Introducing the "One Chip Chorus"!

[anchovie]

I wondered if it would be possible to make a chorus effect using a PT2399 and no external op-amps and the answer is yes! Instead of using the internal amplifier intended for the output mixer, I've given it a new role in a phase shift oscillator. I originally intended to use the LFO output to modulate delay time, but the nice surprise was that there was no need - the oscillator uses the chip's Vref pin (famously exploited by frequencycentral in the Little Angel as a place to inject modulation) as its ground point and, just by being there, it makes the PT2399 wobble.

The LFO is in no way symmetrical, which I think gives this One Chip Chorus a nice character. The single knob has a subtle effect on both rate and depth - be aware that the LFO needs a little bit of settling time when adjusting to a new setting. R8, R9 and the pot value set the range of the control - with R8 too small I found that a pretty awful pulsing distortion was introduced in time with the LFO. Increasing R9 (or removing it completely) gives a greater downward range but LFO settling times when going from one extreme to the other became quite big. YMMV - we know that there are variances in PT2399s out there, especially in non-datasheet applications!


Schematic:

(Edit: There should be a 10uF cap between the junction of the 10K mixer resistors and the junction of the 100K and output)

Unverified vero layout (haven't taken it off the breadboard yet!):

(Edit: C13 on the vero layout is 10uF)

Sound sample: http://www.supload.com/listen?s=BNs8kw
         

[tiges_ tendres]

Beautiful!!

What are the range of sounds like?

[Govmnt_Lacky]

Absolutely loved the sound clip.

I'm also interested in the range of the "lushness"

Also, a pcb layout would be nice! I guess I better get crackin' 

[Taylor]

Very cool! One-chip anything is what's up in my book.

It's been a while since I looked at the PT2399 pinout, but as I read it, the two 10ks are the mixing resistors for clean (right resistor) and delayed (left resistor), is that correct? So vibrato could be coaxed out of this by switching out the right 10k.

[wavley]

Awesome, I really have been meaning to order some PT2399's because of all the awesome simple projects everyone is coming up with.  Now if we can just convince my wife to let me make a parts order!

[Ben N]

Brilliant and beautiful! Nice work. The sound clip reminds me of a small DOD chorus that I loved, but pulled from my board because of ... I can't remember why. But this is great.

[frequencycentral]

Amazing!

[Paul Marossy]

I like it! 

[oliphaunt]

Sounds really nice, and it's great that it is built with common parts.  I would love to hear a demo with a full range of control and the bypassed sound.

[arma61]

Love it! I can imagine it ..... NKC@  .... a no-knob chorus@  into a 1590A.... (setting as per the demo clip, love that sound!)

Thanks for sharing

[anchovie]

What are the range of sounds like?

Not a huge range but all nice! It doesn't get wild and warbly. I'd be interested to see how others get on with playing with the R8/R9/pot values, seeing as some people had to tweak a few parts in the Noise Ensemble owing to chip variances.

It's been a while since I looked at the PT2399 pinout, but as I read it, the two 10ks are the mixing resistors for clean (right resistor) and delayed (left resistor), is that correct? So vibrato could be coaxed out of this by switching out the right 10k.

Correct - I used a passive mixer to free up the op-amp for the oscillator. Vibrato from this shows up the funny LFO shape; rather than a regular up/down waveform, it's more like pressing down on then letting go of a whammy bar.

[Perrow]

I've had the LA on my build list for quite some time now, will have a deeper look at this and see which one to actually build.

[Scruffie]

Quickly thrown together onboard pot PCB layout with plenty of spacing for easy building... i've learnt i'm not too great with laying out efficiently for PT2399s today.

Will fit a 1590B with a battery no problems, PCB mounts to the back of the board.

Unverified of course.





49.53 x 48.26mm

The Clip Sounds Great James!

Edit: Just seen the note about the 10uF Output cap, now added to the PCB transfer (It's just before the output, not updated the SS)
Shrunk the board size down a bit aswell otherwise it's exactly the same.

[Paul Marossy]

Vibrato from this shows up the funny LFO shape; rather than a regular up/down waveform, it's more like pressing down on then letting go of a whammy bar.

That must be why I like it. I like to do subtle wavering of chords with my whammy bar. This circuit kind of has that feel to it.

[Brymus]

That sounds nice.
I wonder if my 2399's will work in this circuit.

Anchovie,that clip sounded great reminded me of Radiohead.

[Scruffie]

While searching for something entirely unrelated just now I came across this post and thought it was quite funny 

I dare anyone to design a lower parts count chorus.   

You two better fight it out! 

[Earthscum]

Dude, this is too awesome. I have an idea for a badazz enclosure for it... 

Hmm... should call it the Cherub Chorus   

[glops]

Man, that's awesome!  Thanks for the work.  The noise ensemble is one of my favorite pedals.  Can't wait to play with this one...

[iq01221]

Awesome!!! Very nice clip! 

[Seven64]

building it right now....