HM2 Gyrator + weight control confusion

[SprinkleSpraycan]

I have a Boss HM2 on the breadboard right now and am exploring changes to the eq control.
I want to get rid of the %^&*ed wah sound when the High control is dimed (sacrilege i know!). I thought altering c15 outta do it. i replaced C15 with a typical "weight control" as drawn and like how it sounds when the weight control is at around 15n. 15n is around 4' on the pot and measured when the leads are disconnected from the gyrator. But when i put a 15n cap into its place on the breadboard it sounds totally different then when coming from the weight control. It sounds more wah like somehow.


Hopefully that all made sense.    What do i not understand about the weight control and how it interacts with the gyrator? If it measures 15n at the pot and the connected ends of the 1n/47n then its 15n. But apparently not...

[FiveseveN]

when the weight control is at around 15n

Umm... how do you figure? With a multimeter, I gather, but a multimeter is set up to measure caps, not RC networks. A "weight control" is not a variable capacitor.
In any case, the resistance is lowering the gyrator's Q, making it less "wah-like". Just leave it in if it helps you get the sound you like.

[ElectricDruid]

In any case, the resistance is lowering the gyrator's Q, making it less "wah-like". Just leave it in if it helps you get the sound you like.

This is the crucial bit, I think. Both of those gyrators have a pretty "peaky" Q, about 3.5, iirc. One is set at about 950Hz, and the other about 1300Hz, so "mids" and "high mids".
Redesigning the gryrators for lower Q would help make the sound a lot less "ice picky". However, lowering the q will mean the two responses overlap even more than they do already, so it might be wise to separate them a bit more. I'd be inclined to move the high-mids one up an octave to say 3KHz or so. That makes it more of an actual "highs" control, and keeps it more out of the way of the mids.
ymmv though - I haven't tried it, only studied the circuit.

This is the tool you need for the gryrator design:

http://www.muzique.com/lab/gyrator.htm

Start by putting in the existing values, have a look what you get, and then tweak from there.

[Rob Strand]

I have a Boss HM2 on the breadboard right now and am exploring changes to the eq control.
I want to get rid of the %^&*ed wah sound when the High control is dimed (sacrilege i know!). I thought altering c15 outta do it. i replaced C15 with a typical "weight control" as drawn and like how it sounds when the weight control is at around 15n. 15n is around 4' on the pot and measured when the leads are disconnected from the gyrator. But when i put a 15n cap into its place on the breadboard it sounds totally different then when coming from the weight control. It sounds more wah like somehow.

I think the HM2W in custom mode kind of does this.

Revoicing a pedal like that is a fairly open task.

Here's some ideas (none are verified).   Start with one then push the tone closer to the original or further away.

Schematic:


Response Plots:

[SprinkleSpraycan]

Thanks so much guys. This is all very helpful. Is there a calculator for this with a visual element like the muff stack calculator?

[ElectricDruid]

Is there a calculator for this with a visual element like the muff stack calculator?

LTspice like Rob used is about the only way, as far as I know. I haven't seen an online gyrator calculator that shows you frequency responses, I don't think.