Author Topic: A simple tweak to delay circuits/builds  (Read 696 times)

Mark Hammer

A simple tweak to delay circuits/builds
« on: March 16, 2021, 01:45:10 PM »
I'm a fan of adding some gentle treble-trimming to the feedback/repeats path of delays, whether analog or digital.  I do this by adding a cap to ground after the feedback pot.

Well, a local guy was selling his attempted (unfinished) PT2399 delay build, plus a non-functioning MXR Commande Flanger for $25, so I figured what the heck.  I finished up his unfinished PT2399 delay (which seems to be identical to a Rebote 2.5, and thought I'd try out something.  Instead of JUST the switch-selectable treble-trim in the feedback path, I thought I'd use a 3-way toggle and insert a "grit" function by using the other toggle position to insert a pair of Schottky diodes (1N5817) to ground to clip the feedback signal.

Not especially elegant, but adds some bite and grit to repeats that goes a little beyond the grit occurring without any such clipping when aiming for many repeats at longest delay.

The Schottkys are essential, since they have the lowest forward voltage (mine were around 160mv).  If one uses germanium or silicon types, it is likely that the amplitude of the feedback signal will not be sufficient to clip.  Note that repeats, in general, tend to decline in amplitude.  So if one needs to use very low forward-voltage diodes in the first place, how would 3 and 4th repeats end up being high-enough amplitude to result in clipping, unless one dimed the repeats control?  Easy.  The tonal qualities of the first repeat are retained for subsequent repeats, and the first repeat is generally more than sufficient to produce the intended grit.

Not earth-shattering, but a simple and cheap mod to add some variety to delay sounds.

Ripthorn

Re: A simple tweak to delay circuits/builds
« Reply #1 on: March 17, 2021, 01:08:17 PM »
Keeley does this in the mag echo as well. I'm a fan of doing things with delay signals, be it filtering, clipping, general destruction, love it all.
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Mark Hammer

Re: A simple tweak to delay circuits/builds
« Reply #2 on: March 17, 2021, 04:14:27 PM »
Keeley does this in the mag echo as well.
I did not know that.  He's a smart fella!  ;) :icon_mrgreen:

FingerBlisters

Re: A simple tweak to delay circuits/builds
« Reply #3 on: March 17, 2021, 10:08:02 PM »
Could someone smarter than me explain off which pin this is occurring? Or draw a rough schematic to help those more visually inclined?

I guess you're using an on/on that adds the diodes in when flipped?

niektb

Re: A simple tweak to delay circuits/builds
« Reply #4 on: March 18, 2021, 03:08:15 AM »

11-90-an

Re: A simple tweak to delay circuits/builds
« Reply #5 on: March 18, 2021, 04:57:21 AM »
Could someone smarter than me explain off which pin this is occurring? Or draw a rough schematic to help those more visually inclined?

I guess you're using an on/on that adds the diodes in when flipped?

Depends on the certain pedal, but for example the Magnetron Delay that niektb posted above...
Schematic (without power supply):



delayed output is taken from pin 12, then filtered by an internal opamp in the pt2399, which is accessed with pin 13 and 14. After that it has some other components then the signal is sent in 2 ways, the "level" pot and the "feedback" pot. The feedback is what we are interested in right now. as you can see in the schematic, there are 2 diodes, D1, and D2, which clip the signal. Other than those diodes you can add some filtering, etc., to condition the sounds of the repeats...

Hope that makes sense... :icon_lol:
flip flop flip flop flip

bluebunny

Re: A simple tweak to delay circuits/builds
« Reply #6 on: March 18, 2021, 05:14:01 AM »
I thought "delay" in the thread title was a verb, and this was going to be advice on curtailing my build addiction...  :icon_rolleyes:
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Mark Hammer

Re: A simple tweak to delay circuits/builds
« Reply #7 on: March 18, 2021, 07:59:50 AM »
Could someone smarter than me explain off which pin this is occurring? Or draw a rough schematic to help those more visually inclined?

I guess you're using an on/on that adds the diodes in when flipped?

Depends on the certain pedal, but for example the Magnetron Delay that niektb posted above...
Schematic (without power supply):



delayed output is taken from pin 12, then filtered by an internal opamp in the pt2399, which is accessed with pin 13 and 14. After that it has some other components then the signal is sent in 2 ways, the "level" pot and the "feedback" pot. The feedback is what we are interested in right now. as you can see in the schematic, there are 2 diodes, D1, and D2, which clip the signal. Other than those diodes you can add some filtering, etc., to condition the sounds of the repeats...

Hope that makes sense... :icon_lol:

Thanks.  Yes that's how to do it.  Personally, I'd recommend Schottky diodes like the 1N5817, instead of the silicon 1N4148 shown.  The forward voltage of the silicon units may be too high to result in audible clipping.  Any clipping one hears may well be a result of trying to push the PT2399 too hard, rather than from the diodes.

jubal81

Re: A simple tweak to delay circuits/builds
« Reply #8 on: March 18, 2021, 10:15:43 AM »
Could someone smarter than me explain off which pin this is occurring? Or draw a rough schematic to help those more visually inclined?

I guess you're using an on/on that adds the diodes in when flipped?

Depends on the certain pedal, but for example the Magnetron Delay that niektb posted above...
Schematic (without power supply):



delayed output is taken from pin 12, then filtered by an internal opamp in the pt2399, which is accessed with pin 13 and 14. After that it has some other components then the signal is sent in 2 ways, the "level" pot and the "feedback" pot. The feedback is what we are interested in right now. as you can see in the schematic, there are 2 diodes, D1, and D2, which clip the signal. Other than those diodes you can add some filtering, etc., to condition the sounds of the repeats...

Hope that makes sense... :icon_lol:

Thanks.  Yes that's how to do it.  Personally, I'd recommend Schottky diodes like the 1N5817, instead of the silicon 1N4148 shown.  The forward voltage of the silicon units may be too high to result in audible clipping.  Any clipping one hears may well be a result of trying to push the PT2399 too hard, rather than from the diodes.

I'd advise against the 1N5817. They are EXTREMELY leaky and will have a big effect on the filter. Bat41s have a similar FV but don't leak much at all.

Mark Hammer

Re: A simple tweak to delay circuits/builds
« Reply #9 on: March 18, 2021, 10:53:44 AM »
Fair enough.  I have some of those I think.  I'll try them out when I have a moment.