Delay modding

[primalphunk]

What all is required to do tap tempo delay?  Additionally...I really dig that smeared gliss kind of sound you can get by monkeying around with the knobs of older delays...How can a pedal be rigged with some kind of expression pedal to do this?  Is there a good web site I could check out somewhere with something like a technology of the delay blurb?

IMHO delays are the coolest pedals in the world and all this talk in another thread about modding an Arion delay has got me kind of wigging...

[Mark Hammer]

At the time of the earliest Memory Man pedals, E-H also produced a bizarre behemoth called the "Hot Foot".  This was a heavy duty foot-treadle type pedal, just like a wah or volume pedal, but it did not carry signal itself.  Rather, rotating the pot inside with the foot treadle and gear mechanism resulted in the rotating of a flexible shaft attached to the pot.  The shaft extended out of the pedal and had a sort of cuff at the end.  The intent was to take the knob off a Memory Man or other E-H pedal, and secure the cuff onto the pot shaft with a little thumbscrew.  That way you could rotate the control with your foot in real time.

In principle, you could probably sick a hole in the side of a delay pedal, run a cable to a foot pedal, and control the delay time for "Whammy"-type effects.  The trouble is that you'll probably acquire some noise problems along the way when the pot adjusting the clock is relocated.  The Hot Foot was a good solution for the time because nothing had to be changed about the location of the delay pot itself.  Unfortunately, it required a big heavy pedal to avoid flipping, and the space between the control knobs had to be big enough to accommodate the cuff.

On the other hand, optoisolators can easily be adapted to mimic delat-time pots.  In most cases, delay pedals will use some Matsushita or Beiling BBD and companion clock chip (MN3101/3102).  The way the delay time is set with the clock chip always involves using a pot as a variable resistor.  There is absolutely no reason on earth why that variable resistor couldn't be paralleled with an LDR whose light source is controlled by a foot-pedal or other changeable source.  This can be as simple as :

a) popping a hole for a mini phone jack in the chassis to connect the foot pedal and delay pedal
b) installing an optoisolator like a Vactrol or CLM6000 or even a homebrew in parallel with the delay-time pot
c) wiring up a volume pedal as a control-voltage source (9v battery fed to pot, wiper output goes to hot output lead)

[primalphunk]

At the time of the earliest Memory Man pedals, E-H also produced a bizarre behemoth called the "Hot Foot".  This was a heavy duty foot-treadle type pedal, just like a wah or volume pedal, but it did not carry signal itself.  
[snip]
On the other hand, optoisolators can easily be adapted to mimic delat-time pots.  In most cases, delay pedals will use some Matsushita or Beiling BBD and companion clock chip (MN3101/3102).  The way the delay time is set with the clock chip always involves using a pot as a variable resistor.  There is absolutely no reason on earth why that variable resistor couldn't be paralleled with an LDR whose light source is controlled by a foot-pedal or other changeable source.  This can be as simple as :

a) popping a hole for a mini phone jack in the chassis to connect the foot pedal and delay pedal
b) installing an optoisolator like a Vactrol or CLM6000 or even a homebrew in parallel with the delay-time pot
c) wiring up a volume pedal as a control-voltage source (9v battery fed to pot, wiper output goes to hot output lead)

Just as I suspected...I am too stupid to understand the part of your answer that looks the most promising to me(option b).  I really need to do some reading about delay units.  Mark, do you have a delay 101 page somewhere?  In the meantime I'm gonna see if there is anything in my Forrest Mims book about optoisolators and stuff...  Thanks for your reply!

[bwanasonic]

There is absolutely no reason on earth why that variable resistor couldn't be paralleled with an LDR whose light source is controlled by a foot-pedal or other changeable source.

This also sounds like a way to incorporate an LFO. In the case of a commercial analog delay pedals, I've often wondered how feasible it would be to add an external *LFO box* via a jack.

Kerry M

[Mark Hammer]

No such thing as stupidity, only mismatches between explanations and learner needs.  I never blame the learner for what the teacher failed to deliver.

