To buffer or not to buffer?

[fiat79]

So, I've got all of my parts for a "special" chip amp project and am getting it laid out and beginning construction. My question is whether or not to buffer the volume control and I would like some advice from more experienced constructors.

1. My project is an integrated amp with a single input, no need for any input switches.
2. It has a built-in 2 way electronic crossover and my volume control feeds it.
3. I selected a 10k Alps v. control due to the input imped. of 20k of the crossover.
4. My single input will be for a SqueezeBox music server.
5. I'm thinking now that maybe I should buffer the volume control and if this is correct, should I wire the buffer before or after the pot?

I'd also like to say I am very impressed with the pcbs I got from BrianGT, they are first rate!

[janneman]

Originally Posted by fiat79

So, I've got all of my parts for a "special" chip amp project and am getting it laid out and beginning construction. My question is whether or not to buffer the volume control and I would like some advice from more experienced constructors.

1. My project is an integrated amp with a single input, no need for any input switches.
2. It has a built-in 2 way electronic crossover and my volume control feeds it.
3. I selected a 10k Alps v. control due to the input imped. of 20k of the crossover.
4. My single input will be for a SqueezeBox music server.
5. I'm thinking now that maybe I should buffer the volume control and if this is correct, should I wire the buffer before or after the pot?

I'd also like to say I am very impressed with the pcbs I got from BrianGT, they are first rate!

Buffering means that you provide the thing that feeds the buffer with alighter load, and the thing that is fed by the buffer with a low impedance so it doesn't load the original source.

Your source is a music server which has can drive anything down to 1k or even 600 ohms. Loading it with a pot of 10k is a very light load and there is absolutely no reason to buffer that.

Your amp has a 20k input load. The pot feeding it has a max output impedance when the wiper is at mid position; at that point the pot 'looks like' 2.5k to the amp. That means you lose about 1/9 of your level (about 1dB); you need to slightly increase your volume control to compensate. You can use a buffer here to avoid this, but remember that any buffer processes your signal and it can only make it less good, never better.

Your call

jd

[fiat79]

Thanks for the well written clarification, I'll certainly skip the buffer in that case.

[jcx]

most active filter crossover circuits will need buffered input to keep varying pot impedance from changing the filter Q, frequency response - does your "built in" circuit have a internal buffer?

[janneman]

Originally Posted by jcx

most active filter crossover circuits will need buffered input to keep varying pot impedance from changing the filter Q, frequency response - does your "built in" circuit have a internal buffer?

Good point; I missed that. I agree; an active filter would be sensitive to pot position (which varies pot impedance).
Some active xovers I know have already an internal buffer at their input to take care of this, but it's a good idea to verify that.

Thanks for saving my rear end jcx

jd

[fiat79]

Thanks - this is why you ask the experts! I'm using the Marchand XM1 and it looks like it has a unity gain buffer, I'll build without the buffer and then check with a scope to see if it's loading it - not too painful to add the buffer after the fact if I have to. Thanks again for the help.