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So I was reading the schematic for the Traynor TS-50B to try find out what the tone controls do. I have kinda always been confused looking at them but today I realized the bass and treble were quite simple to run by themselves in the tone stack calculator with a Marshall stack if you get creative and well:

That's a Traynor if I've ever heard one! The trouble is the mids, however...

I just cant wrap my head around it. My first thought was either some notch filter or BMP-tone but a quick check on AMZ confirmed it is not. I looked at some schematics of RC-bandpass filters and I kinda can see the semblance but I am not really sure. What i do understand is that the pot "switches order" of the high and low-pass sections but I have no idea how this would graph. That is if I'm even right at all, I have extremely limited knowledge of electronics but I would like to learn a bit more. Any ideas? i would love if someone who knows about this kinda thing could help out. These traynors sound awesome and it would be super cool to know a bit more about why they do.

It's a variation on the Twin-T style filter.

You can play around with it here Use the resistance slider on the right side to sweep the pot.

DRodriguez wrote:It's a variation on the Twin-T style filter.

You can play around with it here Use the resistance slider on the right side to sweep the pot.

Thank you very much! very interesting filter

I used LTspice and I got the same result. I wanted to see if the phase was rotating 360. it actually goes through a 180 phase rotation depending on input frequency. the phase also changes direction twice from input to output.


if we measure after the first high pass filter, you can see what is going on with the phase before we put it all together at the output.

I found another circuit like this but more sophisticated in the Bass JCM 800, very different topology than other Marshalls with a baxandall tone stack instead of the typical FMV. The controls are very interactive but are misleadingly labeled as if it was a semi-parametric EQ. Also worth noting is the placement of the mid control in relation to the EQ.
https://drtube.com/schematics/marshall/1992.gif