Multi-band distortion pedal

[PBE6]

Just thinking about an easy way to implement multi-band distortion, along the lines of Craig Anderton's Quadrafuzz but more streamlined.

Has anyone tried using a state variable filter (SVF) to split the signal into low/med/hi bands, then send them for separate distortion processing before mixing them back together? The SVF seems simple enough to implement and modify to add a mid frequency selector control. The only downside seems to be the number of buffers and opamps needed (5 minimum for a buffered SVF with mixer stage, 3 more if each band needs to be buffered before processing).

Is multi-band distortion significantly different sounding than single-band distortion with (pre-gain) EQ? And does it really reduce nasty sounds inter-modulation distortion?

[amptramp]

The nice thing about state variable and biquad filters is that all outputs are outputs from an op amp, so they are all low impedance, the equivalent of buffered.  They will need an input buffer, so one TL074 can do that.  You can add a distortion stage like a DOD250 with antiparallel diodes with no additional active stages and use resistive combiners at the output, but it would be best to have another op amp stage for the mixing since the virtual ground at an op amp's inverting input ensures that controls from each of the three inputs do not interact.  Also, you could add a clipper stage similar to a tube screamer, so three of those plus the output mixer would be another TL074.  I am used to complicated analog electronics, so maybe I don't see a couple of TL074's as a big circuit.

In addition to a switch used to control the mid frequency, you may want another control for the Q of the circuit which sets the midrange bandwidth.  The approach you suggest should help with intermodulation - you may find you need little or no clipping for the high end since the clipping produces even higher harmonics.  The low end could get muddy with too much clipping, so I think you should build separate clipping controls for each frequency band and you will need a tone control at the output (maybe like the tube screamer).

[PBE6]

Brilliant! Thanks amptramp, wonderfully helpful post. I will give this a whirl on the weekend and see what happens.

[nocentelli]

I'm working on something similar at the moment. but rather than reducing unpleasant distortion artifacts, i'm aiming for an extreme multiband fuzz to approximate the Fairfield "Four Eyes fuzz" (check youtube for clips).

I've got something on the breadboard at the moment with a quad opamp to provide an input buffer and 3 opamp SVF: i've got a dualgang pot wired as two linked variable resistors between the  stages to control the cross-over frequency. The LP, BP and HP outputs pass signal to three separate fuzz circuits, each with their own gain/level control. It works pretty well, and i'm currently tweaking the feedback/resonance network to try to get it to self-oscillate a little earlier.

Something else i mean to try is using opamps as gyrators to produce a multiband parametric eq.

[Vallhagen]

Just thinking about an easy way to implement multi-band distortion, along the lines of Craig Anderton's Quadrafuzz but more streamlined.

Has anyone tried using a state variable filter (SVF) to split the signal into low/med/hi bands, then send them for separate distortion processing before mixing them back together? The SVF seems simple enough to implement and modify to add a mid frequency selector control. The only downside seems to be the number of buffers and opamps needed (5 minimum for a buffered SVF with mixer stage, 3 more if each band needs to be buffered before processing).

Is multi-band distortion significantly different sounding than single-band distortion with (pre-gain) EQ? And does it really reduce nasty sounds inter-modulation distortion?

Hi. I have played quite a bit with multiband dirt stuff. Blüe Monster and the PïnKing is 2- resp 3-bands OD/Distortions. Both are posted here in some of the contests (Turkey and 10y) and in the pictures thread, but it might be easier to read up on them over at madbeans in their dedicated project threads if you are interested (hope its ok to link from here to there): http://www.madbeanpedals.com/forum/index.php?topic=6968.0 , http://www.madbeanpedals.com/forum/index.php?topic=11523.0.

Feel free to borrow any ideas or inspiration from my projects. If you like a Blüe Monster pcb i still have them for sale.

Cheers
/Bengt

[teemuk]

How Traynor amps did it.

Is multi-band distortion significantly different sounding than single-band distortion with (pre-gain) EQ? And does it really reduce nasty sounds inter-modulation distortion?

Yes and Yes.

[caspercody]

Would this be similar to the Malekko - Wolftone Sloika?

[Mark Hammer]

One of the things to keep in mind is that the different bands that one might split the signal into will likely have different average amplitudes.  That, in turn, means that one either has to:

• pick your bands and bandwidths such that the output levels are approximately equal
• pick your split-points arbitrarily, and apply different amounts of post-split gain to each band in order to bring it up to similar clipping thresholds
• alter the clipping thresholds for each band to compensate for differential amplitude, or
• treat the differential clipping of bands having the same gain applied and clipping threshold as simply "the way the thing sounds"

My own tendency would be to work out whatever I could ahead of time to sidestep endless tweaking from the control panel.  So, nothing adjustable about the split bands themselves, apart from their post-clip output level.

