Ugly Face - what's going on here?

[soggybag]

I'm trying to wrap my head around what is going on in the Ugly Face with the goal modifying my variation on this.

If I understand correctly, in the Ugly Face the 555 is setup in the astable configuration. For the circuit to oscillate pin 4 of the 555 needs to be at least 0.7v. The guitar signal from the 386 through pulls pin 4 above and below 0.7v. when it's above the we get a square wave and when it's below there is no signal.

The Threshold control "biases" the voltage at pin 4 between 3v and 0.25v. When the "bias" is above 0.7v we get self oscillation.

Looking the astable circuit the frequency is calculated as:

f = 0.72 / C1 * R2

and the output is taken from pin 3.

It's my guess that it's working similarly but it's a little murky. R2 + Frequency Pot (and the LDR), and C6 replace R2 and C1 in the astable circuit.

Looking at the article here: https://www.nutsvolts.com/magazine/article/555-astable-circuits

It says that R1 should smaller than R2. In the Ugly Face R1 is almost always larger!

It also mentions that the ratio of R1 to R2 sets the duty cycle. Which seems like it might be part of the sound?

The question I'm looking to solve is how to get a more useful range out of the Frequency pot. It works pretty but the biggest change is in the last 25%. I get best results with a C100K pot.

Another difference with the Ugly Face and the standard astable circuit is pin3. In the standard circuit pin 3 is the output. In the Ugly Face pin 3 is tied to the wiper of the frequency pot. Which has to affect the timing of the circuit since this is charging C6 the timing cap (as I user stand it.)

I'd like figure out what's going on here so I can make more informed choices for R3, R2, C6, and the Frequency pot.

[Rob Strand]

If you look-up the datasheet and application notes for the NE555 you will see detailed equations for the NE555.

It might help to go one step deeper than the equation you gave
For the astable case in your simplified pic:
- The time the output is high/on is ton = 0.69*(R1 + R2)*C
- The time the output is low/off is toff = 0.69*R2*C
- The frequency would be f =  1/(ton + toff) ; best to calculate frequency in two parts like this.

When R2 is large compared to R1 the output is more or less square.  When R2 is small you have a very small off-time but the on-time levels off at 0.69*R1*C.  Notice how the on-time is always larger than the off-time.  So when R2 is small the waveform is mostly staying high and has very narrow off periods.

When as the on-time and off-time become more different the sound becomes more buzzy.

As for part choices:
- R2 controls the frequency but R1 sets the highest frequency.   
- When R2 is on maximum, it sets the lowest frequency.

A couple of other points about the NE555:
- when pin 3 is low (ie. the off-time) pin 7 is pulled to ground
- when pin 3 is high pin 7 is open circuit.
- when pin 4 is below the threshold both pin3 and pin are low.
   (The chip is held in a reset state.)

So now for the circuit:

It's not straight forward.

The first thing to notice is the output is taken off pin 7.   During the off time this point is held low.  During the on time the cap changes up.  So the at pin 7 isn't on/off it's a rising ramp which gets pulled to 0V during the off-time.

The presence of C7 (100n) and The volume control stuff-up the form of astable circuit.   The fact R3 is a large value like 100k and is comparable to the 100k volume pot also complicates things.  That makes it harder to analyse.  You can write equations but they will be big and mess and not useful for humans.     What we can say is the presence of C7 and the volume pot will slow down the time it takes to charge up the timing cap C6.   That means ton is going to be longer than expect.   When pin 7 goes low it will short C7 to ground so that means the off-time is unaffected.

Anyway all said and done, use the above formulas to get an idea of the range of frequencies,  but expect the on-time ton to be longer than calculated.

The input keeps kicking the astable into reset and that's going to affect the output frequency.  Which is the magic of the effect.

Something I didn't mention is because the signal resets the astable the observed frequency can be much higher than the calculated frequencies.  The input signal basically overrides the timing.

[anotherjim]

The input keeps kicking the astable into reset and that's going to affect the output frequency.  Which is the magic of the effect.

In the synth world, this is oscillator sync. One "master" VCO provides the keyboard pitch but it is not (usually) audible in the mixer. A second detuned "slave" VCO has its start phase reset by the master VCO and provides the audio. Depending on the frequencies of the VCOs, the slave either never completes a full cycle or produces odd-length bursts of normal cycles but the played keyboard pitch is heard in the slave reset rate. The slave waveform prefered is usually a sawtooth and a timers capacitor charge/discharge waveform can approximate that.

[Rob Strand]

Depending on the frequencies of the VCOs, the slave either never completes a full cycle or produces odd-length bursts of normal cycles but the played keyboard pitch is heard in the slave reset rate.

Since the reset comes from the analog input the reset behaviour is quite complex.   There's some YT videos of this pedal, it produces some fairly radical sounds.  Some settings sound like a completely different pedal.   Not sure if the pedals I saw had mods.

Here's an explanation and video of the case jim mentioned,
https://www.keithmcmillen.com/blog/simple-synthesis-part-7-oscillator-sync/