210. I have amended the BOM to show this.
   
  The 10R resistors aren't in the signal path, they're in the PSU CRCRC filters, thick film are probably adequate there. I used a Panasonic part from Farnell, described as an anti-pulse or anti-surge resistor, rated 0.5W 5%. There are pads on the board for 2 in parallel so you could use 2 * 22R, The actual value is dependent on the wallwart you use (how much voltage you can afford to drop) and the dissipation (how hot they get). I've got 1 * 10R in each position, the amp should draw ~50mA quiescent which is 0.5V dropped across each, you can feel that they're above ambient but they're not hot.
   
  R17, 18, 23 - 28, the voltage setting resistors round the regulators again don't need to be anything special, 1% is nice but I just selected some matching values from 5% thick film. I have a selection kit, 50 each of numerous values, only a few dollars from ebay. The values shown, 150R and 1k5, result in +/- 13.75V rails. You could parallel a 10u 1206 cap with the voltage-setting resistor to reduce ripple by piggy-backing it, but I found the hum inaudible once I cut the board. Similarly, the voltage dividers for the protection circuit and the switch-on delay can be thick film. This just leaves the gain-setting resistors in the amplifier proper and the integrator. I used Panasonic ERA series metal film 0.1% for this first build. These are rated low current noise, excellent non-linearity and are 27 pence here, not too bad.
   
  All the ICs in the amp are rated +/-18V, so you can run it this hot if you feel the need and you have a suitable wallwart.  I have some 16V ones. These provide ~22V after the diode drop. Lose 2V in the CRCRC filter, the LM317/337s have a dropout voltage of 2V, that would allow 100mA before the supply would begin to sag, a good compromise keeping the dissipation in the regulators from getting too high. You can see that you could get 200mA by reducing the 10Rs to 5R at the cost of some supply ripple, or go for an 18V wallwart at the cost of having the regulators run a bit hotter. You might want to bolt them to the enclosure and bring wandering leads to the board in that case.
   
  w


----------



## ujamerstand

Quote: 





avro_arrow said:


> I thought they were all sold out of Raspberry Pi...


 
   
  OT, but they are on a rolling production schedule. I'm set to receive mine near the 28th.


----------



## wakibaki

3 boards sent this morning.
   
  w
   
  They told me to place my order for a Raspberry Pi about a week ago, and that I'd get it in about 5 weeks.


----------



## tcpoint

Thanks for the board.  Look forward to the build.


----------



## J-Pak

Quote: 





tcpoint said:


> J-Pak,
> 
> I probably have 35% of the parts, already.  I'll have to check.  Some of the resistors can be thick film (esp. in power supply section).  I'll see what parts I have in the next day or two and I'd be happy to go in on an order.


 
   
  Would you mind sending me a PM when you've figured it out?
   
  Quote: 





wakibaki said:


> OK, there's an error there (mine), the 10R resistors are 1210. I have amended the BOM to show this.
> 
> The 10R resistors aren't in the signal path, they're in the PSU CRCRC filters, thick film are probably adequate there. I used a Panasonic part from Farnell, described as an anti-pulse or anti-surge resistor, rated 0.5W 5%. There are pads on the board for 2 in parallel so you could use 2 * 22R, The actual value is dependent on the wallwart you use (how much voltage you can afford to drop) and the dissipation (how hot they get). I've got 1 * 10R in each position, the amp should draw ~50mA quiescent which is 0.5V dropped across each, you can feel that they're above ambient but they're not hot.
> 
> ...


 
   
  Very helpful, thank you.


----------



## qusp

Quote: 





wakibaki said:


> 3 boards sent this morning.
> 
> w
> 
> They told me to place my order for a Raspberry Pi about a week ago, and that I'd get it in about 5 weeks.


 
  pity they took i2s off the header =(


----------



## wakibaki

Here's the resspin, so far. I've improved the legend legibility, modified the ground planes, added decoupling, ripple rejection caps on the regs. None of it's critical apart from the ground plane split, I'm just taking advantage of the available real estate.
   

   
  w


----------



## J-Pak

I received my board today, thank you wakibaki. Now to put in an order for the rest of the parts, and awaiting the 49600's from the DiyA group buy.


----------



## wakibaki

Good. Now you have something to cross-check the footprints of the switch and sockets. Let me know if you have any problems identifying which parts will fit.
   
  w


----------



## Nebby

Just wanted to say my board showed up a few days ago. Finally an excuse to pull out my dremel


----------



## J-Pak

Are R2 and R6 the gain resistors?
   
  And what gain is the build spec'd for per the BOM?
   
  I'm sure I'll have more dumb questions as they come to me


----------



## tcpoint

Board arrived today.  Getting ready to pull the dremel out.


----------



## wakibaki

OK, looking at the top channel in the diagram;
   
  R14 is the feedback resistor from the output to the input, it works in conjunction with R13 from the inverting input to ground to set the gain. Despite the fact that there are 2 chip amps in the loop, this can be simply treated as an inverting opamp circuit with a gain of (1+(R14/R13), which works out to a gain of 2.
   
  Looking at the circuit again, the 2 resistors R6 and R4 in series from the integrator output control the servo gain. The R4 and R6 are both 4k7 for a series resistance of about 10k. Since the feedback resistor R14 is 1k the servo authority is R14/(R4 + R6) or approximately 10%. That means that to correct an offset of 1V the servo opamp must put out 10V. This is about as much output swing as the servo opamp can manage with +/- 13V rails, which is about what you get with the resistance values shown (R17, 18 etc.) round the regulators. (13.75V actually)
   
  The output of the servo opamp is a virtual ground for AC purposes, so R6+R4 are effectively in parallel with R13. This makes the gain (1+(R14/(R13 || (R6+R4)) or 1 + (1k/(1k || 10k).
   
  1k || 10k = 1/(1k+1/10k) = 1/(11k/10k) = 10k/11k = 0.909k
   
  ...so the gain of the circuit becomes 1+ (1k/0.909k) = 2.1
   
  In addition to this R12 can be paralleled with R13 by the addition of a jumper. This makes the gain 1 + (R14/(R13 || (R6+R4 || R12) = ~ 4.2. The gain, with the values shown, is approximately 2 without the jumper installed and approximately 4 with the jumper in place.
   
  Obviously these values are not set in stone. Few modern sources exceed 2V output so the gain can be increased without the risk of clipping and there is room to increase the rail voltages too. If it was desired to approximately double the gain (to x4 and x8), I would increase R14 to 2k2, R4 & 6 to 10K and omit one of C28/C30. Again these measures would need to be taken with both channels.
   
  Increasing R18 etc. to 1k8 will increase the rail voltage to 16.25V. This may be useful depending on the voltage of your (AC) wallwart, as the less voltage dropped across the regulators (down to 2V, which is the dropout voltage) the cooler they will run. A12V (AC) wallwart is good for 13.75V rails, a 15V wallwart for 16.25V rails, but check the voltage at the regulator input is 2V greater than the required output. The values in the CRCRC PSU filter (R33, 34, 35, 36) can be tweaked to trim these values.
   
  w
   
  Edit: Previously said 'R4 & 6 to 1K' corrected to 'R4 & 6 to 10K'


----------



## J-Pak

Thank you for the explanation. I'm no designer and prefer not to tweak a prototype so I'll stick with a gain of 2.


----------



## Rodeodave

Lovely project, and just what I have been looking for since I have got some LME49990 and LME49600 samples from TI recently. Would not have done much differently myself.
  I guess it's too late for a prototype board and too soon for a re-spun board? Ah, well, at least I'm not like a year late as with The Wire amp...


----------



## Avro_Arrow

I'm doing up a Digi-Key based BOM for this build.
   
  Is R20 supposed to be 47K or 1m?
  It's 47k in the schematic and 1m in the BOM.
   
  Also R25, R17, R27, R23, R26, R18, R28 and R24 are missing from the BOM.
   
  Is this the correct relay?
  http://www.digikey.ca/product-detail/en/IM03GR/PB1061CT-ND/1209989
   
  Cheers!


----------



## Avro_Arrow

Does Q3 have to be pre-biased or can I just use a MMBT3904?


----------



## wakibaki

Hi, Rodeodave.
   
  I'm hoping someone who has had a prototype board will get back to me regarding any problems getting any of the fit-critical components such as jack sockets, switch or pot, before I do a respin. I have a limited number of these parts on hand, I don't want anybody left hung out to dry if they're having trouble getting them, and I want to keep my options open with regard to using different parts before I order the new boards.
   
  I have another board I'm working on, I get a discount if I order both together, the turnaround is about 2 weeks, so it will be 3 weeks at least before I have respun boards. I'm sorry, I hope you'll be able to hang on 'till I get it done.
   
  Avro_Arrow
   
  I wrote the above before I saw your last post.
   
  R20 is 47k. I had 1M in the original schematic, it didn't work out.
   
  R17, R23, R25, R27 are 150R
   
  R18, R24, R26, R28 are 1k5 These values give 13.75V rails. See the last paragraph of #147 for some comments on this.
   
  That's the correct relay.
   
  Yes, you can use MMBT3904, pre-biassed is not required. I tried to eliminate the anomalies in the BOM which arose as the result of drawing in haste due to my mind running ahead of my fingers, but evidently not with complete success.
   
  w


----------



## sek@

While I don't have time to research for convenient parts sources myself right now, I would really appreciate a DigiKey BOM.
   
  Thanks,
  Sebastian.


