Thoughts about Tri Chorus, Dimension D/C and Stereo Chorus Setup?

[Mr. Lime]

Yes, that'd be possible. I'd thought to do it "the other way around" by simplifying the triphase LFO into a single processor, and then using three of the simplest analog clocks I could find (either MN3102s or 4047s, probably). That way avoids the limitations of the NCO-as-clock solution, but the *real* reason is because Microchip don't make a chip with three NCOs on it, so I couldn't wrap all three clocks up onto a single chip even if I wanted to! And you know I'd have *had* to have a go if such a thing existed.
Doing it as a "multiprocessor solution" like you suggest is another entirely valid way to approach it.

Taking Mark's recommendation into account and looking for a solution with as little effort as possible, it would probably be Ripthorn's Attiny841 tri phase LFO feeding the "Manual" inputs of three 4-knob flanger PICs. With the Attiny841 depth turned to zero someone still could use the tap tempo input to sync all three clocks for a single LFO swing.
When the Atty841 swings the depth of each clock could be used to mix different LFO settings to the tri phase LFO which causes a lot of unsynced modulation - or do I miss something?

[Mark Hammer]

Thinking about the challenge on the way back from the dentist's today, I had an idea.  Although the idea of running two unsynced high-frequency clocks in the same circuit is a recipe for trouble, even though it might produce a lush sound, one does not have to use a high-frequency clock to produce unsynced stereo chorus.

I'll borrow something from the EHX Flanger Hoax.  Imagine we have two lag-type 4-stage phasers.  Typically, for musical effects, we use lead-type, in which each stage provides more phase shift for higher than lower frequencies.  This is easily recognized as the form in which there is a capacitor going to the non-inverting pin of the op-amp, and a variable resistance to ground.  The lag type flips the parts around, such that the cap goes from non-inverting pin to ground and the variable resistance goes to op-amp input.  This produces more phase shift the lower the frequency.  In both instances, the minima and maxima of phase shift can be calculated and set to be wherever you want.

In what is called "theta processing", the wet signal (typically of a flanger) is passed through a number of additional fixed lag-type phase shift stages.  From what I read in an Electronotes paper, this has the net effect of spreading the notches out differently in the bass region, reducing the highly resonant "boxey" sound that many flangers can have at the low end of the sweep.  Okay, hold that thought.

Imagine we have one swept time delay that is split and fed to two parallel sets of lag-type phase-shift stages.  Could be 2, 4, 6 or more stages.  The two sets of phase-shift stages are swept by their own unsynced LFO.  Setting the location of the minima and maxima in optimal fashion, the lower end, or even half, of the delay signal's bandwidth, is spread out differently, prior to being combined with the dry signal. 

BUT HERE'S THE GOOD PART.  The result out of each path will sound different, because the notches produced by modulating the delay will be spaced differently, however there is only ONE HF clock involved.  Two unsynced LFOs sweep the phase-shift stages, and the difference between that and the HF clock driving the delay chip results in no audible heterodyning, while still providing a stereofied output.

The older Diamond Halo Chorus also includes phase shift as a path parallel to the delay path from what I could tell (please correct me if I'm wrong), but the phase shift and delay are modulated by the same LFO.  What I'm proposing here is essentially 3 LFOs: one for the delay line, and one for each of the phase-shift networks.

[Mr. Lime]

I ordered a Mutron Bi-Phase clone some weeks ago and looking forward to that stereo phase shifting effect.

Since I had a BYOC Mega chorus laying around I put the DC-2w with it in series and that's quite nice sounding. It's not miles away from the TRI chorus sound samples I heard.
The Mega chorus has a tone control too but I don't find it very effective. It's similar to the Arion SCH-1 EQ but is only effecting the delayed signal while the Arion is pre delay and before the clean bypass.
Never played the Arion but in the YT videos it seems more effective to me. I think it's beneficial to have the bass rolled back a bit when multiple chorus pedals are stacked.

@Ripthorn, is your TinyLFO putting out the second PWM 180 degree phase shifted?
https://scientificguitarist.wixsite.com/home/tinylfo

[Ripthorn]

I ordered a Mutron Bi-Phase clone some weeks ago and looking forward to that stereo phase shifting effect.

