# Itsy Bitsy TPA6120 Amp



## error401

Well, someone just _had_ to make the suggestion that I try this chip out, actually as part of my PGA2311 preamp project. I've got to hear it first before I commit to the boards for the other project, so I've done up a little board to try it out. Could potentially make a decent portable amp, but battery life will not be fantastic (TPA6120 eats up 23mA, OPA551 another 8.5mA). Should sound great though, assuming I haven't made any design errors...

 I'd also like to get at least one successful fab job under my belt before going all out on the preamp boards.

 It's set up for a gain of 5, with buffered virtual ground by OPA551. The plan is to fit a Hammond 1455C802 (50mm x 80mm) with room to squeeze 2x9V batteries in. I'm not totally certain this will be possible; the current layout is 32mm in length, and 9Vs are nominally 48mm long - leaving little room for the connectors. It may be possible to squeeze it in though. Unfortunately the TPA6120 isn't specified to run at +/-4V, but may work anyway, for single 9V operation. I don't think I can squeeze any more space in that dimension due to the necessary connectors. The ground plane is unbroken except for one pesky V- trace I couldn't find a way to reroute. It doesn't split things too badly though.

 No coupling caps are onboard, and 10R output resistors are placed as suggested in the TPA6120 datasheet. Because these are for opamp isolation, I don't think they serve their purpose if placed inside the feedback loop, so they are outside the loop. They shouldn't impact available current when driving low impedance phones.

 This chip is supposed to be one of the best sounding single-chip amps around, I have high hopes for this simple design. I'll be placing an order with Olimex this afternoon for one eurocard, which should fit 8 boards. If anyone wants boards, I'll eat the shipping and send to US/Canada for $6.

 As everything I do, this design is open. If anyone wants gerbers or the Altium files, let me know.

 Comments before I go to fab?

*The main info post is down the thread a bit and I'll be updating it as changes are made.*


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## holland

2 Batteries won't fit in the 1455C802, FWIW. The width is not big enough, look at the schematic for the case. The flutes take space, as does the screw hole. You need to go wider. I've got about 7 of these cases for different things, and I've already tried. 

 Nice choice on the opamp. I was going to get some to try myself. Keep us posted.


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## error401

Quote:


  Originally Posted by *holland* /img/forum/go_quote.gif 
_2 Batteries won't fit in the 1455C802, FWIW. The width is not big enough, look at the schematic for the case. The flutes take space, as does the screw hole. You need to go wider. I've got about 7 of these cases for different things, and I've already tried. 

 Nice choice on the opamp. I was going to get some to try myself. Keep us posted._

 

Yea, misread the dimensions of course. Not a chance of getting two 9V in there, and I'd rather not go to the next longer case, the 1455C1202 (though this should definitely work). Some of the component values are overly optimistic too, 680uF and 120uF are both too large. 330uF and 100uF will fit.

 A DigiKey BOM: http://spreadsheets.google.com/pub?k...j8iJOBIDEoN9Mg totals to $21.77 CAD, including stepping up to min qtys. I adjusted 200K voltage divider resistors to 100K to reduce line item count (uA of current don't really matter when Iq is so high), and subbed cap values that will actually fit the board.


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## error401

I want to go ahead with this, but I'm niggled by the fact that I can't find any appropriate enclosure that can accommodate 2x9V without being way too large for a portable amp. I could potentially switch to AA's plus a DC-DC converter, and I was looking at the Serpac H-45 or H-65 with battery holders, but the technical drawings are awful and I couldn't deduce the available board size from them. The Hammond _might_ squeeze a 3xAAA holder with the current board, but it would be tricky to fit the necessary DC-DC in the minimal available room.

 Any suggestions on portable enclosures?


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## MisterX

Quote:


 Any suggestions on portable enclosures? 
 

Newark #26K8988 

 Would look kinda like this---->


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## Seaside

error401, nice design.
 Is it virtual ground circuit using op, and cmoy style amp with 6120, right?
 And what is supplied voltage?

 As a noob, I do not think I have a lot to tell you about the amp. But I have made an amp using that chip before, and that experience suggests me there's few things to consider when I have to deal with that chip. 

