How to implement a "tight" control in a distortion circuit

[Xavier]

Hi all. I find myself changing the input caps in most of my distortion builds in order to remove pre-clipping bass, and I was wondering if I could build a variable hi pass filter instead, in order to not being limited to what the fixed cap provides.

Someting like the "tight" control that the Friedman BE OD has.

I found this circuit, not sure if this would be suited to guitar  or I can actually use a simple, passive design where you only cut bass. It should actually be like a tone control, but instead of removing treble, it removes bass



Any idea ?  Specially for metal-type distortion corcuits I find this really useful ....

[iainpunk]

Hey,

i recognize this problem, i had it in an amp i am building.
I solved it by getting a pot, (1M, reverse log) wired as a variable resistor, in parallel with a small cap (3n3) this led in to a resistor to ground (100k). The G&L PTB tone circuit has works the same, only the volume pot is replace dwith a fixed resistor.

This works well for me.

Good luck, iain

[GibsonGM]

You could use the opamp type if you want, Xavier.  I'd re-do it (or find one already designed) to run on 9V, but if you're using 18V already, you could use that supply.      There are probably easier, more basic designs that don't require the dual-gang pot, but that  one has a good steep slope; it is probably more what you are interested in.   I think if you look around more, you'll find some stuff already optimized for guitar.

The passive idea Ian posted might work, too, and only takes a minute to try out.  I'd suspect the slope isn't sharp enough, but could be wrong...

[iainpunk]

The passive idea i used works quire well for me personally, but its cutoff frequency is at around 700hz, and the filter is shelved. I have a schematic of what i used.



Since clipping kind of exaggerates the lower frequency over the higher ones, cutting with a full slope is a logical thing to do, but, you also lose a bunch of bass that you can't get back in the same quality, that's why i chose a shelving filter, with an adjustable shelve depth in stead of a slope, so you can keep a bit of bass, with a lower amplification factor so the bass doesn't get distorted, and also not cut.
But every one has their own preference.
The 1 uf cap is to take out the DC from the previous stage.

And my name is spelled iain, not ian. (Scottish way of writing) no bad feelings tho 

[GibsonGM]

I have a buddy named Ian, LOL.....sorry.  You hit around 45, the fingers start to get fat on the keyboard...

[Mark Hammer]

Joe Gagan solved this issue brilliantly many many years back, when this forum (and many current members) were quite young.  Here's one example.  The input signal, wther padded down by the 250k pot or not, passes through a .005uf cap to Q1's base.  But it also passes through the 100k pot and 3u3 cap as well, just not as much of it.  Reduce the resistance setting of the pot, and you introduce more low end, until at zero ohms pot setting, the entire input signal, bass and all, gets fed to Q1.  Very simple.  Very clever.

The principle is essentially identical to what companies like G&L and Reverend use for their "contour" bass cut controls on their guitars.  In those cases, a small-value cap is always in place between the pickups and volume pot.  The contour control allows for varying amounts of bottom to bypass that cap.

[thermionix]

^+1.  I have the "input cap blend" on my Fuzz Face and it works like a mofo.

[Kipper4]

I concur with Mr Hammer it's a great simple approach.

[Xavier]

Many many thanks for all the replies ! I will apply to a couple of circuits I am working on.

[rankot]

Joe Gagan solved this issue brilliantly many many years back, when this forum (and many current members) were quite young.

Similar circuit can be found in one of the Bazz Fuss incarnations.