----------



## Avro_Arrow

this is the audio jack I usually use, I'm pretty sure it will fit.
  http://www.digikey.ca/product-search/en?lang=en&site=ca&KeyWords=sj1-3543&x=0&y=0
   
  I have the BOM about 95% complete.
  Digi-key does not have the alps pot...you will have to Mouser that.


----------



## Avro_Arrow

The power jack:
  http://www.digikey.ca/product-detail/en/PJ-058BH/CP-058BH-ND/1644606


----------



## Avro_Arrow

L3 and L4 were a little tougher, not much information to x-reference from.
   
  How about this?
  http://www.digikey.ca/product-detail/en/28F0121-1SR-10/240-2436-1-ND/806796
   
  Edit:
  Changed my mind...this one:
  http://www.digikey.ca/product-detail/en/HI3312X101R-10/240-2498-1-ND/810915


----------



## wakibaki

The dimensions are good, but the one I have is 78 ohms @ 1-25MHz, which was the lossiest at the lowest frequency I could find. They're there to assist in preventing spikes from the rectifier getting back onto the wire from the wallwart.
   
  They _could_ be superfluous, and they _could_ be doing very little good. I just noticed that the O2 had some hum problems which were cured by a grounding wire. I suggested to the designer that this might indicate that there was some radiation from these wires, especially since he commented that the amp was audibly interfering with some other equipment when brought near, but he dismissed the suggestion, and anyway he had his fix as far as the amp itself was concerned. I haven't noticed any problems, so maybe they're working.
   
  I'll see if I can find some lower frequency ones on Digikey.
   
  w
   
  I think the power and 3.5 jacks are OK.


----------



## Avro_Arrow

Preliminary Digi-Key BOM
  There could be errors or omissions.
   
   

 Digi-Key Part Number Manufacturer Manufacturer Part Number Customer Reference Quantity 1 Description P10.0AACT-ND PANASONIC ELECTRONIC COMPONENTS (VA) ERJ-14NF10R0U   8 RES 10.0 OHM 1/2W 1% 1210 SMD P150FCT-ND PANASONIC ELECTRONIC COMPONENTS (VA) ERJ-8ENF1500V   4 RES 150 OHM 1/4W 1% 1206 SMD P1.00KFCT-ND PANASONIC ELECTRONIC COMPONENTS (VA) ERJ-8ENF1001V   13 RES 1.00K OHM 1/4W 1% 1206 SMD P1.50KFCT-ND PANASONIC ELECTRONIC COMPONENTS (VA) ERJ-8ENF1501V   4 RES 1.50K OHM 1/4W 1% 1206 SMD P4.70KFCT-ND PANASONIC ELECTRONIC COMPONENTS (VA) ERJ-8ENF4701V   4 RES 4.70K OHM 1/4W 1% 1206 SMD P47.0KFCT-ND PANASONIC ELECTRONIC COMPONENTS (VA) ERJ-8ENF4702V   1 RES 47.0K OHM 1/4W 1% 1206 SMD P100KFCT-ND PANASONIC ELECTRONIC COMPONENTS (VA) ERJ-8ENF1003V   4 RES 100K OHM 1/4W 1% 1206 SMD P120KFCT-ND PANASONIC ELECTRONIC COMPONENTS (VA) ERJ-8ENF1203V   2 RES 120K OHM 1/4W 1% 1206 SMD P390KFCT-ND PANASONIC ELECTRONIC COMPONENTS (VA) ERJ-8ENF3903V   3 RES 390K OHM 1/4W 1% 1206 SMD P470FCT-ND PANASONIC ELECTRONIC COMPONENTS (VA) ERJ-8ENF4700V OPT. 2 RES 470 OHM 1/4W 1% 1206 SMD 399-1167-1-ND KEMET (VA) C0805C104K4RACTU   14 CAP CER 0.1UF 16V 10% X7R 0805 399-3525-1-ND KEMET (VA) C1206C106K4RACTU   8 CAP CER 10UF 16V 10% X7R 1206 399-1140-1-ND KEMET (VA) C0805C331K5RACTU   2 CAP CER 330PF 50V 10% X7R 0805 718-1525-1-ND VISHAY SPRAGUE (VA) TR3D476K025C0150   4 CAP TANT 47UF 25V 10% 2917 P12388-ND PANASONIC ELECTRONIC COMPONENTS EEU-FM1E471   7 CAP ALUM 470UF 25V 20% RADIAL LM7805CT-ND FAIRCHILD SEMICONDUCTOR LM7805CT   1 IC REG 1A POS 5V TO-220 296-13869-5-ND TEXAS INSTRUMENTS LM317KCS   2 IC VOLT REG POS ADJ 3TERM TO-220 296-21577-5-ND TEXAS INSTRUMENTS LM337KCSE3   2 IC VOLT REG NEG 1.5A ADJ TO-220 LME49600TS-ND NATIONAL SEMICONDUCTOR LME49600TS/NOPB   2 IC AMP BUFFER AUD HI FI TO-263-5 296-17157-1-ND TEXAS INSTRUMENTS (VA) LM2901QDRQ1   1 IC QUAD DIFF COMPARATOR 14-SOIC LME49990MA-5-ND NATIONAL SEMICONDUCTOR LME49990MA/NOPB   2 IC OP AUDIO LOW NOISE 8SOIC LTC1150CS8#PBF-ND LINEAR TECHNOLOGY LTC1150CS8#PBF   2 IC OPAMP CHOP-STBL W/CAPS 8SOIC ES1A-TPCT-ND MICRO COMMERCIAL CO (VA) ES1A-TP   3 DIODE FAST REC 1A 50V DO214AC CP-058BH-ND CUI INC PJ-058BH   1 CON PWR JCK 2.5 X 5.7MM HIGH CUR CP1-3543-ND CUI INC SJ1-3543   2 CONN JACK STEREO R/A 3PIN 3.5MM CP1-3543N-ND CUI INC SJ1-3543N ALTERNATE 2 CONN JACK STEREO R/A 3PIN 3.5MM CP3-1001-ND CUI INC PP3-002B MATE FOR POWER JACK 1 CONN 2.5MM FEMALE PLUG 5.5MM OUT PB1061CT-ND TE CONNECTIVITY POTTER & BRUMFIELD (VA) IM03GR   1 RELAY TELECOM DPDT 2A 5V 450-1594-ND TE CONNECTIVITY ALCOSWITCH MSS4200R04   1 SWITCH SLIDE 2POS 0.276"BLCK T/H


----------



## Avro_Arrow

Missing:
   
  Q3
   
  L3/4
   
  J1/2
   
  Case and Knob are builders discretion.
   
  Volume pot on Mouser:
  http://ca.mouser.com/ProductDetail/ALPS/RK09712200MY/?qs=fQ8kcZGzY5WCdIeX8tuPzA%3d%3d


----------



## J-Pak

Thanks for all the work A_A. If anyone has a link to a suitable case with PCB rails that would be nice since there are no mounting holes on the proto board. I have been unsuccessful looking through Mouser.


----------



## wakibaki

Brilliant!
   
  Q3 is any general purpose SOT23 NPN transistor
   
  L3/4 can be shorted, put a ferrite on the wire from the wallwart, close to the amp.
   
  J1/2 are 2-pin headers (for jumpers), or run wires to a DPST switch.
   
  Volume pot looks good with 15mm shaft, I had to take a diamond cutting disk to mine.
   
  US supplier for box:-http://www.alliedelec.com/search/searchresults.aspx?dsNav=Ntkrimary|B2%2f-080SI|3|,Ny:True,Ro:0&dsDimensionSearch=D:B2%2f-080SI,Dxm:All,Dxp:3&SearchType=0
   
  UK (Farnell):- http://uk.farnell.com/multicomp/mcrebs80/case-aluminium-80x108-5x30mm/dp/1549967
   
  w


----------



## Avro_Arrow

I have everything I need now to start building...except for the toaster oven.
   
  Actually, I have a toaster oven, but we are still using it. My wife wants to buy
  a new one but does not have it picked out yet.


----------



## tcpoint

I'm placing my order tomorrow.  Not sure if I'm going to go the oven route or the solder iron.  I have an old oven that I use for hardening paint.


----------



## Misterrogers

wakibaki - I'd love a couple of boards if/when there are any available. Cheers!


----------



## MrSlim

Quote: 





wakibaki said:


> US supplier for box:-http://www.alliedelec.com/search/searchresults.aspx?dsNav=Ntkrimary|B2%2f-080SI|3|,Ny:True,Ro:0&dsDimensionSearch=D:B2%2f-080SI,Dxm:All,Dxp:3&SearchType=0
> 
> UK (Farnell):- http://uk.farnell.com/multicomp/mcrebs80/case-aluminium-80x108-5x30mm/dp/1549967
> 
> w


 
   
  Newark is the NA arm of Farnell: http://www.newark.com/jsp/search/browse.jsp;jsessionid=BIZMBQR34T2S4CXDUZ1G13Q?N=422&Ntk=gensearch&Ntt=b2-080&Ntx=mode+matchallpartial&exposeLevel2Refinement=true&suggestions=false&ref=globalsearch&_requestid=117051
  and has better shipping policies(at least to my neck of the woods.. )
  It's the same box as the O2 used.


----------



## wakibaki

Quote: 





misterrogers said:


> wakibaki - I'd love a couple of boards if/when there are any available. Cheers!


