Wednesday, May 27, 2020
Monday, May 25, 2020
Little Phatty overload is just soft clipping
From the time I studied the Little Phatty to build my moog filter, I have wondered if the LP does pre-filter distortion of the signal or if it uses feedback for its overload.
From the Little Phatty schematics it is clear that overload is just soft clipping, and the circuit is extremely similar to what I did for my distortion with in-feedback-loop OTA :-D
The LP:
There are a couple of differences: The moog circuit uses both inputs of the OTAs, and also the same CV (though inverted) for both increasing the amount of distortion and reducing (?) the output gain. A nice trick to keep output at the same level I assume.
The overload CV also controls gain/attenuation of the filter output it seems.
UPDATE: I missed something - in addition to the soft clipping, the output from the distortion is fed back to the audio mixer, thus the distortion also has feedback.
UPDATE: I missed something - in addition to the soft clipping, the output from the distortion is fed back to the audio mixer, thus the distortion also has feedback.
TIL: The little phatty has an analog signal path from pots to synth circuts
I read a reddit thread this morning about a guy that is doing a synth controller front panel. He had the novel idea of switching between using the pots directly to feed an analog voltage to the synth circuit (filter cutoff cv etc) and using voltages from a DAC to do the same. By simultaneously reading the voltage from the pot for patch storage, it would then be possible to load a patch and have a dac generate the required voltage to control the synth, but switch to direct-from-pot cv when the user turns the pot to prevent stepping.
The original poster was immediately beaten down by the knowitalls of Reddit, saying that this was overly complicated and that noone would do it like this.
He countered by claiming that he had looked at the circuit board of a Moog synth and seen analog multiplexers close to each pot, and rationalised that these did what he was thinking about. He also claimed that a video told him that this was the way Moog did it.
Another poster then proceeded to post the schematics for the Slim Phatty, explaining how it was clear to see that the circuits were indeed digitally controlled.
I had a closer look at the schematics and discovered that the original poster was indeed right (though possibly not about exactly how it was done) and the one posting the schematics wrong - the Slim Phatty (and by extension Little Phatty) DO actually have the ability to switch between direct control and digital control. Have a look at this:
When I went back to post my findings on Reddit, the original poster had already done so. He pointed out that this is called RAC (tm) in the Slim Phatty manual - "Real Analog Control".
The discussion went on to saying that this must be a marketing trick more than having real value, and funnily enough saying that it had do be something only the phattys use because "the memorymoog and prophet 5 does not do it". I call bullshit on this.It may be true that it is only found on the phattys, but comparing it to 25 years older synths is no way of proving it...
The original poster was immediately beaten down by the knowitalls of Reddit, saying that this was overly complicated and that noone would do it like this.
He countered by claiming that he had looked at the circuit board of a Moog synth and seen analog multiplexers close to each pot, and rationalised that these did what he was thinking about. He also claimed that a video told him that this was the way Moog did it.
Another poster then proceeded to post the schematics for the Slim Phatty, explaining how it was clear to see that the circuits were indeed digitally controlled.
I had a closer look at the schematics and discovered that the original poster was indeed right (though possibly not about exactly how it was done) and the one posting the schematics wrong - the Slim Phatty (and by extension Little Phatty) DO actually have the ability to switch between direct control and digital control. Have a look at this:
 |
| The filter pot. Its value is buffered by an opamp (U29), which is then both tapped directly (FILTER_CV) and sent to an ADC through AINO.2 for digital processing/storage. |
 |
| Digital control: CV from the DAC is buffered by a sample and hold circuit and output as FILT_CUTOFF_BUF |
 |
| A switch then selects between the CV tapped directly from the pot (FILTER_CV) and the digitally controlled voltage (FILT_CUTOFF_BUF). Notice how FILTER_CV is sent to multiple outputs, depending on what mode is currently selected for the pot. |
 |
| The selected voltage is sent to the board connector as FILT_CUTOFF... |
 |
| ...and received on the voice bort as FILT_CUTOFF (pin 33) |
 |
| There, it is mixed with CVs from other sources (modulation etc) into the filter cutoff CV FILT_CONTROL |
 |
| Finally, FILT CONTROL controls the cutoff frequency of the ladder filter. Thus there is a completely analog signal path from pot to filter. |
When I went back to post my findings on Reddit, the original poster had already done so. He pointed out that this is called RAC (tm) in the Slim Phatty manual - "Real Analog Control".
The discussion went on to saying that this must be a marketing trick more than having real value, and funnily enough saying that it had do be something only the phattys use because "the memorymoog and prophet 5 does not do it". I call bullshit on this.It may be true that it is only found on the phattys, but comparing it to 25 years older synths is no way of proving it...
Sunday, May 24, 2020
Noise tested
I tested the first board tonight, the triple noise. Two external parts are needed, a 2N3904 with the collector unconnected, the base connected to GND and the emitter connected to IN, and a 50k trimpot for adjusting the gain of the input.
It works perfectly. Rainy white, oceany pink and deep, rumbling red noise. Mmmm... I breadboarded this back in 2017 and had a lot of fun playing around with it with my then 3 year old daugter. "Daddy, can we go listen to the rain?"