 I think you already knew this chip is not for portable amp. That chip is very nice and powerful chip, but sort of dangerous, which means it can draw quite large amount of current swing/spike to the point where it can demage your phones (Talking about over 2watts of current spike each channel if something goes wrong). So, be conservative when you set the gain... let's say... max 5 or 6? For the same reason, i recommend bigger electrolytic caps and transformer, if that is possible. I am not sure if that opamp based virtual ground can handle the current draw without crapping out or not. I used a power section similiar to that of C2KIII. 

 It can produce fair amount of heat when supplied voltage is high. Supplying this chip more than +12/-12 might not be a good idea. 
 And just like other picky OPs, this chip can be picky too. You may need a small ceramic capacitor along with that chip. Place ceramic caps and resistors very close to that chip to minimize possible interference. That will also help smooth somewhat harsh sound of this chip.

 Just my suggetions.


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## error401

Quote:


  Originally Posted by *Seaside* /img/forum/go_quote.gif 
_error401, nice design.
 Is it virtual ground circuit using op, and cmoy style amp with 6120, right?
 And what is supplied voltage?

 As a noob, I do not think I have a lot to tell you about the amp. But I have made an amp using that chip before, and that experience suggests me there's few things to consider when I have to deal with that chip. 

 I think you already knew this chip is not for portable amp. That chip is very nice and powerful chip, but sort of dangerous, which means it can draw quite large amount of current swing/spike to the point where it can demage your phones (Talking about over 2watts of current spike each channel if something goes wrong). So, be conservative when you set the gain... let's say... max 5 or 6? For the same reason, i recommend bigger electrolytic caps and transformer, if that is possible. I am not sure if that opamp based virtual ground can handle the current draw without crapping out or not. I used a power section similiar to that of C2KIII. 

 It can produce fair amount of heat when supplied voltage is high. Supplying this chip more than +12/-12 might not be a good idea. 
 And just like other picky OPs, this chip can be picky too. You may need a small ceramic capacitor along with that chip. Place ceramic caps and resistors very close to that chip to minimize possible interference. That will also help smooth somewhat harsh sound of this chip.

 Just my suggetions._

 

Yes, this is basically a CMoy with a buffered virtual ground; though CMoy is really just 'non-inverting amplifier based on a single opamp', even an LM3886-based amp looks like a CMoy on paper 
	

	
	
		
		

		
		
	


	




. Given the specs on this chip I don't really feel that separate input and output stages are necessary. I plan to supply 18-20VDC, though I think I will now add a linear regulator onboard and increase board size slightly (32mm -> 40mm), since I can't find a way to make this portable (and power draw is unreasonable to do so anyway). I took a look at going to 3xAAA in the larger Hammond enclosure, but the power supply got too complicated for my liking, and added too many variables for me to 'hear' the 6120, which was the point of this exercise at the start. Final boards will probably accept DC input of ~20V, regulate it down to ~18V via an LDO and have additional capacitance onboard. I'll modify the board accordingly this evening, and then I'll probably be satisfied going to fab.

 Thanks for the advice, gain is set for 5 but if I do increase board size that will give more room for capacitance as well. I've followed the instructions in the datasheet regarding layout of the ground plane for stability, I've placed V+ and V- decoupling for both sides of the TPA6120 (and for OPA551), and the layout is nice and tight around the chip so I think it will be okay stability-wise. I'm hoping that it will be possible to solder the power pad to the ground plane by using the vias I placed underneath it to heat sink it.

 I'm not sure about the buffered ground though, I've never built one before and I'm wondering if input/output caps are required in this situation. I'm thinking that input caps may be required, but I don't fully understand the implications. I think OPA551 current should be enough, and I don't know any way to build a buffer that's anywhere near as good with a small parts count. A full-on diamond buffer would take as much room as the rest of the amp itself. At full 80mW output of TPA6120 it would be asked to sink/source 50mA RMS, so I think I will leave this chip.


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## error401

I've made some changes and I think I can consider this final:
Board widened from 32mm to 40mm
940uF FM rail capacitance (not 100% sure this will fit, may end up being 660uF or 16V caps instead)
Virtual ground with TLE2426 (SO-8 with Cnr) isolated and buffered for output by OPA551
Onboard 3-terminal fixed regulator (78L15)

 I plan to supply approximately 18V from a cheap switching wallwart. My simulations on the power supply indicate that the 180mV of ripple will be easily quenched by the simple supply I implemented (the sims show a ruler-flat output for the passives I've used), and this will not require finicky unregulated supplies to be used. For critical listening I'll probably hook my σ22 directly to the provided test points, but really even this should not be necessary.

