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Old 1st September 2011, 11:00 PM   #21
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Originally Posted by hwfanatic View Post
I want to use PC PSU. Line out and PSU share ground. If I'm not missing something I don't think I can use voltage splitter in this application.
You don't need to. The PC power supply has +12, -12v, and ground. The molex has +12, +5 and ground. You can tap the -12v from the board connector.
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Old 5th September 2011, 01:39 PM   #22
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Originally Posted by Rembrant View Post
The PC power supply has +12, -12v, and ground.
I wouldn't use that -12V, since it can supply a couple of 10's of milliamps only and is left out completely in modern PCs.
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Old 5th September 2011, 04:49 PM   #23
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I guess it all depends on the PSU. The one I have here says -12v is good for 1amp. That could be a lie also. PSU manufacturers are know to fudge the facts a little.
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Old 5th September 2011, 08:07 PM   #24
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I've abandoned the ground channel for two reasons:
1) Op amps don't seem to share the load, as in the ground channel doesn't contribute to output power.
2) Power supply noise was a ground loop problem after all.

I have a couple of questions:

1) Adding a small resistor between PSU and signal ground solved ground loop noise. Why?
2) I'm pretty sure op amps don't like a low load impedance such as my Sennheiser HD201 (24 Ω @1 KHz). Anythign I can do to improve this circuit? Except paralleling more op amps, that is.
3) Output coupling capacitor is so large (-3 dB @ 2 Hz). Do you think the op amp sees it as a capacitive load and should I be concerned about it?

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Old 5th September 2011, 08:25 PM   #25
jcx is online now jcx  United States
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you really should just use higher output current op amps - the O2 project's NJM4556 are fine if you want to avoid surface mount parts - TL072 have particularly poor output stages for such low Z load

the 47 Ohm current sharing R are too big - single digit Ohm R drop less supply V, reduce problems with C load from the cable

and why inveritng?
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Old 5th September 2011, 08:35 PM   #26
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I have a very limited selection of op amps, unfortunately. Only basic and rather old models. I've picked NE5532 over TL072. But that's about as good as it gets.

I will reduce current sharing resistors value.

Inverting saves a few components, I guess. Does it make any difference audio-wise?
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Old 5th September 2011, 08:39 PM   #27
jcx is online now jcx  United States
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inverting gain loads the source more heavily which can cause distortion, reqiuires bigger DC blocking C for the same frequency response

too high a value of feedback R can allow another destablizing effect to become a isssue - the input parasitic C of the op amp

noise V and in the case of bjt input bias current errors are bigger with higher values of feedback R
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Old 5th September 2011, 10:11 PM   #28
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Would you consider this an improvement over the last circuit? How low should I go for feedback resistor? Lower resistors implies larger capacitors, which is the reason I aimed high initially.

Also, if I can get my hands on a pair of NJM4558 or OPA2134 should I prefer them to NE5532?

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Old 5th September 2011, 10:36 PM   #29
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I think virtual grounds are perfectly OK to work with, but you don't want to sink any serious currents into it. This you would do in the headphone application shown a couple of posts before.

There is an elegant way out, though, which would also increase headroom when driving from a single low voltage supply: of the four output opamps, connect two in positive and two in inverting mode, and bridge the outputs. No drive currents would flow back into the virtual ground, and presto.

Disadvantage is that you halve the available drive current, so you might wish to have some more opamps in the output if you worry about not having enough.

An FFT of my life would show a couple of peaks and some low level harmonics, but mainly random noise.
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Old 9th September 2011, 12:40 AM   #30
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Originally Posted by Conrad Hoffman View Post
[Virtual grounds] can be made to work ok, but it's also one more thing that can be unstable or insufficient to the task, and it's one more thing that can reduce reliability. I ultimately eliminated it from whatever I was working on because I was never comfortable with it. FWIW, I'm more of a fan of class A headphone amps, since the power requirements are modest- that's what I build for myself.
I just wanted to add my support for Conrad's position, on both counts.

A virtual ground is a approximation of a real ground, that approximation is as good as the performance of the amplifier vs. the work it has to do. The impedance at high frequency may be an issue, stability may be an issue, noise may be an issue, and it is for sure more complex and less reliable than the real thing.

It's a valid trick for creating a split power supply from a single supply, and it is useful for battery powered or low budget headphone amp applications where true split supply is not feasible.

However, for hifi application, just use a transformer with two secondary windings. Problem solved: real ground obtained (circuit common, COM, as I prefer to call it.) with no mucking around.

Second point: parallel op amps will lower the noise and increase the output current, but will not get around the fact that you are using op amps to drive headphones: ultra high feedback (in unity gain config), very lean output bias current - i.e. mostly class B operation. A discrete output stage (single ended, or heavily class-A biased push-pull, or any of the power-follower circuits) is hardly much more effort to build, and can be properly tailored to drive low impedance, inductive/capacitive loads.

RJM Audio (phonoclone.com / G+)

Last edited by rjm; 9th September 2011 at 12:43 AM.
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