# A muting delay & DC offset protection circuit from amb



## amb

I present to you the ε12 ("epsilon 12"), a turn-on delay muting and DC offset protection circuit designed specifically for headphone amps. It prevents the turn-on "thump" and turn-off noise present with some amps, as well as provide protection to the headphones in case the amp's output DC offset goes awry during a malfunction.

 The ε12 can be used with a traditional 2-channel amp, a three-channel amp (M³, PPA, Pimeta, etc.), or a four-channel amp (fully balanced stereo outputs), and will provide for DC offset protection on all available channels. This circuit is intended to be powered from the amp's DC rails (up to +/-30V) and has its own voltage regulation and virtual ground reference, which is adjustable.

 The headphone outputs from the amp is routed through a relay, which keeps the headphones disconnected until after ~3 seconds delay after power up. The circuit will disconnect the headphones if any of the channels being monitored produces in excess of 70mV or DC offset (positive or negative), and will reconnect the headphones if the offset drops to a safe threshold. The 70mV trigger point is chosen to be sensitive enough to protect most headphones from damage under normal circumstances, but not too sensitive to get false triggering. Compare this to many DC offset protection circuits for speaker amps, which would not trigger until over 600mV. The relay as specified is a high quality Tyco/P&B RT series unit with 90-10 silver-nickle contacts rated at 8A, but you can use another 12V DC relay as long as the coil current comparable.

 The adjustable virtual ground reference pot let's you set the "zero" reference upon which all channels are compared.

 The circuit is designed to be simple, low in parts count and cost, yet provide all the functionality desired.

 I have a working prototype on breadboard, and I invite your feedback. If you wish to build the circuit, feel free to make your own PCB or use Point-to-point wiring on a perfboard. I have no immediate plans to offer pre-made PCBs, unless there is sufficient demand to justify it.

 The name "ε12" has no special significance, other than that I tend to name my DIY projects with a greek letter and a number.


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## amb

A bit of circuit description is as follows.

 D1, Q1 and Q2 form a simple voltage regulator to provide the +/-12V rails for use by the circuit. U1, a BUF634, is used as a virtual ground rail splitter. This chip is used for its high current capability, and is needed because the relay coil current flows through it. VR1 is a trimpot that sets the zero reference voltage.

 Q5 and Q6 are two transistors in a Darlington configuration for driving the relay. The time delay is accomplished by the charging of C7 through R11 after turn-on.

 U2 is an inverting summing amplifier with a voltage gain of 10. It is also limited in frequency response with a low-pass corner frequency of 1.6Hz. This amplifies the DC offset signal from all the channels being monitored, which is then fed to Q3 and Q4. These two transistors provide positive and negative DC offset detection, respectively, and shunts C7 if the offset exceeds their turn on thresholds, causing the relay to disengage.

 The schematic shows four inputs and four outputs (L+, L-, R-, R+) which corresponds to a four-channel (balanced output) amp configuration. For a three-channel amp, just use one of the "-" inputs and outputs for the shared ground channel. For a traditional two-channel amp, connect the "-" lines to the amp's ground.


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## rickcr42

Will we be seeing the pc boards offered in a "group buy" ?

 because if yes be aware that other than present "buys" all the rest are being put on hold for now until this gets sorted out and an "easily understood" policy is put in place concerning selling in DIY

 thanks

 rickraptor


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## BrokenEnglish

that's great, amb! many thanks! 
 a simple question:
 could one replace u2 with tl071?
 thanks!

 /edits: ah... you updated your first post in the meantime....


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## n_maher

:warpath:

 amb,

 Nice circuit, even if I can't understand it 
	

	
	
		
		

		
		
	


	




. 

 Nate


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## gsferrari

*[size=small]Can someone double check parts list for me:[/size]*



*Parts List:*


Resistors:


 R1, R2 = 3.32K
 R3, R4, R5, R6, R7, R8 = 10K
 R9 = 100K
 R10 = 1K
 R11 = 221K


Capacitors:


 C1, C3, C4 = 22uF/25v Tantalum
 C2 = 0.1uF Multi-layer Ceramic (NP0)
 C5 = 1uF MKP 
 C6, C7 = 100uF/16V Non Polarized Electrolytic


Diodes:


 D1 = 1N5252B
 D2, D3, D4 = 1N4148


Semiconductor Devices:


 Q1, Q3, Q4, Q5, Q6 = 2N3904
 Q2, = 2N3906

 U1 = BUF634P
 U2 = TL081



Others:


 Relay : Tyco RTE44012F / 12V dc / DPST-NO
 VR1 : 1K ohm multi-turn cermet trimpot

 Perfboard
 Wire
 I/O contacts

*Additional Notes By amb*
  Code:


```
[left]1N914 can be substituted for the 1N4148 diodes, and the transistors are all non-critical (2N4401 can be used instead of 2N3904, 2N4403 for 2N3906, etc). For power supply voltages higher than +/-18V, put small TO-92 clip-on heat sinks on Q1 and Q2. The supply voltage should not be lower than +/-12V. Depending on ventilation you may also need a small heat sink on the BUF634P.[/left]
```


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## dsavitsk

Quote:


  Originally Posted by *gsferrari* 
_C5 = 1uF (cant make out the type!)_

 

Looks like MKP to me. Wima perhaps?

 redacted due to not caring anymore


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## Garbz

AMB I what happens at poweroff? Does the relay colapse fast enough to prevent poweroff thump too?
 It's about bloody time someone made something like this!

 Also Rick my Dynalo wouldn't be where it is without AMB, it happens to be his area of expertise eventhough not that many people build them.


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## amb

Quote:


  Originally Posted by *BrokenEnglish* 
_could one replace u2 with tl071?_

 

Yes. Many different opamps could be used here, but I suggest FET input opamps only, and stay away from anything "cranky". There is no performance requirement here other than low intrinsic DC offset, so a jellybean TL081, TL071, LF351, etc., are all ok.

  Quote:


  Originally Posted by *gsferrari* 
_Can someone double check parts list for me:
 ..._

 

Yeah, your list looks fine. C2 should be a 0.1uF multilayer ceramic (NP0) cap, and C6 should be non-polarized electrolytic (also called "bi-polar electrolytic"). C7 doesn't need to be non-polar, but it's easier to just get the same type as C6.

 VR1 is missing from your list, and it should be a 1K ohm multi-turn cermet trimpot.

 1N914 can be substituted for the 1N4148 diodes, and the transistors are all non-critical (2N4401 can be used instead of 2N3904, 2N4403 for 2N3906, etc).

 For power supply voltages higher than +/-18V, put small TO-92 clip-on heat sinks on Q1 and Q2. The supply voltage should not be lower than +/-12V. Depending on ventilation you may also need a small heat sink on the BUF634P.


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## amb

Quote:


  Originally Posted by *Garbz* 
_AMB I what happens at poweroff? Does the relay colapse fast enough to prevent poweroff thump too?_

 

The relay will open as the voltage from the supply rails drop. Depending on how much rail capacitance you have on your rails there will be a delay, but hopefully the relay will disconnect the headphones before the amp begins to emit any last gasping noises. 
	

	
	
		
		

		
		
	


	




 One neat by-product of having its own virtual ground reference, this circuit will draw the same amount of current from each rail (the relay current is sourced from one rail, and sunk by the BUF634 and redirected to the opposite rail), so it will not cause an imbalance in the drain-down time of the positive and negative supply rail caps after turn-off.


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## amb

Quote:


  Originally Posted by *SHLim* 
_I for one would prefer a pcb and I don't want to spend time tracing my work to make sure the wiring is correct._

 

If someone wants to layout a board and do an order, be my guest. I have no financial interest in this, so do whatever you want with the schematic. As I said, it's my gift to the community, and if someone has good ideas to improving the circuit, I'm all ears!


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## DaKi][er

Quote:


  Originally Posted by *amb* 
_If someone wants to layout a board and do an order, be my guest. I have no financial interest in this, so do whatever you want with the schematic. As I said, it's my gift to the community, and if someone has good ideas to improving the circuit, I'm all ears!_

 

it wouldnt be hard to make a perfboard template layout for this and just p2p it on that 

 just i dont have much of an interest in building one, so i'm not doing it


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## BrokenEnglish

amb... as far as i understand, i could connect the voltage-splitter part directly to the amp's psu jack and it hasn't to be a dual-psu, that's feeding the amp, am i right? could you please elaborate on how to adjust "zero reference". thanks!

 oh... if only vksy could jump in to draw a nice p2p-layout as he did with the steps... things would be a lot easier... 
	

	
	
		
		

		
		
	


	




 ...i see myself sticking part for part in a perfboard, arranging and rearranging it a whole weekend through...


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## amb

Quote:


  Originally Posted by *BrokenEnglish* 
_amb... as far as i understand, i could connect the voltage-splitter part directly to the amp's psu jack and it hasn't to be a dual-psu, that's feeding the amp, am i right? could you please elaborate on how to adjust "zero reference". thanks!_

 

Yes, just take the V+ and V- rails from your amp to power this circuit. No need to connect the ground from your amp to it (whether it's a "real" or "virtual" ground). If you have a separate PSU for each amp channel, you should use a separate ε12 circuit per channel too. Do not use a separate PSU for this circuit, otherwise the zero reference cannot be correctly established.

 The zero reference adjustment pot establishes what this circuit considers as "0 DC offset".

 For a 2-channel amp (e.g., unbalanced dynalo/dynahi, SDS Labs, Eaton, etc.), just adjust the pot until the voltage between the amp's ground and pin 6 of the ε12's BUF634 is zero.

 For a 3-channel amp (e.g., M³, PPA, Pimeta), adjust the pot until the voltage between the amp's ground channel's output and pin 6 of the ε12's BUF634 is zero.

