I have been tinkering with passive tonestacks on Ltspice and the breadboard for a few weeks, trying to work around the quirks that irk me in the conventional FMV contraptions.
I came up with this after a few failed or mediocre attempts, and I am quite happy to say that it fits my current needs to a T (which was to find a simple tonestack for a simple Jfet & opamp guitar amp). I do not know if it has been done before, but I certainly haven't seen it around.
In essence, we have what is commonly referred as Framus mids control, somewhat tweaked and scaled, followed by an also tweaked Voigt Treble-Bass control. One of the goals was to have the possibility of a flat response, which can be achieved with Treble and Bass halfway and Mids dimed. I kinda based the center frequencies around the Marshall tonestack, which is what I am used to, but there is plenty of room to tweak and achieve different freq responses.
With the current component values, ranges as follows:
Treble: 18dB @8kHz
Middle: 16dB @700Hz
Bass: 20dB @100Hz
Which is way more than I'd normally use in any given guitar amp tone stack, but it's still good to have.
Pros:
- Manipulating one control does not affect the operating frequencies of the others.
- Possibility of completely flat response, which is useful when using pedals/preamps with their own tonestack.
- Low number of components (6 resistors, 4 caps, 3 pots).
Cons:
- High insertion loss (around -10dB at flat settings)
- NEEDS a buffer before it, ideally opamp-based, and possibly a buffer after it too.
- If RL is much less than 1Meg, high frequencies start to drop (although lowering/removing RT will bring some back).
Hope someone will find it useful :) Comments, observations and suggestions are more than welcome.
Enjoy,
/Alex
(https://i.postimg.cc/NK8FLFJ5/Tonestack-V9.png) (https://postimg.cc/NK8FLFJ5)
(https://i.postimg.cc/bDrw6Tzt/Tonestack-V9-Plot.png) (https://postimg.cc/bDrw6Tzt)
If it needs a buffer ahead of it, and a gain stage after...why not make it into an *active* tonestack and avoid all these problems in the first place?!?
You'd have a better signal level (so less noise), you'd have more flexibility as to boost/cut, and you could still keep the same frequency response if you wanted.
I've never really understood why people are so keen to go with passive designs.
Interesting. And you find it musically useful?
The midrange slopes are not-steep, which is often good, but isn't exciting in the showroom. (Best mid-knob I ever met, seemed dead at first, but became my go-to for layering the mix.)
The input impedance drops to <600r pretty quick!! Certainly not a splice-in-anywhere network. The amplifcation path has to be designed around this.
Quote from: Umlaut on September 16, 2024, 04:47:34 PMIf RL is much less than 1Meg, high frequencies start to drop (although lowering/removing RT will bring some back).
That implies the output is inductive, which seems strange. (Audio without coils or amplifiers tends to be capacitive.)
Arghk. New PC, I have to teach the spell-chucker all my audio jargon.
Quote from: PRR on September 16, 2024, 09:58:25 PMThat implies the output is inductive, which seems strange. (Audio without coils or amplifiers tends to be capacitive.)
I see it that the output impedance of the treble side is higher than the bass side so the load has an asymmetrical effect.
Quote from: ElectricDruid on September 16, 2024, 06:44:04 PMIf it needs a buffer ahead of it, and a gain stage after...why not make it into an *active* tonestack and avoid all these problems in the first place?!?
In my use-case, I do want the volume drop of passive TSs: R2R clipping stage -> tonestack -> volume pot -> active presence and resonance control. Helps to make sure the following gain stages do not go into rail clipping even at maximum volume :icon_biggrin:
I've never been too fond of active tonestacks for distorted sounds, while I do obviously acknowledge their superiority when it comes to tweakability. My old Peavey Heritage had a wonderful & very useful semi-parametric on the rhythm channel, yet I found it quite an annoyance to adjust when using different flavours of distortion upstream.
Quote from: PRR on September 16, 2024, 09:58:25 PMThe midrange slopes are not-steep, which is often good, but isn't exciting in the showroom.
It has less notch but a bit more range than the average FMV. I liked Big Scoop(TM), oil-can-full-o-hornets for maybe 5 minutes when I was 18, then I learned the importance of keeping the useful frequencies on guitar tracks :)