Help with IC Big Muff with James tonestack

tonymcbony · Post by **tonymcbony** » 21 Jun 2016, 20:13

Hi folks,

I've been working on this for a bit - an IC Big Muff with a James Tonestack, and an output buffer.
I literally just clapped a generic buffer on the end but I think I arsed it up - nothing comes through.
Can you spot any glaring errors around there?

I feel that R13 is not required and didn't include it in the build.

Nocentelli · Post by **Nocentelli** » 22 Jun 2016, 04:50

Buffer looks suspect: For an inverting buffer, you want a resistor in series with the inverting input, a resistor of equal value in the feedback loop, and the non-inverting input connected directly to VB (or VR).

tonymcbony · Post by **tonymcbony** » 22 Jun 2016, 06:01

Nocentelli wrote:Buffer looks suspect: For an inverting buffer, you want a resistor in series with the inverting input, a resistor of equal value in the feedback loop, and the non-inverting input connected directly to VB (or VR).

Ah cool.

I was using this:

But I think I should use this (without the 5pf):

(both from http://www.muzique.com/lab/buffers.htm)

That seem the way forward?

Also, if I may pick your brain, in which cases would you prefer an inverting buffer over a non-inverting buffer?

tonymcbony · Post by **tonymcbony** » 22 Jun 2016, 08:56

Right so that was the issue.

However I think I need to get the right input impedence:

With 1M resistors, there is a squeal that's created when the volume control is at zero
With 22k resistors, there's no squeal but the tone controls are ineffective until the volume is turned up a tad. Is there a way to calculate the required impedence setting for the buffer?

Nocentelli · Post by **Nocentelli** » 22 Jun 2016, 15:26

tonymcbony wrote:
Ah cool.

I was using this:
[ Image ]

Almost, but your schematic has the input connected to the inverting (-ve) input instead of the non-inverting.

I'm not really certain why you would choose one arrangement over the other, but I think it would depend on what input impedance you wanted. Non-inverting can be higher impedance with less noise, so migjt be chosen for a standalone buffer, or as the input stage. Inverting uses fewer parts so might be chosen if the buffer follows another stage and high input impedance is not critical.

As for the squeal: You could try shifting the volume pot to after the buffer stage, so the volume pot setting has no influence over the tonestack. The input impedance of an inverting buffer is set by the series input resistance, so 22k is pretty low, and it is really just an inverting gain stage with the gain set at 1 (gain = loop resistance/input resistance). You could also try switching it to a proper non-inverting buffer to see if that helps.