A half-dozen years back, I whipped up something for myself I called the Flexidrive, that simply used a 2-way split: middy-mids and highs, and lower-mids and lows.  The split point was fixed, and I used one pot to adjust the gain of each side in reciprocal fashion, and another pot to adjust the relative balance between channels.  That, and a master output control were all the knobs it had.  Eventually I figured a toggle to set different levels of drive on the input stage would be a good idea.  It had a lotta different sounds available.  Can't speak to the intermodulation distortion, though. 

I made some changes to what you see below, but this was the only drawing I could find and post at the moment.


I think there might be some value in applying a similar strategy to a dual-band circuit, but use crossover distortion for the upper band.  That is, place a back to back diode pair in series with the output of the gain stage.  Personally, I find that x-over distortion likes bridge pickups, so feeding a series diode pair only mids and highs, combined with a clipped-and-filtered low end, might sound decent.

In retrospect, rather than a master drive control and a single pot producing reciprocal gain adjustment of the two clipping stages (i.e., more gain for one means less for the other), it's just smarter to have a drive control for each section, and be done with it.  The nice thing about using those individual pots and a balance/mix control is that you can dial in relatively clean bottom OR top, and blend it with however dirty, or however much, you want of the other band.

Obviously doing that with more than two bands starts to get rather complicated, not least because you'd need two pots (drive and level) for each band, whereas a two-way split permits use of a single blend control: one less knob to find space for.

[deafbutpicky]

Another thing to consider is the amount of phase shift applied to each band with filtering.
This gets some interesting effects when the bands are mixed. Vallhagen's approach is to
eliminate any undesired shift with some allpass filters (and some more overhead), while I
went for some good sounding in between, tweaking the bands by ear and with a little help
of an oscilloscope in my Thrice I & II which gives it some grittiness at some settings. Maybe
you should decide first on how complicated and detailed you want to get and what sound you want to achieve.
If you're just curious and want to try different approaches I'd recommend you to get a nice and big
breadboard at first. This is not for the impatient...
I don't know if too much calculations before hand will get you far though. If you know what you are doing
(say have some experience with building overdrives), and where you want this to be sound wise (a reference
sound is always a good idea as you can get lost in tweaking easily and your ears get tired quick) make some
guesses for the split points and go ahead and remember: too many pots, too many options for bad settings

[tca]

And does it really reduce nasty sounds inter-modulation distortion?

IM distortion isn't all that bad, if you keep it under control, IM can provide some and very warm sounds to your distortion (not good for hi-fi, but good for guitar).

P.S.

Here is some good reading about it: https://passlabs.com/articles/audio-distortion-and-feedback

[teemuk]

^ This.

Usually the idea of "bad" distortion - or any particular aspect of distortion being bad, whether it's softness of clipping, intermodulation, generated harmonic content, etc. - comes from HiFi design and is practically worthless in applications where you intentionally want to generate distortion.

I'm pretty sure most of the inconic distortion tones of guitar amps and effects are plagued by IM to quite large extents.

But multi-band clipping has a slightly different tone and different outcome than clipping the entire band (kinda like single-band vs. multi-band compressors are also different), and as that it's worth trying out, experiencing and experimenting. If not for anything else, to find out what the fuzz was about (no pun intented).

Obviously it didn't catch in a major scale to guitar amp or effect design so that should be tell tale sign of something. I don't see a lot fo folks stepping up to claim their cranked XZY amp or whatever sounds great, except for that hideous intermodulation distortion. If you look at the designs in those applications they are usually everything else but what HiFi design rules recommended to begin with.

[tca]

> Usually the idea of "bad" distortion - or any particular aspect of distortion being bad, whether it's softness of clipping, intermodulation, generated harmonic content, etc. - comes from HiFi design and is practically worthless in applications where you intentionally want to generate distortion.

I'm pretty sure most of the inconic distortion tones of guitar amps and effects are plagued by IM to quite large extents.

To some extend, yes. But in order to remove or generate/control IM (or any kind of distortion) one needs to understand it, and the more effective way of doing that is by measuring it, something that hifi hackers frequently do and guitar effects hackers should do more often. It is some what a dual task. Pass's article is worth reading and testing.

> ... worth trying out, experiencing and experimenting. If not for anything else, to find out what the fuzz was about (no pun intented).
Not taken.

> Obviously it didn't catch in a major scale to guitar amp or effect design so that should be tell tale sign of something.
Yep, a kind of natural selection phenomenon.

Cheers.

[PBE6]

Again, extremely helpful posts!! I love this forum

My plan was to bit the bullet and use 8 opamps to split the signal into lo/mid/hi, with a variable freq knob, and either having separate gain knobs for each band or setting the gain of each band equal (maybe at 1x, maybe more) with separate saturation pots in line with the clipping diodes. After reading Mark's posts, I may just leave the frequency set and go with separate gain pots. Simple is best, but it's hard to edit sometimes - doesn't feed my need to tweak!!