Op-amp circuit question (DAC o/p) - diyAudio
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Old 4th November 2009, 12:55 PM   #1
Glenn2 is offline Glenn2  United Kingdom
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Default Op-amp circuit question (DAC o/p)

Hi,

I've been looking at the schematics of the Cambridge 740C CD player, but the circuit is similar in the in the 640C, 640CV2, 750C, and also the new Dacmagic which is what I have.

(Note I asked this already but it was buried in a Dacmagic thread.)

What is the purpose, or indeed benefit of this configuration of op-amps?
There's a kind of inverted output signal into the non-inverting input ground of the first op-amp.

I guess this is the "Dual Differential Virtual Earth balanced filter topology" they talk of their blurb. As though they are trying to create the same conditions for both phases of the differential DAC signals.

Does this configuration have a name? I have not seen it before.
Is the second op-amp considered to be in the signal path, or rather, is this more like a single op-amp application?
How much affect would the qualities of the second op-amp have?

I'm a bit confused as I've not seen anyone else do it this way.

Why is it better than the more usual grounding of the +ve signal (see edits in red).

Glenn
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Old 4th November 2009, 05:37 PM   #2
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That's the negative feedback. Instead of applying the neg feedback only on one input in an assymetrical way, they take the output, invert it and apply the feedback on positive input too. In this way the main Op-Amp has symetrical signal and impedances on the inputs. Maybe hepls with interferences via ground since it will isolate the ground plane, but I didn't tested nothing like that.
I would use the second half of a dual OpAmp for that.

Last edited by SoNic_real_one; 4th November 2009 at 06:06 PM.
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Old 4th November 2009, 05:56 PM   #3
Mooly is offline Mooly  United Kingdom
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The second opamp keeps the output at zero volts DC. It's non inverting input is ground referenced
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Old 4th November 2009, 06:17 PM   #4
Glenn2 is offline Glenn2  United Kingdom
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Quote:
Originally Posted by SoNic_real_one View Post
I would use the second half of a dual OpAmp for that.
That's how they do it.
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Old 4th November 2009, 06:18 PM   #5
Glenn2 is offline Glenn2  United Kingdom
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Quote:
Originally Posted by Mooly View Post
The second opamp keeps the output at zero volts DC. It's non inverting input is ground referenced
The output is ac-coupled though. Why bother?

(Although I've actually bypassed the output caps as they only block 1-3mV)
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Old 4th November 2009, 06:22 PM   #6
Glenn2 is offline Glenn2  United Kingdom
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Maybe it evens the load on each rail too?

There are no decoupling electrolytics within 6 inches of PCB track of the op-amps, and only 10uF way over by the 7815/7915 on Dacmagic. (1000uF before regs.)

so, 1000uf -> regs -> 10uf -> lots of pcb -> dual opa x6
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Old 4th November 2009, 06:28 PM   #7
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Glenn2 View Post
The output is ac-coupled though. Why bother?

(Although I've actually bypassed the output caps as they only block 1-3mV)
I can only see the circuit you have posted can't see what comes next, muting etc.

AC coupling is an industry fashion anyway, as a manufacturer you don't know what someone might connect it to. If you modded the circuit as you propose I think you would find significant and potentially damaging offset voltages at the output. The reason you only see 1 to 3 mv is because of the second opamp behaving like a servo.
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Old 4th November 2009, 06:35 PM   #8
Mooly is offline Mooly  United Kingdom
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Further thought... how does it mute ?
If it uses transistors these need to switch at zero volts or you will get a massive click when the muting operates.
And similarly if it uses relays too. You have to switch audio at zero volts DC and preferably at the zero crossing point in the signal for it to be totally silent.
Also only a few millivolts DC is needed to make a volume control "noisy" as you turn it, so if it were DC coupled with an offset and fed to a passive or DC coupled preamp that too would be a recipe for disaster.
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Old 4th November 2009, 06:39 PM   #9
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There is no "servo" there. It is virtual ground and the DC voltage is almost zero per OpAmp equations for DC signal, but not zero since no integration it is done (like on a true DC servo). See PID theory.
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Old 4th November 2009, 06:44 PM   #10
Mooly is offline Mooly  United Kingdom
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If you want to decouple the opamp, a small 10mfd low esr cap soldered directly to pins 4 and 8 is best. Never add decoupling from supply to ground unless you are sure that any rail disturbances can not enter the signal ground. Rail to rail at the opamp pins is by far the best You could even add a smaller bypass cap of around 0.1mfd in series with a 1 ohm (to avoid creating any unwanted resonant peak with the self inductance of the 10 mfd)

I would love to hear how that design sounds with a different IC to the NE5532
If it were me I would try an OPA2604 FET input device which should be totally stable in that configuration.
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