Oddity / hen's tooth filter - Holmes Spectrum H-X6

[Strategy]

A friend of mine has one of these and is currently not using it (though please note, not up for grabs)

http://www.effectsdatabase.com/model/holmes/hx6

At first glance you'd expect it to be a cheap parametric EQ. But it's much weirder -- like a mutant cross between a parametric EQ and a resonant bandpass and highpass filter mixed in. Totally unlike any other filter I know of. Totally cool vowel-like sweeps and squeeks very synthetic Even though it's static (e.g., no modulation), still incredibly musical as an extreme EQ. Just begging for CV inputs to be kludged in with a vactrol, LFO daughter board, etc. Imagining separate LFO's for each sweep!

I have access to the pedal for a while and can photograph it, document parts, meter it, I'm really still not any good in the reverse engineering department but this little thing is probably worth documenting and duplicating provided that it ins't full of obsolete parts. I haven't even gotten to crack it open yet.

Questions:
1 - anyone familiar with these? came under a Coron label as well.
2 - any interest in reverse engineering?
3 - what information do I need to gather to reverse engineer?

I'll try to post up sound samples but I'm getting married in a week so I haven't had the time for DIY that I normally do...

Strategy

[deadastronaut]

looks cool, yeah its screaming out for lfo's/expression by the looks of it...

look forward to hearing clips...

[micromegas]

sounds like an interesting one.

[Mark Hammer]

The gut shots of the Coron version show a trio of transistor circuits, that have just about the right parts count to be three gyrators, for a 3-band EQ.  The op-amp part is likely some state-variable filter.

[Strategy]

EQ and filter under the same hood! that's in keeping with the sound the bottom-most knob is level, above that is eq - middle row is low, middle, high, then top row is "reso", "bpf" and "hpf"

Will get some high resolution guts and sounds up ASAP

Strategy

[DiscoFreq]

Nice find, I have the Coron version, one of the rarest (and most expensive) pedals of that (usually cheap) brand :-)

[Mark Hammer]

I keep meaning to get around to drawing one up and building it, but for my money, the most flexible arrangement with the fewest possible controls would be a bass shelving control (baxandall type), two bands of semi-parametric (boost/cut, centre frequency), and a sweepable 2-pole lowpass to shave the top end.  Maybe, the lowpass would have a hi/lo Q toggle, but I think the 6 controls listed would likely be enough.  The option to tweak the Q of the semi-parametric sections via internal jumpers might be useful.  I can see where a person might want a lower and higher semi-parametric section to have different Q.

I find that, with the exception of the Empress pedal, most parametric stompboxes offer a single resonant cut/boost, which isn't enough control.  Six and 7-band EQ pedals offer more control, but it isn't necessarily the control that YOU need.  Bass boost/cut controls tend to be more universal, in the range they cover, but mid and treble controls tend to be a little more rig-specific in terms of what they need to control.  A number of pedals will give you a fixed bass and treble control and sweepable mids.  That's certainly more flexible than a simple 3-band, but I don't think it handles the top end well.  Top end control benefits from a steeper rolloff than Baxandall-type controls provide.  All of which is what leads me to the suggested control complement.

The Holmes/Coron unit provides much of that tonal flexibility, though.

[Strategy]

I am mid-process on building a couple of the Serge resonant equalizers, with one of them as a standalone unit that can take guitar and line levels. It is more like the grapic EQ but with some off the beaten path approach to the frequeny spread. I have found that  never use EQ pedals in part because they do not work across all applications. when I have used them it's more for an extreme (rather than "pragmatic") use: e.g., creating super middy fuzzes using EHX Graphic Fuzz, etc., using extreme high pass filtering for experimental funk rhythm guitar, etc. The EQ I use most often is a Krohn-Hite dual bandpass filter - an ancient tube thing designed for ham radio and laboratory test equipment - not something you want to gig with (weighs easily 20 lbs.) but good in the studio for creating extreme filters.

