PT-80 build : works but something is wrong...

[keith71]

This is the Pt80 from GGG, I build it without a problem and it had worked at first fire-up, but something is strange :

First, their were a strong feedback loop at high repeat settings (past 12 o'clock at the knob), to fix it I put a 10k resistor before 1µF cap that is wired to pin 6 of the sa71. The problem  : I get no more than 5 or 6 repetiton with the pot maxed out... is it normal with pt80 ?

Second, the tone of the repetiton goes bad past three repetitons (distorded and muddy), is this normal ?

Thank you for any help

[Mark Hammer]

It is often the case that regeneration loops need some fine adjustment.  A number of commercial flangers and phasers have a trimpot in series with the regen level pot.

I would suggest doing the following (which is how it gets done in commercial effects):  

1) Insert a 25k trimpot in between the 22k feedback resistor going to pin 12 on the compander and the 1uf capacitor before it, setting it to maximum resistance.

2) Set the delay time for shortest delay.

3) Set the echo for maximum delay level.

4) Set the Repeats level for maximum.  This will now produce annoying resonant feedback.

5) Slowly reduce the resistance of the trimpot until the oscillation stops.

Another approach to try is something I often recommend as a way of producing more natural sounding repeats.  In the real world, earlier repeats have more treble than later repeats.  If you place a simple lowpass filter in the feedback loop, this will have no effect on the first echo, but will take a little bit more treble off each repeat afterwards...just like in the real world.

So, replace the 22k feedback resistor with two 12k resistors in series.  Run a .01uf to .015uf cap from the junction of those two resistors to ground.  This will have two effects:

a) It will roll off the treble at around 1300hz (880hz with .015uf).  With a simple 6db/octave filter the rolloff will be just noticeable the first time, and become more noticeable with each repeat.

b) You will be able to set the Repeat level to maximum without much or possibly any oscillation.

It works like this because the oscillation depends on having a constant feedback level.  Because the filter eats away a little bit of signal each repetition, and also changes the bandwidth, oscillation is prevents.

To my ears, this produces a more natural sounding echo, and also comes closer to what reverberation sounds like (even though delay lines don't do a very good job simulating reverb).  I understand that it isn't "le gout de tout le monde", but it may be a more pleasing solution to your problem.

[keith71]

Thank you.
I replaced 22k feedback resistor with a 33k, and this improves the sound of the repeats. I'll soon try the cap to ground mod in the feedback loop.

[geertjacobs]

My PT-80 also had this problem with the massive feedback on certain settings.
Yesterday I finally tried the 12k-12k-10nF mod (Mark had already described it earlier) and it seems to work great.
Thanks Mark!
Next thing I want to try is a stereo mod as discussed in this thread.

[Mark Hammer]

What both you gentlemen illustrate so well is that in the world of 5% tolerance components, many schematics are essentially "best guesses".  It also illustrates how helpful it is to:

a) Understand a bit more theory so you can make the circuit behave as intended/desired.

b)  Have a webforum like this with folks who "have been there" before you. :wink:

Glad these changes work for you

Incidentally, Geert, you might even try reducing the second 12k resistor (the one tied directly to the compander chip, since the lowpass filter takes away some of the signal.  I don't expect the circuit will be very friendly towards anything smaller than 8-9k, but you can experiment with either combinations of small values or with a trimpot so that you can get maximum regen without oscillations.

[lmkv15]

Hi,
The repeat resistor in the layout is a 27K and greater then in the normal layout from the PT2399 (22k). The repeat funktion Is also dependent on the level at the Poti level (Dry/wet). If the the level knob in the max. (right postition) the full level goes to the repeat pot. Then is normal that the delay oszillated.

regards Uwe

[stallik]

Only my personal opinion here but I really like to test this pedal and the degradation of repeats when playing to backing music with other pedals (distortion etc) active.

Reason for this is that, to my ears, the pedal may sound wrong on its own but completely natural when in real use. Its normal for me to prefer my digital delay on single clean 'test' notes but much prefer the crappy PT-80 sound for real playing

Until I need a 2 second delay that is