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Old 1st July 2012, 05:43 PM   #1
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Default Single ended DAC + TPA6120, where to put the pot?

Hello,
I am building a custom designed DAC that, for what concerns this discussion, follows quite closely the reference PCM63 design (see attached image). I want to integrate it with a TPA6120 headphone amplifier.
I have a major doubt on where to put the volume potentiometer (see the 4 red circles):

Click the image to open in full size.

(i) before the final dac buffer?
(ii) in its loop (altering its unity gain)?
(iii) after it (before entering the TPA6120)?
(iv) in the tpa6120 loop?

Any help would be really appreciated.
Thanks,
Primiano

Last edited by primiano; 1st July 2012 at 06:38 PM.
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Old 1st July 2012, 05:47 PM   #2
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Your image link is bad, it does not go anywhere (404)
Try again but attach the image this time.
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Old 1st July 2012, 06:19 PM   #3
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Mm the url is good I checked after posting.
It's not a 404 error. I guess the server is just temporary unreachable.
Hopefully should come back in a while.
Cheers,
Primiano
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Old 1st July 2012, 06:54 PM   #4
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(iii) after it (before entering the TPA6120)?

Why bother with that blob of opamps?
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Old 1st July 2012, 07:18 PM   #5
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I can't see the link either, but a google search shows that the website exists, it's just down.

Either way, you do not want to put the volume control inside the feedback loop of any part of the circuit, especially not of the TPA6120, which is current feedback and very fast and quite prone to instability.

You need an I/V converter after the PCM63 and the TPA6120 needs to be driven from a low source impedance. If you place a potentiometer after the I/V converter then you need an opamp following it before the TPA6120. You do not want the pot after the TPA. So really what you want is

PCM63>I/V>Pot>Buffer opamp>TPA6120.

In my set up I go

PCM1792>I/V>CS3318>TPA6120.
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Old 1st July 2012, 09:34 PM   #6
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Attatched the TS pic.
Attached Images
File Type: jpg dac_headphones.jpg (61.7 KB, 159 views)
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Old 2nd July 2012, 04:47 PM   #7
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Quote:
Originally Posted by 5th element View Post
So really what you want is

PCM63>I/V>Pot>Buffer opamp>TPA6120.
Seconded.
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Old 2nd July 2012, 05:20 PM   #8
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So, final answer seems to be (i), the red leftmost small circle before the last PCM63 opamp.
What about its value? I don't know exactly the source impedance driving that buffer. Will a 50k be fine?
Thanks,
Primiano
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Old 2nd July 2012, 07:58 PM   #9
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The resistor and the cap before the red circle form a potential divider that acts as a low pass to filter the signal coming out of the DAC. If you place another resistor (the pots trace) in parallel with the cap then at DC/low frequencies this will act as a further divider in combination with the 1.33k resistor to lower the overall output.

In other words at low frequencies, where the caps impedance is very high, all of the signal normally enters the opamp. If you were to use a 1.33k pot, even with the wiper in the maximum position, you would end up with half of the signal entering the opamp compared to if the pot wasn't there. In the maximum position the 1.33k pot would still form a potential divider with the 1.33k resistor, cutting the signal at the opamps input in half.

Using a larger value pot obviously reduces this effect. With a 50k pot, if the maximum signal amplitude would be normally be ~2.1Vrms, it would now be reduced to ~2.07Vrms. This isn't going to be a problem. You can use lower value pots if you want to but you might need to increase the gain @ the TPA6120 if it wont go loud enough (I highly doubt that this will be the case).
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Old 3rd July 2012, 07:22 AM   #10
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Ehm... Why load futher the IV/filter output, won't FET opamp be the best option to use there (call it buffer), throw a pot between THS and TPA (TPA will benefit from additional "buffer" or composite-opamp thing). Keep the DAC's buffer powerful, keep pot's impedance low (10k)...

Don't forget that the pot should be placed at front panel, with long wires involved. Highish output impedance of DAC's filter plus longish wires = pretty awesome noise-picking antenna.
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