Since I had a BYOC Mega chorus laying around I put the DC-2w with it in series and that's quite nice sounding. It's not miles away from the TRI chorus sound samples I heard.
The Mega chorus has a tone control too but I don't find it very effective. It's similar to the Arion SCH-1 EQ but is only effecting the delayed signal while the Arion is pre delay and before the clean bypass.
Never played the Arion but in the YT videos it seems more effective to me. I think it's beneficial to have the bass rolled back a bit when multiple chorus pedals are stacked.

@Ripthorn, is your TinyLFO putting out the second PWM 180 degree phase shifted?
https://scientificguitarist.wixsite.com/home/tinylfo

Honestly, I haven't looked at the TinyLFO in a long time, as I got sidetracked by several other projects. I would have to look and see what's up there, though I recall that the last time I did stuff with it, I think I tracked down the synchronization issue to a weird quirk in the Arduino tiny libraries that had a weird idiosyncrasy around how the timers were referenced. I'm actually about to head out of town for a couple weeks, so I won't be able to look into it until after I get back.

[johngreene]

I didn't think the stereo tri chorus was separate tri chorus on each channel. I thought you had three total delay lines and they were split stereo. Is this not correct?

That's what I've seen. The classic stereo tri chorus circuits come initially from the string synths of the 70s, with the Arp Solina probably the most famous. The sound was so popular that later single-oscillator polysynths like the Roland Juno106 and Korg Polysix used similar circuits to help fatten things up (check the schematics of those synths for details!). The general plan in these circuits is two LFOs with triphase outputs (so 0°, 120°, and 240°) with one LFO set for deep and slow, and the other for shallower and faster. Rate and depth are not variable, typically. Each of the three delay lines gets modulated by a mix of two of the LFO phases, one fast and one slow. Then you've got three complicated wet signals and one dry signal to mix back together however you like to get some crazy-lush stereo image. Maybe put a Dry and a 120° wet in the centre, with  0° and 240° left and right? There are loads of possibilities.

There were other options too, like Roland's "harmonic chorus" which used three LFOs at x0.5, x1, and x2 rates, iirc, but the typical tri chorus is as described above.

I've experimented with various possibilities using a microprocessor to generate the modulation signals for analog clocks and delay lines, since it's easy to make two LFOs in firmware and generate different phases and add them together. Since you've got three delay clocks, you only need three PWM channels, not six - do the adding in the digital domain, not afterwards in analog. In truth, I was somewhat underwhelmed. There's a definite point of diminishing returns, and it soon all becomes very lush and your ears/brain can't really tell one type of lush chorus from another type of lush chorus. I'd hoped to design a chip that could cover all the classic circuits and make them more practical to build (and I have a prototype of such a thing) but it's not actually that fantastic, which I hoped it would be. The different LFO options are not sufficiently different in sound. You're better off choosing one option and going with it, in my view.

HTH

What you describe is pretty close to what the Tri-Stereo does. Two LFOs, tri-phase, one fast of small amplitude and the other slow with high amplitude, there is almost no dry signal at all. Only the very high frequencies are mixed in that are above the anti-Alias cutoff frequency. There is a third tri-phase LFO that is mixed in when manual mode is selected. It's frequency is determined by opto-couplers that are contolled via the "manual" knob on the front panel or by an external expression pedal, if it is a later model. In "stereo" mode it mixes the center delay channel with one LFO for one side and with the other for the other side, if that makes sense. The design is very similar to a Roland design from back then. I can go through my files and post which one if interested.

[Mr. Lime]

Honestly, I haven't looked at the TinyLFO in a long time, as I got sidetracked by several other projects. I would have to look and see what's up there, though I recall that the last time I did stuff with it, I think I tracked down the synchronization issue to a weird quirk in the Arduino tiny libraries that had a weird idiosyncrasy around how the timers were referenced. I'm actually about to head out of town for a couple weeks, so I won't be able to look into it until after I get back.

No hurry, work and family keeps me busy too but I really dig that microcontroller stuff and would love to learn more!

What you describe is pretty close to what the Tri-Stereo does. Two LFOs, tri-phase, one fast of small amplitude and the other slow with high amplitude, there is almost no dry signal at all. Only the very high frequencies are mixed in that are above the anti-Alias cutoff frequency. There is a third tri-phase LFO that is mixed in when manual mode is selected. It's frequency is determined by opto-couplers that are contolled via the "manual" knob on the front panel or by an external expression pedal, if it is a later model. In "stereo" mode it mixes the center delay channel with one LFO for one side and with the other for the other side, if that makes sense. The design is very similar to a Roland design from back then. I can go through my files and post which one if interested.