 For a 4-channel amp (amp with fully balanced outputs), this is a bit trickier. Since there are two active "ground" outputs, one per channel, each having a small DC offset during normal operation, you should adjust the pot until the voltage between pin 6 of the ε12's BUF634 and both of the amp's "ground" outputs are the lowest you could make them.

 This assumes a correctly-functioning amp that does not have a problem with high DC offset. The ε12 is there to protect your headphones in the event of a malfunction.


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## BrokenEnglish

thanks for your answers, amb!
 another question: since i have lots of ha-5002 on my hands (thanks to glassman! 
	

	
	
		
		

		
		
	


	




 ), could they be used as replacements for u1 (after rerouting the pin-connections)?
 thanks!


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## amb

Quote:


  Originally Posted by *BrokenEnglish* 
_since i have lots of ha-5002 on my hands (thanks to glassman! 
	

	
	
		
		

		
			





 ), could they be used as replacements for u1 (after rerouting the pin-connections)?_

 

I don't have any working experience with the HA-5002 serving as a rail splitter. In theory it should work, but I don't know whether it would be happy with large-ish 22uF caps from its output to the rails. The BUF634 has been known to work well in this scenario.


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## peranders

Quote:


  Originally Posted by *amb* 
_I present to you the ε12 ("epsilon 12"), a turn-on delay muting and DC offset protection circuit designed specifically for headphone amps. It prevents the turn-on "thump" and turn-off noise present with some amps.._

 

The idea is surely good but complex. Why not use precision parts instead?

 My QRV-04 headphone amp with OPA627 or AD8610 is totally click/thump free.


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## gsferrari

Quote:


  Originally Posted by *peranders* 
_The idea is surely good but complex. Why not use precision parts instead?

 My QRV-04 headphone amp with OPA627 or AD8610 is totally click/thump free._

 


 This will be useful in Tube amps and high power amps (Dynahi for example). Most tube amps that I have used had the on/off thump. Some had it loud and some had it long.

 Has anyone built this already? My parts are coming in...but I am tempted to run to Radioshack and use generic parts for the time being just to play with the circuit.


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## peranders

These thumbs can be reduced in the design stage if you have those on the agenda but I admit that sometimes it's hard to realize if you will have such problems.

 I'll guess that you'll have more problems with amp which are AC coupling in one way or other and also single ended amps.


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## amb

Quote:


  Originally Posted by *peranders* 
_The idea is surely good but complex. Why not use precision parts instead?_

 

Sure, but even normally thump-free amps might malfunction and take out your expensive headphones with them, and the fact that some amps out there are _not_ thump-free means that an inexpensive circuit like this is probably good insurance. I am not saying that a protection circuit like this is a must, each DIY builder will have to make his/her own decision.


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## amb

Quote:


  Originally Posted by *gsferrari* 
_This will be useful in Tube amps and high power amps (Dynahi for example). Most tube amps that I have used had the on/off thump. Some had it loud and some had it long._

 

Since the max supply voltage for the ε12 as designed is +/-30V and must come from the amp's rails, it is not a good fit for most tube amps. However, if a tube amp is in the traditional 2-channel configuration (which should cover 99+% of them) you could build the ε12 without the BUF634 rail splitter portion, use a separate +/-15V PSU to power it, and tie the ε12's "VG" to the amp's real ground and it will work well.

  Quote:


 Has anyone built this already? My parts are coming in...but I am tempted to run to Radioshack and use generic parts for the time being just to play with the circuit. 
 

LOL, I had just started this thread yesterday! 
	

	
	
		
		

		
		
	


	




 Yes, you can use Ratshack parts but they don't carry the BUF634. If you happen to have one in your drawer then it's cool. Another thing to be aware of is that if you use another relay than the Tyco I specified, be sure that its coil current is no more than around 30mA so that the BUF634 won't get too hot.


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## BrokenEnglish

well... i hope, one of the first builders will provide us with some pics... could be really helpful for us noobs. gsferrari... please...


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## kevin gilmore

I do a very similar thing except i don't like anything in series with
 the audio signal if at all possible. So i use the normally closed contacts
 on the relay and then tie the contacts between the audio outputs
 thru the relay to a pair of back to back shottky diodes. That way the dc level
 limits to +/-.3 volt until the relay pulls in.

 Nice job AMB.


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## amb

Quote:


  Originally Posted by *kevin gilmore* 
_I do a very similar thing except i don't like anything in series with
 the audio signal if at all possible._

 

Neither do I, but there are lots of contacts within the audio chain, from both ends of interconnect cable, to your headphone jack, to possibly yet another set within the headphones (a la Sennheiser). If a high enough quality relay is used the degradation should be minimal.

  Quote:


 So i use the normally closed contacts
 on the relay and then tie the contacts between the audio outputs
 thru the relay to a pair of back to back shottky diodes. That way the dc level
 limits to +/-.3 volt until the relay pulls in. 
 

Yeah, that would do it but it's still possibly up to +/- 0.3V, which is quite a lot. Also, some amps don't take kindly to being shorted out for a few seconds, or in the case of an actual malfunction, possibly a lot longer than that (until the user turns the power off, after the smoke and smell of burning parts... 
	

	
	
		
		

		
		
	


	




 ).

  Quote:


 Nice job AMB. 
 

Thanks.


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## walangalam

Quote:


 U1, a BUF634, is used as a virtual ground rail splitter. This chip is used for its high current capability, and is needed because the relay coil current flows through it. 
 

Can a regular splitter be used instead of the 634, which would supply the opamp? The relay driver does not need to be referenced to virtual ground, it can have a regular 7812 and tied to the original supply directly, right?

 I have some spare 7812 and tle around, that why...


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## amb

Quote:


  Originally Posted by *walangalam* 
_Can a regular splitter be used instead of the 634, which would supply the opamp?_

 

If by "regular splitter" you mean a TLE2426, the answer is no, because the TLE2426 does not have enough current capacity.

  Quote:


 The relay driver does not need to be referenced to virtual ground, it can have a regular 7812 and tied to the original supply directly, right? 
 

While it's possible to use a 7824 regulator instead of the 2N3904/3906 plus zener, I do not recommend that because the 78xx regulator has higher drop-out voltage than the discrete transistors, making the supply rail voltage requirement higher (at least +/-15V or 30V total). You cannot use a 7812 regulator because that would only make a +/-6V supply after the BUF634, and that is insufficient to pull the relay.

 EDIT: After I re-read your question I think I understand what you're asking better. You are wondering whether it's possible to use a 7812 and a TLE2426 to make +/-6V just for the opamp, but change Q3, Q4, Q5 and Q6 to be referenced to -6V instead of the VG. The answer is no, because Q5 and Q6 is triggered by the opamp which normally has an output voltage of less than +/- tens of mV, and if Q3/Q4 is to be referenced to -6V they will turn on and cause C7 never to be charged, and thus the relay will not turn on.


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## Pars

Amb,

 If one were to want to add a fault LED or other indication that it is either a) in delay, or b) a DC offset fault has occurred, I assume that this could be accomplished by monitoring the point between R11 and C7? In normal operation (relay on), this should be at about 1.4V (the base-emitter junctions of Q5 and Q6)? During fault, it should be close to 0V WRT VGnd due to Q3 or Q4 bypassing C7?

 -Chris


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## amb

Quote:


  Originally Posted by *Pars* 
_If one were to want to add a fault LED or other indication that it is either a) in delay, or b) a DC offset fault has occurred, I assume that this could be accomplished by monitoring the point between R11 and C7? In normal operation (relay on), this should be at about 1.4V (the base-emitter junctions of Q5 and Q6)? During fault, it should be close to 0V WRT VGnd due to Q3 or Q4 bypassing C7?_

 

Don't put an LED there because it will drain down the C7 capacitor. If you want a fault LED, wire a LED + 3.3K series resistor from the collector of Q6 to the VG.

 Radio Shack used to sell a 12V-powered red blinking LED (no series resistor necessary). If you could find something like similar, it would be cool to use that here.


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## Pars

Quote:


  Originally Posted by *amb* 
_Don't put an LED there because it will drain down the C7 capacitor. If you want a fault LED, wire a LED + 3.3K series resistor from the collector of Q6 to the VG._

 

Would this be true even if the LED were driven by some TTL such as an OR gate or a pair of inverters? I would think these would have a high enough input inpedence to have no effect on C7?

 Agreed, the blinker would be cool.

 -Chris


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## amb

Quote:


  Originally Posted by *Pars* 
_Would this be true even if the LED were driven by some TTL such as an OR gate or a pair of inverters? I would think these would have a high enough input inpedence to have no effect on C7?_

 

A CMOS logic gate should be ok, although the 1.4V B-E junction voltage there is too low to be a good "true" level.


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## walangalam

Hi amb,

 Just 1 more Question before I build this...

 What happens when one channel is at -70mv and the other is at +70mv? Since its a summing amp, I would assume that the output of the opamp would be at 0mv, or is it something else?

 Thanks


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## amb

walangalam, yes, the scenario you describe will null out at the summing amp. Keep in mind, though, that it is assumed that the main amp does not have such wacky DC offset issues (i.e., this protection circuit is not intended to mask an existing DC offset problem. The amp should normally have near zero DC offset). This circuit is there to protect the headphones in case the amp fails and the DC offset goes haywire. It is highly unlikely that two channels will fail at the same time but the offset goes in opposite directions with the same magnitude.


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## amb

By the way, this circuit now has its own web page.


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## Garbz

Just checking if the 1N4749 24V 1W zener is a suitable substitute for the 1N5252b?