 I'm about to put in a fab order at Olimex for just a single eurocard. Assuming there's nothing heinous wrong with my output I should have boards in a couple weeks.

 The BOM at DigiKey comes out to just over $40 USD, complete including case and wall wart.

 Some files you might like too:
Fab Outputs (gerbers) (these files are not up to date, PM me if you want current ones)
Nice PDF schematic/layout










 Edit: Okay, scratch the order for now. Olimex's annular ring requirements make it pretty much impossible to mount TO-92 parts straight on. How bad is that?! Getting a quote from another fab house while I see if I want to bother offsetting them and redoing most of the pads.

*Update Oct 05 2007*
 See post #22, I've got this working, which required that I change the 100K resistors to 5K due to input bias on the TPA6120 causing excessive DC offset. Other than that, everything is working fine for me with OPA551 on ground and it sounds great! Not as good as my full-fledged mini3, but still drives all my cans quite respectably. I'd say great for its diminuitive size, this would be great to integrate into something like a preamp where a headphone out is desirable, but not the main purpose.

 I don't think I will be using it in my preamp though, due to how finicky it seems to be to work with. Definitely don't want to be plagued by DC offset and the like in that application. Nothing against the sound of it though.

 If you want a board, I have 3 left (Oct 15/07) that I'll part with for $9 a piece, shipped. *If you build this, please verify the parts against the schematic, as I'm not positive the silkscreen matches the BOM.*


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## holland

There seems to be some decoupling caps not listed in the BOM, though the quantity can cover them. C3 and C11 at least.

 Some n00b questions.

 I don't know if you want to run some vias for debugging/probing. Is ground a layer or a trace? Just curious, I'm sure it's fine either way.

 Why do you have 2 parallel caps (C2 and C12)? Size and cost?

 You may be able to save some space with vertical mounted jacks, and possibly volume though I don't know if the power switch will work with it. It'll take about half the space and allow you to squeeze some other things in there, be it some cap or the rail splitter or even a power LED.

 As for portability, I think it's possible. Regulator is fine, but I would add a charging circuit then. FWIW, the Little Dot MK1 uses the TPA6120 as a buffer/driver for the LM4562 with 4x3.7 LI batteries. The MK1 is replacing the LDM+ and is the portable line. Since you're going with a wider case, can't you run 2 9V batteries? Can you fit 10+ AAA NIMH batteries? Sorry, I'm feeling lazy and don't feel like pulling up your case dimensions. 
	

	
	
		
		

		
		
	


	




 Edit: Loaded up digikey and loaded the PN. It's the 1455C802, so nvm.


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## error401

Quote:


  Originally Posted by *holland* /img/forum/go_quote.gif 
_There seems to be some decoupling caps not listed in the BOM, though the quantity can cover them. C3 and C11 at least._

 

Good catch, updated the qty but not the designators.

  Quote:


 I don't know if you want to run some vias for debugging/probing. Is ground a layer or a trace? Just curious, I'm sure it's fine either way. 
 

Ground is the bottom layer. It's broken for a couple of traces, any through-holes not connected to ground, and underneath the TPA6120 and OPA690 to reduce parasitic capacitance (per the TPA6120 datasheet). Also, the output ground is buffered and is therefore a trace. The bottom layer is the blue fill in the images, top is red.
  Quote:


 Why do you have 2 parallel caps (C2 and C12)? Size and cost? 
 

Size indeed. There was room for more capacitance, and I thought the existing 330uF was definitely on the low side, so I increased the diameter of the caps and doubled them up. It also reduces ESR (R || R = 0.5R), though larger caps would have less ESR as well.

  Quote:


 You may be able to save some space with vertical mounted jacks, and possibly volume though I don't know if the power switch will work with it. It'll take about half the space and allow you to squeeze some other things in there, be it some cap or the rail splitter or even a power LED. 
 

Yes, I thought of using Panasonic EVJ series, but then a power switch would be required. It's a wash, and I prefer the switched pot. Vertical jacks aren't a bad idea, but I don't feel the need for space. There's probably enough room between jacks to fit an LED, but I dislike having them on everything, and if I were to place one I'd probably airwire it at the back of the case where the DC comes in.

