modifying rc tone values

Itchy Scratcherson · December 20, 2023, 01:45:05 PM

Hi everyone, I'd like to ask for some help determining a couple new values for the tone controls in this circuit. It's aion's Dryad Transparent Overdrive which is Catalinbread's Silver Kiss: a trimmed-down TS w/3-band EQ.
Basically I'm only using the very top of the bass range and with the treble almost off. It's okay like that but I feel it has pretty limited flexibility for me.

I think I just wanna move the knee of the high end down a bit? Can I also expand the hi bandwidth here to be more interactive w/the Mid control? I guess the bass would be more useful by moving it's knee up & also maybe playing with it's Q.

Of course, AMZ's review of the SWTC is a gold-standard reference, right? https://www.muzique.com/lab/swtc.htm
Also, these rc low-pass & hi-pass online calculators were a real find for me: https://www.trance-cat.com/electrical-circuit-calculators/en/

Not sure where they got it, but freecircuitdiagrams' 3-band EQ circuit seems a good comparable here.

Thanks for any suggestions helping me to put this all together. It's a bit challenging for me to hold multiple concepts in mind simultaneously when I'm trying to create some new brain pathways!

ElectricDruid · December 20, 2023, 03:01:08 PM

The basic principle of all RC filters is that the frequency is inversely related to the product of R and C.
That means that if you double the value of either R *or* C, you *lower* the frequency by an octave. If you halve the value of R or C, you raise it by an octave. If you increase R by five times, the frequency goes down to 1/5th of what it was previously. If you cut the cap value to 1/10th of what it was (so 10n in place of 100n, say) then the frequency goes up by x10...etc etc.

In this situation with a three-band tone control, there's quite a lot of potential interactions, but that general principle still holds. If you want the Treble lower, make C10 larger. Maybe 100n, maybe 220n? As you mention, that might make it start to step on the toes of the mid control.

I agree the freecircuitdiagrams 3-band EQ is a similar circuit. If you look closely, you'll notice they've done the treble control the same way as the mid control, so it's actually a bandpass-type-thing with a roll-off for very high frequencies we don't hear. The Dryad circuit doesn't do that, and potentially adds more boost higher up which would probably contribute noise. Aside from that they're the same, I think. Different values, slightly, but that's tweaking.

HTH

antonis · December 20, 2023, 03:49:51 PM

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Itchy Scratcherson · December 20, 2023, 04:01:20 PM

Thanks guys, that's just the info I'm needing.
Also, my experience this afternoon playing this "stripped TS w/eq" on my (clean) Supro Delta II opened my ears a bit since my OP, which I wrote after auditioning through my Vox AC4HW, clean. I think there are opinions for/against Tube Screamers in bed with Vox amps(?) In any case the Supro literally revealed the interactivity of the 3 tone controls for me. I'll still do some tweaking, of course...
Thanks again!

Itchy Scratcherson · December 20, 2023, 04:35:06 PM

One more question: what's this about? Is that just to get a 5.7μ value? If so then why 2 types? (Same configuration is next-to-last, right before the volume pot).

ElectricDruid · December 20, 2023, 06:26:54 PM

Quote from: Itchy Scratcherson on December 20, 2023, 04:35:06 PMOne more question: what's this about? Is that just to get a 5.7μ value? If so then why 2 types? (Same configuration is next-to-last, right before the volume pot).

I don't know. There are good reasons why you might put a larger electrolytic cap in parallel with a smaller ceramic cap to filer radio frequency noise, but that's not this case. Personally, I think I'd either have decided 4u7 was enough, or put a 10uF in its place. But perhaps they thought 4u7 wasn't quite enough and they had a large surplus stock of 1uFs hanging around? I can only imagine.

What is certainly true is that the potential impedance into that tone control is very low by most standards - with all the controls at one end, we've got three x 2K2 in parallel to virtual ground point. That's only 733R! Baxandall tone controls should always be driven by a low impedance output for this reason. The input impedance is *way* off your usual 1M input! Hence the need for large caps...

That said, 2K2/3 = 733R, and with 4u7 that gives 45Hz or better as the HPF roll-off point, so I'm not sure I really understand why they needed to add another 1uF. The guitar's lowest note is 80hz, more or less, so we're not hitting anything except perhaps a bass guitar on the very lowest notes.

HTH

PRR · December 20, 2023, 10:19:32 PM

In High-Fidelity, naked Electrolytics are scorned. They distort, they go inductive, argh!!!

Bypassing with a "better" capacitor is supposed to take the curse off.

In part this comes from bad old e-caps of the 1930s and 1960s. Recent caps are FAR better.

Also a HUGE value lessens audio flaws. I've seen over 100uFd in similar positions.

But you do whatever your guru says to do.

antonis · December 21, 2023, 05:31:21 AM

Quote from: ElectricDruid on December 20, 2023, 06:26:54 PMWhat is certainly true is that the potential impedance into that tone control is very low by most standards - with all the controls at one end, we've got three x 2K2 in parallel to virtual ground point. That's only 733R! Baxandall tone controls should always be driven by a low impedance output for this reason. The input impedance is *way* off your usual 1M input! Hence the need for large caps...

Good point of deliberation but that R3 sours IC1A output low impedance..

P.S.
I shouldn't call it "transparent" due to R3/C4 LPF corner frequency at 1.6kHz..
(almost identical to Treble control max setting value..)

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modifying rc tone values