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## amb

Quote:


  Originally Posted by *Garbz* 
_Just checking if the 1N4749 24V 1W zener is a suitable substitute for the 1N5252b?_

 

Basically any 24V zener will work. The 1N5252B is rated at 500mW, the 1N4749 is overkill but will work.


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## Garbz

Overkill is only overkill if it's not the only part I can find


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## Garbz

Hey AMB is it possible to modify the design so the DC Epsilon2 can run from a single ended 24v supply? Such is the case with the amplifier where I was hoping to use this circuit.


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## amb

Quote:


  Originally Posted by *Garbz* 
_Hey AMB is it possible to modify the design so the DC Epsilon2 can run from a single ended 24v supply? Such is the case with the amplifier where I was hoping to use this circuit._

 

What amp are you considering? Does it use a rail splitter to get its virtual ground or is it actually single supply with an output coupling cap (i.e., ground is V-)? If it's the latter, the coupling cap will prevent any DC offset from reaching your headphones (unless the cap fails). If the cap is to fail then the DC offset can only go positive relative to ground, not negative. For such an amp, skip the BUF634 on the ε12, omit Q2 and Q4, connect all three pins where Q2 was together, and tie VG to the V- rail. Put a small heat sink on Q1 because it will be dropping 24V down to 12V and dissipating some heat.


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## Garbz

Yeah well the main problem is the coupling cap still produces some offset while it's charging and while the tube warms up. It's is fully single ended with -ve = GND and the output sitting at +12v or something I believe, I'll get exact measurments later.

 The other problem is at poweroff the amp goes DC in a huge way. We are talking 1.2V into my dummy load. I'm hoping the relay will solve this issue.


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## amb

Yep, single supply amps with a big output coupling cap will thump big-time upon turn-on. The muting delay should take care of that.


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## walangalam

In my dynalo, one channel has a positive offset drift, the other has negative. At start up, its around 10mv either way. Without the servo, it climbs to around 35mv -- one channel postive, the other negative. But with the servo, its a cool 0mv.

 The servo is a opa2132 serving both channels. If this chip fails, what I describe could happen, specially since my amp runs very hot. 

 So I plan double up the sense amp, use maybe a tl082 dual, and duplicate everything for U2 (use only 2 10K inputs) including R10 and sum at the base of Q3. Would this work alright? 

 I also found a more sensitive relay in digikey 255-1404-5-ND. Its rated at 4.2ma 12V, so i could use TLE splitter after all.


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## amb

Quote:


  Originally Posted by *walangalam* 
_In my dynalo, one channel has a positive offset drift, the other has negative. At start up, its around 10mv either way. Without the servo, it climbs to around 35mv -- one channel postive, the other negative. But with the servo, its a cool 0mv.
 The servo is a opa2132 serving both channels. If this chip fails, what I describe could happen, specially since my amp runs very hot._

 

If the servo opamp is to fail, chances are its output will drift toward one of the rails, causing a big DC offset at the output rather than in a way like you describe. 

  Quote:


 So I plan double up the sense amp, use maybe a tl082 dual, and duplicate everything for U2 (use only 2 10K inputs) including R10 and sum at the base of Q3. Would this work alright? 
 

No, because the two offsets will still sum at that point and cancel out. If you want completely independent monitoring of the two channels (which I don't think is necessary), you should use a completely separate ε12 circuit per channel.

  Quote:


 I also found a more sensitive relay in digikey 255-1404-5-ND. Its rated at 4.2ma 12V, so i could use TLE splitter after all. 
 

That relay has a low current coil, but its contacts are only rated for 10mA (compared to 8A like the Tyco that I specified). 10mA is really too low...

 Also, using a TLE2426 as the splitter will not allow you to adjust the zero reference voltage via a pot.


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## walangalam

Thanks. I didnt see the 10ma rating.

 I did realize that it will still sum at Q3, but only after I had already posted! I think I will double up the e3. Now to find a way to use only 1 relay...


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## Garbz

you should really have the dynalo running at 0v without servo after it's warm, and let the servo just handle the amp heating up.

 I.e. remove them power the unit up and prehapse adjust the 500ohm resistor feeding the LEDs to null the offset when the unit is on. That way if a servo fails you're safe.

 Still it's unlikely both servo's will fail synchronously.


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## BrokenEnglish

could someone please post some pics of its build?! most of the parts are on their way and i'd like to decide on my parts layout or... if i build it at all. thanks!


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## PinkFloyd

Dr. White was doing much the same thing 10 years ago......... check out his loudspeaker protection circuit http://www.wnaudio.com/ click on "products" and then click on "speaker protection"

 He's been working on a bullet proof DC protection circuit for "headphones" for quite some time and it looks as if PCB's are imminent 
	

	
	
		
		

		
			



http://www4.head-fi.org/forums/showp...&postcount=433

 Nice one!


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## ppl

Over at diyaudio.com there is a thread that deals with this same issue and has plenty of different ideas http://www.diyaudio.com/forums/showt...threadid=57173
 Including a circuit similar to AMB's. http://www.diyaudio.com/forums/attac...amp=1115921488
 and also see http://www.diyaudio.com/forums/attac...amp=1115990451

http://www.diyaudio.com/forums/attac...amp=1116692994


  Quote:


  Originally Posted by *kevin gilmore* 
_I do a very similar thing except i don't like anything in series with
 the audio signal if at all possible. So i use the normally closed contacts
 on the relay and then tie the contacts between the audio outputs
 thru the relay to a pair of back to back shottky diodes. That way the dc level
 limits to +/-.3 volt until the relay pulls in.

 Nice job AMB._

 


 I Believe as Kevin dose that putting unneeded contacts in the signal path is not the Best for Sound quality regardless of how many op amps and switches the signal passed through prior to your equipment as this will just further degrade the signal even more. My recommendation is to turn your amp on first then connect your headphones Soft start circuits and DC protection can be done by turning on the output stage last and the Voltage gain stage is turned on First using AMB’s or similar circuit as the Root cause of turn on and turn off is the fact that the output stage in most Amps turns on prior to the Voltage gain stages so the Output stage is merely passing the turn on and turn off transient caused from the Voltage gain stage turning off first and turning on Last

  Quote:


  Originally Posted by *pabbi1* 
_Did someone move your cheese?

 So, now as an agent of change you are just determined to be a thread crapper - how sad.

 It has been clear from your 'Midnight Millet Attack' you have been aiming at m³ all along. I won't guess who has been stoking you about 'fair', but PPA does come to mind._

 



 Oh and BTW anyone that thinks any member of the PPA Team is responsible for the action of Rick is seriously mistaken as Team PPA had to endure similar accusations from another member of this Forum http://www6.head-fi.org/forums/showt...ght=ballad+ppa


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## rickcr42

Quote:


 Oh and BTW anyone that thinks any member of the PPA Team is responsible for the action of Rick is seriously mistaken as Team PPA had to endure similar accusations from another member of this Forum http://www6.head-fi.org/forums/show...ight=ballad+ppa 
 

Wow ! never had i read that before because I make it a habit to take a pass on the "team" anything threads.I don't build the products and have no interest in the amps so i just cruise on past the threads and go to the more generic DIY posts.

 As far as any action I decide to or not to take and anything said by me in any thread-all me all the time and no human has influence over my thoughts.I call 'em as _I _ see 'em.
 Always have,always will

 personally ?

 I think the DIY forum has gone to hell but since most would rather do *DIY by the numbers * and never figure out how anything works my attitude is FK it until or unless there is a rules change.I refuse to play but hey,I refuse to play in a lot of games I don't care for.........

 and to say no one is making enough to make a living is all BS._ANY profit_, even a tiny one is still a business and if not then try to use that argument with the IRS is they ever audit you and see how far you get.

 nuff said and again I will bow out,just responding since my name was brought up by others even though i had left this thread permanantly........


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## ppl




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## amb

Guys, I would really appreciate it if you would please stop derailing this thread and take this mud-slinging elsewhere.


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## xtreme4099

*yawn* i gotta burritos in the microwave --- be right back ... *Zzzzzz*


----------



## Pars

Quote:


  Originally Posted by *rickcr42* 
_I think the DIY forum has gone to hell but since most would rather do *DIY by the numbers * and never figure out how anything works my attitude is FK it until or unless there is a rules change.I refuse to play but hey,I refuse to play in a lot of games i don't care for.........

 and to say no one is making enough to make a living is all BS.ANY profit, even a tiny one is still business and if not use that argument with the IRS is they ever audit you.

 nuff said and agin I will bow out,just responding since my name was brought up........_

 

Ya know Rick, anyone who has built a Gilmore or probably an M3 or PPA knows (or has found out) that it never is as easy as plugging some crap into a PCB and firing it up. Yeah, its on a PCB and might not meet your definition of DIY as being you design it, and build it Pt-Pt. 

 In doing my first Dynalo (on a commercial board from the prominent Gilmore vendor who it appears got chased off these boards before I got into phones at all), I had to learn Eagle, and design and etch my own rectifier boards to replace the GBU recifiers called for with Schottkys. And I had to learn how to match FETs and transistors (thanks for all the help Amb). And, once it was put together, I had to go thru the schematic and figure out what I should be seeing here and there to get the DC offset into a manageble (i.e. perfect, without the servo) state. So if someone tells me that this isn't DIY, I bristle a bit.