  Quote:


 As for portability, I think it's possible. Regulator is fine, but I would add a charging circuit then. FWIW, the Little Dot MK1 uses the TPA6120 as a buffer/driver for the LM4562 with 4x3.7 LI batteries. The MK1 is replacing the LDM+ and is the portable line. Since you're going with a wider case, can't you run 2 9V batteries? Can you fit 10+ AAA NIMH batteries? Sorry, I'm feeling lazy and don't feel like pulling up your case dimensions. 
	

	
	
		
		

		
		
	


	



 

The 1455C802 is very small, 50mm x 80mm x 20mm. 2x9V won't fit either way, nor will even 2xAAA with holder. All the other enclosures I looked at I didn't like or were too large, so I've given up on making this portable. If you want to take the design and put it in a huge case with 10xAAA, be my guest 
	

	
	
		
		

		
		
	


	




. If you jerry-rigged it, you might squeeze a small DC-DC and this board in a Serpac H-65, but I couldn't read their technical diagrams (they're hideous) to actually design around it.

 PS. I couldn't help myself and fixed the board to fulfill Olimex's requirements, and sent off a request for a quote. DigiKey order placed for a whole ton of stuff. Wee, winter's here!


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## holland

Quote:


  Originally Posted by *error401* /img/forum/go_quote.gif 
_If you want to take the design and put it in a huge case with 10xAAA, be my guest 
	

	
	
		
		

		
		
	


	




. If you jerry-rigged it, you might squeeze a small DC-DC and this board in a Serpac H-65, but I couldn't read their technical diagrams (they're hideous) to actually design around it._

 

Haha! No thanks. 
	

	
	
		
		

		
		
	


	




 I need to get off of portables and building real amps, my Mini3 builds will be my last portable. I am curious to know what case the Little Dot MK1 uses. It's been used on other amps as well, and I had it written down a while ago, but lost it unfortunately.

 If anything, I would build around LI, just to get away from NiMH. Maybe I can get one of those exploding batteries that caused all the recalls for laptops.


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## error401

PCBs have been ordered from CustomPCB. I've ordered 12 pcs, at a rather high cost of $8 each, but they took my Protel files directly and I didn't have to fiddle around. As a bonus they fab and ship much faster so I should have boards in a week or less.


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## holland

May I ask what you used for simulation?


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## error401

Quote:


  Originally Posted by *holland* /img/forum/go_quote.gif 
_May I ask what you used for simulation?_

 

I usually use Protel's built in simulation since it has models for a lot more components, but I also often use SwCAD III since it's easier to put together a simple one-off sim. In this case I used SwCAD.


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## holland

Ah, cool. I never used those tools, so I didn't know. I'll make a mental note for future reference. Thanks! I was initially wondering if one would have to write some RTL and simulate it on a Linux farm (I use this at work).


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## bennet

sweet amp


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## dBel84

just a curiosity, why the soic TLE2426 and not the TO92 package? ..dB


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## error401

Quote:


  Originally Posted by *dBel84* /img/forum/go_quote.gif 
_just a curiosity, why the soic TLE2426 and not the TO92 package? ..dB_

 

They take up about the same board area, but the SO8 version has three advantages - it only affects the top layer, it offers use of the noise reduction capacitor, and it didn't undergo a perceived shortage a few months ago 
	

	
	
		
		

		
			





. I also happen to prefer working with SMD 
	

	
	
		
		

		
		
	


	




.


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## error401

I should really read datasheets before I choose parts. OPA690 is rated for max 12V operation, so it's out.

 I've looked around and I think the THS3091 looks to be a good substitute, it has low noise, high slew rate and can drive 250mA. If you're not comfortable using this chip, OPA551 should be a drop in replacement, but may require lifting/cutting the pin 8 trace pulled high for ~DIS on the OPA690. For now I will change the BOM to OPA551, *just note that pin 8 is not compatible with the board*. Driving FLAG high probably won't break anything, but the trace should really be cut or pin removed. Once I test the THS3091 and find it stable, I will likely make a permanent change due to much improved specs across the board (except possibly noise). If you do choose to use THS3091, R1 should be increased to 1.78K or more to improve stability.

 Images of the board aren't quite accurate, since the layout I sent to CustomPCB was not set up with Olimex's annular ring requirements yet. The biggest change is the TO-92 part, which was previously mounted with pins flat on rather than offset.


