TPA6120a2 CFA with current output PCM63 DAC - diyAudio
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Old 17th June 2014, 08:59 PM   #1
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Default TPA6120a2 CFA with current output PCM63 DAC

Hello guys!
I'd kindly appreciate your help and guidance to design the analog output stage of a DAC / Headphones amplifier.

Here's briefly the plan: I'm making a USB DAC with headphones output.
I'm not getting too much into the details of the digital part here (I'll do on a separate post once the project is done). Briefly, the design is based on a TI USB receiver -> DF1704 dejitter-reclock stage -> galvanic insulator (USB/digital and DAC will have separate power rails) -> PCM63 DAC (which has a current output) -> TPA6120a2 headphones amplifier.
As you can imagine at this point, the driving principles of this project are: simplicity, quality, and don't care excessively about price.

Now, I feel I have solid bases in the digital domain (I've taught FPGA / digital design in university for a bit). As a matter of facts, the entire digital part is done and works fine

What I'm missing now is the analog part. My analog electronics skills are mediocre, especially when it comes to opamps.
I have very little clue on how to couple the output stage of the PCM63 (which has a single ended current output) all the way down to the TPA.

I've the following goals:
- I'd like to get a headphones output. I'm pretty convinced by the tpa6120a2 (Link to schematic)
- I'd like to get a (unbalanced) line level output as well (for connecting a power amp)
- I'd like to have a potentiometer in the middle, to control both the line level and the headphones output.

Here's the problem:
I'm tempted to follow the PCM63 reference schematic for the I/V stage, using the OPA2134p, unless anybody have better ideas, which is the following:
Click the image to open in full size.

The question now is:
I assume I should put the potentiometer after the last current gain stage (before the 3.3v out marker). What should be the impedance of such potentiometer?
Can I use the same output to drive the TPA6120a2? It seems like that has a relatively low input impedance (how much?).
Can i came out with a potentiometer which impedance is enough to drive the TPA (and the line out)?

The other bigger question is: how should I drive the TPA6120a2? In inverting mode? Non inverting mode? I'm afraid I am lost at this point.
Considering that after the 4th opamp in the PCM63 schematic have a 3v out, a unity gain (or even less) for the TPA should be enough, right?

It is my understanding that current feedback amplifier (like the TPA) are pretty tricky and require care.
I would be really grateful if anybody could give me guidance of this.

In change I promise I'll publish the schematic of all the USB DAC once it's done, as I've always done with all my projects on my website (link).

Many thanks,
Primiano
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Old 18th June 2014, 01:21 AM   #2
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If you care primarily about SQ then passive (LC) filtering beats pretty much anything based on opamps hands down. With a single DAC the inductors for that filter will most likely need to be hand-wound which might discourage you though...
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Old 18th June 2014, 02:34 AM   #3
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The DF1704 will be doing 8x OS, so a lowpass filter is not strictly needed, unless some small amount of 176kHz noise will "anger" the TPA6120 chip. The app note LPF is the horrible GIC type that B-B was pushing at the time, but later realised it has unexpected, and pretty bad, problems with actual audio, which is why you no longer see any reference to it in app notes for PCM1704 & forward. If you're married to using opamps, I would recommend you build the stages shown in the PCM1704 datasheet, but with OPA134/2134 opamps. A volume control can be added either before or after the LPF stage.
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Old 18th June 2014, 04:58 AM   #4
jcx is online now jcx  United States
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you have a source of PCM63?

for DAC I/V there are better speced op amps today than the OPA2134 - for what the PCM63 likely cost you might as well use $7-10 I/V op amps

as long as you have a fast 'scope for digital work you could try ADA4898 65 MHz op amp with "highly linear" input stage - quite a departure from 20-30 year old op amps

also has pretty high output current so the highly recommended I/V op amp feedback C should work well soaking up glitch energy without the 5534 asymmetric feedthru zero problem

OPA827 looks good too - at 1/2 the price of the classic 627 it also has modern complementary bipolar output stage

I would go with just a few steps of discrete gain switching for way different sensitivity headphones
if you got the fpga chops then digital volume should be trivial - even with dither
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Old 18th June 2014, 08:21 PM   #5
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Thanks all for suggestions about the OPA in the I/V stage.
stephensank: thanks for the explanation for the PCM63 schematic. I always wondered why the PCM1704 one was so much simpler.

The biggest problems however remain open:
with what impedance should I source the TPA6120a2 CFA?
Should I use it in inverting or non inverting config?

Thanks
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Old 19th June 2014, 02:45 AM   #6
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Quote:
Originally Posted by jcx View Post
you have a source of PCM63?

for DAC I/V there are better speced op amps today than the OPA2134 - for what the PCM63 likely cost you might as well use $7-10 I/V op amps
Specs or not, if you're going to go with opamp i/v & lpf with the PCM63 or just about anything else, NO opamp sounds better than the OPA134/2134.

You know, now that I look through the datasheet for the TPA6120A2, it sounds like it has SERIOUS tendencies to oscillate in the mHz and cook itself(and easily your phones). When the maker of a chip has PAGES devoted to preventing their own chip from oscillating, that is a BIG red flag. I would not touch this chip with a ten foot pole after reading the lit. I would strongly urge that you check out the BUF634T, which I think would make a hugely better phones driver, without any strange & dangerous oscillation issues.
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Old 19th June 2014, 04:13 AM   #7
jcx is online now jcx  United States
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I'm pretty sure you can find lots agreeing with "no op amp sounds better"

and probably not much google fu required to find some op amp rollers dissing OPA134 "sound" - just about any opinion for or against you want to see has probably already been posted somewhere

but when using op amp I/V I doubt you can find many testimonials for these recent chips - certainly no controlled blind listening comparisons - so I think really understanding the I/V application technical requirements and which specs matter, then selecting from the last decade or so of better candidates is likely to work well

the OPA134 is noisy relative to my suggestions, slower in GBW, Slew Rate limit and has less output current capability - other than those parameters I'm sure its fine

the OPA827 specs may not exceed by enough to justify the price, the ADA4898 is really a new class in op amp performance for specs that matter in op amp I/V


the TPA6120/THS6012 (really read both datasheets - its the same chip, but different specs shown for the different markets) like all CFA, and all 100 MHz op amps does require some care - output isolation from 100s-1000s of pF of cable C is needed - but lossy ferrite doesn't impact audio output Z and is still safe against Cload
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