 Personally, I think that some of the people around here that might be termed "manufacturers" under your definition are also some of the most helpful people in terms of knowledge and insight on these boards, and to hinder them and/or chase them away would be a *serious* mistake IMHO. I quite frankly do not understand the paranoia that seems to be prevalent on these boads WRT "manufacturers", unless all they are doing is pimping their products. I can assure you from what I see from people like Amb, ppl, etc. that they respond to specific questions with valid and responsive answers to the question asked, as they are in the basic sense, interested hobbyists just like you or me. I have not read one post yet which was a response to something where I felt anyone was trying to sell me anything or push their product. And I think that making some money off of their talents is defintely not blasphemy. When I see someone post something like "Sorry dude, gotta cut it short cuz my jet is waiting" then I might come to agreement with you. I don't think any of the manufacturers are making anywhere near that kind of money. If they are, could they please swing past Chicago next weekend and drop me off in the Carribean for a weekend of golf? Please be sure to have tee times and accomodations taken care of also


----------



## Sovkiller

Another point that I would like to add, is that most of those "manufacturers", became manufacturers years after they were here, in headwize, and in many other forums and sites, helping people and offering his free DIY support, the case of ppl, tangent, voodoochile, JMT, Meier, and so many others, of course after seeing that many of other "not so helpful people" are making money out of their knowledge (and even out of others knowledge) well why not them??? Inside the rules or not, with a commnercial point of view or not, what I feel at the end, is that they are helping many people to get out, of their problems....


----------



## grasshpr

Hey AMB,

 Quick question. This circuit will immediately switch to the amp channel once the DC offset would reduce to less than 70mV. Also, one of the primary purposes of this component is to remove the popping effect when the amp gets turned on. 

 However, if you already have a signal being played on the amp, wouldn't switching from ground to some input signal immediately produce the same popping effect? I would assume you need some component that will reduce the transient response from ground to the amp signal smoothly. 

 Pretty neat circuit, BTW 
	

	
	
		
		

		
		
	


	




 grasshpr


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## amb

grasshpr, if the DC offset threshold is exceeded and the relay disconnects the headphones, and then the offset somehow drops back to normal, the relay will re-engage after a 3-second pause. If music was already playing then the audible effect of all this is a temporary mute. Granted it's not a smooth fade-out and fade-in, but there shouldn't be a "thump" like what might happen when you first turn on some amps.


----------



## grasshpr

Thanks for info. However, I've had bad experience (not with audio equipment, but with mechanical shaker tables) where switching from ground to some bounded quasi-periodic signal will produce a big thump. I assumed it would be very similar for headphones since the source signal is amplified to provide a good amount of current. 

 Now about fade-in/out, wouldn't it be simple enough to just put an additional stage which fades in/out the input signal (in about a millisecond or two with minimal phase shift) and switch to the actual input signal or ground after a millisecond or two? 

 Just a thought, since if your going to make a circuit that protects headphones, mind as well go all out and crazy 
	

	
	
		
		

		
		
	


	




. That and its probably a nice touch to have a fade in/out circuit for an amp, just because it makes the amp sound a little more professionally built. (this is purely my opinion, BTW)


----------



## amb

This circuit doesn't "switch from ground". It simply disconnects and connects (see schematic) the headphones. This is no different than, say, unplugging and plugging your headphones while the music is playing (actually, it's better because the 1/4" phone jack briefly makes a couple of "wrong" connections as you insert/remove it, but this circuit doesn't do that).

 As for fading, that would require quite a lot more circuitry, and tricky to do if we want it not to add additional noise/distortion or affect other performance criteria. To properly implement fading we would in effect need an electronically-controlled volume control, which should be at the input side of the amp rather than at the output.

 This muting/delay protect circuit is intended to be cheap, simple, space-efficient, easy to integrate into a wide variety of existing amps, and not affect the sound quality. I don't think it's possible to achieve all these goals and add the sort of fading capability that you seek.

 Not to mention that since it's supposed to protect the headphones in the event of a big DC offset; so when that happens we want the headphones to be disconnected *now* rather than playing any fading sound trick.


----------



## grasshpr

Quote:


  Originally Posted by *amb* 
_ As for fading, that would require quite a lot more circuitry, and tricky to do if we want it not to add additional noise/distortion or affect other performance criteria. To properly implement fading we would in effect need an electronically-controlled volume control, which should be at the input side of the amp rather than at the output._

 

I don't think fading in/out would be very difficult to implement, simple solution is to put a low pass filter with a high enough corner frequency and switch out to the unfiltered amp input after a fixed period (using another relay). This would not effect the audio signal path that much (as much as your original dc offset protection circuit since it does include a capacitor in the audio path). This would not require any complicated volume control. 

 Also, isn't it dangerous to have one side connected with ground but the other as open (potentially connected with the body of the relay instead)? When the headphone is unplugged from a component, the ground and hot is at the same potential as air, which appears to be different compared with the circuit you provided.


----------



## amb

Quote:


  Originally Posted by *grasshpr* 
_I don't think fading in/out would be very difficult to implement, simple solution is to put a low pass filter with a high enough corner frequency and switch out to the unfiltered amp input after a fixed period (using another relay)._

 

You lost me here. How does a low pass filter provide fading volume?

  Quote:


 This would not effect the audio signal path that much (as much as your original dc offset protection circuit since it does include a capacitor in the audio path). 
 

Again you lost me. The muting/protect circuit I designed here does not add anything to the signal path except for relay contacts to switch the headphones in/out. What capacitor is it that you speak of?

  Quote:


 Also, isn't it dangerous to have one side connected with ground but the other as open (potentially connected with the body of the relay instead)? When the headphone is unplugged from a component, the ground and hot is at the same potential as air, which appears to be different compared with the circuit you provided. 
 

No, I think you have a basic misunderstanding about circuits. When the positive side of the headphone leads are disconnected by the relay, the circuit is _open_ and there will be no more current flow. There is no "danger" to speak of.


----------



## grasshpr

Quote:


  Originally Posted by *amb* 
_You lost me here. How does a low pass filter provide fading volume?_

 

What I'm saying hear is using a low pass filter to provide smooth transitioning into the proper audio signal. A low pass filter is merely a stable state-space system that has an input and spits out an output. If the input is a quasi-periodic signal, then the output would simply be the same quasi-periodic signal with a phase shift. Only difference would be that this system would have to transition from some initial condition (which would be ground, or the way you have it which is in disconnection mode). 


  Quote:


  Originally Posted by *amb* 
_Again you lost me. The muting/protect circuit I designed here does not add anything to the signal path except for relay contacts to switch the headphones in/out. What capacitor is it that you speak of?_

 

You are right in this case, however, the way you can design the fade in/out would not require you to affect the audio signal path as well. After several milliseconds of timing, use another relay to switch from the filtered signal to the non-filtered audio signal. Sorry for this confusion.

  Quote:


  Originally Posted by *amb* 
_No, I think you have a basic misunderstanding about circuits. When the positive side of the headphone leads are disconnected by the relay, the circuit is open and there will be no more current flow. There is no "danger" to speak of._

 

Your relay switches between two signals, the way you have it, when the DC offset is high, you switch the hot signal to open. However, you still have your ground signal connected to ground (which can supply some current to the headphone right?). This is the same idea as having your open side fictiously at the potential of air and your ground signal supplying a current as a result. Same principle as static discharge, when you touch ground on a metal surface and your body is at another potential. Furthermore, this is not the same situation as having your headphones disconnected from your amp. If your really interested in switching the headphones to an open circuit, then use two DPST relays and open both ground and hot. I'm not saying that your circuit is incorrect (and also in no way am I criticizing your work), I'm saying under extrenuous situations, you could be providing an obtrusive signal to the headphones (e.g., colored noise) which may not be desired. 

 Lastly, I didn't mean to make such a fuss about this design. Maybe my comments are not warranted, however, I am just interested, thats all. Take it with a grain of salt


----------



## rickcr42

I will say this one time and only one time.No discussion,no comments here it is :

 Do NOT direct posts in my direction if you do not want me to come back into this thread and post my feelngs on this topic.I tried to let it die but some of you are far too anal and can not let go.You bring me in and you will have to deal with me.Simple.
 So don't go there or put on your asbestos suit.Cast a stone in my direction and I will come back with a truckload of rocks so chill out.

  Quote:


 Another point that I would like to add, is that most of those "manufacturers", became manufacturers years after they were here, in headwize, and in many other forums and sites, helping people and offering his free DIY support, the case of ppl, tangent, voodoochile, JMT, Meier, and so many others, of course after seeing that many of other "not so helpful people" are making money out of their knowledge (and even out of others knowledge) well why not them??? Inside the rules or not, with a commnercial point of view or not, what I feel at the end, is that they are helping many people to get out, of their problems.... 
 

You need to think before you type.This alone if I so decided could be if I followed the letter of the rules is grounds for banning so walk easy.I say to _you_ right here and directly.Do NOT carry this.
 Let it go right here and right now or it will get ugly and neither of us want to go there.I will also say if you ever meet me in person,please stick your tongue out at me.I would enjoy that.

 Finally-do NOT respond to this post.Keep the damn thread ON TOPIC because if I check my email and I see a bunch of off topic posts I will not only delete them but will take other steps as well since this is a very clear warning to cease.Consider it formal.

 thanks

 rickmonster


----------



## amb

Quote:


  Originally Posted by *grasshpr* 
_What I'm saying hear is using a low pass filter to provide smooth transitioning into the proper audio signal. A low pass filter is merely a stable state-space system that has an input and spits out an output. If the input is a quasi-periodic signal, then the output would simply be the same quasi-periodic signal with a phase shift. Only difference would be that this system would have to transition from some initial condition (which would be ground, or the way you have it which is in disconnection mode)._

 

I see what you're trying to accomplish. Phase shift the signal so that when we reconnect the headphones it occurs at zero-crossing time to avoid the small discontinuity in waveform. Good thought, but...