Both orders from JLCPCB has arrived!
Finally, after waiting for over two months, the second order from JLCPCB arrived yesterday. I now have loads of circuits to test:
- JP6-style SVF
- Juno-style OTA filter
- Moog-style ladder filter (but this requires some soldering)
- 4ch mixer
- 4 way VCA
- Three types of CV/sample and hold
- Noise
- VCO, CEM3340 w/waveshaper
- DCO
- Waveshaper
- Led ring
The quality is overall very good, though I have found at least one solder bridge and and one of the led rings appear to have been slightly reworked (oh, and I've already broken a led, but that was my own mistake).
Some pics:
- JP6-style SVF
- Juno-style OTA filter
- Moog-style ladder filter (but this requires some soldering)
- 4ch mixer
- 4 way VCA
- Three types of CV/sample and hold
- Noise
- VCO, CEM3340 w/waveshaper
- DCO
- Waveshaper
- Led ring
The quality is overall very good, though I have found at least one solder bridge and and one of the led rings appear to have been slightly reworked (oh, and I've already broken a led, but that was my own mistake).
Some pics:
 |
| 4ch mixer and VCA, with room for v2164 or similar |
 |
| Top: Waveshaper. Bottom: Ring modulator and 4ch CV buffer |
 |
| VCO, DCO, two versions of S&H for CV buffering, DAC. |
Thursday, May 7, 2020
LED dial light pipes, second try
I did some more testing yesterday. I moved the connection points for the slits to the inside of the circle, tried engraving to make room for the leds, tried engraving slits to make them not go all the way through and made a black circle for the center.
Unfortunately I fucked up bigtime when cutting so everything is covered in oily brown residue. But I can still test the concepts.
Initial conclusion: Engraving room for the leds does not work at all. The diffuser has to be some distance from the LED, having it on top of it made the result very blurry. It may be a combination of this and light leakage between the cells, but it seems we get the best result when we have completely separate (cut) cells 'projecting' onto a diffuser.
The previous version with connection points on the outside and a simple diffuser still works the best. It may be that 3mm light pipes (i.e. 3mm plexi) is better to focus the beams.
There is also a lot of light leakage between the cells in the unengraved 2mm version. Not sure if this is because of the short light pipes or the moval of connections from outside to inside. Mostly the edges of the slits light up for at least the two neighbouring cells.
As for trying to make the cells bleed a little to get a continous bar, I feel that that did not work very well at all, the edges get blurry very quickly so I probably have to accept that they are slightly separated.
Using clear 3mm pipes with a 0.3mm spacing to the diffuser gives a continous bar but with slight bleeding. May be acceptable if we want a bar. No spacing will give an extremely sharp edge which is also very cool.
Further testing
- Is engraved room for led combined with cut cells working?
a: somewhat but not very well
- Are engraved slits working or significantly worse?
a: not conclusive but may achieve the same with a spacer
- what works best, connection points inside or outside the circle
a: outside seem slightly better.
- will dividers between the cells improve bleeding
a: Definitively, but needs to be dark color
- should we cut dividers even closer to minimise contact points?
- Is 2mm (or close to 1mm really) worse than 3mm for light pipes
- diffuser - should it be a continous ring or start and stop at the ends of the led ring?
- diffuser - try individual cells? Does it look cluttered?
I think that the diffuser may be less visible if it is a full circle and not divided into cells, especially once it is red.
Unfortunately I fucked up bigtime when cutting so everything is covered in oily brown residue. But I can still test the concepts.
Initial conclusion: Engraving room for the leds does not work at all. The diffuser has to be some distance from the LED, having it on top of it made the result very blurry. It may be a combination of this and light leakage between the cells, but it seems we get the best result when we have completely separate (cut) cells 'projecting' onto a diffuser.
The previous version with connection points on the outside and a simple diffuser still works the best. It may be that 3mm light pipes (i.e. 3mm plexi) is better to focus the beams.
There is also a lot of light leakage between the cells in the unengraved 2mm version. Not sure if this is because of the short light pipes or the moval of connections from outside to inside. Mostly the edges of the slits light up for at least the two neighbouring cells.
As for trying to make the cells bleed a little to get a continous bar, I feel that that did not work very well at all, the edges get blurry very quickly so I probably have to accept that they are slightly separated.
Using clear 3mm pipes with a 0.3mm spacing to the diffuser gives a continous bar but with slight bleeding. May be acceptable if we want a bar. No spacing will give an extremely sharp edge which is also very cool.
Further testing
- Is engraved room for led combined with cut cells working?
a: somewhat but not very well
- Are engraved slits working or significantly worse?
a: not conclusive but may achieve the same with a spacer
- what works best, connection points inside or outside the circle
a: outside seem slightly better.
- will dividers between the cells improve bleeding
a: Definitively, but needs to be dark color
- should we cut dividers even closer to minimise contact points?
- Is 2mm (or close to 1mm really) worse than 3mm for light pipes
- diffuser - should it be a continous ring or start and stop at the ends of the led ring?
- diffuser - try individual cells? Does it look cluttered?
I think that the diffuser may be less visible if it is a full circle and not divided into cells, especially once it is red.
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