 
   
  Won't be for less than 3 weeks. I'll PM you when they're available.
   
  w


----------



## J-Pak

I have my parts from Digikey on the way. I was thinking about just soldering with an iron, but given the proximity of the pads would it be easier to just use a toaster oven? I have one I haven't used in years.


----------



## wakibaki

I prefer to use the toaster oven, but I had the advantage of being able to make a solder stencil. Solder paste and reflowing is still possible if you place the solder paste by hand, you can run a bead down all the pads of a chip, but it's more prone to problems, sometimes the surface tension skews the chip instead of squaring it up on the pads. I usually watch through the door and get in there and straighten it up before the solder has a chance to set on the odd occasion that it happens. Doing all the solder by hand with a syringe and needle takes some of the advantage out of doing it by reflow, it adds to the overall work and stress.
   
  I kept the res and caps fairly well spaced because the webs between the pads on the stencil get etched away if they're too thin. 'Fairly well spaced' is only in a manner of speaking, but they are hand-solderable in my experience.
   
  Perhaps a compromise might be a good introduction to reflowing. Some of the parts are thru-hole and require hand-soldering anyway. You could do the res and caps by reflow and place the chips by hand.
   
  It's great when reflow works out, all the components settle down and pull themselves into alignment on the pads because of surface tension. There's very little flux residue and the whole thing looks nice and professional.
   
  Whatever route you choose, good luck!
   
  w


----------



## wakibaki

It occurred to me to mention that you need to dog-leg the pins on the regulators to make them sit down so that everything fits in the case. Be careful not to make them short on the gain setting resistors.
   
  w


----------



## Avro_Arrow

I found a mistake in the BOM.
  In the third line it says you need thirteen 1k resistors...you only need four.
  Quote: 





avro_arrow said:


> Preliminary Digi-Key BOM
> There could be errors or omissions.
> 
> 
> ...


----------



## Avro_Arrow

Opps...just found another one.
   
  R46 is listed as 1k in the BOM and 100R on the schematic...


----------



## Avro_Arrow

SMD all soldered up except fpr R46...I'll have to scrounge one up for there.
   

   
  Now on to the through holes!


----------



## sek@

Looking very good. 
   
  Did you end up using a stencil and an oven?


----------



## Avro_Arrow

Thanks!
   
  I ended up hand soldering everything. I did use solder paste though...
  I put just a little dab of paste on the pads, used the tweezers to hold
  the component and just touched the iron to it. It seemed to be easier
  to control the amount of solder applied that way.
   
  Here is the fully populated board:
   

   
  I had a pot with a switch laying around that needed a new home.
  I just cut off the switch pins and it actually fit!
   
  I used OPA211 for the servo amp. I have not cut the board yet...I want to
  see if mine buzzes too.


----------



## J-Pak

Nice work A_A!


----------



## wakibaki

R46. The value is not critical, it should be equal to the DCR of the coil to get the fastest drop-out on power off. Coil power is 50mW, supply is 5V so current is 10mA and best value is 500R. You could parallel 2 * 1k on the same pads. Sorry about the inaccuracy, I had it somewhere in my head that the current was 5mA.
   
  w
   
  Looking good there A_A!


----------



## Avro_Arrow

Well, it works...sort of.
   
  1. The first problem is the voltage from the 337 regulators.
  The 317s come out pretty close to the 13.75 volts.
  One of the 337s comes out at 14 volts and the other at 13.8 volts.
  2. Next (and maybe related to the first) is it won't come out of protection (I just defeated it for the time being).
  3. The servo takes a long time to settle, maybe 15 to 20 seconds for a sudden shift in input offset.
  One channel seems to settle around -0.5 mV and the other around 2 mV (I used OPA211 for the servo).
   
  All that being said, it sounds fine (other than the buzz, but that was expected).
   
  I will be interested to see other peoples results.


----------



## Avro_Arrow

It also gets pretty hot...uncomfortable to touch.
  I'm using a 16 VAC @ 2.5 A transformer which gets me +- 20 volts at the input to regulators.
   
  The regulators, LME49990, OPA211 and LME49600 all get hot.
  The LME49990 in my DAC gets only slightly warm (it only has +- 6 volts rails though).
   
  After about half an hour, offset sits at -0.4 mV in one channel and 0.4 mV in the other.


----------



## Avro_Arrow

I got the buzz to go away...just cut the board like Wakibaki did.
  I didn't cut mine as much as Wakibaki.


----------



## Misterrogers

Subjectively - how does it sound?


----------



## Avro_Arrow

Well, with the caveat that it might not be working 100%, it sound as good or just a little bit better than my PPAv1 (OPA627, Glassman Buffers).
  I could not hear any side effects if it is indeed oscillating (I can't think of any other reason for it to get so hot...).
  No, I don't have an oscilloscope (I sold my Philips PM3260E years ago).
   
  Quote: 





misterrogers said:


> Subjectively - how does it sound?


----------



## Misterrogers

Thanks mate. Looking forward to building one.


----------



## wakibaki

Mine runs pretty hot too, so I'm concerned that it might be oscillating. I couldn't see anything with the 60MHz scope, and the RMAA results seemed reasonable, and it sounded OK and it didn't destroy itself so I turned my attention to the other board I had to build . Now I'm thinking break the loop either by lifting a pin or cutting a track and see if it runs cooler. With hindsight it would have been a good idea to have had a small res between the output of the 49990 and the 49600.
   
  I've been a bit busy with my PC, it developed a software fault in the browser (Chrome) and I'm also messing with the Raspberry Pi which arrived a few days ago. I'll try this pin lifting experiment soon, but maybe not till tomorrow.
   
  As far as the protection circuit is concerned, you didn't forget that the transistor footprint has a problem?
   
  I'll try to get a simulation up to check the response time of the servo and see if can be tweaked a bit. It's always going to be a compromise between response time and overall frequency response.
   
  w


----------



## Avro_Arrow

I think my voltage divider for the protection circuit is the problem.
  The voltage on one half was not what I expected...I have not had
  time to troubleshoot it further.
  I took the 10uF caps that formed the filter for the chopper op amp
  back out but the settling time did not improve.
   
  I mounted Q3 upside down and sideways...you can see it in the pictures.


----------



## wakibaki

I had another look with the scope, I couldn't see anything. 
   
  Looking at the voltage drop across the 10R in the PSU filters, its about 850 mV, which means ~85mA (+/- *5*%). The calculated current with the BW pin connected to Vee (as it is in my case) is 80mA, not taking the worst case values from the datasheet. I've got an NEC relay in there that draws 10mA more than the Axicom, I had about 20 that I bought to build a switched attenuator. 
   
  The hottest things are the 49990s, but they're not finger-pull-away hot which probably means they're no hotter than 50 degrees C, and they take a while to get there, there's not a lot of cooling available to them.
   
  It would be nice to get a 500M scope on there, I wonder if the university would give me 5 minutes.
   
  w
   
  Edit:- corrected slip of the pen from 10% to 5%


----------



## Avro_Arrow

I'm glad mine behaves the same as yours...at least I built it right 
	

	
	
		
		

		
			




  I have my LME49600 in wide band mode too.
   
  I'll try and get a total current draw measurement when I can, might not
  be until Thursday or Friday


----------



## wakibaki

After a long walk with the dogs in the rain, always a good time for thinking, I came back with a better analysis of the current requirement. The circuit is arranged so that the power requirement is symmetrical. Considering just the positive rail we get:-
   
   

 Device Amps Number Total LME49600 0.015 2 0.03 LME49990 0.009 2 0.018 LT1150 0.0008 2 0.0016 LM2901 0.0008 1 0.0008 LM317 resistors 0.008333333 2 0.016667 Relay 0.02 1 0.02       0.087067
   
  This corresponds very closely with what I am seeing.
   
  w
   
  Using the Axicom relay you should expect 10mA less.


----------



## Avro_Arrow

I'm getting 97 mA with the amp cold.
  Both rails the same.


----------



## wakibaki

That certainly seems a bit high. With the Axicom relay and OPA211 you'd expect ~82mA (87 - 10 + 5.6).
   
  I'm fairly convinced mine isn't oscillating. I'll hit it with a square wave and capacitive loading and see how it likes that. It'll keep me from messing with this raspberry pi, which is driving me mad. 
   
  You could try lifting pin 2 on one of the 49600s. If the current drops that's a pretty good indication that it's oscillating.
   
  w


----------



## wakibaki

Scope traces 1kHz square wave, amp adjusted for a gain of 1:-
   
  With scope load only (1M)...
   

   
  With 16R...
   

   
  With 16R || 1u...
   

   
  With 1u
   

   
  w


----------



## Avro_Arrow

I added some extra bypassing right across the pins of the LMEs.
  Didn't make any difference to the heat or current draw.
  Still sounds great, though!


----------



## wakibaki

It's both puzzling and troubling that the amp is getting so hot though.
   
  Another simple experiment you could try is to reduce the buffer BW by removing the 0R resistor.
   
  Hopefully somebody else will complete one soon...
   
  w


----------



## Avro_Arrow

Just for reference, "The Wire" runs at +- 15 volts and the LME49600 in wide band.
   
  I wonder how hot they run?


----------



## Avro_Arrow

Here is some of my simulating:
   

   
  For an input offset of 100 mV...
  Point A = 2.1 volts
  Point B = 100 mV
  For a signal of 1 Vpp at 1kHz...
  VF1 = 1.4849 Vrms
  Gain = 2.1
   
  Frequency response of the servo:
  Swept from .001 Hz to 1 Hz...
   

