[dsavitsk]

So, I have these two questions that I feel should have obvious answers, but that maybe don't.

The first relates to the ground channel setup in the ppa/pimeta/m3 type amps. Why is there an opamp in the ground channel? What is it doing that the buffer can't do alone? In the L & R channels, the opamp provides gain, but with no gain on ground, what's the point?

And, why does the ground channel opamp not get biased into class A? As I understand it, the real advantage of class A for an opamp is that it eliminates the switching noise of turning on and off. Why not do that for the ground channel? Edit: well, I see that the M3 does do this, just not the Pimeta


The second relates to DC offset. There is always concern about whether to use blocking caps on the input of these amps. The argument seems to be that if there is any offset from the source that it will be multiplied by the gain of the amp. So the question is, isn't this only the case for listening at full volume? Shouldn't the pot shunt most of the offset to ground at lower volumes?

So, what don't I understand here?

-d

 

[Polaris111688]

Answering your last question about DC offset, the potentiometer functions to reduce both the incoming signal and the DC offset. In other words, the ratio of DC offset to signal peak voltage is still the same, regardless of whether a potentiometer is there or not.

 

[amb]

Quote:

Originally Posted by dsavitsk The first relates to the ground channel setup in the ppa/pimeta/m3 type amps. Why is there an opamp in the ground channel? What is it doing that the buffer can't do alone? In the L & R channels, the opamp provides gain, but with no gain on ground, what's the point?

Without an opamp the output buffer will have to run "open loop". It would work, but the overall loop feedback dramatically reduces the output impedance. Since the ground channel is supposed to serve as a low-impedance source and sink for the return current from the other channels, this is a very desirable trait.

Quote:

And, why does the ground channel opamp not get biased into class A? As I understand it, the real advantage of class A for an opamp is that it eliminates the switching noise of turning on and off. Why not do that for the ground channel? Edit: well, I see that the M3 does do this, just not the Pimeta

The PPA has it on the ground channel too. On the M³, the opamp biasing serves double duty, as the current is pulled through the Vbe multiplier transistor for the purpose of biasing the output MOSFETs.

Quote:

The second relates to DC offset. There is always concern about whether to use blocking caps on the input of these amps. The argument seems to be that if there is any offset from the source that it will be multiplied by the gain of the amp. So the question is, isn't this only the case for listening at full volume? Shouldn't the pot shunt most of the offset to ground at lower volumes?

Yes, the pot attenuates the input DC offset just as it does actual signal. The "multiplied by gain of amp" statement refers to the worse case when volume is turned all the way up.

 

[mono]

Quote:

Originally Posted by amb The PPA has it on the ground channel too. On the M³, the opamp biasing serves double duty, as the current is pulled through the Vbe multiplier transistor for the purpose of biasing the output MOSFETs.

Now I wonder if Pimeta would benefit from the Gnd opamp bias too. What do you think amb ?

 

[amb]

Quote:

Originally Posted by mono Now I wonder if Pimeta would benefit from the Gnd opamp bias too. What do you think amb ?

Probably not enough to justify the trouble.