do you like your buffer before or after volume knob?

[darron]

i've made a distortion which i'm almost ready to start designing a board for, but one last thing... i sounds great through my solid stage amp but going into a tube the final tone stack really loads it down and sounds too warm. a buffer restores that to exactly where i want though.


SO...


would you rather the buffer after or before the final volume control? i can see pros and cons...


IF the buffer was very last then the loading from the volume pot wouldn't matter and it would be the cleanest 'reset' for the signal possible.

IF the buffer was before the volume pot then there's the advantage that the pot dialed down would also dial out any noise associated from the active buffer electronics. buffers i have built haven't introduced any audible noise though...



if you think the buffer before the volume pot is a better idea, then what value pot would you suggest? i usually like a higher value log pot with a tiny cap to bleed highs through and prevent loss. i might consider a 100kA just by itself though.




not looking for a massive debate but some preferences with reasoning would be cool (:


Thanks guys

[amptramp]

I prefer to have the volume control within the stompbox electronics so it never sees the next item in the signal chain.  If you have a 100 Kohm volume control, the output impedance of the volume control is zero near the minimum and maximum (if you have a low-impedance output stage like an op amp) and 25 Kohms near the middle resistance (the two 50 Kohm halves of the resistance in parallel).  This can have a subtle or not-so-subtle effect on the following item in the signal chain which may affect the frequency response and therefore the tone.  I put the buffer after the volume control so I have complete control within the stompbox of the effect of the control and I also have a consistent, low output impedance for driving the next stage.  A higher output impedance usually raises the bass and reduces the treble because of cable capacitance and the effect of resistance in series with a coupling cap.

Putting the pot ahead of the buffer also gives you the freedom to use a pot in non-standard ways: if you connect an inverting op amp stage so that the wiper goes to the inverting input, the signal input is at the clockwise end of the pot and the counterclockwise end of the pot is connected to the op amp output, you can use a linear pot and get a usable approximation of audio taper.  You may want to add a resistor to the inverting input in series with the pot wiper and a small cap from output to inverting input for stability, but the point is, you cannot do this with a pot at the output - it would have to be audio taper.  Note that the inverting stage with a volume control in it as described has a consistent, low output impedance, so the volume control can be part of the final buffer stage.

I agree that buffers, whether op amp or discrete, rarely add significant noise.

[darron]

thanks for your feedback amptramp. i'm definitely leading towards your side of having the buffer post-volume for reasons that you have outlined

- you can't always anticipate what will follow the signal - possibly a really long high-capacitance cable or an extra-tone-suck wah. having the buffer last should give more consistent peformance in more setups.

- that loading which i've used from the 500kA output volume control plays a strong part in the tone stack that i have already tuned!



sweeeet. it's good to be able to justify your choices. thanks.

[amptramp]

If you were wondering what I was talking about in the second paragraph of my response, here is a quickie TinyCAD drawing where the pot is a rectangle with an arrow through it and the resistor is a rectangle.  The gain of this circuit is -(right half of pot resistance / left half of pot resistance).  As you can see, this is highly non-linear, but it is not equivalent to a log pot.  The fixed cap and resistor add a feedback lead which stabilizes the response.

[darron]

it looks like a lot of trouble and footprint on the pcb... i like playing with the linearity too, it can make a big difference in how useable something 'appears' for lazy people! damn those lazy people....

but you can usually achieve a lot with a slightly larger pot and using R.G.'s tricks on the secret life of pots: http://www.geofex.com/article_folders/potsecrets/potscret.htm