An optoisolator is any semiconductor where signal coming in one part is translated to light internally, and the light is sensed by something else and "turned into" something.  The fact that the "handshaking" between the two halves is simply light electronically isolates them, hence the name "optoisolator".  There are different categories, some involving transistors of different types, but the one of most interest to people here is simply an LED and a photocell in a light-tight package.  You can buy them or make them yourself.  Vactrol is one brand of optoisolator, but since they tend to be the most widely used and available, people use the brand name interchangeably with the *type* of device, the same way everyone says "Kleenex" instead of facial tissue.

If the time delay is set by a single resistor, then it stands to reason that you can stick a variable resistor in its place and have different delays, right?  And if you can stick in a variable resistor and have different delays, then anything that could duplicate a variable resistor (of the appropriate value/s) without problems, *and* could be controlled remotely, also without problems, could be used to change the delay time.

A vactrol put either in parallel with the existing control or instead of it, would allow you to change the resistance value in the optoisolator (and, by consequence, the delay time) by feeding a voltage/current that would change how brightly the LED inside the optoisolator is shining.  As the LED gets brighter, the photocell resistance goes lower, and the delay time changes.

From a practical perspective, my vote would be for placing the photocell in parallel with the existing control.  The MN3101/3102 clock chip inside the delay pedal will likely have a fixed resistor in series with the onboard control to set the minimum delay time.  Most commercial photocells and optoisolators will have a high enough resistance under dark conditions that in parallel with that existing pot, it will not substantially reduce the effective resistance.  More concretely, if the photocell has an "off" resistance of 10meg and the pot is 100k, then sticking the photocell in parallel won't really reduce their combined resistance very much.  Light up the LED, though, and the photocell might drop as low as 100 ohms, producing a result akin to turning the delay time to absolute minimum with the pot.

One of the ongoing concerns with any BBD-based effect is that  the clock signal has to be prevented from "leaking" into the audio path.  Consequently, running wires from the clock chip away from the chip to somewhere distant, especially if they are not well-shielded, is likely to produce some of that bleedthrough.  The advantage of the optoisolator method is that you could have someone standing in another room controlling how bright or dark the LED is, but all those wires are carrying is control signal, not audio signal, making it perfect for the purpose of remote foot-control.  Indeed, there really needs to be no change to the existing wiring.  You can simply tack on the leads from the photocell half of the Vactrol to the relevant lugs on the delay-time pot.

For any pedal or device that uses a control voltage (i.e., an input voltage that causes something else to change proportional to the voltage), there is often a jack to accept an external CV.  The most common arrangement is to do what I described.  You get a standard volume pedal, stick a 9v battery in there, with positive going to the pot terminal where the audio input signal would normally go, and ground going to..... ground.  That pot now will divide down the 9v to something lesser.  The usual arrangement for such pedals is to have a single stereo output jack.  The jack is wired up much the way you'd wire up a stereo input jack to turn on the battery for an effect, and for the same reasons, although a mono plug and cablecarry the voltage to the receiving device.  The voltage from the wiper of the pot constitutes your "hot signal" going to the tip connector of the jack.  Of course, as the voltage changes, the LED will vary in brightness, and the photocell will vary in resistance.  You'd wire up the pot in the pedal the "normal" way or in reverse, depending on whether you want forward treadle motion to increase delay time or decrease it.  If you want a deluxe controller, you stick in a DPDT toggle and wire it up the same way you'd wire up a pickup to reverse polarity, flipping the connections between the two outside lugs of the pot.

It may or may not be necessary to stick a fixed resistor between the wiper and output jack of the controller pedal.  LEDs can be damaged by too much current, and since you can't actually see the LED in a Vactrol (it is light-tight after all), there is a risk of feeding it too much current without realizing it.  I think the smart thing to do is to stick a 2k trimpot between  the wiper of the pot and the output jack, set for maximum resistance.  With the delay pedal's delay-time control set for maximum time, sweep the controller pedal back and forth and listen for how much variation it produces.  If you don't hear any (and are certain the wiring is correct), reduce the trimpot value to allow a little more current through, repeating the sweeping action.  At some point you'll get a reasonable amount of sweep.  The pot in the controlled pedal should probably be about 10k, with taped depending on your tastes and needs.

That should be enough info to get you started and INTO trouble.  If you need to get OUT of trouble, just post and we'll answer.