 If you're trying to make a phase shift in a uniform way with respective to frequency then you need an all-pass filter, not a low-pass filter. But more to the point, you've got to remember that a relay is a mechanical device and it takes a small amount of time for the contact to close after energizing the coil. To do what you suggest here assumes that we know exactly when that is going to happen and, we don't. And even if we did, transitioning from the phase-shifted version of the audio to the non-shifted version will still incur a small discontinuity in the waveform without doing something really fancy, and another relay is not going to cut it. And relay contacts do "bounce" when closing, how would that affect the signal?

  Quote:


 However, you still have your ground signal connected to ground (which can supply some current to the headphone right?). 
 

Actually, no. Trust me. Do a little measurement on your own. Hook up your DMM in AC milliamps mode in series with the connected ground, play a test tone and watch your meter and you'll get a reading of some amount of current. Now, disconnect the positive side of both outputs. Still got a reading?

  Quote:


 Same principle as static discharge, when you touch ground on a metal surface and your body is at another potential. 
 

Static discharge is tens of thousands of volts or higher, arcing across air. Headphone amp DC offset is much smaller and cannot do that. Have a look at the schematics of almost any piece of audio equipment, and you'll find that the power switch only switches one side of the AC line and there is no need to switch both. Same principle -- it breaks the circuit, and we're talking about AC mains here, which is much higher voltage than any DC offset that this protection circuit is going to encounter.

 At any rate, I think you're obsessing. What you suggest is far, far beyond what this circuit is intended to do.


----------



## grasshpr

Quote:


  Originally Posted by *amb* 
_You've got to remember that a relay is a mechanical device and it takes a small amount of time for the contact to close after energizing the coil. To do what you suggest here assumes that we know exactly when that is going to happen and we don't. And even if we did, transitioning from the phase-shifted version of the audio to the non-shifted version will still incur a small discontinuity in the waveform without doing something really fancy, and another relay is not going to cut it._

 

You are right! Thats why I mentioned building a low-pass filter with minimal phase shift, this would allow you the reduced transition from minimally phase shift signal to the actual signal. This discontinuity will be much much less (depending on the phase shift) compared with transitioning from open to some audio signal directly (which could be large depending on how your potential on the open end is). At worst, you will hear a minuet crackle (which in my opinion is better than a pop. I guess I'm sounding like Rice Crispies). 

  Quote:


  Originally Posted by *amb* 
_ Actually, no. Trust me. Do a little measurement on your own. Hook up your DMM in AC milliamps mode in series with the connected ground, play a test tone and watch your meter and you'll get a reading of some amount of current. Now, disconnect the positive side of both outputs. Still got a reading?_

 

You are right again, however, I was not referring to only normal conditions where the open end will not effect the output to the headphones. I was talking more about cases like when the air is very ionized (usually due to high moisture content in air) where the voltage difference between ground and your open side will measure slight voltages. Given, these values will not be very large, but they exist. Test for yourself, measure ground and leave the other end free from a DMM and see if you get some stray voltage. 

  Quote:


  Originally Posted by *amb* 
_Static discharge is tens of thousands of volts or higher, arcing across air. Headphone amp DC offset is much smaller and cannot do that._

 

Of course I was exaggerating this situation with the point on static discharge (and was in no way trying to compare the magnitude of current flow compared with what you have in the headphone end). I was just pointing out that current may flow in/out of ground if you do have one end open. 

  Quote:


  Originally Posted by *amb* 
_Have a look at the schematics of almost any piece of audio equipment, and you'll find that the power switch only switches one side of the AC line and there is no need to switch both. Same principle -- it breaks the circuit, and we're talking about AC mains here, which is much higher voltage than any DC offset that this protection circuit is going to encounter._

 

Yes, I use the same principle for my amp, however, power supplies are different compared with outputting audio signals to headphones (where the sensitivity to stray voltages, whether it be ever so slight, could be annoying). What makese power supply circuits different from headphones is that power supplies usually have additional regulation components where noise will not effect the output much. Headphones don't have that luxury.


----------



## Garbz

Acutally isn't it common to use DPST switches to switch both L and N in the powersupply? This would be for safety purposes more then anything, especially in european countries like Austria where the power plugs can be inserted in either direction. 

 Also this would allow you to use a balanced powersupply where both wires are live, which was mentioned in the power filtering thread here last week.


----------



## guzzler

Here's a little single layer layout (2 wire links). Only for 24V supplies, as I've missed out the little regulation stage. I've replaced the very expensive BUF634 with just a simple transistor pair, save you a few pennies there 
	

	
	
		
		

		
		
	


	




 Also, I've replaced the pair of 2N3904s in the Darlington configuration with a single Darlington transistor. Looking at a ZTX605 in an E-line package (shown on board as a TO-92) for that position. Costs about 2.5x as much as 2 x 2N3904, but saves on board space. The whole board measures 66x24mm

 2D Layout





 Eagle3D Image: (not rendered relay, pot, or diodes)





 Eagle Layout File: here 

 Should be correct I think, haven't religiously checked it yet. Not built it yet, may etch one in the next few weeks. Enjoy...

 g


----------



## AtomBoy

Guzzler,

 Neat neat neat! Could you post the schematic (.sch)?


----------



## guzzler

It's just the original schematic, with the two transistors, the zener diode and two resistors missed out in the power supply. The transistor pair is just 2N3904/6 with emitter resistors and common base.

 g

 PS, if you can wait a while, I'll try and get one made so you can see what it's doing


----------



## AtomBoy

Thanks, Guzzler. Yes, I can wait. I guess. 
	

	
	
		
		

		
		
	


	




 I'm still learning Eagle, but I've done a version of the schematic here:

http://www.swcp.com/~atomboy/amps/e12.sch

 My parts are sized a little different from yours. I've got an order going at Mouser that I've been working from. I could post it if there's interest.


----------



## amb

Quote:


  Originally Posted by *AtomBoy* 
_I'm still learning Eagle, but I've done a version of the schematic here:

http://www.swcp.com/~atomboy/amps/e12.sch_

 

Your C6 is missing a connection to the base of Q3, and Q4 is wired wrongly (reversed collector and emitter).


----------



## AtomBoy

Quote:


  Originally Posted by *amb* 
_Your C6 is missing a connection to the base of Q3, and Q4 is wired wrongly (reversed collector and emitter)._

 

Thanks, amb! I've fixed it now (I found the mirror button for Q4 
	

	
	
		
		

		
		
	


	




 ). Updated .sch posted to original location. I almost had a .brd layout I wasn't embarrased to post. Maybe in a couple of days.

 Do you have any thoughts on replacing the BUF634 with the transistor pair ?


----------



## amb

Quote:


  Originally Posted by *AtomBoy* 
_Do you have any thoughts on replacing the BUF634 with the transistor pair ?_

 

Looks usable, I think I'll plop those parts onto my breadboarded prototype and see how well it works.


----------



## amb

I checked guzzler's dual-transistor rail splitter on the breadboard. Here is the schematic of just that portion:






 While this splitter basically works, there are some issues.

Since the transistor pair has a rather low input impedance, and the current flow into the transistors' bases is significant, it impacts the accuracy of the splitter operation. I tried reducing the voltage divider resistors from 10K to 1K and it helped a little but still insufficient. Also the lower value resistors increase the current draw of the circuit greatly.
Due to the open-loop nature of this buffer, the emitter resistors increase the output impedance of the buffer, which also affect the splitter's accuracy. I tried dispensing with the emitter resistors and just connected the transistors directly, and again that helped, but not enough.

 The net affect of these shortcomings is that the virtual ground potential changes significantly between just after power-up and when the relay kicks in. I measured a ~800mV "shift". Now, since the circuit is supposed to detect as low as 70mV offset, this large shift in the virtual ground would cause start-up problems with the circuit (i.e., the relay may not close at all after power up due to the skewed "ground", or it may click in and out in an oscillatory manner, depending on how the zero-reference trimpot is adjusted).

 Given this, I did a bit more testing and determined that the BUF634, while better than the dual-transistor splitter, also suffers a bit of virtual ground shift (probably also because it's open loop).

 I replaced the BUF634 with a OPA551 opamp hooked up as a voltage follower and the situation is much better. This is clear evidence that high input impedance, low output impedance, and closed-loop feedback all contributes to improved accuracy of the splitter, which is required for optimal operation in this circuit.

 I have changed the revision of this circuit to 1.1 and the following is the new schematic showing the switch from BUF634 to OPA551. The OPA551 is also less expensive than the BUF634 (8-DIP version $3.94 vs. $6.10 from Digikey).


----------



## guzzler

To lose even more money, at the expense of a couple of extra parts, would it be plausible to use a cheap opamp, say another TL071 (or use one TL072 for the whole thing), and the transistor pair method (thanks for the schematic there amb), and wrap those up inside the feedback loop of the opamp, to gain the high input impedance needed, with the low cost of that method? I’ll try and knock up a quick layout (and schematic, for once 
	

	
	
		
		

		
		
	


	




) later on today

 g


----------



## guzzler

Well, here's my altered version. I don't like it as much as the first one, but nothing comes for free 
	

	
	
		
		

		
		
	


	




 Dimensions are 52x37mm and leaves plenty of room for some mounting screws etc... _edit: estimated cost is £5 including a high quality relay_

 So, layout

 **WRONG**

 Eagle3D






 Revised portion of supply splitter:






Eagle .brd file 

 g


----------



## amb

guzzler, I played with your revised splitter a bit on the breadboard. The two transistors basically operate as a class B output stage, where there is a dead zone in the middle where neither transistor is on. This makes the virtual ground potential "touchy" and a bit unpredictable. I suggest the following:

 1. Eliminate the emitter resistors and tie the two emitters together.
 2. Add a 47 ohm resistor between the bases and the emitters of the transistors. This makes the opamp do a little of the work in that dead zone to keep things stable.

