Building Stereo Pedals? (An Always-On Harmonic Tremolo)

[steveyraff]

Hey guys,

Yea so, I feel a bit out of my depth on this one. Had a guy ask me to build him a harmonic tremolo. His requirements are as so:

"I'm running a stereo rig, and I'm after an always-on stereo harmonic tremolo to sit at the end of my chain and bounce between the amps. It also needs a phase switch to make sure I don't get any phasing issues when I play through different amps. It doesn't need a footswitch, literally just a rate and depth knob should do."

For starters, I've never built a stereo pedal, and I can't find any wiring guides anywhere online for doing so. I presume the circuit itself has to be designed for stereo and its not just to do with the offboard wiring?

Also, I've found a lot of layouts online for Tremolos, but I am not sure if I can also find what would be described as a "Harmonic" (stupid question, but not even sure what that would mean?) - and none of the layouts have a "Phase" switch.

Any help from those more experienced is appreciated. Thanks for the help.

Steve.

[blackieNYC]

Look at the Polarity Reverser at Geofex, the stereo panner something or other also at Geofex, and the Cardinal Harmonic Tremolo here in the forum.

[Mark Hammer]

One of the episodes of Seinfeld had Jerry distinguishing between "good naked" and "bad naked", when he was dating a woman who insisted on walking around his apartment nude.

I think there is a similar distinction to be made between "good stereo" and "bad stereo".  "Good" stereo makes the sound rich, animated, bigger.  "Bad" stereo makes the sound distracting, hard to peg down, difficult to balance.

Harmonic tremolo is essentially like having a soundman play with the tone controls, turning the bass up as they turn the treble down, and the treble up as they turn the bass down, doing so in rhythmic fashion.  That's an oversimplification, but close to accurate.  If the treble part of the unit feeds one amp, and the bass feeds another, that invites the challenge of balancing amp levels appropriately, since the bass will tend to be heard louder.  I suspect there would also be challenges in terms of tone adjustments; particularly if the two amps are not identical.

In some respects, what you proposed also has the caveats that come with auto-panners, and that is that they are bloody distracting.  I've recounted in past posts how the only time I ever stepped up to a mic and completely blanked on lyrics was when I had my guitar going to a slow Leslie on one side of me, and a slow tremoloed amp on the other side.  VERY disorienting, and very hard to ignore.

I do like harmonic tremolo, though.  My own preference - which my be assuming hurdles that don't exist/apply in your case - would be to use a mono harmonic tremolo, and if a wider stereo field is wanted, feed the tremolo's output to a chorus that could feed two amps with different versions of that signal.  That way you get the swirl and spaciousness without the distraction.

[samhay]

>"I'm running a stereo rig, and I'm after an always-on stereo harmonic tremolo to sit at the end of my chain and bounce between the amps. It also needs a phase switch to make sure I don't get any phasing issues when I play through different amps. It doesn't need a footswitch, literally just a rate and depth knob should do."

If this is a true stereo rig, then it has 2 inputs and 2 outputs. Sometimes mono-in, stereo-out is called stereo. Good to check, which is the case.

If it is 2-in, 2-out then think of the circuit as 2 tremolo's in parallel driven by a single LFO (or not if you want to get a little more sea sick). You don't need to worry about phase if this is the case (it would already be an issue before plugging in the tremolo, so is not your problem).

If it is 1-in, 2-out then you want an input stage and a signal splitter, then 2 tremolo's in parallel. In this case, you do need to worry about phase, so make your splitter a bit more complicated by adding a switchable phase inverter.

If you want harmonic tremolo then perhaps your guy wants the left channel to have more bass while the right channel has more treble and vice versa? If so, you need to add a phase splitter to the LFO output.

There are a number of DIY harmonic tremolo designs you can then drop in to this design...

Simple.

Edit - you can make this bypass-able. True bypass for a 2-in, 2-out needs a 4PDT, which is available in various flavours of switch.

[ElectricDruid]

I think Samhay is getting to the crux of the issue here: there's still a lot of uncertainty about what exactly this person is after and what they're trying to achieve.

Mono-in/stereo-out or stereo-in/stereo-out is one question.

In-phase or out-of-phase modulation is another. You could it either "left has more bass while right has more treble and vice versa" or you could do it "left and right have more bass, left and right have more treble" with it in phase. And then you could get very Leslie-speakers on it and have different rates for the treble and bass. Why not add a half-moon switch to drop the rate while you're playing too, a la Hammond organ? Ok, sorry...

Personally, I'd build something based on David Rolo's Twin Peaks (search on the forum) since I think it sounds great. If it's full stereo-in-stereo-out, you'd need basically two circuits controlled by one LFO. If it's mono-in, stereo out, you can share the input and filter stages, and just double up the tremolo stage. I'd also want an in-phase/out-of-phase modulation switch so you could have the two channels working together or opposite. That might even be what they mean about "a phase switch".

HTH,
Tom