Buffer after 1k pot?

holmesc

Banned
2015-11-05 3:40 am
I'm reducing the volume control in my design from a 10k to 1k pot to reduce noise. The pot is driven from an 'enhanced' Schlotzaur buffer with 50mA SE output, sufficient for a nominal 2Vrms line level through a worst case 250 ohm impedance.

The power amplifier's imput impedance is 1.8k ohm. I'm wondering whether it is actually beneficial on the whole to insert a buffer between the pot and the power amp? Worst case source impedance without the buffer is ~1k ohm, which is a little high, but then again a buffer will add its own noise and distortion. Any thoughts?
 
holmesc said:
Worst case source impedance without the buffer is ~1k ohm
No. For a 1k pot driven from a 250R source the worst case source impedance seen by the next item is 1250/4=312.5R. This will be at -6dB, which is near maximum volume.

It is unlikely that the change in thermal noise in going from 10k to 1k will be worth it. Have you done a noise calculation?
 

holmesc

Banned
2015-11-05 3:40 am
Actually the output impedance of the buffer preceding the pot is negligible (about 10 ohm) so the worst case is ~1k / 4 = 250 ohm. I'm not at all convinced it is worth adding a buffer stage between the pot and power amp to negate this.

I haven't done a detailed analysis but have read three books from Douglas Self that touch on the subject, and according to his figures the difference is small, almost certainly inaudible.

The problem is that I'm scratching around for things to improve and this is one stone left unturned.
 
No need for a buffer stage. Many buffers will include a resistor at the output to ensure stability so they could easily be greater than 250R output impedance.

To what extent is an improvement which makes no practical difference a genuine improvement? More likely is that the change in load from 10k to 1k will slightly increase distortion in the pre-pot buffer.
 
...The pot is driven from an 'enhanced' Schlotzaur buffer with 50mA SE output, sufficient for a nominal 2Vrms line level through a worst case 250 ohm impedance...The power amplifier's imput impedance is 1.8k ohm.

Am I correct, in understanding that you plan to have the 1K pot as the final output element? If so, I'd say the issue isn't needing to buffer that output as much as it is the need to increase the power amplifier's input impeance.

I've never seen an input impedance anywhere near as low in any other power amp, the next lowest I can recall being 8.8k. Why is it so low? Typically, the amplifier input impedance can be rather arbitrarily set by an input resistor, is that not the case for your amp?