   
  Does all that jive with your design math?


----------



## jdkJake

avro_arrow said:


> Just for reference, "The Wire" runs at +- 15 volts and the LME49600 in wide band.
> 
> I wonder how hot they run?




Cool as a cucumber. 

Seriously, they run extremely cool. Cool enough that I never felt the need to measure it. The PS regulators are the warmest parts and even they are relatively cool.


----------



## wakibaki

Quote: 





avro_arrow said:


> Just for reference, "The Wire" runs at +- 15 volts and the LME49600 in wide band.
> 
> I wonder how hot they run?


 
   
  That's kind of irrelevant. I was suggesting that changing the BW might have an effect greater than would be expected on the basis of the current in the BW pin, which would be a clue that it was oscillating.
   
  You have to consider then in what configuration it might run OK, say with a LME49710 or, dare I say it, an NE5532. There's nothing to suggest that the OPA211 is any more likely to be the cause. I'll check the math on the servo. 
   
  I've got 4 more 49600s arrived. I guess I'll have to build another one before I respin the board.
   
  w


----------



## qusp

you know it doesnt have to be all or nothing right? you can connect to the rail via a resistor to calculate a bandwidth somewhere between the low iQ mode and wide BW mode


----------



## wakibaki

Quote: 





qusp said:


> you know it doesnt have to be all or nothing right? you can connect to the rail via a resistor to calculate a bandwidth somewhere between the low iQ mode and wide BW mode


 
   
  For the moment I've just suggested this as an experiment which _*might*_ affect the amplifier's propensity to oscillate, which hasn't been demonstrated yet. Avro_Arrow's build seems to be running hot, but we don't know why. Mine isn't running hot and doesn't look unstable within the limits of tests I have been able to apply, my scope only runs to 60MHz.The current drawn by mine corresponds closely to expectations, Avro_Arrow's is ~ +15mA. A more reliable indication of oscillation would be a drop in current draw when the loop is broken by lifting a pin, e.g the input pins on one or both of the 49600s, but obviously this will only have an effect if the amp _*is*_ oscillating.
   
  w


----------



## Avro_Arrow

I removed the jumpers and put the LME49600 into low Iq mode.
  As expected, they run cooler, however, the LME49990s still run hot.
   
  Current draw on each rail (relay energized) is 85 mA


----------



## Avro_Arrow

I ordered an infrared thermometer so maybe in a few days I will be able to give you an exact temperature...
   
  Just for laughs I put my meter on AC and tested across the output with no signal.
  It read zero. Even if it is oscillating, I doubt the frequency response of my meter
  would pick it up anyway.


----------



## J-Pak

I put a little work into the amp yesterday, hoping to get more done next weekend.


----------



## tcpoint

I got all the smd parts on.  Just need another evening to finish it up.


----------



## Nebby

Missed a few parts on my first order, but I'm slowly but surely getting there on my board.


----------



## J-Pak

Quote: 





tcpoint said:


> I got all the smd parts on.  Just need another evening to finish it up.


 
   
  Quote: 





nebby said:


> Missed a few parts on my first order, but I'm slowly but surely getting there on my board.


 
   
  What are you guys using for a power supply?


----------



## wakibaki

Quote: 





j-pak said:


> What are you guys using for a power supply?


 
   
  I have a selection of AC wallwarts, 12, 15 and 16V. The 12V is just sufficient to get 13.75V after the regulators because it is good for 1A and probably runs a bit above the 16.95V peak that it should produce when loaded. (In my case) there is 1.7V lost in the 2 10R resistors in the CRCRC filter in the onboard PSU, and the regulators require 2V to maintain regulation. For a loaded 12V wallwart this would be 16.95 - 1.7 - 2 = 13.25V, half a volt below what the regulators are set to, but as I said, a lightly-loaded wallwart probably runs at least 0.5V hot.
   
  The 16V wallwart still works, but the regulators run pretty hot. One of the things I thought of doing is dismounting the regs from the board and mounting them to the case and bringing wandering leads to the board. Insulating kits (washers) would be required and 3-pin headers on the board with mating plugs would simplify assembly. That said, I've run the amp on 16VAC, boxed up, for several hours. The box gets warm, but it didn't kill the amp.
   
  w


----------



## Avro_Arrow

I have some of these laying around in various voltages. They are pretty common...their normal use is to power security systems and fire alarms.
   
  Quote: 





j-pak said:


> What are you guys using for a power supply?


----------



## Avro_Arrow

OK, here are some numbers.
   
  Ambient temperature 28c.
   
  After about ten minutes at idle (volume control zero),
  LME49990: 51c
  LME49600: 54c
  LM3x reg  : 55c
   
  OPA211   : 45c (probably being heated by the LME next to it.)


----------



## wakibaki

OK, ~25 over ambient is not unreasonable, obviously lower would be better but it doesn't indicate that the devices are likely to fail. The 49990 is rated for continuous short circuit and the 49600 has thermal limiting. The layout is compact and some compromise in terms of running temperature is inevitable.
   
  The principal concern remains that the amplifier as a whole is oscillating, so anything you can do to clarify the situation one way or another would be welcome.
   
  w


----------



## Avro_Arrow

I tried disabling the servo by removing R4, 5 and 6.
  No change.
  Offset without the servo was 0.5 mV.
   
  I tried bypassing the LME49600 by lifting pin 2 and 4.
  Jumper from Pin 2 to R14 to maintain the feedback loop.
  No change.
   
  I would have to conclude by this point that it just gets hot.
  Live with it.
   
  It will be interesting to hear other peoples results.


----------



## wakibaki

Thanks for making the effort. It seems the amp is working correctly, if running a bit hot.
   
  I'm certainly not unhappy with the SQ of mine.
   
  Hopefully someone else will complete one soon.
   
  w


----------



## Avro_Arrow

I've always been very happy with the sound quality.
  It was just the heat that was worrying me.
   
  It's just something to take into account for the next version.
   
  Maybe I'll keep me eye open for a little stick on heat sink for the LME49990s.
   
   
  Quote: 





wakibaki said:


> Thanks for making the effort. It seems the amp is working correctly, if running a bit hot.
> 
> I'm certainly not unhappy with the SQ of mine.
> 
> ...


----------



## Misterrogers

Hey Guys - just wanted to chime in and say I'm up for and excited for a build when it's 'soup'. I think you'll find quite a few chiming in once it's ready. I for one don't mind riding the edge, so toss me a board when you're ready.


----------



## fzman

Quote: 





avro_arrow said:


> I've always been very happy with the sound quality.
> It was just the heat that was worrying me.
> 
> It's just something to take into account for the next version.
> ...


 
  they make small, finned heatsinks for dip-16 chips, which are easily cut with a Dremel tool.  scuff the chip, and use 2-part epoxy to glue them down.  PITA, but works well, and looks kinda cool


----------



## J-Pak

I've been swamped at work, I'm still hoping to complete mine soon.


----------



## tcpoint

I got mine running and did a quick test.  It runs cool and has low dc offset.  I really didn't get a chance to give it much of a listen.  I am going on vacation for a week.  Will give a report on the sound when I get back.


----------



## wakibaki

Great! Looking forward to a more complete assessment.
   
  w


----------



## J-Pak

I finished mine up over the weekend. It works and nothing blew up! I have the hum problem as well, but will rectify that the next time I visit my parents house to get my Dremel. Mine is uncased at the moment in an AC room the 49990 does get warm, but can pass the touching with finger test no problem.


----------



## wakibaki

Thanks J-Pak
   
  I will order some boards in the next couple of days.
   
  w


----------



## Misterrogers

Quote: 





wakibaki said:


> Thanks J-Pak
> 
> I will order some boards in the next couple of days.
> 
> w


 
  That's very good to hear!


----------



## qusp

i'm using little copper bga sinks stuck on with sekisui thermal tape, commonly used by PC modders. the 49990 get hot no matter what they are used in, its normal it seems. for me its the LPUHP that puts out ~16WPC and the 49990 are still the hottest thing on the board. in this case there are 3 in close proximity for an instrumentation input and the ground plane is eroded away around them to lessen capacitive coupling, so they are even hotter; especially given they are running at +/-18vdc


----------



## J-Pak

If there are ever any modular boards made like the one A_A posted I would buy some (so I can build on balanced). Even with the hum I can tell this is a very nice sounding amp. I will post a more critical evaluation when I can get rid of it.


----------



## Avro_Arrow

I've been quite happy with the sound quality of mine.
  I can deal with the heat now that I know thats just how
  it is and that it doesn't indicate a fault.
   
  If there is enough interest in the module I posted earlier, I could
  have some done...but I'm not in a financial position to make them
  just for the fun of it.
   
  What I really need is a partner. I can design, they can handle
  the distribution and finance. All I'd ask is a couple of boards
  for each design.


----------



## wakibaki

Thanks qusp for the input. The 49990s in this case have the groundplane removed in their immediate area for precisely the same reason, to reduce stray capacitance. It's a difficult balance to strike, making a compact board while at the same time having adequate cooling with a layout that doesn't create inadvertent feedback or crosstalk.
   
  I ordered some Sekisui tape, and I've found some v. small heatsinks on Farnell.
   
