How does a chorus work exactly?

[Mark Hammer]

Disagreed...

for me, Chorus is more about pitch than delay (though a little time variance won`t hurt...) 

Oh I probably agree with you, but what gets included in the category of "chorus" includes things where the pitch sticks out way more than any delay, and things where there is both pitch and delay.  Some people would probably say the second type is really ADT and not chorus, but other people would probably still call it chorus.  It's a bit like the boundary between "distortion" and "fuzz"; hard to pin down.

All rested up from Musikmesse?  I see that your friend Bootsy is coming to my city for Bluesfest. 

[puretube]

Disagreed...

for me, Chorus is more about pitch than delay (though a little time variance won`t hurt...) 

Oh I probably agree with you, but what gets included in the category of "chorus" includes things where the pitch sticks out way more than any delay, and things where there is both pitch and delay.  Some people would probably say the second type is really ADT and not chorus, but other people would probably still call it chorus.  It's a bit like the boundary between "distortion" and "fuzz"; hard to pin down.

All rested up from Musikmesse?  I see that your friend Bootsy is coming to my city for Bluesfest. 

Yes, I prefer the old-fashioned (pre-BBD-days) way of pitch-altering like it was done in the organs of the 50`s & 60`s,
and then having 2 of those, oppositely driven, for chorus...
(there`s a small delay in those vibe-like thingies, too).

Not rested yet, but I managed to take this year`s first open-air easter-swim on monday...
Didn`t have much time to rap with the booted one,
but the more R`n`R with him...   

[YouAre]

Ive got a synth book by Barry klien called "musical electronic circuits" there is a good section in there that describes the differences in a more mechanical sense, I wouln't call it definitive but maybe gives an idea about how different elements of the effects are created. I imagine most circuits exploit more than one of these properties thus the blurring of what a chorus / Flanger is. I have described them.

Vibrato = Low frequency sine oscillator modifying the delay speed no feedback or wet/dry summing
frequency shift = Saw / Ramp modifying the delay speed no feedback or wet/dry summing
Flanging = Feedback from the output to input (regeneration) no wet/dry summing
Chorus = Sine wave moifying pitch with wet/dry summing
Echo = Both feedback and wet/dry summing big delay

Ah that's interesting! So There's no dry signal with flange....hmmmm, something to look into! Thank you!

[12Bass]

Ah that's interesting! So There's no dry signal with flange....hmmmm, something to look into! Thank you!

Actually, flanging generally does mix dry and delayed signals, preferably exactly 1:1 to create the deepest notches.  Feedback can be added to increase resonance.

Oddly, against folk wisdom, my RTA testing shows that added feedback does not lead to deeper cancellation*, though it does make the sound more resonant, and will actually turn into a recognizable pitch generator, if feedback is high enough.

*My tests have shown that deepest cancellation occurs when dry/delay are mixed at exactly the same level, without added feedback.  With pink noise sent into my A/DA clone, I'm seeing a drop from around -20 dB nominal to -72 dB at the deepest (fundamental) notch.

[Mark Hammer]

One of the interesting aspects of the A/DA is that there is that there isn't a low of lowpass filtering on the delay path overall, but there IS an additional 2-pole lowpass filter on the feedback signal that would appear to hem the feedback signal in at around 4.8khz or so.  I would imagine that extra filtering is primarily intended to keep any hiss in the delay path from multiplying, but the fact that it is only applied to feedback signal has the effect of leaving the overall flanging sound more "raw" and unrefined.

And yes, a proper flanger has a wet/dry mix control so that maximum notch depth can be achieved.

[YouAre]

Actually, flanging generally does mix dry and delayed signals, preferably exactly 1:1 to create the deepest notches.  Feedback can be added to increase resonance.

Oddly, against folk wisdom, my RTA testing shows that added feedback does not lead to deeper cancellation*, though it does make the sound more resonant, and will actually turn into a recognizable pitch generator, if feedback is high enough.

*My tests have shown that deepest cancellation occurs when dry/delay are mixed at exactly the same level, without added feedback.  With pink noise sent into my A/DA clone, I'm seeing a drop from around -20 dB nominal to -72 dB at the deepest (fundamental) notch.

Which is the better defining characteristic of a flanger in your opinion? Cancellation, or deep notches?

One of the interesting aspects of the A/DA is that there is that there isn't a low of lowpass filtering on the delay path overall, but there IS an additional 2-pole lowpass filter on the feedback signal that would appear to hem the feedback signal in at around 4.8khz or so.  I would imagine that extra filtering is primarily intended to keep any hiss in the delay path from multiplying, but the fact that it is only applied to feedback signal has the effect of leaving the overall flanging sound more "raw" and unrefined.

