Working out the varitone circuit, or sort of...- created 07-17-2009

Wybierala, Mark - 07/17/2009.08:27:52
Full Time Instrument Tech

A vendor called Torres Engineering offers what they call a Varitone kit and I've installed about a dozen of these handy little circuits in various instruments. Its a six position rotary switch that has one off position and five filter positions. The filters are five different selectable capacitors that are in series with a choke that goes to ground. Wiring up the circuit is just a matter of connecting to the signal output at the output jack or, you can connect to a potentiometer to vary the filter's effect. For a completely passive circuit, it provides a decent pallet of tonal changes on both electric guitars and bass guitars.

The original Gibson Varitone is a bit different in that it uses both caps and resitors and a 1.5 Henry choke. A 1.5H choke is rather large and heavy physically and not readily available. I have the original wiring diagram provided to me from Gibson in a jpg file if anyone is interrested. I don't believe I can post it here. I'll email it to anyone who wants it.

The Torres circuit uses the primary side of a miniature audio transformer for the choke -- Mouser part# 42TL021-RC. I've been looking for alternatives to get a deeper frequency cut or play around with the resonance but I'm at a loss for knowledge in the method to select an inductor/choke. I did study this briefly in school but this was long ago. For the modern inductors available, they do not intend them to be used in this fashion. There is little data available concerning the actual properties of the components in "henrys".

Does anyone have some insight to make the selection of alternative inductors easier for the layman who gets headaches from math? Just general conceptual info is about all I can handle.

I'm more than happy to share anything I've learned from using this circuit.

More turns is more Henrys, all else being equal, which of course, it ain't! But... "more is more" should serve well as a rule of thumb.

Transformers are more concerned with turns ratios than inductance, so you're not likely to see inductance ratings for such things.

Coils are not so popular these days because it's so easy to replace them with a capacitor and an op-amp -- smaller, cheaper, and fewer issues.

(One option, considering all the successful pickup makers out there these days, is to go boutique and wind your own. Maybe use a ferrite core, slap a couple end plates on it, chuck it in the drill and off you go! Experiment with number of turns. I have some Bill Lawrence pickups that are 1.5 henries, but they don't have a very high-efficiency core, so they're big! Smaller wire... a nice core...)

If either the inductance goes up or the capacitance goes up, the frequency goes down (by the same ratio). However, increasing inductance decreases the bandwidth, whereas increasing the capacitance will increase ("soften") the bandwidth. Increasing series resistance also increases bandwidth.

If you can work through a little

<henries> * <farads> =

Mark, you might be interested in a circuit in Craig Anderton's "Electronic projects for musicians". It uses a Mouser inductor, #42TM-019. It's a center-tapped inductor and if you add a switch you can switch between the full coil or the half coil

Great info -- thanks. I have a pickup winder son winding my own coils is not a problem. The little audio transformer used by Torres has 1320 turns over what I belive is about a 3/16" bobbin. The straight resistance is 148 ohms. This is a nice idea to wind my own and its not a big deal to install a few taps to experiment with -- a very cool experiment...

Rather a fun example of a Varitone built

Http://www.vintagehofner.co.uk/britamps/selmer/schematic/rsel.html

Look for L1 - S2 - C15 thru 19 to find the filter stage.

>I've been looking for alternatives to get a deeper frequency cut

You know the inductor in the Gibson L-6S was a 1/4" bolt, three washers and a nut! It was wound in two halves as to be humbucking. I think it was 36AWG wire.

In the book, Electronics Projects for Musicians, Craig Anderton uses the primary from Mouser part# 42TM-019, and has has a switch to switch in one half or both.

I've also used the auto output transformer from Radioshack.

Yeah David, I said that in post #3. <G>

Oh you sure did Mark. That's what I get for skimming! <g>

Quote; A vendor called Torres Engineering offers what they call a Varitone kit and I've installed about a dozen of these handy little circuits in various instruments ; End Quote

Artec makes a solid state "Varitone" style switch. It's calleed a "QTP" Circuit. It's a 5 way rotary.

I understand we're trying to keep this limited to a passive circuit Varitone, but I'm becoming interested in a low-power Analog Devices op-amp chip - the ADA4691-2 - that can be wired up as a gyrator (an active circuit that simulates the properties of an inductor) and dropped into a Varitone circuit in place of the coil.

There are tons of op-amps out there, but this one can run off a pair of AAA-batteries if necessary, eats up only a tiny amount of space and isn't as susceptible to hum as a coil. I haven't done anything with this part yet, but soon.

A big aspect of my joy of guitar technology is in seeing how much circuitry and junk I can fit into as small a space as possible. ICs like this always make me stop and go "wow".

AP

Alan, you bet! As much as I appreciate simplicty its great fun to have wires explode out of the control cavity when the screws are removed. I'm still playing with the varitone and have acquired a range of four of those audio transformers in the mouser catalog with different specs. I'll be installing all four into a victim guitar to see what the effects are between them. This is such a great little circuit. The greatest variety of tonal pallet I've seen is with a Gibson P90 and it is just the perfect match where all six positions have their own tasty tones. So many projects--so little time... At the moment I'm also looking into solidstate switching using the TI TS5A2236

You can also put the LC filter between two op amp stages to make an active filter of sorts, or at least a recovery amp after the Varitone

(Mark Sez:) At the moment I'm also looking into solidstate switching using the TI TS5A22362.

I just looked that up. Very cool little chip! The internal shunt to ground sure solves the problem of that nasty thump when switching sources. And its faster than a mechanical switch. Let us know your results.

It's dangerous letting me near a chip catalog. I've spent long hours doodling all kinds of crazy stuff over the years: A 4051 analog multiplexer as an 8-throw Varitone (!), a common flip-flop tied to a quad-bi switch to allow touch-plate control over pickup switching (no more toggle)... really wacky ideas I've had since forever but never pursued.

I'd love to see what you come up with.