The Waveulator: Two VCA-based Clipper Saturators

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Re: The Waveulator: Two VCA-based Clipper Saturators

Post by mediatechnology »

plantylife wrote: Fri May 12, 2023 12:29 pm I see!.. I thought that I saw somewhere that you've did both odd/even separate in the past.. my bad.

If I understand correctly you planning on crossfading between odd/even with a pan pot. Is there a reason for this?
Im curious why because the option to have both separate (like for instance the VSM-3) makes also sense, at least i think.

Looking forward Wayne!
I did do an even/odd version earlier but abandoned providing even-order since my interest was in processing an overall mix.
Even was just too aggressive and introduced nasty IM.
For individual tracks it would be useful.

There are two polarities of even-order distortion available.

One polarity compresses (or expands) only the bottom half of the waveform, the other compresses (or expands) the top.
They do sound different.

We'll call one "-even" the other "+even."
When +even and -even are equal in level the result is odd-order.

The first control would pan between -even at full CCW to odd in the middle (+/- equal) to +even at full CCW.
This allows the user to set the ratio of even to odd and the polarity of even.
This sets the type of color.
If you were looking at it on the 'scope the signal would go from saturated only on the negative peak to both peaks saturated in the middle of rotation to saturation only on the positive peak as the control is rotated to full clockwise.

A second control would set the level of modulation which is actually a Ratio control.
This control adjusts how much color or saturation.
Ratio comes after the -even/odd/+even control.

The Ratio control can be made to provide compression to expansion over the full rotation range or just compression with a switch to change it to expansion mode. (The Dave Matthews demo had compression/expansion on a single rotation.)
I sort of favor the later since the usable ratio for compression can be quite large but a usable range for expansion is small.
Ratio can be described as "hardness" of the saturation curve.

A third control sets the Threshold which is the level where bending occurs.
Ideally this control could be calibrated in the user's dBFS scaling so bending could be set at -3 dBFS for example for protection limiting or much lower to introduce audible compression.

A fourth Makeup Gain control is required to bring the squashed output back up to level or in the case of expansion to lower output level.

To summarize if there were two or three controls to separately add in even and odd they would interact and if only two controls were used a single polarity would have to be chosen for even-order.
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Re: The Waveulator: Two VCA-based Clipper Saturators

Post by plantylife »

Thank you Wayne for the super extended and clear explanation.

I confused it indeed! I thought maybe it was possible to nul the signals but by reading
and thinking about the above I think it isn't

Looking forward to progress.
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Re: The Waveulator: Two VCA-based Clipper Saturators

Post by mediatechnology »

I decided to try a super-simple rectifier circuit I borrowed from Cordell.

Take a look around IC4, IC5 and Q2-Q3.

Build A High Performance THD Analyzer Oscillator Cordell Audio June 1981.

It's really important that the modulating waveform not have any sharp hooks, corners or aberrations particularly as it comes out of baseline. Most opamp-based rectifiers have this because they run open loop at zero crossing. I've always thought Cordell chose this topology to reduce internal harmonics in his oscillator. Q2 pre-biases the transistors to linearize rectification.

If you look at the linear rectifier in earlier posts (shown below) you'll see that the pre-distorter has a shunt resistor across the diodes. The absolute value stage that follows it doesn't have diodes within the feedback loop. The result is pretty clean rectification.

The threshold circuit that follows it does however. U5B has a pretty hefty compensation cap to provide rolloff and minimize hooks and sharp edges.


The threshold circuit shown above clips out the lower portion of the modulating waveform and clamps it to the threshold voltage. The modulation however needs to have a 0V baseline so it has to be AC-coupled and DC-restored by the following stage at U8A.

I decided to try the Cordell circuit because it can be modified to provide the threshold function by biasing the rectifier off. Replacing Q2 with a variable voltage source sets the threshold voltage. An added advantage is that the output is referred to baseline so it does not require AC-coupling and DC-restoration.

Switching out of threshold is super clean.

The only disadvantage I see is that the transistor reverse B-E voltage limits the peak-to-peak input to around 8V. (4V peak). The threshold voltage ranges from about +550mV to -3.75V with a rectification range of about 100 mV p-p to 8V p-p. This can be worked around by scaling.

I'll post a circuit fragment soon but its basically IC4, IC5, Q2 and Q3 in the Cordell circuit with a couple of added op amp buffers for the Threshold voltage and output.
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