  I've just been checking out my TPA6120 design respin as I get a reduction for a bigger order, everything is looking good so far so I'll probably place the board order tomorrow.
   
  w


----------



## wakibaki

Avro_Arrow,
   
  I've just been looking at your circuit for the modular boards you were working on, the servo opamp has the inverting and non-inverting inputs swapped, if I'm not mistaken. The amp output should connect to the non-inverting input as the servo feedback goes to the amplifier proper's inverting input.
   
  w


----------



## qusp

no worries wakibaki, I thought it best to chime in, as it worried me quite a bit initially too. I tried all sorts of wiring layouts to make sure I wasnt causing oscillation from RFI pickup with no success I conferred with opc and he confirmed its normal to get hot. lowering the heat in all honesty is probably more about us making ourselves feel better, going by the datasheet i'm pretty sure the chips are still well within SOA even when toasty to the human touch, the opa1632 is pretty hardcore like this too.
   
  its still worthwhile checking that though if you havent already, are the inputs left floating when they are getting hot? ie. is this just during initial testing or in normal use with signal connected or inputs grounded? I did find that with the LPUHP one amp channel had a fair bit of offset (~3v) at the output (21db gain) with 30mV at the input. this completely disappeared when I grounded the inputs or applied signal and it seemed to be cooler to the touch too subjectively. the other channel was fine with only a few mV at the output whether floating or not.
   
  both function perfectly with grounded inputs or signal, only when left floating (particularly because the LPUHP is completely floating including the PSU). I have some of those unfortunate high efficiency lightbulbs in my office, great for the electricity bill, **** for DC coupled floating wideband amplifier inputs...  it may well be that I just did the first channel test in the daytime, as ive struck this before in here.
   
  I really need to get a scope …


----------



## Avro_Arrow

Your right!
  Thanks for catching that...
   
  Quote: 





wakibaki said:


> Avro_Arrow,
> 
> I've just been looking at your circuit for the modular boards you were working on, the servo opamp has the inverting and non-inverting inputs swapped, if I'm not mistaken. The amp output should connect to the non-inverting input as the servo feedback goes to the amplifier proper's inverting input.
> 
> w


----------



## wakibaki

Ordered 10 boards.
   
  I went for minimal changes, just put the slots in the groundplane, fixed the relay transistor footprint and moved the silkscreen around a bit to improve legibility, some of the legends were missing bits because they were on top of vias.
   
   

   
  w


----------



## Misterrogers

I'll take one of those when they land


----------



## J-Pak

I got the hum issue fixed. The amp sounds great. Rather than go into a description (since I am not sure I could describe it), I'll just say I doubt I would be able to tell it apart from a Gilmore Lite (Dynalo) if I was blind folded. It has more than enough gain with the prototype DAC I'm using with my HE-500.
   
  Still using it uncased, and not finding the heat is much of an issue. I will probably case it up in a single box with an ODAC or something similar and have myself a pretty nice laptop amp/DAC one box solution.


----------



## wakibaki

Excellent, J-Pak. Thanks. Praise indeed.
   
  I'm expecting the new boards in a bit over a week. Naturally I'll want to build one to verify them before letting any go, but this is unlikely to take more than a day.
   
  w.


----------



## wakibaki

Boards came back yesterday, unfortunately I spent most of the day in hospital.
   
  About 3 weeks ago I had radioactive iodine treatment to kill off my overactive thyroid, but it seems that despite that thyroxine has built up in my body to the point where it is causing an irregularity in my heartbeat (atrial fibrillation).
   
  I'll try and get one built later today.
   
  w


----------



## Misterrogers

Sorry to hear about your health issues Wakibaki; take care of yourself first.


----------



## J-Pak

Sorry to hear that wakibaki, hope you get sorted out and feeling better. Hopefully you won't need anything long term for the A. Fib


----------



## wakibaki

Thanks for the good wishes, guys, a couple of days of beta-blockers and I'm feeling mostly recovered. It's just a been bit exhausting because it's hard to sleep when your heart keeps saying 'listen to me, listen to me'
   
  w


----------



## fzman

i am recuperating from a quadruple bypass, and remember having a-fib pretty regularly when i was in hospital, post-op.  not much fun..  hope you feel better


----------



## wakibaki

Thanks fzman. Good to hear you are recuperating. I'm back to what passes for normal for me.
   
  I have built up one of the new boards, it works fine, hum has been eliminated. I'll post some measurements soon.
   
  I wrote a query via the feedback form inquiring about selling boards, but I haven't had a reply. Does anybody know how this works?
   
  Boards will be £5 flat rate (5 Pounds UK) including worldwide P & P (and Paypal charges). This is ~$7.85 US at current exchange rates. If you send me a PM I will send you a paypal invoice, but please wait until I confirm that I am have fulfilled any forum requirements. There are 10 available in this batch.
   
  w


----------



## fzman

Thanks for the kind thoughts.  Your design looks interesting, and i have a bunch of those National Semi parts already, as I had tried to build some regulated power supplies with them, and could not get them to settle down - it was about 2 years ago, and i would probably do it a bit differently now, but i have not had the need to. That said, it looks like a bit too much surface mount for me right now, as i would solder each part individually, not by reflow....
   
  oncve again, i hope you feel better and things come back to an acceptable level of normal...


----------



## Misterrogers

Hey All - finished my board to day and fired it up, no output. I'm using the latest batch. A couple of questions; 
   
   
  1) Does Q3 still need to be flipped? I didn't. I did switch the polarity of C27 as recommended.
  2) How do I bypass the protection?
   
  Any other troubleshooting suggestions would be greatly appreciated. I'm a noob. Also, I assume J3 is input - yes?


----------



## wakibaki

Yes J3 is input. Q3 no longer requires to be flipped. The simplest way to defeat the protection is to short across the collector and emitter of Q3. Holding the board so that the legends read correctly from left to right, the topmost pin of Q3 is the emitter and the pin on the left is the collector. You should hear the relay click (quietly) if it hasn't flipped already.
   
  w


----------



## Misterrogers

Thanks mate. What would cause the need to defeat the protection?


----------



## Misterrogers

So I bypass protection, and have music! Letting the servo stabilize, I have 0mV for offset on both signal lines. (much) more listening to follow but woohoo! Thanks wakibaki for making this possible. It would be nice to 'fix' the protection. Any thoughts?


----------



## wakibaki

Did you orient the cap on the protection circuit as per the written note included in the package?
   
  w


----------



## Currawong

Quote: 





wakibaki said:


> Thanks fzman. Good to hear you are recuperating. I'm back to what passes for normal for me.
> 
> I have built up one of the new boards, it works fine, hum has been eliminated. I'll post some measurements soon.
> 
> ...


 
   
  No problem doing these at cost.  I just built a Japanese amp using the LME49600 and the results have been good.
   
  I suggest starting up a Google Docs spreadsheet. It also has a form building option so people can add themselves. That might make it easier for you to keep track. I used this system for a group buy. I'd suggest though that you pass on your contact details and make it possible for someone to contact us if, for example, you have to go into hospital suddenly or have some other kind of emergency. That's happened in the past and caused all sorts of drama.


----------



## J-Pak

Congrats misterrogers, let us know how it sounds
   
  Quote: 





currawong said:


> No problem doing these at cost.  I just built a Japanese amp using the LME49600 and the results have been good.
> 
> I suggest starting up a Google Docs spreadsheet. It also has a form building option so people can add themselves. That might make it easier for you to keep track. I used this system for a group buy. I'd suggest though that you pass on your contact details and make it possible for someone to contact us if, for example, you have to go into hospital suddenly or have some other kind of emergency. That's happened in the past and caused all sorts of drama.


 
   
  What Japanese LME49600 amp is this? Cheers


----------



## Misterrogers

yep, thanks w - I did (reversed polarity on C27). J-Pak - sounds very good! Just now giving it a good listen.I definitely think you should do a run of this amp. The tight smd may be a bit tough for some, but it isn't too hard or expensive, and performs well.


----------



## agdr

Quote: 





wakibaki said:


> Boards will be £5 flat rate (5 Pounds UK) including worldwide P & P (and Paypal charges). This is ~$7.85 US at current exchange rates. If you send me a PM I will send you a paypal invoice, but please wait until I confirm that I am have fulfilled any forum requirements. There are 10 available in this batch.


 
   
  I'm in - PM sent for the invoice. 
	

	
	
		
		

		
			





  I just discovered this thread.  I've built a couple of the OPC "Wire" amps, will be fun to compare.  Looks like you developed a good solution to that broken DC servo design on the National design.  I hand built that design a couple of years ago and discovered the defective servo (fairly high output offset).  I tried LTSpicing it at the time and the sim results pretty much matched up with the real world results - broken servo design.


----------



## Avro_Arrow

Geez...I should have noticed that...I had my C27 in backwards too.
  Now my protection works too.
   
  That's why we prototype...


----------



## Misterrogers

I want to figure out why mine isn't energizing the relay. It'd be nice to remove the jump. I've got to take a closer look at Q3 - I may have it flipped.


----------



## wakibaki

Quote: 





avro_arrow said:


> Geez...I should have noticed that...I had my C27 in backwards too.
> Now my protection works too.
> 
> That's why we prototype...


 
   
  Sorry about that Avro-Arrow, I should have made it more widely known. Oddly my first build worked OK with the cap reversed??? This is not a serious issue, you just have to be aware that the polarity indication on that particular cap is reversed, even on the most recent board. I will fix this in future if the demand for boards exceeds those currently available.
   
  w


----------



## wakibaki

Quote: 





misterrogers said:


> I want to figure out why mine isn't energizing the relay. It'd be nice to remove the jump. I've got to take a closer look at Q3 - I may have it flipped.