And yes, a proper flanger has a wet/dry mix control so that maximum notch depth can be achieved.

I read the ADA clone project schematic. I went cross eyed. Referencing the Moosapatamus ADA Clone Project, is this 2 pole filter made up of r39-40, c18-19, and ic1d? Also, is the Enhance knob a combination feedback resistor/high pass. It seems to me that as you lower the knob, you increase feedback, but then you also increase the frequency of the high pass cutoff, that seem right?

[12Bass]

Which is the better defining characteristic of a flanger in your opinion? Cancellation, or deep notches?

Genuine tape flanging uses no regeneration and sounds wonderful.  Not sure about your question.  The notches represent cancellation.  The best cancellation occurs if the delayed signal is full bandwidth and mixed exactly 1:1 with the dry signal.  Deep notches means deep cancellation at the notched frequencies (which are linearly spaced).  Adding feedback makes the notches more nasty, metallic, and "tubular" sounding.  BTW, my figures above were incorrect - the notches are not as deep as suggested above.  If time permits, I'll post some RTA captures showing what the notches look like at various settings.

As for the LPF on the regeneration path, yes, it is the section with C18/C19.  FWIW, I reduced those values on mine so that the feedback is less bandwidth limited.  This allows for more of a "swoosh" in the highest audible frequencies.  BTW, the flanging effect starts to disappear once the fundamental notch exceeds around 2 kHz, which would probably be at less than 0.5 mS delay.

[Mark Hammer]

I read the ADA clone project schematic. I went cross eyed. Referencing the Moosapatamus ADA Clone Project, is this 2 pole filter made up of r39-40, c18-19, and ic1d? Also, is the Enhance knob a combination feedback resistor/high pass. It seems to me that as you lower the knob, you increase feedback, but then you also increase the frequency of the high pass cutoff, that seem right?

No.   Whatever bandwidth is set by the IC1d stage, and C9, is retained throughout the entire range of "Enhance" settings.

[YouAre]

No.   Whatever bandwidth is set by the IC1d stage, and C9, is retained throughout the entire range of "Enhance" settings.

Remember when I said I went cross eyed? I dyslexia'ed the circuit and thought it was a high pass...i see it's just a coupling cap....

But what purpose does the reference to Vb serve? i.e. why isn't the Enhance pot a straight variable resistor or just a voltage divider to ground?

[Mark Hammer]

Many effects that use feedback will employ a trimmer as a series resistance, coupled with a voltage divider.  During setup/calibration, the pot is set to max, and the trimmer adjusted so that max control-setting gets you that point juuuuuuusssstt below where it starts to oscillate or get otherwise objectionably resonant.

Why is this necessary?  Because there are several active stages before and after the BBD, and in the world of 5% resistors, there is no guarantee that what you want to be unity gain WILL be unity gain, once you start feeding back signals.  The trimmer compensates for whatever unintended additional gain there may be in the overall wet path.  Runaway feedback sounds cool in an analog delay when the delay time produces audible repeats.  Not so much when runaway feedback simply sounds like a mid-frequency howl.

[YouAre]

Many effects that use feedback will employ a trimmer as a series resistance, coupled with a voltage divider.  During setup/calibration, the pot is set to max, and the trimmer adjusted so that max control-setting gets you that point juuuuuuusssstt below where it starts to oscillate or get otherwise objectionably resonant.

Why is this necessary?  Because there are several active stages before and after the BBD, and in the world of 5% resistors, there is no guarantee that what you want to be unity gain WILL be unity gain, once you start feeding back signals.  The trimmer compensates for whatever unintended additional gain there may be in the overall wet path.  Runaway feedback sounds cool in an analog delay when the delay time produces audible repeats.  Not so much when runaway feedback simply sounds like a mid-frequency howl.

Sorry, my question wasn't clear. I get why the enhance trim pot is there, it's a limiter. But why is the enhance knob a voltage divider going to Vb? Why not just ground?

[Mark Hammer]

It's actually a very common thing.  The volume control on the Tube Screamer goes to Vb.

I'm ashamed to say that when it should go to ground and when to Vb is a bit above my head.

[YouAre]

It's actually a very common thing.  The volume control on the Tube Screamer goes to Vb.

I'm ashamed to say that when it should go to ground and when to Vb is a bit above my head.

heheheheheheh. I've stumped the Hammer! I guess I can test the difference myself and see.

[merlinb]

Sorry, my question wasn't clear. I get why the enhance trim pot is there, it's a limiter. But why is the enhance knob a voltage divider going to Vb? Why not just ground?

Because if it went to ground there would be significant DC current in the pots, which is generally avoided in audio because it can cause scratchiness when turning them. The alternative would be to connect it to ground via a big cap to block the DC.