XR2206 Tremolo/Vibe/Leslie circuit [documentation]

[phatt]

Revisiting a circuit I dreamed up years back and to the best of my knowledge this has not been done this way before so I thought folks might like to try it.
This does a fairly good job of emulating a spinning speaker effect.It's based on a XR2206 function generator chip and with a few added parts can do a Tremolo/Vibrato effect as well as a fake Leslie spin. Most of this is accidental discovery not brain power so I'm not sure how/why this works
but there is obviously some inter-modulation effect happening. Better minds may wish to add comment or improve it.

I've researched utube clips and there are obviously some clever circuits around that do similar tricks but I've not seen this done in this manner.
Anyway adding the schematic so folks can go test it out and may find even better ways to use this chip.


Note Tr3 & S2 are not used, I've left pins 13 & 14 open as that is for sine wave,
I'm only using the triangle wave but you can try it out.

I'm using a 18Volt supply into a LM7815 reg, you need at least 12 volts as the data sheet states a 10Volt lower limit.
The bias trim pot (Tr1) is a critical adjustment, a normal pot will likely drift and cause the dreaded click to wonder around so you need a
quality trim pot or even a 10turn trim. (that is where I went wrong in my first build)
The output (pin3) also needs some fine tuning just enough level to

balance the dry level with the FX level, too much and it will distort.
The rest is fairly straight forward pedal stuff that most will recognize.
Nothing better than a video demo of the effect so you get an idea of the effect.
Have fun, Phil.

Video link

[peterc]

Sounds very nice Phil, great project.

Peter

[Ben N]

Hey, Phil, that looks and sounds incredible. Two questions: Have you actually laid one out, and if so, any layout suggestions? And is there any reason you regulated down to +15V -- the datasheet says it can take up to 26 volts single supply, so 18 ought to be doable, right?

[phatt]

Hi Peter, thanks for the feedback.
Hey Ben,
Good to hear you like the effect.
Yes you can run it on 18V as long as it is well regulated. My plugpak/wall wart is an old Cannon PSU and not well filtered so I used the reg chip to keep supply noise down.
I will be designing another PCB when time permits, meantime I'm still trying to work out the mix setup as there are 2 pots at the mo, one Wet, one dry.
The original just had the dry fixed and a single pot to mix wet but you can't get all the in between sounds that way, so work in progress.
As for layout;
It's likely the Led and BJT flashing might introduce some clock noise if near the audio path but as long as the trim pot and output level is set right then noise is low.
There will be some swishing at higher volume but you would only notice it when you stop playing. (Heck, no more than a real Leslie cab which swishes when no signal is present)
Wise to insert this at the end of a pedal chain, that helps to keep noise down. Maybe stay away from really higain amps.
Cheers, Phil.

[phatt]

Just an update on the Schematic.
Note a mistake on VR1, it needs to be 250k. Sorry
You can go bigger for even slower speed but 250k delivers a good balance between Fast/Slow.

Regards using a pedal;
Unlike the old rack and pinion systems which deliver full 270deg rotation of the pot these cheaper lever setups only give about 90deg rotation. So I'm using a 2meg pot which gives me about 250k. This is enough to give the desired result but if you can source a Rack & Pinon type pedal then you only need a 250k pot.
Big thanks to those who left a message on utube
Phil.

[Hotrats]

great design idea and design Phil, congrats ...

so if I get this right you're using the 2206 as a multiplier
with the internl LFO multiplying the incoming audio (of course)
and this 180 shifts phase of audio alternating with LFO cycles

so, I get that running without phase shift stages produces signal cancellation
and therefore trem ...

I get that running into fixed phasor will split audio band
into in-phase and out-of-phase (harmonics) portion of the band
producing a cancellation in the low band and enhancement in the upper
and therefore harmonic vibrato with phasing

I got a similar thing happening by doing the harmonic vibrato thing by
adding the outputs of two ota's and by being able to invert the phase on one side
and therefor harmonic trem with or without the phasing component

I'll be giving your approach a shot fer sure ...
thx & cheers

[phatt]

Hi Hot rats,, sorry been very busy,
I believe this thing does a sum and difference trick,, still not sure of exactly how but I don't believe this chip has ever been used in this way.

Sorry the schematic has gone but no matter as I've changed a few things.
Update 17-8-20
1/ Resolved the tick,, thanks to a knowledgeable teck.
The ground of the XR has to be a single track back to PSU comm. I've drawn this in the Schem for clarity.

2/ Removed a pot,, Wet output is now taken directly from pin3 of XR2206 via VR2.
Note needs to be 5k max,, a 10k is too much and clock noise will become audible again.

3/ R6 and R11 have been altered to balance gain,, I had too much gain at input (R6).
R11 balances the max dry level, this gives a nice even progression through the 3 basic effects from
Tremolo,, throb trem or leslie to a modulated Chorus. (for want of better descriptions)
With Dry at max and backing off the wet you loose the flip/spin effect but gives a subtle modulation.

I'm working on a PCB but other things are slowing progress,, (I think it's called life) lol
So if you try this be aware the ground path return for the XR chip needs to be separate.

Also note,, Running Q1 from V1 18volt instead of the reg 15V helps reduce clock noise even further.
Pin 11 is the square wave out which I've used to drive the flashing led. potential noise so watch your track layout
At high volume through my gear all I hear now is the swish which is to be expected.
Cheers, Phil.

[phatt]

LessLee update;

Removed C12, C17 & R19. (Was no need for the Caps)

Added R26 4k7 across VR2.
Max resistance is now ~3k. Unlikely to find a 3k pot so a 10k with 4k7 gives a good balance.

R23 up to 47k. (needed to reduce clock noise bleed through)
Note Q1 supply must be taken before R25 otherwise it will tick.
Note, Pin 12 of XR2206 needs to have a separate ground path back to supply Com,, again reduces tick bleed through.
Still have not got round to a PCB as that will need careful layout.



This is working well through 3 amplifiers without issue.
Also running Keyboards is working with no Ticking.

Also been running this from a 12Volt supply with No Regulator and seems to be ok.
Though clean head room might be an issue for big signals.

[phatt]

Well Finally I've resolved the tick and the unit is working just fine.
So I've updated the final Schematic, except for front panel art work it working perfectly.

For those trying this, just know that pin 11 (sync Out) Must have a pull up resistor to Supply otherwise it will tick no matter what you do,in this case R23 (2k2).
I should have read the data sheet long ago,, Durh?
I got side tracked because somewhere it was mentioned to ground pin 11 which does nothing to stop the tick.
Data says pin 11 should have a pullup but no mention of stopping the tick bleed through. Oh well I got there in the end.

I added a 470R (R25) across pin 13&14 this seems to help with any residual ticking. There is no audible difference between Tri wave and Sine wave except it does reduce signal level a tiny bit but makes little difference to the effect. lower values of R25 will have and effect on the outcome.

Changes are noted on the Schematic for you.

So I'm closing the lid on this and I'll call it a success.
Phil.