 
   
  Of course. I'm sure we can get to the bottom of it. Presuming that Q3 is correctly placed, have a look for any solder strings on the associated comparator U10. It's unlikely that you've got Q3 incorrectly placed, it requires quite an effort to get it that way on the earlier boards. The junction of R20 and R41 should be sufficiently above (>=0.6V) the negative rail (the tab on U11) to turn on the transistor. Be aware that there are 2 negative rails, one for each LME49600. The one you are interested in is the one closest to U10.
   
  w


----------



## Misterrogers

Listing to 'Smokin' while working today; I really think you've got something here. I don't know if it's 'brain train' or burn in, but at about 3 - 4 hours I noticed an increase in sound stage and dynamics. Very nice little amp. Also of note; it's absolutely silent. Even when cranked all the way up, not a peep. The stock build/BOM has plenty of gain for my HE-5 too.


----------



## wakibaki

agdr asked me for a more detailed picture of the schematic, I'm going to post a .jpg made from a 600dpi bitmap here, I won't know how successful it's been until I see it on the forum, however
   
   

   
  Unfortunately it doesn't seem to work, I will send a copy to anyone who needs it if they PM me an email address...
   
  w


----------



## Currawong

I suggest getting a Dropbox account (very handy for backing up important files) as it can create a URL for any file inside for sharing. Then you can post the url for the file here.


----------



## wakibaki

Thanks Currawong, I'll do something about that this afternoon.
   
  w


----------



## wakibaki

Here's the dropbox link to the high-res .jpg of the schematic.
   
  https://dl.dropbox.com/u/107775480/d56233_10x10_1.jpg
   
  w


----------



## agdr

Quote: 





avro_arrow said:


> this is the audio jack I usually use, I'm pretty sure it will fit.
> http://www.digikey.ca/product-search/en?lang=en&site=ca&KeyWords=sj1-3543&x=0&y=0


 
   
  The 5 pin version might be a little better for the input jack, with the internal switch for disconnect grounding:
   
http://www.digikey.com/scripts/DkSearch/dksus.dll?WT.z_header=search_go&lang=en&keywords=%09CP1-3545-ND&x=0&y=0&cur=USD
   
  I'm using your Digikey BOM - extremely helpful! - and I noticed that was a 3 pin jack.  Having that input jack grounded when nothing is plugged in has worked out really well on the O2, using the internal switch.  I was going to suggest going with a 5 pin if there was another board re-spin, but on the bigger schematic I see it is already set up for a 5 pin input jack with grounding.  Best I can see on the layout it looks like there are 5 holes.  Needs to be confirmed, though. 
	

	
	
		
		

		
			





  On the schematic pin 1 of the input jack is a signal line and pin 2 is the grounded sleeve, but both the 3543 and 3545 jacks have those pins flipped, so it might be necessary to do trace cut and swaps on pins 1 and 2 to use these jacks.
   
  wakibaki: thanks for the higher res print!  Works great.


----------



## wakibaki

This looks to be the right socket if you can't identify one from a regular supplier:
   
http://www.ebay.com/itm/5pcs-1-8-3-5mm-Stereo-Socket-Headphones-Jack-PCB-Panel-Mount-Connectors-2411-/320960009847?_trksid=p2045573.m2042&_trkparms=aid%3D111000%26algo%3DREC.CURRENT%26ao%3D1%26asc%3D27%26meid%3D2258704038311425622%26pid%3D100033%26prg%3D1011%26rk%3D2%26sd%3D221023868653%26
   
  Obviously I can't be certain without having one in my hand, but there are numerous other 5-pin 3.5mm sockets on ebay, all with a chrome ring round the hole, unfortunately they protrude too far from the front of the board to permit it to fit in the enclosure, so be warned. I may change the footprint at the next iteration, but that doesn't help here and now.
   
  Failing any other source, I do have a limited number of the correct sockets in hand, but I think you will find it cheaper to buy 5 from that ebay source than pay the postage from here, I get away with sending the boards letter mail (£2) because they are flat, but package postage is twice that.
   
  w


----------



## Avro_Arrow

On the board as it is now, signal is on the outside pair and the rest connect to the ground plane.
   
  Thanks for pointing that jack out...I will see about incorporating it into my future projects.
  I'm glad you found my BOM helpful.
   
  Quote:


agdr said:


> The 5 pin version might be a little better for the input jack, with the internal switch for disconnect grounding:
> 
> http://www.digikey.com/scripts/DkSearch/dksus.dll?WT.z_header=search_go&lang=en&keywords=%09CP1-3545-ND&x=0&y=0&cur=USD
> 
> ...


----------



## agdr

That eBay source for the jack is probably is the best way to go!  I'll get some on order.
  
  I'm exploring building mine up as near unity gain (just a current buffer), with +/-6Vdc rails.  I worked up LT Spice sims of the power supply and amp/servo section this afternoon at +/-6vdc all all seems to work very well.  Especially the servo!  I tried adding a +50mV offset to the input signal, then again with a +150mV and -150mV DC offsets.  The output of the sim corrected well, to within +/-0.5mV or so.  Will be very interesting to see how the actual circuit matches up.  There is no sim model for the LME49600 and LME49990, so I'm using the models for the BUF634 and LME49720 instead.  I have a LT1007 in for the servo amp. 
   
  I'll try posting the circuit here, but will probably have that same issue of being too small to read.  To get (near) unity gain I'm using the values in the Wire amp, 10K to ground with the 1K feedback resistor.
   
  I've wanted to try a reduced voltage version with both the O2 and Wire but never did.   All the chips involved here are specified at 5Vdc, according to the data sheets.  The performance will likely be slightly less on the smaller rail voltages, according to the sheets, but so will the dissipatiion.  I might be able to get away with LM317L and LM337L (the TO-92 version) regulators.  I'm doing the sims with a 12Vac source (transformer) with 5R series resistance.  My big reason for the unity gain version is my particular headphones don't need a lot of voltage gain.  The AKG K550s are full volume at just 80mV rms (114dB/V sensitivity) and the Shure SRH940 at about 1.8V rms.  My sources put out 2V rms.
   
  In the plots, green is input and blue output.  The first is 200mV input at 1.2kHz into a 32R load with 0Vdc input offset.  The second is the same thing with +150mV input DC offset.  The third is an ac plot from 5Hz to 500kHz.  The 100K resistor to ground before the input of the LME49600 is a sim model issue.  I found out from sims of the Wire amp the LME49600 model will not converge without some small DC path to ground.  The sim model predicts clipping at around +/-3.9Vdc with a 44R load, with the +/-6Vdc rails.  Will be quite interesting to see if real measurements are close, or if the sim turns out to be off by a mile!


----------



## agdr

One more interesting sim plot that seems to show the importance of the relay in addition to the servo.  This is the step response, with the 150mV input DC offset occurring at 1.0 seconds.  The servo settles after about 34 seconds.  The relay would kick in until the servo gets the DC back under control.
   

   
  ould kick in until the servo gets the DC back under control.


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## wakibaki

Ah, I need to run some sims or tests on the servo/relay circuit myself. When does the relay actually drop out in response to a step input? I don't think it does until the DC offset at the servo output gets quite large (quite a few seconds), since the take-off point feeding the comparators is at the servo output, at the midpoint of the filter intended to keep switching spikes from the chopper-stabilized servo opamp out of the audio.
   
  This is not the only function of the relay, it prevents thumps at switch-on and switch-off. Obviously you can speed the whole thing up, but only at the expense of increasing the impact on the FR of the amplifier as a whole. Switching off the output comparatively late is better than never switching it off. This way is better than a (another) simple low-pass filter at the output. Output offset protection is always problematic, there's always a compromise between response time and false triggering due to low frequency program content, I always intended to return to the issue and that of the servo authority at some point, but I've allowed myself to be distracted.
   
  w


----------



## agdr

I haven't tried simulating the relay circuit yet.  I'll give that a shot this weekend.  Looks like a really good implementation.
   
  Since all the chips involved seem to go down to +/-2.5V rails from their data sheet graphs (LME49720/10, LME49990, LME49600, LTC1150, LT1007, OPA277) it should actually be possible to use the amp directly off two lithium batteries at +/-3.5Vdc -> +/- 4.2Vdc!  Going with the "transportable" model rather than "portable", and given the quiescent draw, those would probably have to be (larger sized)18650 or 26650s in an external battery holder.  I have both here I use in high power LED flashlights.  The 26650's are about the size of "C" cells at 3900mAhr:

http://www.opticsplanet.com/4sevens-rechargeable-lithium-ion-battery-fs-26650.html?utm_source=google&utm_medium=cpc&utm_campaign=plusbox-beta&gclid=CKDtlfHd0LICFXCmPAodeyUAeg
   
  Assuming a 200mA max draw from the amp, that would be 3900 / 200 = 19.5 hours! 
	

	
	
		
		

		
		
	


	




  Then assuming "music power" draw vs. sine waves would extend that at least 2x, and probably 3x, the amp might make it 2 full 24hr days on one set of 26650s.  The 18650's go up to around 3100mAhr these days

http://www.lighthound.com/AW-18650-Protected-3100-mAh-Rechargeable-Lithium-Battery-_p_3954.html
   
  so would go nearly as long at a smaller size.  With batteries the regulators may not be needed, saving some more voltage overhead, or maybe a pair of 3.3V LDO regulators could be used.  I wouldn't be brave enough to design a charger for lithiums, but they could be charged externally on a commercial wall charger if the battery pack was separate and plugged into an amp somehow.