From CGS synths, the people who have licensed many Serge circuits for legitimate reproduction as pcb's:
"The RESONANT EQUALIZER (EQ) is a unique ten-band filter designed specifically for electronic sound synthesis and processing. Except for the top and bottom frequency bands, all other bands are spaced at an interval of a major seventh. This non-standard spacing avoids the very common effect of an accentuated resonance in one key, as will be the effect from graphic equalizers with octave or third-octave spacing between bands. Spacing by octaves will reinforce a regular overtone structure for one musical key, thereby producing regularly spaced formants accenting a particular tonality. The Resonant Equalizer's band spacing are much more interesting, producing formant peaks and valleys that are similar to those in acoustic instument sounds."

http://www.cgs.synth.net/

What strikes me about this Holmes EQ is that it really wants to be more like a funky filter - the BPF sweep for example is perfect for envelope, treadle, LFO type control. It seems like a very tweakable wah/filter circuit that never fully evolved into a treadle pedal but stayed stuck in this more static format...

Strategy

[UKToecutter]

Strategy
I don't suppose you could mail that thing to digi2t to reverse engineer it?
Could you....?

[Strategy]

It's not mine to mail out, but I can ask!
I assume this "borrow to duplicate, then return" is done relatively frequently on this forum!

[Strategy]

Good news, if digi2t has time to reverse engineer it and can guarantee safe passage on the thing's return flight so I can give it back to it's owner, we can do this!
Strategy

[digi2t]

Good news, if digi2t has time to reverse engineer it and can guarantee safe passage on the thing's return flight so I can give it back to it's owner, we can do this!
Strategy

Hmmm..... I'm all backed up, project-wise, right now. I mean, I would love to do it, It's just that it may take several weeks before I can get to it. If you guys don't mind waiting a bit, then yeah, I'm in. PM me for details.

As for "safe passage", I use Canada Post, with tracking. Besides being the most economical way, I've sent a Ludwig Phase II to Texas, and a Mutron Flanger to Australia, and many others in between, without a problem.

[Strategy]

I wouldn't be able to send this out for a week or so anyways, with all the wedding hubbub and overlapping work deadlines (ugh)
Canada post sounds good...I will PM ya soon
Strategy

[digi2t]

I received the pedal last week, and decided to dust off this topic.

This pedal is really cool. Like strat says, a three band eq, with BP and HP filtering. It has a sort of MS-20 filter edge to it, like the Resonator, but with much less complexity.

I traced it out, with my super-duper-pooper schematic skills (not), here it is;



Here's the trace side reversed, with the components drawn in, in case anybody wishes to double check it.



If anyone wishes to clean up the schematic, by all means...  

I'm going to vero this, maybe stick it into a Crybaby shell, just controlling the BP, which can really rock the tone on it's own. All in all, it's one of those circuits that acts like a subtle seasoning on a nice juicy steak. I mean, the steak is great on it's own, but the seasoning adds that little extra zing to push it to the next level. Tone seasoning. Yeah... that's it!  

Many thanks to Strategy for lending me this pedal for tracing. Another feather for the Hen's Tooth Cafe. I'll try to get a HTC video out soon.

[Strategy]

Digi2t thanks for tracing this out. I'll give the vero a try, I've had a bad track record with vero but eager to replicate this! (maybe this would be one to learn doing my own layouts with...)

consider posting a photo of the pedal itself, crazy color scheme!

I found that using the filters I could get my guitar to sound like a really dubby, reggae style bass guitar tone. Pretty serious filtering for a goofy little EQ pedal.

Strategy

[digi2t]

Finally got a quick video done. Just a fly-over of the controls, and noodling with the BPF.

Thanks again to strategy for the opportunity to play with this wonderful little piece of kit. I'll have a vero for this in the short term.

[stm]

Nice tracing job digi2t!

I took the liberty of rearranging the schematic for clarity in order to understand how it works.  As drawn the circuit is suitable for true bypass implementation.  I also added some extra comments in red.

The circuit is basically a state-variable filter with flexible frequency and resonance controls.  The EQ part reminds of the Craig Anderton SuperTone.

[tubegeek]

Finally got a quick video done. Just a fly-over of the controls, and noodling with the BPF.

SUPER-BITCHEN! I love it. Thanks to everyone involved for their kind efforts.

[Strategy]

Too cool! Thanks Dino & friends for shedding some light on this odd one
The original belongs to a friend, so I'll have to return it to him and get on with my clone!
Strategy

[digi2t]

Immense thanks to STM for the rearrangement. I was hoping someone would clean it up. My tracing skills are getting better, but my flow skill still leave much to be desired, especially where complex circuits are concerned.