 Edit: this is not good enough, see post 97 below.


----------



## guzzler

thanks amb, I'll update the layout sometime

 g


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## AtomBoy

I hope y'all don't mind me following allong here. I've updated my copy of the schematic here:

http://www.swcp.com/~atomboy/amps/e12.Jul15.sch


----------



## amb

Quote:


  Originally Posted by *AtomBoy* 
_I hope y'all don't mind me following allong here. I've updated my copy of the schematic here:

http://www.swcp.com/~atomboy/amps/e12.Jul15.sch_

 

Looks fine.


----------



## guzzler

Final version (replaces one above, which was actually wrong). Actually quite like it now, except the long wire to the darlington

Eagle .brd file 

 Changed: updated to latest schematic (except 24V regulation stage), made ground continuous between channel tracks to reduce crosstalk.

 Beware of the Darlington transistor, I don't think the pinouts are standard. On the silkscreen, the corresponding pins are:

 1. collector
 2. base
 3. emitter

 I'd be interested to see some pics if someone etched one of these, I'm out of chemicals just now


----------



## amb

guzzler and Atomboy, before you go too far with this, I did more testing with the TL072 + dual transistor ground splitter and I have to say that it really doesn't work well. The TL072 has a very high output impedance (see its datasheet, it's got a big resistor in series with its output) and as a result the ground potential shift under load is still significant, and unstable (drifts all over the place). I tried substituting with a NE5532 (which has much better output current capability than the TL072) and it was better but the shift was still too big. To "fix" this would require making the two transistors operate in class AB rather than class B. This means adding diodes and resistors to bias them, and therefore also increasing quiescent current draw.

 I am convinced at this point that this is simply not a good way to go. The two transistors and biasing resistors/diodes will eat up more board space than a single DIP-8, so I really recommend staying with the OPA551 opamp like in my rev. 1.1 schematic. It works great and steady as a rock.


----------



## guzzler

Pain, that.... I'll have a further dig about for a more suitable opamp. If not, it'll be back to the amb rev 1.1 
	

	
	
		
		

		
		
	


	




 fitting in some biasing diodes wouldn't eat much space, but is starting to lose the cost advantage (not that diodes are expensive)


----------



## guzzler

Back to Rev 1.1, after some discussion it seemed the easiest way out of the impedance problems. Anyway, the Eagle .brd files are available here and here is a picture of the finished boards (on a rather shockingly red carpet):






 hope you like them


----------



## Pars

Nice Guzzler! You gonna have any extras left over or make more up? If so, I'd be interested.

 Chris


----------



## Pars

Amb, all,

 I began my own layout/schematic of this with the following enhancements, directed towards use of my Gilmore (or whatever) as a preamp:

 1) Manual mute switch bypassing the delay... a simple SPST switch in the relay drive leg

 2) Replacement of the R11/C7 and Q5/Q6 darlington pair with a 555 timer circuit driving the relay directly. This should allow for a wider range of delay with smaller caps (i.e., 1M resistor, 10uf tantalum for 11 second delay, etc.). The 555 will reliably drive one relay (has in my Counterpoint pre for >15 years). The 555 output goes low after the time delay, so is opposite the scheme used here, but is a drop in replacement. If a darlington pair to drive the relay is desired, a third 2N3904 functioning as an inverter is required.

 3) Reversing the relay, and substituting an RTE24012F DPDT with both NC and NO contacts accessible. By including jumper pads in the proper sequence(s), it should allow for both 

 a) the original audio thru the relay, or optionally, 
 b) the use of the relay to short the amp output to ground thru the NC contacts and a pair of Shottkys as Dr. Gilmore suggested, thereby not routing the audio thru the relay itself. 

 Additionally, when used in the normal fashion of the audio going thru the relay, a pair of jumper pads would be included to connect the NC contacts to ground, thereby shorting the outputs to ground (but not the amp itself). I would think this would be a good idea, but comments welcome.

 4) Inclusion of LED drivers (2N 3904 darlington pairs) for both delay/fault state and on state.

 Comments/criticism welcome. Also, I assume when Dr. GIlmre states: Quote:


  Originally Posted by *kevin gilmore* 
_I do a very similar thing except i don't like anything in series with
 the audio signal if at all possible. So i use the normally closed contacts
 on the relay and then tie the contacts between the audio outputs
 thru the relay to a pair of back to back shottky diodes. That way the dc level
 limits to +/-.3 volt until the relay pulls in._

 

that he means the Shottkys are in parallel but reversed from one another and connected to ground? And I would assume that you choose the Shottkys based upon the lowest forward drop? Any suggested Shottkys to use here?

 Thanks,

 Chris


----------



## amb

Quote:


  Originally Posted by *Pars* 
_1) Manual mute switch bypassing the delay... a simple SPST switch in the relay drive leg_

 

That's fine, or you could put the SPST switch in parallel with C7. When the switch is closed, it mutes immediately, but when the switch is opened, it goes through the delay interval before un-muting.

  Quote:


 2) Replacement of the R11/C7 and Q5/Q6 darlington pair with a 555 timer circuit driving the relay directly. 
 

That's fine too. There is no magic in the darlington transistor drive in my circuit, it's simple and does the job. Make whatever mod you desire in your version.

  Quote:


 3) Reversing the relay, and substituting an RTE24012F DPDT with both NC and NO contacts accessible. By including jumper pads in the proper sequence(s), it should allow for both 
 

The RTE24012F will work, and the smaller-footprint Omron G6A and G5V2 series relays should work well too. The Omron relays will fit the PCB layout from guzzler (but the Tyco RT series won't). While I am on the subject about guzzler's layout, he combined the Q5 and Q6 into a single darlington transistor. I recommend using a MPSA14 for that application. 

  Quote:


 a) the original audio thru the relay, or optionally, 
 b) the use of the relay to short the amp output to ground thru the NC contacts and a pair of Shottkys as Dr. Gilmore suggested, thereby not routing the audio thru the relay itself. 
 

The schottky scheme will still allow 0.3V or so of output, so it's not really a short. Since this circuit will protect against DC offset deviations as little as 70mV, that leaves a range of voltage from 70mV to 0.3V where the circuit has activated but still allows offset through to the headphones. Also, that 0.3V gap is still going to pass a thump of turn-on transient.

  Quote:


 Additionally, when used in the normal fashion of the audio going thru the relay, a pair of jumper pads would be included to connect the NC contacts to ground, thereby shorting the outputs to ground (but not the amp itself). I would think this would be a good idea, but comments welcome. 
 

This won't hurt anything, but isn't necessary either.

  Quote:


 4) Inclusion of LED drivers (2N 3904 darlington pairs) for both delay/fault state and on state. 
 

You can simply hook up an LED + resistor in parallel with D2 to indicate 'on' state, and an LED + resistor across the C-E junction of Q6 (or your 555 timer IC output equivalent) to indicate 'mute'.


----------



## Pars

Quote:


  Originally Posted by *amb* 
_That's fine, or you could put the SPST switch in parallel with C7. When the switch is closed, it mutes immediately, but when the switch is opened, it goes through the delay interval before un-muting._

 

Yeah, but I wanted the switch to be immediate (unless there is a fault).
  Quote:


  Originally Posted by *amb* 
_The RTE24012F will work, and the smaller-footprint Omron G6A and G5V2 series relays should work well too. The Omron relays will fit the PCB layout from guzzler (but the Tyco RT series won't). While I am on the subject about guzzler's layout, he combined the Q5 and Q6 into a single darlington transistor. I recommend using a MPSA14 for that application._

 

I'll take a look at those. Thanks!
  Quote:


  Originally Posted by *amb* 
_The schottky scheme will still allow 0.3V or so of output, so it's not really a short. Since this circuit will protect against DC offset deviations as little as 70mV, that leaves a range of voltage from 70mV to 0.3V where the circuit has activated but still allows offset through to the headphones. Also, that 0.3V gap is still going to pass a thump of turn-on transient._

 

Yeah, I realize that. Some people have commented that they didn't want a relay in the audio chain (one person who contacted me about doing boards backed out after learning that the audio was going thru a relay). I personally would rather have the protection in case I was running Grados or something at the time, as I would think 300mV would toast them at least in time. Having the fault LED might help save you though. Without just dead shorting the output, I don't see how this can be avoided. 0.3V is the lowest quoted value I have seen so far. Just drawing it up on paper (the configurable relay section) results in 3 jumpers per channel to do all of that. And, of course, you can use whatever wire you want to for the jumpers 
	

	
	
		
		

		
		
	


	



  Quote:


  Originally Posted by *amb* 
_You can simply hook up an LED + resistor in parallel with D2 to indicate 'on' state, and an LED + resistor across the C-E junction of Q6 (or your 555 timer IC output equivalent) to indicate 'mute'._

 

Where's the fun in that? 
	

	
	
		
		

		
		
	


	




 My goal was/is to make a more or less universal board that can be configured to suit whatever thoughts and/or biases the user might have.


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## Garbz

hey amb this thread just reminded me, this circuit couldn't be modified to work from a balanced source could it? my guess is the input of the comparitor would screw with something.


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## amb

Quote:


  Originally Posted by *Garbz* 
_hey amb this thread just reminded me, this circuit couldn't be modified to work from a balanced source could it? my guess is the input of the comparitor would screw with something._

 

If by "balanced source" you mean an amp with balanced outputs (like dynamid or dynamight), then the answer yes, the ε12 supports them without need for modification. See my web page for this circuit (or the beginning of this thread).


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## Pars

Quote:


  Originally Posted by *guzzler* 
_Back to Rev 1.1, after some discussion it seemed the easiest way out of the impedance problems. Anyway, the Eagle .brd files are available here and here is a picture of the finished boards (on a rather shockingly red carpet):_

 

Guzzler,

 I may be daft, but looking at your .brd file while doing my layout, for Q1 and Q2, are the transistors reversed? If the layout file is from the top (this is how I think that Eagle works... could be wrong), it would appear that the V+ and V- board inputs are connected to the emitters of the NPN and PNP when they should be connected to the collectors? Comparing the board file to your pics of the finished boards, that would at least appear to be the case with std. 2N3904s/3906s (std. E-B-C pinout)? 

 Thanks,

 Chris


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## guzzler

Looks alright to me Chris. Which way up is your standard pinout? The pinout I have here from the Rapid Electronics catalogue is with the D facing down the way (like a bowl), with E-B-C left to right along the top. With the layout, the D is facing up so pins should be the other way round. I've attached the relevant portion of the board; check the other transistors as well


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## Pars

Quote:


  Originally Posted by *guzzler* 
_Looks alright to me Chris. Which way up is your standard pinout? The pinout I have here from the Rapid Electronics catalogue is with the D facing down the way (like a bowl), with E-B-C left to right along the top. With the layout, the D is facing up so pins should be the other way round. I've attached the relevant portion of the board; check the other transistors as well_

 

According to your .gif, they are backwards. See the chop of an ST datasheet attached, but with the flat of the "D" down, the pins are E-B-C across left to right.


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## amb

(guzzler's layout) Yup, Q1 through Q4 are reversed. The darlington is correct for a MPSA14 or ZTX604/605 though.


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## Nisbeth

Quote:


  Originally Posted by *amb* 
_(guzzler's layout) Yup, Q1 through Q4 are reversed. The darlington is correct for a MPSA14 or ZTX604/605 though._

 

<slightly OT> Are there any other options for that darlington beside these mentioned?


 /U.


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## Pars

Nisbeth: A pair of 2N3904 (NPN) or 2N3906 (PNP) (or countless equivalents) as in the original schematic... but you probably already knew that


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## Nisbeth

Quote:


  Originally Posted by *Pars* 
_Nisbeth: A pair of 2N3904 (NPN) or 2N3906 (PNP) (or countless equivalents) as in the original schematic... but you probably already knew that 
	

	
	
		
		

		
		
	


	


_

 

Yup, I did 
	

	
	
		
		

		
		
	


	




 I have a PCB for the circuit so I need a darlington that fits and I can't seem to find the MPSA14 or any of the ZTX-variants anywhere near me 
	

	
	
		
		

		
		
	


	





 /U.


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## amb

Any TO-92 NPN darlington with EBC pin assignments and Ic > 100mA, Vce > 25V, Hfe > 5000 should be fine. Such as:
 KSP13, KSP14, KSP25, KSP26, KSP27, MPSA13, MPSA27, MPSA28, MPSA29, 2N6426, 2N6427, 2N7052, 2N7053, BC372, BC373

 These are also good, but the pin-out is reversed (CBE):
 BC517, BC618


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## guzzler

Sorry about that folks. The catalogue listed it as TO92B, which is (from your point of view, Chris) C-B-E. The only thing I can think of is that the one Rapid lists is made by Continental Devices, but I should have thought the 2N3904 pin out was standard.... Thanks for pointing that out


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## Nisbeth

Quote:


  Originally Posted by *guzzler* 
_Sorry about that folks. The catalogue listed it as TO92B, which is (from your point of view, Chris) C-B-E. The only thing I can think of is that the one Rapid lists is made by Continental Devices, but I should have thought the 2N3904 pin out was standard.... Thanks for pointing that out_

 

Does that mean that the pinout on the board is correct when using BCxxx devices?


 /U.


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## guzzler

Yes, they should be fine. Just check the GIF above to make sure, it's the same for the NPN and PNP


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## Pars

Quote:


  Originally Posted by *guzzler* 
_ The only thing I can think of is that the one Rapid lists is made by Continental Devices, but I should have thought the 2N3904 pin out was standard._

 

Yeah, you would think you wouldn't even have to think twice about tossing in some bog standard device like this! Really nice tight layout guzzler (which was why I was using it as a reference to begin with 
	

	
	
		
		

		
		
	


	




 )... should be an extremely easy change, and if these things actually are available in a TO-92B package with C-B-E pinout, then any boards you had made up could still be used, or std. TO-92 pkgs could just flip the transistor and bend the base leg to fit OK.


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## Pars

Per my previous posts on additions to Amb's e12, see the attached schematic and board images. Any comments/suggestions are welcome (be kind as I'm kinda new to Eagle and board layout/design). Thanks to guzzler for starting me in on this
	

	
	
		
		

		
		
	


	




 And of course to Amb for the design.

 Chris


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## amb

HI folks, I finally found the time to assemble and test this circuit on one of guzzler's boards (shown in post #99 above). Aside from the reversed Q1 through Q4, I found another minor error in the layout. The negative power rail is wrongly connected to pin 5 of U1 (the OPA551) instead of pin 4. Fortunately pin 5 is unused on the OPA551. So, to fix this, you can just solder a short jumper wire and bridge pin 5 to pin 4.

 I also found that the Omron relay seems to have a lower pull-in voltage than the Tyco unit I previously tested with, so this results in a rather short delay time after power-up. Thus, I suggest changing R11 from 221KΩ to 475KΩ.

 So in summary, if you are building on guzzler's board, you need to note the following deviations from the "officlal" v1.1 version found on my web site:

Instead of the Tyco RTE44012F relay (which doesn't fit), I recommend either the Omron G6A-234P-ST-US-DC12 or G5V-2-H1-DC12. Both are available from Mouser and Digikey.
Q5 and Q6 have been replaced with a single darlington transistor. I recommend MPSA14, but you can also use any of the alternatives listed in post #113 above.
If you use 2N3904 for Q1, Q3 and Q4, and 2N3906 for Q2, you must install them in exactly the opposite orientation than as marked on the board. If you use other transistors, check the pin-out against the layout.
Install a jumper wire between pins 4 and 5 of U1 (OPA551).
I suggest changing R11 from 221KΩ to 475KΩ.
Parts on the board are packed very tight, so be sure to choose parts that will fit their outlines and hole spacings.

 Here is a picture of the assembled board:






 Here is the bottom of the board showing the added jumper:


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## guzzler

What a silly mistake about power pins, wonder what kind of person would do that 
	

	
	
		
		

		
		
	


	




 Thanks for picking that up amb, much appreciated (and the design too 
	

	
	
		
		

		
		
	


	




)


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## Pars

Per some previous posts, I have still been working on a layout for an enhanced version of Amb's board and would request any comments on this. The board is appoximately 3" x 2.2", and features the following additions:

 1) Provision for a manual mute switch bypassing the delay

 2) Addition of fault/mute and on LEDs

 3) Use of a 555 timer instead of the resistor/capacitor time constant for driving the relay and setting the delay. A 1M Rt and a 4.7uf Ct gives a dalay of about 5.1 seconds, etc.

 4) Will accomodate either the newer Omron relay package (smaller) or the original Tyco form factor relays, making it easier to source

 5) Does not emply the pre-packaged Darlingtons, but rather 2N3904s and (1) 2N 3906 to make parts sourcing easier (E-B-C TO-92 package). Packaged Darlingtons could certainly be used if you have them laying around.

 6) Configurable relay: Per some concerns (not mine) about running the audio signal thru a relay, there are jumpers which allow the relay to be configured to short the output, with the audio not going thru the relay (unless shorted). Provisions for paralleled back-back BAT43 Schottkys are there if you don't want a dead short. As Amb noted, this will limit the offset seen on the output to 0.3V (300mV), so low impedence cans will not be completely protected in this configuration.

 Schematic:





 Board:





 Eagle files here:
board
schematic


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## amb

Pars, your changes look ok to me. Did you try breadboarding it to see if it works as intended?


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## Pars

Thanks Amb. Yes, I have breadboarded everything except the power supply portion (don't have an OPA551 laying around at the moment). I had questioned the need for a couple of the diodes (D4 and D6), and although I don't think they're really needed I left them in. I home-etched a board last nite and will be prototyping it... may have a few commercially done also (if so will send you one). PM me if you have any concerns, etc.

 As far as a choice of Schottky goes, I just picked something in a reasonable pkg. with the lowest Vf I could find and the BAT43 seemed to meet those criteria. I personally won't build mine that way though.


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## Nisbeth

Nice work, Chris 
	

	
	
		
		

		
			





 A little nitpicking though, just because I can't help myself 
	

	
	
		
		

		
		
	


	




 Your groundplane renders nicer if you:
 - Put another angle in the trace running from LED2 to Q8 so that it just slinks around LED1
 - Turn Q7 so that it faces the same way as Q1
 - Put another angle in the trace from D5 to R12 (so that it runs closer to the groundpad on C6)

 Also, I think you should move the bottom right mounting hole to the corner of the board. You might have to move LED2 a little but it's not much (and if you think in 3D it's probably not necessary at all, since the LED will be above the board surface) 
	

	
	
		
		

		
		
	


	





 /U.


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## Pars

Thanks Uffe. I'll take a look at it. I intended to leave plenty of space around the LEDs for some Lumex lightpipes...


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## ottopig

Okay, now that all of you who really know what you are doing have done all the heavy lifting and made it workable for us noobies, are there any plans for a group buy or another way to obtain an "e12" board besides etching or ordering our own? 

 I'm really interested, with an M-H and and M3 each at home and work, and four M-H waiting for cases and then to be given away. That's six. PM me if your answer is too "commercial" for the DIY section.......

 ottopig


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## Pars

Quote:


  Originally Posted by *ottopig* 
_Okay, now that all of you who really know what you are doing have done all the heavy lifting and made it workable for us noobies, are there any plans for a group buy or another way to obtain an "e12" board besides etching or ordering our own? 