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## wakibaki

I have this:- http://www.head-fi.org/t/586042/what-i-did-next-with-texas-headamp-chip-a-digital-control-desktop-portable PGA3120/TPA6120 design running from 6 * 16340/CR123A lithiums, but I didn't incorporate the constant current/constant voltage charger in the 49600 amp. I use protected cells in combination with the charger, it requires accurate setting of 2 trimpots, but thus far I've had no problems.
   
  I decided to go with the charging scheme after reading a few datasheets and some articles about lithium battery charging and standby use. The batteries are continuously in circuit even when the wall supply is plugged in.
   
  You'd have to drill an extra hole in the backplate and bring out a wandering lead if you wanted to plug an external battery pack into the 49600 amp, but you'd have to think hard about how you wired it in since there's currently separate regulation for each channel
   
  w


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## agdr

Hey that is an interesting amp!  Those 16340s are small enough to get a few in like that, to get the voltage levels up.


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## Rodeodave

Okay, I'm joining you guys in building this amp. Naturally I will have a couple of questions, so let's see...
   
  Why was the LTC1150CS8 chosen? I see it's a zero drift chopper amplifier with low offset and +/-16V supply capability. Are there any other requirements for an opamp in this position? Any alternatives, like would the ADA4638 do as well?
   
  That'd be it for now, but there will be more to come.


----------



## Avro_Arrow

Quote: 





rodeodave said:


> Okay, I'm joining you guys in building this amp. Naturally I will have a couple of questions, so let's see...
> 
> Why was the LTC1150CS8 chosen? I see it's a zero drift chopper amplifier with low offset and +/-16V supply capability. Are there any other requirements for an opamp in this position? Any alternatives, like would the ADA4638 do as well?
> 
> That'd be it for now, but there will be more to come.


 

 I used an OPA211 in mine and it worked fine.
  It settles  to 0.1mV after about three minutes.


----------



## wakibaki

The DC servo integrator offset is critical in determining the overall amplifier output offset, so the offset voltage of the opamp used is critical. Since the integrator uses quite high values of resistance to keep the size of capacitors used manageable, the input offset current has quite a big effect on the output offset and it's necessary to calculate its effect when choosing an opamp. 
   
  It's a close call in performance terms. The two chips are very close in terms of offset voltage and current, and the ADA4638 is cheaper by a good margin (less than half the price). I'd probably use it in future over the Linear part, particularly as it doesn't require additional filtration at the output, although the filter in the amplifier as drawn serves a dual purpose, and should be left as-is if the ADA4638 is used. Other than that, they're so close that in testing either chip might win, depending on the individual examples.
   
  I has a pretty good search round for low offset parts at the time I drew up the design, and ran a spreadsheet to calculate overall offsets, but I didn't see the ADA4638 at the time. Low offset and +/- 16V with a good output swing are the principal requirements, so if you find anything exceeding the performance of the ADA4638 then please let me know.
   
  w
   
  Of course the actual output offset may in practice be perfectly satisfactory with a range of different parts, and in fact it has been argued that the LME49600 amplifiers with no DC servo are entirely adequate. The amount of offset tolerable is open to question, but nobody would really dispute that a low offset is preferable in principle.


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## wakibaki

I believe the Mouser BOM which Avro_Arrow posted in the thread is accurate, but I prefer 565-1577-ND for the electrolytics as they are 15mm tall. This will ensure that they clear the extrusions in the recommended case. I will move the footprint slightly in a future iteration so that taller caps will also fit.
   
  This is now a well-proven design with some desirable features.
   
  1) It fits an attractive off-the-shelf enclosure and is completely self contained (PSU is on the board).
   
  2) It runs off a readily available wallwart.
   
  3) It has a switch-on and -off relay to protect headphones from any transients (thumps or clicks) and any excessive (beyond correction) input DC offset.
   
  4) It is fully DC coupled throughout with a DC servo and vanishingly small output offset (<0.1mV).
   
  5) It employs active parts enjoying the highest reputation for quality and can be built with the very best passives in the signal path.
   
  No claims are made for the performance of the amplifier beyond the RMAA and oscilloscope results shown, but if they are to be believed (and I think there is little reason to seriously question them) the noise, distortion and crosstalk are all well below the threshold of audibility, the FR is flat, the output impedance is extremely low (<1R) and the output voltage swing is sufficient to drive all but the most demanding of 'phones, and there is even the possibility of building for higher power rails.
   
  There are 3 boards remaining, although I will probably order more when they are gone. Price (non-profit) including worldwide shipping is £6.50 (UKP).
   
  w


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## chalkandtalk

Hi Wakibaki
   
  Congrats - great work on this. You have PM.
   
  chalk


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## chalkandtalk

Thanks for the board Fred. I'm looking forward to the build. I don't have any SMD parts in my parts bin, so I'm checking out Farnell. But before I order, does anyone have spare SMD components that would like to sell?  
   
  Chalk


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## Goobley

I built this amp about a month back and have been very impressed by its performance. I haven't however been able to listen to it as much as I'd like due to my main cans being away for repair. I've got them back now though and turned my amp on again today after a couple of weeks. While it's as good as I remember it (extremely good and perfect complement to my high distortion starving student variant), I still have a slight problem with the build and would like some advice.
  My issue is with the time that the build takes to turn the relay on. It took 5.5 minutes with the standard configuration (I set my power supply up for +\-15V rails). My tests however show that offset is below 1mV within 10s of the amp turning on. the simulations I ran on the servo part of the amp indicated that the relay should have clicked in well under a minute (probably closer to 30s), but as this wasn't the case I decided to reduce C27 to 100uF today. Doing his has reduced turn on time to 4 minutes but I still feel that it's longer that necessary. So my question is, how long do your amps take to turn on and what can I do to speed mine up as I can easily measure that after 10-20s there's not going to be an offset issue. I was thinking of changing the resistor divider that biases C27, but would like advice before I do it as what's happening in reality doesn't quite add up to the sims and I didn't design the offset comparator (not quite sure what/if/whether the opamps are still configured as AND and if so what effect this has).
   
  Thanks again for such a good design, it always amazes me the sound that comes out of such a small package.
   
  Cheers.
   
  (hope you're feeling at least a bit better by the time you read this waki).


----------



## wakibaki

OK, mine doesn't take that long to turn on.
   
  Did you see the note I included in the package about the polarity + on C27 is on the wrong pin? This is the first thing to check. Most turn-on problems have been associated with this or the transistor that drives the relay.  If configured correctly the delay should be a few seconds, so there is no necessity to change any values. Electrolytic caps have quite a wide tolerance so there may be a little variation in delay times.I fixed the transistor a couple of iterations back, I'll fix the cap the next time I order boards, which will be soon.
   
  I'm glad that you feel the amp's performance is good. Obviously the more people who report good results, the more popular the design will be. Which any designer would hope for, it's the ultimate verification that you did your job right. 
   
  The performance is extracted at the cost of running some of the components quite hot. Engineering the cooling was the biggest challenge in the design next to grounding and these were in competition for PCB. Some components moreover require the removal of planes (copper) around them to reduce stray capacitance, the chips have such high bandwidth that these measures are required to reduce the chances of oscillation.
   
  Unlike the TPA6120A however, no recommendation is made in the datasheets regarding special measures to prevent oscillation with the LME49600, such as resistors or inductors at the output, and no provision is made for fitting them.
   
  This amplifier should compete with the O2 in every parameter except cost. It is capable of greater output voltage swing and delivers greater current.
   
  I have been debating with myself whether any improvement could be made by going to a 4-layer board. I think it's already a very good compromise between cost, cooling and signal integrity.
   
  I've been looking at buying a 3D printer. This would allow me to make a couple of big improvements. I could engineer a clamshell case with a heatsink.
   
  Still no date for surgery.
   
  Fred


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## Goobley

Yeah I checked that the capacitor was installed as per the note and I checked the transistors data sheet, the schematic and the the traces on the board. I did go with precision OPA277s instead of chopper amps for the servo due the the price difference allowing me to complete the entire build for under €100 (which does actually compete with the O2 on cost in Europe as far as I'm aware, I also ABed them with a friend's O2 and we both thought the 49600 was clearer and punchier).
   
  With regards to the biasing of the relay transistor, after a few seconds the base sits at just under 0.65V, which is certainly very close to its minimum on voltage. However it remains at the same voltage when the relay is energised suggesting that it is sufficient. I'll give the board another in depth going over later but the only thing that comes to mind that's different to the schematic values is R46 (resistor in series with the diode that's parallel to the relay's coil). I used a 470R here as I seem to recall you mentioning that it was a) a non-critical value and b) supposed to match the impedance of the coil if possible (I used the IM03 which as I just found from checking the datasheet again has an impedance of 178R and not the 500R I thought it had. Must've confused the 03 I ave and the 23 you used... Gah silly mistake!). Any chance that this could be part of the problem? If the answer is nay then the board'll be going back under the loupe. I can't quite work out the purpose of R46 either, I'd imagined that the diode was there for flyback or something but Im confused now, it's not like the coil is polarised or anything...
   
  With regards to heat, mine doesn't really get hot. I'm driving 250R cans so obviously less current is being solicited from the buffers than with lower impedance phones, but the hottest component the LME49990 barely makes my finger tip tingle (and I had it one for at least 4 hours straight yesterday afternoon). I'd say this two layer board is pretty much spot on, and as others have done in the thread we can always stick on little baby heatsinks ourselves. It is fair to say that a case designed for this amp would be very cool though... Particularly as the half finished MDF box I have it in at the moment is not the most elegant. My woodworking skills leave a lot to be desired. 
   
