Guitar stutter pedal . Muting Problem

Thread Starter

Chnopf

Joined Apr 11, 2023
5
Hi Everyone.
I try tu build a stutterpedal for my guitare similar to this one: (https://duckduckgo.com/?t=ffab&q=St...i=https://www.youtube.com/watch?v=AFxujKNzebo)

So i have a 555 timer chip to control the frequency.
To cut the signal i tried it with a optocopler. it workd but decreesd the Volume. so i tried it with a vactrol. at first this does not workd so i put a resisstor in sereis with the output, becouse the LDR always have a spesific amount of resistance and i thought maybe the LDR has a higher resistance than my amp. With this resistor its working but it also does not completly mute my signal.

1715959327563.pngAny ideas how i can muti my signal without decrees the volume?

Thanks For the help
 

LowQCab

Joined Nov 6, 2012
4,290
It's not going to be that simple to go from "Zero-Attenuation", with complete "pass-through", to "Full-cut-Off".

It could be done with 2 small back-to-back FETs setup as an AC-Switch,
with their Gates driven by a Photo-Voltaic Gate-Driver, which is then driven by the 555.

This may be the most practical way to do it for strictly "On-Off" chopping of the signal.

For more versatility, the Schematic below shows a Compressor using Diodes to attenuate the signal.
But it can do a lot more than that.
All You would need to do is remove the Feedback from the Output of the Compressor
and then connect it to your 555.
You can also use the Triangle-Wave-Output of the 555 Oscillator-Circuit to control it
resulting in quite a few different and unique modulation possibilities.
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Diode Compressor .png.
 

MisterBill2

Joined Jan 23, 2018
19,366
It will provide a better reduction if that resistor is put in series with the source.
I is also likely that the LDR can use a bit more drive to the light source. What are the specifications for that opto-coupler??
 

Alec_t

Joined Sep 17, 2013
14,398
The post #4 circuit is fine in principle, but if the ideal Q1 is replaced by a real-world transistor (bjt or jfet) there is undesirable break-through of switching noise due, I think, to parasitic capacitances of the transistor.
Here's the circuit I simulated:
1716390198076.png
This break-through persists even if the chopping pulse is slugged by C2.
This is the resulting output :
1716390404067.png
Chopping starts at ~3 sec and the break-through shows from ~7 secs onwards. This is distinctly audible in the attached audio file (stutter.mp3.txt) derived from the v(out) trace. (Delete the '.txt' file extension).
 

Attachments

Ian0

Joined Aug 7, 2020
10,232
The post #4 circuit is fine in principle, but if the ideal Q1 is replaced by a real-world transistor (bjt or jfet) there is undesirable break-through of switching noise due, I think, to parasitic capacitances of the transistor.
Here's the circuit I simulated:
View attachment 322920
This break-through persists even if the chopping pulse is slugged by C2.
This is the resulting output :
View attachment 322921
Chopping starts at ~3 sec and the break-through shows from ~7 secs onwards. This is distinctly audible in the attached audio file (stutter.mp3.txt) derived from the v(out) trace. (Delete the '.txt' file extension).
I wonder if the current through the transistor base and C2 is upsetting the virtual earth.
I‘m intrigued now. I‘ll try a few other things and I’ll see if my guitarist friend wants one.
First would be the JFET.
Second would be to move the 555 so it is powered between the virtual earth and 0V, so that there is no current through the diode.
I’ve used the bipolar circuit many times, mainly to mute music when a microphone is used, and the breakthrough is not noticeable, probably because it happens at the same time as a load of microphone handling noise.
 

Alec_t

Joined Sep 17, 2013
14,398
I didn't find a jfet worked any better than a bjt, but using an N-channel MOSFET with a low Rdson (AD6408) improved things.
1716394470787.png
I’ve used the bipolar circuit many times, mainly to mute music when a microphone is used
The occasional faint click would likely go unnoticed, but the tip and toe clicks for multiple chopping pulses are definitely noticeable.
 

Ian0

Joined Aug 7, 2020
10,232
That's definitely worth a try as well. The bipolar circuit ceases to work if the base-collector junction gets forward biassed, so the collector has to be kept very close to zero, hence the virtual earth amplifier.
 

MisterBill2

Joined Jan 23, 2018
19,366
This comes across as an extreme version of the vibrato that wasavailable on the tube type guitar amps back on 1960 and earlier. An analog switch could also be used for chopping the audio. But I am trying to think if I have ever heard that effect used. I recall a noise-gate fake drumbeat on some really talent-free stuff some stations were playing about 25 years ago. Totaly devoide of anything that would cause anybody to choose to hear it.
 

Ian0

Joined Aug 7, 2020
10,232
I would suggest trying a version that can be adjusted to Chop to some Gain greater that "Zero".
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You mean so that it doesn’t go completely off?
That’s reasonably easy - a resistance (possibly variable) inthe transistor collector.
 

Alec_t

Joined Sep 17, 2013
14,398
This looks promising :-
1716460393201.png
Chopping noise is now virtually inaudible (wav file attached). I included the gain stage to test if high amplitude audio would partly turn on the fets when they were supposed to be off. Doesn't seem to be an issue.
 

Attachments

Last edited:

Ian0

Joined Aug 7, 2020
10,232
This looks promising :-
View attachment 322963
Chopping noise is now virtually inaudible (wav file attached). I included the gain stage to test if high amplitude audio would partly turn on the fets when they were supposed to be off. Doesn't seem to be an issue.
We need to be a bit careful here as we are dealing with a source impedance that is 6kΩ+6H, not a SPICE true voltage source.
 

Alec_t

Joined Sep 17, 2013
14,398
Yes. I was thinking a buffer stage might be needed between a guitar pick-up and whatever circuit is used to give the stutter effect.
 
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