I would be happy for any design insights. 


Just bought a Arion SCH-Z as it's described to be Landau's second choice of chorus pedals. Looking forward to hear how that one in series with the Dimension D will sound like compared to the Tri chorus. Technically it's different but as discussed before, it might be close enough for my ears.
In the fx loop I find the DC-2 even on the lowest LFO setting almost a little too much.
BTW has anybody come across a MXR M134 Stereo Chorus schematic?

[johngreene]

I was thinking if the chorus section in the Polysix by Korg. It's similar.

The manual LFO in the TSC in addition to using Vactrols for the frequency adjustment (a dual) it uses a single vactrol to make it a sine wave oscillator.

[Mr. Lime]

The Polysix was mentioned earlier in this thread and indeed gives an overview, how the 120 degree phase shift is made.

I'm still wondering what the benefit of that 120 degree phase shift really is. In a stereo setup for guitar we have only 2 channels: L/R.
Do we hear much of a difference if two LFOs modulating the delay time of two chorus in 180 degree  anti-parallel vs. three chorus in parallel shifted 120 degree?


Did I get it right that in manual mode, the TSC only has one LFO swinging? The depths of all 3 delay lines can be dealed in independently which makes the difference probably more recognizable. In preset mode the depths all maxed so the two LFOs modulate the delay lines evenly, just 120 degree phase shifted.

[Chillums]

ARP Solina String Ensemble Chorus.... Always wanted to try and recreate this beauty....
https://www.synthxl.com/wp-content/uploads/2018/01/Arp-Solina-Ensamble-String-Service-Information.pdf

[Mark Hammer]

ARP Solina String Ensemble Chorus.... Always wanted to try and recreate this beauty....
https://www.synthxl.com/wp-content/uploads/2018/01/Arp-Solina-Ensamble-String-Service-Information.pdf

I think Behringer beat you to it.

[Chillums]

ARP Solina String Ensemble Chorus.... Always wanted to try and recreate this beauty....
https://www.synthxl.com/wp-content/uploads/2018/01/Arp-Solina-Ensamble-String-Service-Information.pdf

I think Behringer beat you to it.

Yeah but do you think they used these?   

[Mark Hammer]

Not likely, especially for the production volumes they would anticipate.  But I can't imagine that a trio of Xvive MN3009s would sound terrible different.

[Mr. Lime]

ARP Solina String Ensemble Chorus.... Always wanted to try and recreate this beauty....
https://www.synthxl.com/wp-content/uploads/2018/01/Arp-Solina-Ensamble-String-Service-Information.pdf

There's also the Jackson Audio New Wave Stereo Chorus which is kind of inspired by the ARP Solina keyboard.

Seems like it got two delay lines but in 80s mode there are two LFOs modulating the BBDs so it's close to what has been discussed in the thread so far and maybe has some TSC vibes. It's expensive but has an analog signal path paired with some nice controls that are rare on other chorus effects. Probably worth a try.

[Mr. Lime]

I now got myself a RT Electronix Threeway chorus pedal, which is based on the TSC circuit.

It's more subtle than the Boss DC-2w but to me that's a good since I have problems finding useful settings with the Boss pedal.
I still have the DC-2w on my pedalboard and run it in series with the Threeway which gives a nice lush 80s chorus sound.
The Threeway can be left on in preset mode without sounding too dominant.
Tap Tempo and division switch are unnecessary features and I had rather prefered different wave shapes or a tone control like the Arion chorus has.

[Mark Hammer]

Though it can run into some cumulative noise issues, there is no requirement that a chorus employ more than one swept delay, and no prohibition against running several in series.  Keep in mind that the pitch-varying delayed sound of pedal A automatically becomes part of the "clean" sound of pedal B, and that both the original clean tone AND the pitch-varied signal from pedal A will again be modulated by pedal B, such that one now has THREE delay-modulated versions of the original input signal.  Now THAT's "thick".

[Mr. Lime]

As I currently look through different schematics I again got interests on how a simple but lush DIY stereo chorus could look like.

I found the DOD FX 67 Chorus to be a dual MN3007 BBD circuit with simple filtering similar to Tom's Flangelicious kit. One downside of the Multiflange PIC is the waterdrop sounding ticking that occurs sometimes in my build. Otherwise it would be a nice clock substitue.