  Cheers


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## wakibaki

You know it fits the same box as the O2? A box is shown on page 6. You do need a drill press and needle files to make it fit. There are sources for the box on page 11 and thereabouts.
   
  A lower resistance relay in the coil means the relay is pulling more current. This probably means that the delay circuit takes a bit more time to deliver the current. You could try reducing rhe the value of R41 currently 390k by my diagram. This will pull up the voltage at the base of the resistor, make more current available and charge the cap quicker. Try 270k.
   
  Fred


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## Avro_Arrow

I noticed my relay sticks.
  I had the same problem of the amp taking forever to kick in.
  I found that if I waited for ten seconds, the gave the relay
  a flick with my finger that it kicked in right away.


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## Goobley

Yeah, I had seen the box, but I'm not overly keen on it. I may still end up going that way though as my Dad has all the kit I'd need to make it fit (might have to get him to do it or I'll end up destroying the anodising or something).
   
  I changed my R41 yesterday, unfortunately I didn't have any SMD values between 120k and 390k in the house so I've paralleled 2*390k in series with 100k (giving 295k). The relay now engages within 5 seconds at which point the offset is around 5mV in one channel (worst spike I measured on cold turn on was 8mV - there are plenty of amps that run consistently with more than that!). This causes a very slightly noticeable pop, but is hardly gonna cause any damage. Thanks for you help in getting my build up and running!
  I also scoped the output yesterday looking for the possible oscillation that was mentioned up-thread somewhere. Nothing was visible with my 60MHz scope and I'm not even convinced that any of the chips get more than 20-25°C over ambient which is a real win! The 7805 gets a bit toasty but nothing that really worries me, however I finally box it I want to keep ventilation holes in the top.
   
  I'm gonna be going off to uni next school year with this amp and I have a question that someone with a bit of knowledge of electrics in the UK should hopefully be able to answer: All my electrical devices will have to undergo PAT testing for my accommodation, do you have any idea what class this amp comes under? I'd guess that since it's floating and runs from low power 15VAC it's probably class 3. If it's not then I'm gonna have a bit of trouble because I can't claim that it's double insulated, so I'd have to earth the case or at least all exposed metal. I'm also not using a proper wallwart but instead a boxed toroid with fuse that I'm somehow going to have to make pass commercial wiring requirements O.O And that's before I get to my point to point valve amp...
  I'd be really grateful if anyone knows their stuff well enough to offer me some advice on this.
   
  In the meanwhile I'm gonna chill hear listening to this really sweet piece of engineering and curse the fact that I have school tomorrow!
  Cheers,
  Chris


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## G.Trenchev

Hello guys,check this out.2 buffers per channel in parallel and lots of filtering.It outs 2 watts without effort.Zero mV offset.
   
   

   

   
  Also 0.03 ohm output resistance!


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## Goobley

I've almost finished the casing on this amp using a slightly taller box so I don't have to worry about the height of the to220 packages. However I have some hum since putting the amp into a metal enclosure, the noise level increases a fair amount if I touch the pot, but I can't really see a way to ground the pot. Any ideas?

If I short the barrel of the ac adapter to the case then the noise disappears. Should I keep it isolated and then connect a wire from the input jack to the case (like the O2's designer recommends for his amp)? I'm not worried about a slight but of noise when I change the volume but I'd like to get rid of the permanent noise as it wasn't there when I had it in a dodgy wooden box.

Cheers


----------



## wakibaki

It's difficult to know for sure, but I've always thought that the lead from the AC adapter to the amplifier is a potential cause of problems, hence there are ferrites just after the power socket put there to mitigate any problems arising. Evidently they are not sufficient in this case. You could try passing the wire through a clamp-on or toroidal ferrite positioned as close to the box as possible. You can take the wire through a number of times to increase the effect.
   
  I have had no issues when using the smaller case.
   
  If connecting the input jack to the case as in the O2 _works_, then that is an option. Since shorting the barrel of the AC adapter to the case works, then I would be tempted to take this straight-line solution. You can find a soldered connection to the barrel on the board.
   
  As far as the pot is concerned, I would expect the body to be in galvanic contact with the case when the retaining washer and nut are in place since drilling the hole in the fascia usually exposes bare metal. It's possible though that the front panel is not truly connected to the rest of the case, because most aluminium parts (smart-looking ones that is) are anodized. The coating produced by anodizing is definitely non-conductive, and may need to be scraped away in some places to ensure that there is a galvanic connection between the parts of the case, I can't tell though, because I haven't had a chance to inspect your case. If the AC adapter barrel is shorted to the case, and there is galvanic integrity throughout the case, then the pot body will be grounded.
   
  Since the amplifier actually _is_ quiet in some configurations, my recommendation is that you should fiddle around with it until you get the hum to go away. I appreciate that this is not a very scientific-sounding solution, but we're just really concerned with results here.
   
  w


----------



## Goobley

It's done.
   
  I just connected from the power jack to the case by connecting a jumper to the side of the connecter and clamping it between the case and rear panel. The amp is silent all the way to the noise floor of my usb powered DAC  I did a quick RMAA and all it really shows is that my kit isn't good enough to quantify the performance of this amp, it just measure's the same as my DAC: ( https://www.dropbox.com/s/ysyx6qhhzcl646i/LME49600.pdf ). I'm running at 4x gain and decided to do the face of the amp in nice thin leather. Glamour shots below:
   

   

   

   

   
  Thanks for the cool project waki, and the others who got involved


----------



## NuClear235

I am starting my project of semiportable headphone amp with OP1611 + LF411 as DC servo + 2x buffer paralel LME49600 /channel. Powered by 6x AW cells 750mAh -12V and +12V.
 http://www.lighthound.com/AW-RCR123a-Protected-750-mAh-Battery_p_114.html
 I hope it will play some time long- 750mAh is not much!
 One relay as undervoltage (3.0V/cell) switch off device for protect deep discharge accus.
 Second relay delayed switch on headphones to output for protect crack to headphones after start.
 Blue ALPS pot, output JACK 6.3 mm and Neutrik XLR NC4 (pseudo balanced).
 I hope this is way for my Senn HD800...


----------



## wakibaki

Very nice, NuClear235.

How are you charging the batteries?

w


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## NuClear235

I am RC modeler, so I have model hobby charger for Li-Ions. I think is OK constant voltage (6x3.1V) with limited current (750mA) supply too. But modell chargers are very cheap and very universal.
 Here is some solving with accus too:
 http://www.head-fi.org/t/586042/what-i-did-next-with-texas-headamp-chip-a-digital-control-desktop-portable


----------



## wakibaki

I am an RC modeller too, but an external charger is not always what is wanted.

You have found my TPA6120 amplifier. I have built 2 of these with protected li-ion batteries. One set of batteries has survived being left on continuous charge, but one set died, I don't know whether the batteries were at fault, or if the charger is wrongly adjusted. The batteries are from dealextreme.com, I buy them for my LED flashlights, I have had a few fail. Anyway, be careful if you use the charging circuit, do not set the end-of-charge voltage too high. Unfortunately I am ill, and I will probably not be able to follow up this issue.

w


----------



## NuClear235

I am using service connector to every pack. Is perfect for balanced charging and for diagnostic of every cell separate. On my picture the long one.


----------



## NuClear235

Finished.


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## wakibaki

Very nice. Pleased with it?

w


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## NuClear235

Yes, I am, but I am not objective because is it my child 
I will do some measurement and I will see.
Now I am in phase of testing other OPAs.
In game are LME49710 and LME49990. But the OPA1611 is probably the winner.


----------



## GxxANC

Hello!
  
 I built a hedphone amp with this LME49600 amp IC. I bulit the circuit whta you can see in the user manual in the Figure 28. But unfortunately it doesn't work. There is the positive supply voltage on the output.
  
 The PCB plan is OK, but I think, I damaged the IC under the soldering.
  
 Or the other problem maybe, what I think is, there is a JU4 and JU8 (maybe switch?) in the Figure 28, what connect the 7th leg to the GND. And I didn't bulit this section to the circuit. Is it a problem?
  
 Thanks in advance!


----------



## Misterrogers

Hello and Welcome!
  
 You'll want to post more information (photos, schematic - even if it's a sketch on a napkin). There are bright, amazing and helpful people on this forum, but they'll need more to go on.


----------



## GxxANC

Hello!
  
 I send the shematic from the TI's LME49600 user manual, but with modification. I cancelled the HP31, JU1, JU17, JU4, JU8, JU15, JU14, because they have not be built to the circuit. I try, that the 7th leg of the LM49720 opamp soldered to the GND, what is ihe JU4, but the problem is beeing now.
  
 And is send the PCB plan, I made it in Sprint Layout 5.
  
 This ithe shematic's picture:
  
 https://drive.google.com/file/d/0BzKO-EKpwCC6S3hFa1BOZlRoT28/edit?usp=sharing
  
 And the .lay file from the PCB:
  
 https://drive.google.com/file/d/0BzKO-EKpwCC6alR3R09OczZxT1U/edit?usp=sharing
  
 Thanks in advance!


----------



## imran27

Has anyone tried the circuit without that opamp low pass filter (Butterworth LPF? If I'm not mistaken)?


----------