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## error401

I received the boards today from CustomPCB and somehow there is no ground plane on the bottom layer, so they're unusable 
	

	
	
		
		

		
		
	


	




. I hope they'll do something to rectify the situation, as it'd really suck to be out $100 on useless boards. There's an error or two of my own (PowerPAD area is solder-masked for example), but nothing as serious as the missing ground plane. They also sheared one of the boards incorrectly. How disappointing!

 Here's what they look like anyway, with the incorrectly sheared board:





 I suppose I could air wire ground, but then what's the point of having a nice small PCB anyway...


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## DaKi][er

May I suggest you try BatchPCB.com, they're a bit slow on the turnaround but they are very flexible with what you give them and will do a single PCB for a very good price and the quality is as good as any other top fab place. 

 If I had of noticed, I would have told you to steer clear of custompcb.com, my first fabed board were through them and while I didn't have ground plane problems and the PCB's I got actually looked like the files i send, the actual quality of the board was pretty poor. The soldermask wasn't your normal one and when i went to wash the boards after soldering in metho it actually dissolved away the soldermask

 Though defiantly get them to redo the lot or a full refund, that is completely unacceptable


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## error401

So finally an update on this project. I received the redo (done and shipped at CustomPCB's expense), and it is of acceptable quality. The soldermask is 'weird'; it makes a plastic-y smell if you get it too hot, and it seems very weak as there are some spots on my board where it scratched off rather easily. I wouldn't recommend this fab house, but the boards are working.

 Due to the (unknown to me at design time, since it doesn't seem to be in the datasheet) rather large input bias current for this amp, it has a pretty serious DC offset as designed. Swapping 5K resistors for the 100K ground reference resistors in the initial design gets the DC offset down to approximately 20mV, which I find acceptable. Input impedance is definitely on the low side, but both my portables and AlienDAC seem to do just fine driving it.

 Summary of issues I ran into building this board:
100K resistors should be replaced with 5K to get DC offset to reasonable levels
Drill holes for RK097 pot are too small, the part will fit but must be forced fairly hard. I recommend straightening the crimped pins first or they will likely crumple instead of being forced into the hole. I was able to fit the pot properly without modifying the board.
I forgot to remove the soldermask from the ground plane under the thermal pad on the TPA chip; you could scrape this away if you wanted to try to solder it down.
Pin 8 on the OPA551 should be removed, if you use this chip

 Other than that, I think it sounds pretty fantastic considering the size. It's smaller than the last protoboard CMoy I built was, and sounds far better. I don't detect any hint of oscillation, but I don't have a scope to verify this for certain.

 If anyone wants to build this up, PM me. I have a few boards I'll part with at my cost + $1 shipping = $9/board.


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## dBel84

congrats & glad to hear. do you think that there would be any benefit in buffering the inputs? ..dB


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## error401

Definitely, that is IMO the main problem with this amp. Input impedance is far too low to present an easy load to most sources, so a buffer would be beneficial. After reading more on other people using this chip it seems it's the best way to use it if you want to use it with any arbitrary source.


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## NelsonVandal

Error401, I soldered everything into place. I used very arbitrary resistor values, 1 k everywhere except 2 k as feedback resistor, even from input to ground. Still there's about 30 mV DC-offset at max volume. I've only hooked it up to a 8.4V battery, so I can't really tell how it sounds (but it sounds without any flaws, very clear - maybe overly clear, and the slam/speed/timing is very good) and how it works at a proper supply, just wanted to see that everything was allright.

 I tried it first with the TLE as IG and OG = ground channel opamp bypassed. Then I was at ELFA to buy a LMH6642, only to find that they only sell the SOT23, so I tried an OPA690 I thought was fried, but it seems to work allright even though it runs quite warm. Current reading is what's expected. DC-offset is the same. I have a THS4221 to try if the OPA runs too hot at 12 V, but I don't really like the sound of it. I don't think OPA551 is an appropriate match in a high tech amp like this. It sounds slow and mellow. I don't think OPA690 is a very good opamp either, even though it's used in the highly regarded Mini3, because I think my PINT-clone with LMH6654 as ground channel amp sounded better.

 I guess a DC-servo is the best option now. There's no room for high quality output caps (and input caps, there's 5 mV at the input), and they would probably degrade the sound more than a servo.

 If I get it to work I'll probably box it up in a mint tin with 9 AAA's. 10 won't fit. Or maybe a shortened Hammond J1201 with 10 stripped cells from 9 V's. If I use OPA690 and an OPA2134 for servo, the current draw would be about 35 mA, the use of 250 mA batteries would be OK for semi-portable amp.