 I'm really interested, with an M-H and and M3 each at home and work, and four M-H waiting for cases and then to be given away. That's six. PM me if your answer is too "commercial" for the DIY section.......

 ottopig_

 

After I finalize the layout and build a prototype, I might consider having some boards run. The extras would be at cost plus shipping, so I don't think that would violate the "commercial" aspect of things. I'll let you know. Note that it will probably take me at least a month for the prototype as I don't have all the parts and the wife seems to have plans for any leftover money presently


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## n_maher

Any thoughts of Group Buys should be run past a mod before opening it up for public discussion. See Rick's post in the Dynalo thread.

 HTH,

 Nate


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## Pars

Work is progressing on the home-etched prototype. The board came out OK (first time doing a 2-sided board at home). I currently have the 555 timer/relay drive circuit functioning (works great!), with the mute switch and the two LED drivers. A few things I found were:

 1) Didn't have enough space between the pot (VR-1) and the OPA551. Rectified in latest board layout.

 2) I was using the R207/7 resistor layout in Eagle which provides 7.5 mm of space. This is pretty tight with anything other than small resistors such as generic metal film, Yageo, etc. My preferred BC 0.4W will barely fit. Don't even think about Dale RN60s. I am thinking of increasing the resistor package to R207/10 with 10mm spacing. This should allow RN60s. I figured most people would build these with whatever they have laying around, and RN60s are popular. Any thoughts? Board size would not increase.

 3) Relay problems: I hadn't noticed that the Tyco/P&B relay originally used (RTE44012F or RTE24012F) has a *different pinout* than the Omron G6A or G5A-V2. The P&B has NC-C-NO whereas the Omron is C-NC-NO. Gut instinct tells me the Tyco/P&B pinout is more common. In skimming thru the Digikey catalog it doesn't appear that anyone other than Omron uses that pinout. Also, I noticed that Omron had some specific Telecom grade relays that are even smaller than the G5/G6 ones. I was under the impression that most audio manufacturers used Telecom grade for their reliability? At any rate, should I dispense with trying to accomodate more than one relay type, or if I continue with this, what relays should I try to support?

 Thanks for any thoughts/recommendations.

 Chris


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## steinchen

2)
 my vote: increase the resistor package to R207/10, since the board size stays the same it adds versatility without a drawback

 3)
 I already bought Tycos, but Omrons are available in Germany in contrast to the Tycos. All other parts should be easy to source round the world, so I think it would be great if the board supported both pinouts to make part sourcing easy for DIYers throughout the world.


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## amb

Hi folks, just as a FYI, I am adopting guzzler's modifcations as version 1.2 of this circuit, and will be updating the schematic and parts list on my web site very soon.


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## Dougigs

That's a very impressive PCB design, and a nice demonstration of communal circuit-design work. I currently have two of my amps 'protected' with a single-transistor + relay delay/muting circuit I hacked together on protoboard, and this is definitely going to replace it. Over the course of this thread I've gone from yawning disinterest in yet another power-on-delay-muting circuit to a real interest in this one, which is very nicely tailored to the world of headphone amps.

 One teeny concern: If I'm hooking this up to an amp that has a bipolar supply (rather than simple + and -), do you think I'd be better advised to power it from both rails (i.e. skip the virtual-ground circuit)? My concern being that throwing its current draw on the + rail but not the - could create some asymmetry... this would not be a problem if the amp's supply is hugely overspecified with plenty of extra VAs available on both rails, but could be a problem if it's anywhere close to its limit, in which case this could pull down the voltage on the + supply without doing same for the -.


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## amb

Quote:


  Originally Posted by *Dougigs* 
_One teeny concern: If I'm hooking this up to an amp that has a bipolar supply (rather than simple + and -), do you think I'd be better advised to power it from both rails (i.e. skip the virtual-ground circuit)? My concern being that throwing its current draw on the + rail but not the - could create some asymmetry..._

 

The beauty of the virtual ground circuit is that it _is_ symmetrical with respect to current draw on the rails. Since the circuit has no connection to "real" ground, all draw from one rail must go to the other rail. In this circuit, the relay would have caused more draw on the positive rail than the negative rail if it was powered the conventional way. But the virtual ground neatly solves that problem.


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## Pars

Quote:


  Originally Posted by *Dougigs* 
_That's a very impressive PCB design, and a nice demonstration of communal circuit-design work. I currently have two of my amps 'protected' with a single-transistor + relay delay/muting circuit I hacked together on protoboard, and this is definitely going to replace it. Over the course of this thread I've gone from yawning disinterest in yet another power-on-delay-muting circuit to a real interest in this one, which is very nicely tailored to the world of headphone amps.

 One teeny concern: If I'm hooking this up to an amp that has a bipolar supply (rather than simple + and -), do you think I'd be better advised to power it from both rails (i.e. skip the virtual-ground circuit)? My concern being that throwing its current draw on the + rail but not the - could create some asymmetry... this would not be a problem if the amp's supply is hugely overspecified with plenty of extra VAs available on both rails, but could be a problem if it's anywhere close to its limit, in which case this could pull down the voltage on the + supply without doing same for the -._

 

Not sure which layout you're referring to (Guzzler's or mine), but I'll reply anyhow





 Regarding your concern when used with an actual dual-rail supply, this was my intended use also. My observations in building both a circuit on Guzzler's board and on the home-etched board that I am currently building is that I do see some pulldown on the +V side, at least on the muting board itself. I would be very surprised to see any pulldown on my Gilmore PSU from this, but I haven't tried that just yet. Please note that I do not have any OPA551s handy, so was just testing it with a TL071 in the splitter portion (which is inadequate, as had been noted previously). With the relay energized on my board, the rails on the muting board drop from nearly balanced at around +/- 11.6V to +8.6/-13.4 or so (this was on my board as I also don't have the small form factor Omron relay to finish testing Guzzler's board). Also, from my initial testing, it appears that it is not possible to balance badly mismatched rails via the pot, but the general idea with the pot is to match the board ground to the amp ground as closely as possible. I am using a bench supply setup for +/- feeds and just feeding it the +V/-V sides without tying its ground in (i.e., set for ~+/- 15.5Vdc and feeding it ~31Vdc between the rails). Perhaps its something wrong with my test setup. 

 Please correct me if I'm wrong on any of this Amb. The circuit in my testing on a breadboard really does work as advertised WRT offset detection and shutting things down. I am in the very early stages of testing, and obviously am still without some parts


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## amb

Pars, if you use a OPA551 you'll find that the problem you encounter to essentially disappear. The TL071 does not have low-enough an output impedance to serve as an effective virtual ground splitter that is being asked to sink many tens of milliamperes without some voltage shift.


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## Dougigs

Quote:


 The beauty of the virtual ground circuit is that it is symmetrical with respect to current draw on the rails. Since the circuit has no connection to "real" ground, all draw from one rail must go to the other rail. In this circuit, the relay would have caused more draw on the positive rail than the negative rail if it was powered the conventional way. But the virtual ground neatly solves that problem. 
 

Sorry, I think I wasn't clear enough. I am indeed now a believer in rail-splitter circuits (from my good experiences with the M3 and the MINT). I had been concerned with the possibility of attaching this thing to the + and ground rails of an amp that has + and ground and - rails -- its current draw with the relays on seemed to be enough to load one rail. I have some amps and DACs with true bipolar supplies that I'd want to do this with.

 But my question is moot if it is indeed possible to hook this up across the + and - rails rather than the + and ground. The question is, is this circuit happy with 36 volts across it? The OPA551 has a stunning 60V supply capability, so maybe that's the case (and maybe you've discussed this already and I've missed it).


----------



## amb

Dougigs, this circuit is intended to be powered from the + and - rails of the amp. Its V- is not intended to be connected to the amp's ground, whether real or virtual.

 As for voltage handling, yes, the OPA551 can handle lots of it, but even before that, the pair of 2N3904/3906 serves as a voltage regulator so the OPA551 never even sees any more than +/-12V.


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## amb

I will now discontinue participation in this thread. Here is the
new ε12 discussion thread at Headwize.


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## Pars

Quote:


  Originally Posted by *amb* 
_As for voltage handling, yes, the OPA551 can handle lots of it, but even before that, the pair of 2N3904/3906 serves as a voltage regulator so the OPA551 never even sees any more than +/-12V._

 

Not to mention the 24V zener which also serves as part of the voltage regulation and limiting circuit. I believe that Amb had stated in his initial posts for the design that it could handle up to +/- 30Vdc, or 60 Vdc cumulative, but that if you were powering at +/- 18V or more to put TO92 heatsinks on Q1/Q2.

  Quote:


  Originally Posted by *amb* 
_Pars, if you use a OPA551 you'll find that the problem you encounter to essentially disappear. The TL071 does not have low-enough an output impedance to serve as an effective virtual ground splitter that is being asked to sink many tens of milliamperes without some voltage shift._

 

Yes I did note that it was inadequate for this duty in my post and what I saw was not unexpected. 
	

	
	
		
		

		
			





 I really need to get an order in to Digikey...


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## Voodoochile

Quote:


  Originally Posted by *amb* 
_I will now discontinue participation in this thread. Here is the
new ε12 discussion thread at Headwize._

 

Do you want this one closed also? Just let me know if you do.


----------



## amb

Hi voodoochile, go ahead and close the thread. If other members want to talk about this circuit here they can open new threads.


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## rickcr42

I would leave it open Mark so there is some continuity rather than 25 threads on the same topic


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## Voodoochile

...cool...


----------

