I am trying to replicate the sound of the lovely Electro Harmonix Small Stone phaser guitar pedal. Using CombC I feel I can get halfway there. I found this diagram of the Small Stone but unfortunately I do not have the knowledge to decipher what is going on. When messing with CombC it is obvious that the exact delay times and phase offsets are crucial. Any help will be much appreciated.
Hi Nathan, thanks a lot for the thorough answer.
The slightly curved LFO sounds about right. I have the hardware pedal, but it is broken at the moment (for the xxx time, old Electro Harmonix stomp boxes sound amazing but are very poorly built) so I canāt check right now. The other ācolorā on the color-switch sounds like a square wave and as I recall it also makes the phasing more pronounced.
The nonlinearities are not my main concern to begin with, more the overall design in regards to how the LFOs modulate the signal and the exact (or close to exact) values used and also the ācolorationā of the sound in terms of limiting, saturation, clipping etc. I suspect the ladder will be harder to determine as those things can be a side effect of different parts of the analog circuitry.
I have been struggling to find a good VST or AU plugin phaser for years - I used to have one (forgot the name) back on OS9 (!) which I liked. One thing I noticed about a lot of the digital version I have heard, including all the implementations in SC I have tried, is that I hear the resulting signal as a combination of wet and dry (which it is), but when I play through the analog stomp boxes the guitar sounds fully āsubmergedā in the fx - even when intensity is not at max. I donāt perceive it as being a mix of dry and wet.
I read you post on FOS, very interesting. I donāt really possess the math skills to fully understand the calculations, really wish I did (btw. my dad is a math professor and probably, for me, the worst person on earth to ask for help on this:)). ToDo list: Learn More Math. However, I did try the resulting SC code but I canāt figure out how to create any phasing when modulating the coefficients.
What is the difference between using FOS for an allpass and using the AllPass ugens?
When setting up the four LFOs, how should they be timed? All the same rates and phases but with different waveshaping for each? All different?
btw a modulated CombC is not a phaser, itās a flanger with feedback
It did sound more flanger-like 
It is often stated that chorus, flanger and phaser are all the same - only difference is the delay time. Any thoughts on that?
AFAIK: Chorus is a modulated delay line without feedback, mixed with the original signal. The effect can be intensified by using more delays, often in parallel and panned across the stereo field. A flanger is the same as chorus but with feedback (a modulated comb filter).
Iām not 100% certain this is correct but Iāve often implemented a phaser by substituting a Schroeder allpass in place of the flangerās comb filter. Considering that most phaser plugins have a feedback parameter, I suspect the plugins are doing it this way.
hjh
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The flanger sound is decidedly that of modulated comb filters ā so I perhaps said it a bit wrong. Flanger is structurally like a chorus but substituting feedback delay lines (comb delays) in place of simple delays.
āChorus with feedbackā likely means that the chorused signal is fed back, which is structurally very different. In any case, to qualify my original statement, by āwithout feedbackā I was referring to the nature of the delay lines only. (That is, a chorus using comb filters has changed into a flanger and isnāt a chorus anymore.)
hjh
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What a wealth of useful information I got from both of you, thank you for that. @nathan, the feedback loop is definitely a plus, almost a must I would say. On the guitar sample I just tried it on, it sounded drastically better when the block size was 16 rather than 64, especially when on the verge of self-oscillation. When I distorted each iteration in the 4.do{ } a bit (I used SmoothClipQ for the test) and brought the feedback loop to self-oscillation I could hear what I imagine is the concept of the Roland Jet Phaser - self oscillating, distorted filters with a good amount of lo cutting. The Small Stone is not really able to self oscillate, but close.
How sweet that tape flanger sound is!
I will keep working and explore the concepts suggested.
Haha, phasing, vaping and cooking all at the same time:) Thanks for posting this one!
Can I ask about what block size you guys are using? Are you changing block size per project or do you keep it fixed? Do you find there is an overall optimum samplerate/block size ratio for the type of effects we are discussing here: chorus, flanger, phaser and related. Are there any potential ringing or other artifacts related to the samplerate/block size frequency?
I am working at 48k, canāt go higher than that because of other software running in tandem. Also, delaying the original signal is not an option, I am dependent on realtime response.
I should have phrased my question differently, the optimum balance between block size and performance. So far I have only been able to use a block size of 1 for fairly small projects. Anyways, thanks again for all the insight. Good to hear that single-feedback Ugens can be made and thanks for the heads on the freelance work. I will keep working a bit on the phaser, made something with the FOS approach which is pretty good.
Now after hearing the tape flanger and got excited about this slow flanging which I find hard to achieve in SC. I tried slightly modulating the playback speed one of two PlayBufs running in parallel in the hope of emulating the tape flanger. I added a little saturation too on the modulated PlayBuf to mimic the degradation of the signal when recording onto tape and back to another tape. I have a hard time getting it smooth and slow even if the modulation is slow. Anyways, I will keep working at it.