 Edit: there's no dramatic difference (or really no difference at all at modest listening level with 300 Ohm phones) in sound with 9 or 12 V supply. This is a fine sounding amp, but a bit "overly clear".


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## NelsonVandal

BTW, if you're planning on using an opamp as input stage with TPA6120 as buffer, you should try ADA4899-1. I can see no point in using some crappy sounding opamp in front of it. It would make your amp become like all of the rest.


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## error401

What I ended up doing with my amp was putting an LME49720 unity-gain buffer in front of the amp, and lowered the input resistor to 1k. In my case, this brought the output DC offset to a reasonable couple mA.

 I'm using OPA551 in output ground as I'm running my amp at 15V and OPA690 can't handle that voltage. I'd be interested to hear your comments on other ground opamps, but I'm quite happy with OPA551. I'm not that picky when it comes to sonics though...

 A DC servo would certainly clear up the problem, and maybe I even have room for one on my buffer stripboard. If you get something going, be sure to post details and maybe I'll try it out too.

 Thanks for the comments, and I'd be interested to see if you manage to get a semi portable setup going. Pics!


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## NelsonVandal

More to come if and when I get a servo to work. ELFA doesn't sell SOIC TLE's, that's why I use TO92. You see there are 4.7 Ohm resistors on the output. I'm a cheap person, and that's what I had laying around.


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## NelsonVandal

OK, now it's up and running with a DC-servo, an ugly prototype one. It's an OPA2132 in inverting mode. 0.22 uF ceramic in the feedback loop, because that's what I had at hand (maybe I should make this larger, please help me on this). 470 k from output of TPA connects to inverting in of OPA, and 47 k from OPA out to TPA in. Noninverting input of OPA connects to ground. I changed the 1 k resistor from input to ground to a 10 k resistor to get an acceptable input impedance. The DC-offset is about 2 mV at max volume. The sound seems to be the same.

 It takes a little while for the DC to settle, I think this is normal - is it? I've noticed the same thing when using DC-servo in other amps.


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## NelsonVandal

... a Hi-Fi bug
	

	
	
		
		

		
		
	


	




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 Now it's only waiting to get boxed up. I haven't decided yet - a pretty C801 or an ugly mint tin with better run time.

 It sounds good, very detailed, great slam. It's very low noise as well when using IEM's, but there's better synergy with HD650.

 I increased the gain to about 4, still only 2 mV offset.

 Edit: I have to listen some more to tell how it really sounds, but I think this a real butt-kicker. I can't understand how you could rank it below Mini3. Try it with OPA690 sometime. I think I have to take back what I said about OPA690 before, but the sound still is slightly edgy (but without harshness and sibilance). The soundstage is great, the clarity is great, the PRAT is great, it's exciting withot being annoying and fatiguing, it's very powerful, it's transparent. Why haven't we seen this opamp in more commercial amps? They're still trying to sell us "new" and "revolutionary" portables with AD823, OPA2134 and other crappy opamps, they should use this one instead. The world is crazy, don't tell me I am!


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## peranders

Before you send awy the gerber files it may be a good idea to import them into som gerber viewer. Can't you use Altium for this?


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## holland

It definitely is a nice little headphone amp, the TPA6120A2. I like the sound as well. It's utilized in my DAC/headphone combo.

 I think it likes higher voltages though, higher than the OPA690 can handle.


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## NelsonVandal

Quote:


  Originally Posted by *holland* /img/forum/go_quote.gif 
_It definitely is a nice little headphone amp, the TPA6120A2. I like the sound as well. It's utilized in my DAC/headphone combo.

 I think it likes higher voltages though, higher than the OPA690 can handle._

 

Have you actually tried it at different voltages and found a better sound at higher voltage? What's the supply in your DAC? Can't you please describe the sound of it and what you compared it to. How do you handle the DC offset?

 I've seen the graphs in the data sheet, and the spec's are better at higer voltage, especially with a low impedance load. You could be right. Maybe I should use the 7812 to the OPA690 only and use 2 x 9 V to the TPA. I've tried it with 9 and 12 V and there's no audible difference unless driven to distortion. I use a 300 Ohm HD650 and I never listen at crazy levels.