Comparing again the Hollis Ultra Flanger and Zombie Chorus, I see a CD4046 used with a CD4069 as parallel inverter buffer. Is there a chance to get rid of that waterdrop ticking when the clock signal is buffered by 4069 inverters?

Another option that comes to my mind would be the Attiny 85 tinyLFO with two PWM outputs that drive two 4069 clocks for a parallel BBD setup.

One LFO signal has to be inverted but the result would be a simpler DC-2..

Mark seemed to have got some experience with de-paralleling the antiparallel LFO swings in a Behringer DC-2 clone so maybe even a second LFO (which could be syncd with the first) might be beneficial.

Something like the digital control of the Spaceman Delta harmonic trem would be cool:

[ElectricDruid]

One downside of the Multiflange PIC is the waterdrop sounding ticking that occurs sometimes in my build. Otherwise it would be a nice clock substitue.

Comparing again the Hollis Ultra Flanger and Zombie Chorus, I see a CD4046 used with a CD4069 as parallel inverter buffer. Is there a chance to get rid of that waterdrop ticking when the clock signal is buffered by 4069 inverters?

Probably not. While anything that reduces the amount of clock bleed in the final output will help make the noise less obvious (and buffering *may* help there, a bit), the source of the problem is the NCO used to generate the clock. An NCO generates an *average* frequency by jumping between two different values - jittering about. Although the final output frequency might be way too high to hear, the "jitter frequency" (e.g. the rate at which the NCO switches between the two different frequencies it's generating) might well be audible. That's what the weird watery noises are.
You can design an NCO that would minimise these effects to the point of inaudibility, but since we can't change the PIC's hardware, we're stuck with what we've got. The 4KNOB and MULTIFLANGE chips are an attempt at squeezing a lot out of a very little chip, and some flaws come along with that.

Another option that comes to my mind would be the Attiny 85 tinyLFO with two PWM outputs that drive two 4069 clocks for a parallel BBD setup.

Yes, this is the obvious solution - use the processor to generate only the LFO, and use an an analog VCO for the clock. This then avoids the jitter problem and eliminates the source of the noise. But you need more chips.

Something like the digital control of the Spaceman Delta harmonic trem would be cool:

That's a "0 to 180 degrees phase shift" control between the two outputs. That's a very simple thing to program, once you've got a digital LFO done.

[Mark Hammer]

I wonder if a simple lag allpass on one of the LFO outputs of a DC-2 (or workalike) could introduce usable "de-paralleling" (to use Mr. Lime's term).

Remember that more familiar "lead" phase-shift stages apply up to 90 degrees of phase shift for content above some minima, as dictated by the cap feeding the non-inverting pin, and the vaiable resistance to ground from that node.  A "lag" form feeds a resistance to the noninverting pin, with a cap going to ground.  This applies up to 90 degrees phase shift for frequencies below some maxima.  If one employed a standard P90-type allpass stage, but used 15K resistors, instead of 10k, and had a 470nf cap to ground, such a stage would apply up to 90 degrees of phase shift for low frequency content, starting around 22hz.  Assuming such a stage truly was unity gain, it would be the sort of thing one could insert between the LFO and one of the clocks in a dual-BBD chorus, to alter the resulting sound, by making the counterswept delays 270 degrees apart, rather than 180.

Just an idle thought.

[Mr. Lime]

Seems like a almost fitting LFO has already been done and shared to do some "de-paralleling" 
Night Shift
Not sure if it would work with the 4046 straight away as it is but with additional depth control this would basically bark up the right tree for an easy stereo chorus that's somewhere between a Tri-STC and a DC-2..

If two BBD lines are parallel, would adding a feedback loop with resonance control to one of them mess with the balance?
Like volume mismatch? I just find interesting how the Hollis Ultra Flanger is made with the odd/even feedback switch and it seems easy to add as a MN3007 is used as well.

[Mr. Lime]

Not exactly the same (digital instead of BBDs) but here is a through-zero-flanger which does pretty much what I was asking for.
Two parallel delay lines with one having a feedback loop for regeneration.
Interestingly the TZF is archived by simply keep one delay line unmodulated and remove the dry path - looks like a neat and easy to add feature on a stereo chorus! 

Here the schematic of the Daredevil Flanger: