Opto (La2a) and FET (1176) in same box

Okay. What would be involved to build, in the same box, a compressor that you can switch between an opto La2a-style circuit and a FET 1176-style circuit?

For example...

A double pole potentiometer - each pole controls compression amount (la2a fashion, rather than increasing gain in fixed 1176 compression).

Makeup gain is controlled on the next potentiometer (again, like the la2a) - either one pole or two, depending on impedance. Makeup gain will be tube-based.

The FET could be fixed ratio at 4:1.

OR...is there a simple way to use the La2a design and somehow switch between the T4B circuitry and some sort of FET circuitry?

Has anyone gone down this rabbit hole?

Thanks in advance.

Or...

Is there some substitute for the T4B that is a great deal faster. I know there are "fast" T4Bs, but they're nowhere near as fast as a FET. Is there some way of building into the La2a FET-style speed?

Using an 1176 into an LA2A is surely a well known great combo. 

I don't see a lot of advantage in having both in a single enclosure -  it sure can be done, but would take reasonable skills to squeeze both into a rack.

Each really need a lot of attentive building to keep the hum managable - spacious contruction is one's friend when trying to make low noise!

So I would say one of each, and keep seperate. But a resounding yes, as far as chaining them, to get the 'fast'n'slow' thing.

I do it all the time - works good in either order - I like to do :    neve-e preamp -> mnats rev d -> drip la2a v1

I have all the pieces shaken down for noise, so it all works without a noise gate  -  but I often do use a really good gate too  ...

Bump.

Any thoughts?

The EL panel sets the attack/release for the LA2A, so the 1176 swap-in would need its own attack and release controls. To avoid duplicating too much (and setting up a new PSU or using high-voltage solid-state) I wonder if you could more or less control the FET with the voltage from the LA2A  sidechain's 12ax7-- so  LA2A's C4 connects to 1176's rectifier just before the attack and release controls. The 1176's FET gets a good chunk of feedback from the next stage, but the next thing you know you've duplicated the whole make up amp. I'm too much of a hack to know how it would act without breadboarding while making sad, anxious sounds first; here's just where I would start. Plenty of FET-based compressors don't have feedback there, study other designs.
Then  you're just swapping out the LDR and FET connection from the voltage divider. I don't know how sensitive the whole thing would be to the first leg of the divider (R6/R7 for the LA2A, R6 for 1176), I would speculate wildly that the FET would be a little more forgiving, so try the 68k to begin with. Worst case, you could swap out the dividers with a DPDT like a true bypass.
The FET's drain will probably want to have some kind of voltage reference when disconnected?
I am kind of ignoring the ratio (the dividers again, I think), if I thought much more about this I will feel compelled to make it and I am aready *busy*.
Do try it, and I am excited to hear it!

BluegrassDan said:

Bump.

Any thoughts?

Click to expand...

Yes, an 1176 uses very little in common with an LA-2A, other than the I/O jacks and the fact that they both are built into a chassis. The transformers will be different, the amplifiers require different power supply voltages, the circuits share the same basic feedback limiter topology, but are made with completely different amplifier circuits, GR devices, and amplifier components. Why not just bolt an 1176 and an LA-2A together, or make a custom front panel to house both devices?