Tim Escobedo's Simple Square Wave Made Simpler question

[gigimarga]

Hello,

I reloaded an older project, Tim Escobedo's SSWMS (http://www.jiggawoo.eclipse.co.uk/guitarhq/Circuitsnippets/snippets.html), built using Matt Casey's PCB (http://www.aronnelson.com/gallery/main.php/v/Matts-layouts/SSMASD.gif.html)...thank you very much both of you!

It works OK and it seems to me that it sounds as in the soundclip, but the 500K pots seems to be too big because: 
1. they changes the sound dramatically only in the first 10-20%
2. with them minimized I have a huge output
3. with them maximized I have a low output (muffled too)

So, my question is if a pair of 50K pots isn't enough for it?

Best regards!

[cpm]

well, you have the circuit at hand, you could try yourself

[gigimarga]

well, you have the circuit at hand, you could try yourself

You're right cpm...as usually, due to my poor English, the question was badly formulated.
More clearly (I hope), I wanted to ask:
1. if anyone else which built it can confirm me that's the normally behaviour of the circuit
2. if I lowered the values of the pots to 50K I will obtain the same shape of the signal

Thank you!

[cpm]

those pots are there as variable resistors. so with the 50k you wont get the other 450k.. but you said only the first steps really make a difference, the wort that may happen is that 50k is not enough for that useful range.

[Mark Hammer]

This is purely a matter of pot taper.

Some things can have linear changes that are noticeable to the human ear, but many things require exponential changes in order to be detectable to the human ear/brain.

The square-wave shaper is simply a lowpass/lag filter for the rising and falling edges of the square wave.  If you look at many schematics for attack/decay envelope generators for synthesizers, they will look very much the same.  The difference here is that the range of attack and decay times needs to be much much shorter because you are dealing with waveforms in the audio range, rather than blips that last for a few seconds, as on a synthesizer.

The other thing I will mention is that the cap value of .02uf (.022uf, realistically) is optimized for some notes but not others.  Remember that your guitar will produce notes over a wide range.  The combination of pot setting/resistance and cap value may produce a specific slope for a higher note that will be almost impossible for you to hear for the lowest notes.

This circuit may prefer an anti-log pot.  Since it doesn't matter, and won't confuse anybody if the pot works opposite, just use a log pot and wire it up backwards.

[gigimarga]

Thx a lot Mark Hammer!
Which other values would I test instead of the 22nF cap?

[Mark Hammer]

Try .033 for lower note range, and .015 for higher notes.

[gigimarga]

Try .033 for lower note range, and .015 for higher notes.

Thx a lot Mark!
As I can see with my mathematically eyes, the 22nF cap is almost the average of the two values indicated by you

[Mark Hammer]

You will often see "compromise" values for caps in real-world-to-synth-world conversion circuits like this.  So, some pitch-to-voltage convertors will use cap values for time constants that may do an excellent job for 80% of the frequency range to be covered but not be ideal for the top and bottom 10%.  Same thing, sometimes, for compressors.  The time-constant cap may be great for the middle and higher notes, but inappropriate for the lowest ones.

[earthtonesaudio]

I built something like this, but replaced the two pots with one 10k, and swapped one diode for a zener (Vz < square amplitude).

I had to fiddle with the cap value to find a good range.

[gigimarga]

I built something like this, but replaced the two pots with one 10k, and swapped one diode for a zener (Vz < square amplitude).

I had to fiddle with the cap value to find a good range.

Thx a lot earthtonesaudio!
Can you tell me what Zenner have you used? I think that it could have a low value (my lowest value is something like 3.3V).
 

[earthtonesaudio]

This is what I used:
http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=41246&g2_serialNumber=1

The zener value I used was maybe 5.1V, I don't remember.  The important thing is that it's voltage is less than the voltage of the square wave you're trying to shape.  That way, you can turn the pot all the way to the zener side and get a fairly square wave output.  In the middle of the rotation it's more of a triangle and toward the non-zener diode it's a ramp/sawtooth type of wave.
An alternative to the zener could be one Si diode (cathode pointing right) and one LED (cathode pointing left).  All that really matters is that the discharge path back through the zener be lower impedance than the path through the resistance and other diode.


I had the same amplitude problems with this that you mentioned.  The more you shape the wave with a passive shaper like this, the less amplitude the signal has left over.

One solution to control amplitude could be a couple diode clippers (actually just one should suffice) located after the zener/square side.

[gigimarga]

This is what I used:
http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=41246&g2_serialNumber=1

The zener value I used was maybe 5.1V, I don't remember.  The important thing is that it's voltage is less than the voltage of the square wave you're trying to shape.  That way, you can turn the pot all the way to the zener side and get a fairly square wave output.  In the middle of the rotation it's more of a triangle and toward the non-zener diode it's a ramp/sawtooth type of wave.
An alternative to the zener could be one Si diode (cathode pointing right) and one LED (cathode pointing left).  All that really matters is that the discharge path back through the zener be lower impedance than the path through the resistance and other diode.


I had the same amplitude problems with this that you mentioned.  The more you shape the wave with a passive shaper like this, the less amplitude the signal has left over.

One solution to control amplitude could be a couple diode clippers (actually just one should suffice) located after the zener/square side.

Thx a lot earthtonesaudio!
I will try this mode as soon as possible