[balou]

Hi,
what are the advantages of having an opamp in the ground channel when there's already a buffer there? I've heard that the opamp can help reduce the DC offset and lower the output impedance, but I have no idea how exactly that should work. Could anybody explain this to me?

 

[majkel]

Op-amp makes sense only when it has negative feedback going from the buffer's output. This way you provide the ground channel being truly 1/2 battery voltage. The buffer itself is a static device which cannot react on the voltage drop on its output because the input knows nothing about that drop, caused by higher volume passages for instance. The op-amp makes it visible for the buffer and provides a compensation of the input to maintain the output level referred to the + op-amp input.

 

[tangent]

A buffer's output impedance is not zero. V=IR, so any current variance (I) shows up as a voltage variance (V) as a function of the output impedance (R). This means that as the current load changes in an amp using a plain buffer for virtual ground, virtual ground changes voltage. This increases crosstalk, because one channel can affect the other through virtual ground.

Wrapping the buffer in an op-amp actively corrects for this. Its effectiveness is limited by how good the op-amp is.

 

[balou]

Quote:

Op-amp makes sense only when it has negative feedback going from the buffer's output.

I've should written it that I assumed that - like in the PPA or Pimeta

Quote:

This way you provide the ground channel being truly 1/2 battery voltage. The buffer itself is a static device which cannot react on the voltage drop on its output because the input knows nothing about that drop, caused by higher volume passages for instance. The op-amp makes it visible for the buffer and provides a compensation of the input to maintain the output level referred to the + op-amp input.

thanks, this makes sense to me

Quote:

A buffer's output impedance is not zero. V=IR, so any current variance (I) shows up as a voltage variance (V) as a function of the output impedance (R). This means that as the current load changes in an amp using a plain buffer for virtual ground, virtual ground changes voltage. This increases crosstalk, because one channel can affect the other through virtual ground.

so, just a plain buffer would render the whole virtual ground/ground channel thing useless/less useful

Quote:

Its effectiveness is limited by how good the op-amp is.

this leads straight to the next question... what makes an opamp good for this purpose?

 

[majkel]

Quote:

Originally Posted by balou /img/forum/go_quote.gif I've should written it that I assumed that - like in the PPA or Pimeta thanks, this makes sense to me so, just a plain buffer would render the whole virtual ground/ground channel thing useless/less useful this leads straight to the next question... what makes an opamp good for this purpose?

Any "audio" op-amp will be OK but they actually add their sound signature to the whole amp, so you need to experiment and find the best duo: L/R channels dual op-amp against ground channel op-amp.

 

[NelsonVandal]

You should try AD829 in the ground channel, comp pin out like in LISAIII. It's very detailed neutral and transparent. Less colored sound than AD744.

I've not tried it for L/R since I only have one of them, but I'm about to order some more. I think you should try it before settling with 5534 or 744.

 

[balou]

thanks for the tip. I knew that the AD829 also had a compensation pin, but I dismissed it for reasons I can't remember. The drawback of it is that it is rather pricey through farnell/newark, and I can't really get it otherwise. The 5534A is low-cost and is available through virtually every supplier.
Oh, about comp out: the amp (see here) is designed to use the comp pin as default. that's why I've chosen the 5534 and AD744