LA-2A compressor as pedal?

[DaveTV]

Has anyone built a compression pedal based on the LA-2A compressor? I've actually never heard one before, but for some reason I downloaded the user manual yesterday (did someone mention the LA-2A in a post recently???) and I noticed it had a few schematics in it.

The LA-2A is a tube-based compressor that uses an electo-optical cell for gain reduction. The cell reminded me of the one I put in the Tremulus Loon and so it got me wondering how this circuit might sound as a transistor-based pedal. Any thoughts?

[MarkB]

I think there are some comp pedals based loosely on the design of the LA2A... but the sound of the LA2A is really from the opto element, the proper transformers, etc...

I think the Menatone JAC is based on the LA2A design.
"-)

[Mike Burgundy]

try the LA2A-light! (do a search in the archives and/or schem2). I think Moosapotamus also has an La-light inspired pedal/modified version up.
This does away with tubes and high-voltage supply, but works nicely.
Keep in mind that the original LA2A excels at *subtle* effects, especially for vocals. Try the "light" and if that really tickles your fancy, build a real LA2A.  I'd put that one in a rack, though. It's more than good enough for top-notch studio use.

[DaveTV]

Thanks guys. I was curious about how these opto-compressors sound for guitar. I'm pretty happy with my Ross and DynaComp clones. Would an LA-2A type compressor add anything different?

[Mike Burgundy]

definitely different feel. If I was correct about Mossapotsamus' site, he'll probably have some soundsamples as well.

[bwanasonic]

I was curious about how these opto-compressors sound for guitar.

The Demeter Compulator is an opto compressor that purportedly sounds a bit like a vintage studio tube comp. The original DOD 280 was an optical design, but I think later ones are not. Never tried either, but I'd like to get my hands on the Compulator.

Kerry M

[Johan]

yes, the schematic for the LA-light is on moosaputamus site, under fat'n'pretty..after Charlie was nice enough to put it out there i did a few changes to it. I got rid of the miniboosterstage and did the gain recovery in the output buffer instead, and also now feed it with 9 volts instead of 18volts

the input still has that low impedance that changes with the attenuation ( 30-100kOhm's), loading the the previus devise just like a real LA2a. if you dont like that, just put a buffer( or a buffered pedal) infront.

if you look at the schematic for a LA2a, its clear, there really isnt any reason to use tubes. the only stage in the signal path that uses tubes, is the gain recovery stage, and that one is heavily feedbacked to keep the  frequency responce intact, and the distortion is very low thanks to the attenuation being before the amp, so there really isnt any tubesound in the unit...its all in the optical attenuation and the dynamic loading of previus stages...I never got close anough to a real one to compare, but everytime I bring the LA-light to the studio, its the one that gets used, with the dBx's and symetrixes looking sorry in the corner......

it seems I'm not clever enought to figure out how to post schematics. but I drew a new one for how it looks now...if anyone wants it, I can mail it and maby you ( whoever that is  ) can post it....

...and now, back to my conversation with Captain Morgan ( black Label)....

Johan

[moosapotamus]

Hi Johan,
I'll be glad to post your updated LA-Light scheme. Send it on...
cbarth-AT-welchs-DOT-com

Thanks!
~ Charlie

[moosapotamus]

Here's Johan's updated LA-Light schematic...


And, his additional comments...

if you just going to use it for guitar, lift the two resistors on the input (68k and 100k ) from ground and replace the 1k in the sidechain with a 500ohm. then the loading will be decreased and the threshold lowered. ( or even beter...make it switchable..)

Regarding that little bias arrangement and the input/output caps...

Something like 10K, 10K and 10uF? yeah something like that..I took what I had at home, I think it was two 22kohms for the spliter and 1uF for the input and output caps..

Thanks, Johan!

~ Charlie

[bwanasonic]

I'd be interested in anyone's experiences building this. Also like to see the updated schem.

Kerry M

[moosapotamus]

Kerry - Johan's updated scheme is in my previous post. See my experience with the previous version here...
http://www.moosapotamus.com/Fat-n-Pretty/fatNpretty.htm

~ Charlie

[casey]

what chip is used in this circuit, and what is Q1 and Q2?  THanks.

[moosapotamus]

Opamps like TL072 or NE5532 or similar should be fine.

What Q1 & Q2? There aren't any trannies in that scheme, above.

If you mean my modified LA-Light (fat 'n' pretty), they're marked on the scheme.

~ Charlie

[jochris]

Charlie, do you have some information about the LDR used in this circuit.
In the orginal LA2A the attack/relise time of the EL panel and the LDR is considered to be very critical.

jo

[Johan]

when I built mine I tried many different LEDs/ldr's and vactrols.
of the commersial vactrols I found VTL5c2 to be the best one.the others seemed slow/sluggish. When using separate LED and LDR it all seamed to work best with the kind of LEDs that gets dimmed when the voltage drops, some just turn on and off and those make the responce very uneven. the ldr should be one that can go down to a few k-Ohms or less. its a simple circuit but it works great for recording vocals and bass

Johan

[moosapotamus]

Thanks for the opto info, Johan.

Got another question about filtering in the sidechain...

That 0.2uF cap in the sidechain is acting as a high pass filter, yes/no?

Have you tried different capacitor values in the sidechain, or maybe a low pass filter (cap to ground)?

~ Charlie

[Johan]

yes, there is a 3dB/octave rolloff at roughly 800hz. I didnt really intend for that to happen, but it turned out to be a god thing I think..pushes voices up right to your face without being rude..makes you sound very big and warm...also great for bass..if you dont like it, you can make the cap bigger or the resistor smaller....or replace the resistor with a 1k pot to make it variable...I like it as it is....

Johan

[casey]

a couple of more questions....

what does Vr1 and Vr2 control?

and...what is a bi-polar power supply?

[Johan]

vr2 is the threshold and vr1 just sets the output volume
....mount the threshold LED through the chassis and you will see when its working..

Johan

[Johan]

...and bipolar powersupply is when you have booth + and - voltage...if you run on a single 9volt battery you will need the two resistors at the far right in the schematic above. with a bipolar supply, thats allready taken care of...