PT-80 delay build report: crazy runaway feedback

[STOMPmole]

If your PT80 oscillates at the last 10% of pot travel you probably don't have a problem with it.  It's normal for a delay to go into oscillation when the repeats pot is cranked...and desirable if you want that Tommy Bolin phaser sound effect.

If it goes into oscillation TOO easily though (like mine did before the mod) it makes it impossible to set a nice ambiance echo.

[Atodovax]

The problem of excessive regeneration is common enough that even the best designs include some way of "taming" it.  One often sees a trimpot in the regeneration path of phasers, flangers, and delays.  I suspect the reason is that when you throw in 5% tolerances on this resistor and that one, it can be the case that the level of the regen signal can, in some cases, be high enough to keep it going.

Here is how it works.  At the factory, the regen pot is set for maximum.  The trimpot is then adjusted until you reach a point where oscillation (runaway feedback) stops.

If you look through the schems of some flangers (remember, chorusses, while they might use a VERY similar circuit, do not usually employ recirculation, so you will typically only see this in flangers), you will often see an op-amp stage two or three semiconductors "into" the circuit with a pair of diodes (or more, in the case of the A/DA flanger) in the feedback loop to form a crude limiter that keeps the addition of new and recirculated input from exceeding a certain level and turning into runaway feedback.  A resistor in series with those diodes "softens" the clipping so that this mixing stage doesn't just turn into a fuzz (although the extra harmonic content probably makes the effect sound more interesting).

In the case of delay/echo boxes, there is often a compander chip to do this, instead of an op-amp clipper circuit.  Still, the compander doesn't really set an absolute limit.  As far as I know, it just reduces input proportionally, making it still possible to insert a regen signal that stays alive longer than desired. 

In the case of the PT-80, probably the simplest fix, and most sensible thing to do is to to insert either a well-selected fixed resistor, OR a trimpot, between the 1uf output cap of the compander and the 50k REPEAT pot.  If, for example, a 4k7 resistor is placed in series with the input lug of the pot, the pot "behaves" as if it is a 54k7 pot that can never be turned up more than about 90% of the way.  In a sense, it is as if you have converted the pot so that 100% of its rotation is used for only 90% of its range - i.e., the pot now affords greater precision.  I suspect some sort of series resistance of 25k or less, in either trimpot or fixed form will allow you to nail maximum regen up to the point of oscillation.

From another perspective, when it comes to delay lines, I am a big fan of adding more lowpass filtering in the regen path.  Normally, everything going through the regen path also passes through all the assorted lowpass filtering before and after the delay chip (digital or analog) all over again.  But this filtering is intended to provide maximum usable bandwidth at all delay times so the cutoff frequency remains the same, but the effectivee steepness of the filter above that cutoff increases with each repeat.  In the "real world", long delays and secondary/additional reflections lose a lot of high frequency energy and are mere bandwidth shadows of their former selves.  As such, additional lowpass filtering in the feedback path tends to increase the "realism" of longer delays with multiple repeats by shaving off more and more highs and mids below the cutoff frequency of the initial lowpass filters.

I mention this here because additional filtering in the feedback loop will effectively reduce the feedback signal amplitude over repetitions such that it can't keep summing with itself to the point of runaway feedback quite as easily.  It CAN happen, but with extra filtering, a regen setting that might normally start to produce howls will not do so.  I might point out that this particular mod will probably eliminate the need to do the one mentioned earlier, since it attacks the same problem, just in a different way.

Probably the simplest way to do it in this case is to take the 22k resistor that feeds pin 12 of the compander chip, and replace it with a 10k and 12k resistor in series to achieve the same series resistance.  Now, from the junction of the two resistors, run a small nonpolarized cap to ground, and voila!  I would suggest a value, but unfortunately the computer I'm working on has the Windows applets like the calculator locked out.  I am guessing, though, that with a 10k resistor, something like a .015uf to .022uf cap might be sufficient to do the deed.

The nice thing about this is that you can easily install a toggle to lift the ground end of the cap and restore full bandwidth if you so choose.

Finally, while it doesn't sound at all more "natural", cutting the low-end of the regen signal will also reduce the accumulated signal level and reduce howling.  Here, what you would do is stick another cap in series with the 1uf feedback cap to create a lower effective value series capacitance.  A value of .047uf to .1uf is probably a good place to start.  This will shave off several octaves from the bottom of wherever the passband starts.  If you want to defeat it, simply install another SPST toggle than shunts the added cap and you'll be back to original bandwidth.

I recommend both these latetr twomods.  I've used them before and you'll find it opens up a lot of sonic possibilities and textures, at the same time as solving a usability problem.

Sorry to bump this thread, but i really need help on this MOD.. According to GGG layout i see a 36k resistor on pin 12 on the compander chip. Am i wrong?

[Mark Hammer]

No, you're not wrong.  But there are two resistors tied to pin 12.  The 36k resistor is certainly the one closest to pin 12, so I can see how you got confused.  But if you follow the trace back from pin 12, you will see that there is also a 22k resistor, situated near (but not connected to) pin 16.  THAT's the one I discussed in the mod, thirteen years ago.

[Atodovax]

I Saw it! Thanks again! ))

[ElectricDruid]

I love that it's still possible to get clarification about something like that after that long!

[vivek.rama]

Hi Mark, All

Thank you for this tread, I recently built the PT-80 and experienced the feedback issue, implemented the two resistor with a cap in between to ground mod, now it's a lot more manageable now.

Question i had was is this feedback issue present in the standard build or is it due to an error that might be present with my build? Do other have no issue just with the standard BOM?

Thanks

Vivek