Poor Man's LA2A

[LeftyStrat]

In another thread about a clean amp, Miles mentioned something along the lines of why not build an LA2A into the amp.

That got me thinking (danger, danger) about how a lot of classic clean tones are a combo of a nice tube amp, and a nice studio quality compressor.

I'd love to own an LA2A, but even homebrew is still pricey (~$800). But if you were building a dedicated circuit in a tube amp or in the clean channel of a multi-channel preamp, perhaps you could simplify things. You could dump the pricey transformers for one.

Now some might ask why not use a stompbox compressor. I'll tell you why, they all suck. This is coming from someone that owns an original DynaComp, and has built a Ross clone, an Anderton compressor, and a slew of others.

I think this might have to do with the fact that every stompbox compressor circuit I've ever seen uses a feedback side chain, whereas almost all of the classic studio compressors use a feed-forward side chain (I could be wrong on this, but the LA2A does).

So I want to come up with a simplified feed-forward optical compressor that is filled with loads of yummy thermionic goodness.

I figure dump the tube-based envelope follower in favor of a digital solution. This article suggests an intriguing design that could be implemented cheaply with an MCU:

http://www.edn.com/design/analog/434465 ... low-ripple

Maybe combine this, the LED/LDR combo, and two of the makeup stages from the LA2A:

http://www.recproaudio.com/diy_pro_audi ... a_1968.jpg

Notice those two stages don't use bypass caps, but have high plate resistors. Perhaps this is part of the magic?

Looking for thoughts or suggestions. I've never even got to try an LA2A.

This is the sort of compressive sweetness I'd love to find:

[Leo_Gnardo]

In another thread about a clean amp, Miles mentioned something along the lines of why not build an LA2A into the amp.

That got me thinking (danger, danger) about how a lot of classic clean tones are a combo of a nice tube amp, and a nice studio quality compressor.

Notice those two stages don't use bypass caps, but have high plate resistors. Perhaps this is part of the magic?

Looking for thoughts or suggestions. I've never even got to try an LA2A.

I'm afraid it was me that put that bug in Miles' ear. He said he started to break out in a sweat IIRC. heh heh heh......

What you've been missing!

Hi plate R's & no bypass = compressive on its own. Resembles a grid-leak and that has audibly some squash to it.

IMO the LA2A's optical cell with its unusual "glow" element of electro-luminescent material is part of the magic. It tends to continue glowing for a little while after signal has been applied to it and that tends to integrate (average) the compression effect over time giving a long "decay" time or a signal-averaging effect. Of course I could be full of beans but I can't think of another comp that has that sort of setup.

There are sources for EL strips - I don't have ready to hand - but google away and you'll soon find. (One of my customers found replacements for the EL glow strips in his late 60's Magnatone amp.) There's also a photocell involved, and for that I'd start with the one sold by CE as a replacement for Morley pedals. It will take a bit of mad scientist experimentation, or you might be able to locate a cell built to replace the LA2's ready-built and that will save you time fiddling around.

That said, some of the classic compression tones have been created by multiple applications of compression. First, an overdriven guitar amp is a heck of a compressor/limiter on its own. Then a comp may be in the signal chain prior to recording. A slightly overloaded signal applied to the record head will get a bit of squash onto the tape, and tape itself can absorb only so much signal so there's yet another comp. If that's not enough a comp can be applied at mixdown on the channel, and another across the whole mix.

I've had the privilege of watching ace engineer Chris Lord-Alge at work in the studio and he had a 4-compressor chain typically used in mixdown on vocals that makes them sound terrific and pop through a mix. He termed it
"CRUSH". That would be hard to duplicate, and my foggy memory recalls a dbx160 plus a special "sound good box" that had the compressor section of an SSL console bus compressor in it. This last would be hard to duplicate - as I recall only 40 were ever made. I think there was an 1176 involved too and can't remember the fourth. What order and how they were set - too many parameters I know - but it made for an effect that got Chris a lot of mix engineer gigs and maybe a couple of Grammys.

[LeftyStrat]

I'm afraid it was me that put that bug in Miles' ear. He said he started to break out in a sweat IIRC. heh heh heh......

