Another Little Angel-derived PT2399 chorus

[samhay]

Recently, while fed up with a never-ending breadboard experiment, I was looking for a diversion. Jack posted his Little Angle - NYE, which was enough of a shove to finally put the Little Angle on the breadboard. It is a clever circuit - well done Rick - but there were things I didn't like, and it mutated into something else. Some of these modifications have surely been done before, and I claim no propriety, but I figured it might be of some interest. As this circuit is now quite different to the LA, I figured I would start a new thread.

The input stage is non-inverting, which should be both quieter and higher-impedance than the LA. As the circuit is fairly minimalistic, I figured I could bias the op-amp from the 5V regulator. I intended to decouple this, but it was so quiet on the breadboard I haven't bothered.

After reading some more of Rod Elliot's musings (http://sound.westhost.com/project26a.htm) I trimmed some of the filtering off the PT2399, as I figured we are not going to be too bandwidth limited with the minimal delay times employed.  At this point, I realised I could re-purpose the PT2399's output stage as a mixing/output buffer. This could cause issues with headroom with the clean signal, but I have kept this unity gain throughout and haven't managed to clip the PT2399 with a moderate booster in front of it.

Initially, I didn't intend to play with the LFO as it was, quite surprisingly, very quiet (I'm pretty sure you can't hear it in the clip below). However, the depth adjustment was almost useless and the max depth wasn't cutting it for me. Eventually I figured it might be cool to have the depth and rate vary at the same time - more depth as the rate is decreased. This is what we have now, and it works reasonably well. You still get a fairly extreme ray gun at max rate, but it is a bit more even, and it now lends itself to a one-knob effect. As an afterthought I added a mix potentiometer, but I think if I build this I will leave it out (and play with the value of R2 to find something is not too sea sickening). Note that the mix pot works as a volume control in Vibe mode.
Edit - the original LFO/depth pot arrangement is a drop-in replacement here, which should make it sound almost indistinguishable from a LA.

The circuit includes anti-latch up protection from Q1. Note there is no series resistor from there to pin 6 on the PT2399. I figured that there is enough resistance through the BJT to prevent the PT2399 from melting (which it hasn't yet).  As an aside. I did try modulating the delay time at the same time as Vref by connecting R18 to pin 2 of the PT2399. The results were underwhelming. Either wobbling Vref or the delay time works great - and sounds similar - but getting this to work together will require some further effort. I guess somebody must have got this to work by now?

It's not built yet, but I think I like it enough to box it up.

Edit - it occured to me that the mix pot could be configured differently to mix between fully wet and dry signal. This way we can do away with the 'vibe' switch. The schematic has been updated.

The schematic:


A rough audio clip straight from the breadboard, recorded with a compressor in front and then straight into my audio interface.
The mix pot was at max, so it isn't very subtle.
https://dl.dropboxusercontent.com/u/11996927/Cherub_chorus.mp3
0:00 - 'vibe' setting with switch open
0:14 - 'chorus' setting with switch closed
1:10- playing with the LFO rate
1:50- clean

p.s. to whoever created this Eagle PT2399 component - many thanks.

[deadastronaut]

cool, you clever cherub you...

why the one pot for depth/speed?...

i like a full depth, slowish chorus...

[samhay]

Why one knob*? - When I increased the depth (by decreasing the value of R16, which is 33k in the LA) the LFO freaked out. It really should be buffered and is a little bit temperamental. This way you can get something like maximum depth across the whole range of LFO speeds. The idea is to increase depth as speed decreases. This only works to a point, and I could not get it to give a really long slow sweep like a BBD chorus.  

The one-pot option is not set in stone. If I can't figure out how to fit it into a 1590A, then I will probably add either the mix pot and/or add a LA-style depth pot.

* I also like big knobs, and you can only fit one on a 1590A (how's that for a signature line).

[deadastronaut]

 

fair enough man..

[samhay]

i like a full depth, slowish chorus...

Is this any better?
https://dl.dropboxusercontent.com/u/11996927/cherub_v2.mp3

[deadastronaut]

Yeah, that sounds much lusher...

way better than the original. Nice work man.