Cherub - another PT2339 pseudo chorus and vibe

[samhay]

Introducing the Cherub pseudo chorus and vibe (for the last few soles who haven't built a Little Angel, or who never managed to get one to work).

Not long ago, I started playing with the Little Angel chorus on the breadboard (http://www.diystompboxes.com/smfforum/index.php?topic=106680.0).
With a little prodding from Rob, it mutated from a LA-derived circuit, to something a bit further from the tree. This is where I ended up - sorry for the thread proliferation, but I figured a clean slate might be helpful.

The input stage is non-inverting for lower noise and higher input impedance. As the circuit is fairly minimalistic, the op-amp is biased from the 5V regulator, like the 1590A layout of Rick's for the LA. This doesn't seem to cause any problems, and might save a couple of parts.
There is minimal filtering around the PT2399, which allows the use of the PT2399's output op-amp (pins 13,14) as a mixing/output buffer. The mix pot lets you pan between fully dry and fully wet signal, so you can have a vibe effect at one extreme and can use it as a pseudo depth control at the other.
The LFO is essentially lifted from the LA, but has been biased a little hotter and can go a little slower. We get modulation by playing with the resistance from pin 6 of the PT2399 - we've all seen this trick before, but I found it worked better at slower LFO speeds than modulating Vref (pin 2).
Q1 is our variable resistor. The choice of transistor is not too crucial here, but you may have to play with the value of R13 to get a reasonable amount of modulation. If R13 is too big, the delay time can get long enough to be a problem. Further, there is some regeneration via R2, which will give multiple 'repeats' and gives the effect a bit more depth at slower LFO speeds. The trade off is that the regeneration makes the delay more obvious if you play staccato as you can hear the multiple slap back. The value of R2 can be lowered for more regeneration, but it will begin to oscillate below ~18k.

A quick and dirty clip recorded direct.
https://dl.dropboxusercontent.com/u/11996927/Cherub.mp3

The schematic:





I built this on perf using the below layout. It worked ok, and it should translate to a PCB fairly well.





Here is an untested PCB layout based on the perf layout above...

https://dl.dropboxusercontent.com/u/11996927/Cherub_PT2399_Chorus_PCB_transfer.pdf

[bluebunny]

Nice one, Sam.  Yet another one for "the list".   

[samhay]

Thanks Marc.

One thing I should note. As it seems that PT2399s are a little variable, it might be good to try this on the breadboard first.

[slacker]

Looks cool Sam. Quick question what does D4 do?

[samhay]

I was wondering when somebody would ask about D4.
It is mostly just a relic from when I was trying to modulate the depth in a slightly more aggressive way - the idea was to offer some protection to the PT2399.
I left it, because it didn't hurt and I would otherwise have needed a jumper in the layout I used. It can be left out, or could be replaced with a small resistor or a 1-10k pot to give variable delay.
I tried to use an LED here instead, so that it could double as a LFO rate monitor. It almost worked, but didn't dim enough to quite cut it. The delay still modulated fairly nicely.

[arma61]

Looks cool Sam. Quick question what does D4 do?

just to be updated on this project....

looks like D4 is a LFO monitor.   
Ciao

[samhay]

It would have been cool if D4 was the LFO monitor. As it is, that job falls to D3 (which is optional of course).

[deadastronaut]

great, you finished it...sounds much better/more full than the original, nice work man...

[samhay]

I thought it was finished...
I quite like the vibe effect (fully wet), and am starting to wonder if a depth control might be a good idea, as it gets a bit sea-sickening at faster speeds.

[bluebunny]

. . . it gets a bit sea-sickening at faster speeds.

Just avoid using after a kebab...   

[samhay]

Thanks Rob.
Marc - where would be the fun in that?

[bluebunny]

^ True!   

[samhay]

Another case of mission creep:
I added a depth control last night. It is more useful than I thought it would be, and is a recommended addition. The 3 controls - rate depth and mix make for a rather versatile little circuit and the vibe effect (fully wet) is now quite useful right across the range of LFO speeds. I guess I better make a proper demo...

The schematic has been updated and the layout will need tweaking at some stage.

[deadastronaut]

Cool, yeah depth is always a good option...look forward to hearing the update, 

[slacker]

Only just had a listen to the clip, sounds really good. Some of the more extreme sounds were particularly interesting.

[samhay]

Thanks Ian.
I added the depth control so that when wet it didn't always sound quite so 'interesting'. You can always increase the value of the depth pot for more extreme settings - an A1M pot could work ok.

The way Q1 is biased is a bit of a low-parts-count bodge that works much better than I thought it would. Less base resistance (R13 + depth pot) = lower base voltage = shorter delay time, but less wiggle. If you make the depth pot too big then the delay starts to get quite noticable / 'interesting'.

[deadastronaut]

we all love 'interesting' 

[samhay]

Rob - I bet it would sound awesome on the breadboard next to your X Fuzz...

[deadastronaut]

i haven't got a spare breadboard  ,   xfuzz/synthbox/spitty/ and some weird 4017 led thingy are occupying them at the mo...


(note to self, get another board) 

[samhay]

Fair enough.
Why am I not surprised you have at least LED 'thingy' on the breadboard?