 The problem is portability. I've almost decided to use AAA's. With a higher supply it will be a huge amp.


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## NelsonVandal

I've noticed an instability problem with low impedance load and with the braided SPC cables I use. There's probably oscillation above the audible frequencies. The current draw raises to 80 mA. With a cheap shielded cable to the Senn's, the current draw returns to normal. I have some beads lying around, think I'm going to add them to the output. I wonder if its the OPA or the TPA that's oscillating.


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## holland

I haven't done extensive listening with it at different voltages, that's why I said I think.

 My DAC is a DAC 301 (which is a bit problematic IMO), and I got tired of tweaking it. I have tried 14V and I currently run it at 24V. I have an external regulated supply based on the Twisted Pear board, it's similar to dual TREADs or STEPs.

 The sound seems a bit crisper, and more cleanly defined at 24V, but it's subtle, nothing earth shaking. I have about 10 headphones I used, ranging from 32 ohm (AKG K81DJ, Beyer DT660) to 250 ohm (Beyer DT990) and a few others in between like the AKG 271s. The sound is heavily influenced by the opamp used for I/V, which is where I spent most of my time tweaking.

 I compared it to itself at the different voltages, but again only briefly. Compared to my X-Fi to CK2III or Mini3, it doesn't quite hold up because of background noise (not totally black). Since it's an integrated device, it's hard to pull out just the TPA6120A2 contributions. However, I've not used it in a 3 channel configuration and the CK2III is a 2-channel discrete configuration.

 DC offset of 30mV is nothing to concern yourself with. Your HD650 won't even flinch at that. My situation was different, I had to balance the voltages coming out of the I/V stage by better matching resistors (going from 1% to 0.1%) of the differential outputs of the DAC. The TPA6120A2 was used for gain (2) and differential to single ended conversion to drive headphones. I believe mine is at 14mV now, due to the tolerance of the opamps and the TPA6120A2.

 When you're measuring DC offset, try shorting the inputs to input ground. You may be picking up some stray stuff on an unconnected input.

 See if you can account for the DC offset by looking at the measuring resistance of your resistors * gain.


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## NelsonVandal

I added a ferrite bead to the OPA690's output, and now the amp is totally stable with the braided cable and HD650. Thank you amb for inspiration and wisdom.

 It's funny to notice that a very serious electrical error like this doesn't affect the sound at all. The amp sounds the same.

 Thank you for your reply Holland. I'm going to try to short the imput when measureing DC offset in the future. Right now DC is no issue thanks to the servo circuit.


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## error401

Quote:


  Originally Posted by *peranders* /img/forum/go_quote.gif 
_Before you send awy the gerber files it may be a good idea to import them into som gerber viewer. Can't you use Altium for this?_

 

I didn't send gerbers, and that was the problem. They took the Protel files directly, as they requested, and then botched the conversion. I'll just stick to gerbers now since they're a known quantity; there are apparently version migration issues with the Protel files.


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## amphead

Nice work guys! Love to see SMD projects like this.


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## NelsonVandal

I went the long-play, but cheap and ugly, way and boxed it in a mint tin, with a trickle charger. Theoretically I should have about 30 h of playing time. I'm charging it now, and I will burn it in for a while. The impressions are still that this amp is detailed and almost neutral. There is some kind of shiny or silky coloration. Like I said before, exciting and fast without being harsh and sibilant. What more could one ask for? A tad meatier (or for me a veggier) sound, it lacks some body compared to e.g. JISBOS. It's still not as exciting and detailed and lacks that fabulous sound stage compared to Analog opamp based amps like my LISAIII clone and AD8599/AD825 amp.

 I've read about Channel Island VHP-1 and 2 that use TPA6120. The impressions of it seem to be about the same. This should be better though with an active ground channel and no coupling caps (or how do they handle the DC-offset, it looks like caps), but with a DC-servo that could mess up the sound, but I don't think it sounds worse since I applied the servo. The voltage supply is lower in this amp. I can't really tell tell how important the supply is since OPA690 can't be used above 12 V. There's no audible difference between 9 and 12 V. But most important of all, this is portable.


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## tcpoint

I, finally, got around to putting my amp together. Sounds great. I will have to give it a serious listen. I will compare it against my Mini^3. My first impressions are that it is very neutral and pleasant to listen to. It isn't harsh.

 Thanks for the fun project.


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## peranders

Pictures?


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