What you've been missing!

Hi plate R's & no bypass = compressive on its own. Resembles a grid-leak and that has audibly some squash to it.

Well attribution where attribution is due. Great idea. And yes, I've always been a fan of high plate with no bypass.

The decay on the electro-luminescent is similar to what gives character to the Univibe and it's lamp/LDR combo. I've managed to model the Univibe sweep in digital, something similar to this:

[LeftyStrat]

Perhaps a bit more representative:

[Leo_Gnardo]

Lefty you are the digital KING! To model the Chris Lord-Alge "CRUSH" - if it could be done - would be absolutely terrific. Guarantee you're hearing it on early 90's Tina Turner hits or anything else he mixed.

[LeftyStrat]

Let me explain what I have done. The classic wave forms are Sine, Triangle, Square (and various PWM), Ramp and reverse Ramp.

I discovered a way to morph between them all using a Rational Cubic B-Spline.

I've been calling it a Cubic LFO, because all of those waveforms and be produced with three parameters: Skew, Width, and Sharpness.

Along the way I realized I could repro that a-lub-a-lub-a-lub of a univibe. I honestly believe I can mimic any LFO response with this mathematical curve.

I'm a total bitch for analog path, digital control.

[VacuumVoodoo]

In another thread about a clean amp, Miles mentioned something along the lines of why not build an LA2A into the amp.

This guy did. The sustain circuit in his amps is based on a simplified bare bones LA2. I don't know the exact details (and didn't ask). It sounded better than just good.

It's possible to mimic to LA2's electroluminescent light source + LDR response with OPTOFETs in feedback loop around an opamp. There was a EDN Design Idea covering this some time in not so distant past. It would also need some (piecewise linear?) approximation of S-shaped step response. The main difficulty IMO is in emulating hysteresis that LA2's control element has. I guess that's where your digital math skills would be invaluable. This could be an interesting project.

[Leo_Gnardo]

In another thread about a clean amp, Miles mentioned something along the lines of why not build an LA2A into the amp.

This guy did. The sustain circuit in his amps is based on a simplified bare bones LA2. I don't know the exact details (and didn't ask). It sounded better than just good.

Leave it to you VV! I never heard of Tomaszewicz but your brief review "better than just good", well isn't that what we're all striving for?

DANG nothing new under the sun... But amp builders can take up the comp/limiter challenge. I tried doing this in an Ampeg V4 many years ago, trading out the reverb send/return for compression and repurposing the tubes involved, but I got nowhere & long since sold the amp as stock.

[Reeltarded]

Or you can just put the LA2 in your effects loop.

It's REALLY noisy with noise. No, REALLY noisy. No, REALLY REALLLLLY noisy.

[Leo_Gnardo]

It's REALLY noisy with noise. No, REALLY noisy. No, REALLY REALLLLLY noisy.

So unacceptably noisy anyone who has one should send it to Miles, that's Miles, Savannah GA. It'll get to him. If you have more than one, send 'em all. You'll cover the postage, right Miles?

[Reeltarded]

Just sayin'..

It's my favorite amp in limit, but give it 80db of noise and it will show you 110db of noise.

The lite-brite lamp is more interesting and hard to make work as an emulation than you guys know yet. That is the whole shebang. Without a good one an LA2 is an anchor.

[LeftyStrat]

Have any experience with the Altec 436? That's another interesting one.

For my application I'm actually looking for coloration or "character."

[Reeltarded]

No.

If you are really into a compressor scheme check out a Fairchild 660 or 670 for ideas. Vari-Mu. The knee changes with the level in a very cool way. No idea how to make that work, but there it is.

[LeftyStrat]

No.

If you are really into a compressor scheme check out a Fairchild 660 or 670 for ideas. Vari-Mu. The knee changes with the level in a very cool way. No idea how to make that work, but there it is.

Thanks for the info. The Altec 436 is vari-mu and what was used at Abbey Road. I'm thinking it and a REDD.47 for channeling my inner Beatle.

[Reeltarded]

Anything 47 is fine. heh