LA2A parts, T4B spare

[karsten21]

Hello dear friends,

I play with a possible project to build a realy good true tube compressor. I found a resonalbe old project for a LA2A in the net. Looks good and seemed to be a good sounding true tube gear.
Well, transformers can be get from SOWTERr (UK ) but where to get a T4B without to pay hundrets of EURO for it?
Does someone of you know if there is a spare part with modern technique ( LDR/LED ) available.

Or is this LA2A just obsolate and you know an equivalent project/schematic??

Informatins of this LA2A can be found here
LA2A Hombrew Page

Thanks a lot for any hints and suggestions.

Best regards
Karsten

[kevinkr]

Look at the lightspeed attenuator thread, perhaps you can use the LED/LDR device discussed there.

[Tape Echo]

Drip, of course. I believe that you can also order matched pairs if you wish.

Drip Electronics. com (ver 5.0.0)

Many projects and support threads at

www.prodigy-pro.com/diy

[Tape Echo]

New t4B are available at Drip, and IIRC, you can also order matched pairs.

Drip Electronics. com (ver 5.0.0)

There is a LA2a build thread in The Lab at www.prodigy-pro.com/diy

Best luck on a top-notch piece of gear!

[Michael Koster]

Originally Posted by karsten21

Hello dear friends,

I play with a possible project to build a realy good true tube compressor. I found a resonalbe old project for a LA2A in the net. Looks good and seemed to be a good sounding true tube gear.
Well, transformers can be get from SOWTERr (UK ) but where to get a T4B without to pay hundrets of EURO for it?
Does someone of you know if there is a spare part with modern technique ( LDR/LED ) available.

Or is this LA2A just obsolate and you know an equivalent project/schematic??

Informatins of this LA2A can be found here
LA2A Hombrew Page

Thanks a lot for any hints and suggestions.

Best regards
Karsten

Do you want to build an LA-2A copy or an LA-2A like compressor?

I've purchased t4b units from a number of sources but not tested them yet. Search the prodigy pro DIY board for ideas. They are available for about $100 US, which is IMO not a bad price in the context of building an entire LA-2A.

I believe one could build a nice compressor using an LED+LDR optocoupler but it won't sound like a LA-2A without some tweaking of the circuit driving the LED rather than the usual EL panel. I've been playing with this idea but haven't built a circuit yet.

Being a parallel sidechain, the control amplifier driving the LDR will determine the dynamic characteristics but the signal amplifier is not in the sidechain loop, thus is not critical to the dynamic response.

Cheers,

Michael

Note that the LA-2A input circuit puts a real 600 ohm load on the thing driving it.

[leadbelly]

Originally Posted by karsten21

I play with a possible project to build a realy good true tube compressor. I found a resonalbe old project for a LA2A in the net. Looks good and seemed to be a good sounding true tube gear.

Not that the LA2A isn't legendary, but since you keep emphasizing "tube compressor", are you sure you aren't confusing it with a vari-mu compressor, or mabe that you just want a tube compressor? A vari-mu is even more associated with tubes and a lot cheaper to build! Not knocking the LA2A, I know it has a huge following.

[Michael Koster]

Good point.

The LA-2A isn't a "true tube compressor" in the sense that the gain element is not a tube. The LA-2A is an optical compressor with a tube makeup gain amplifier (and a right nice one at that...) and a parallel tube sidechain.

Fairchild 660, Altec 436, etc. are compressors that use a remote cutoff tube in a balanced circuit as the gain element, using the bias voltage for gain control. They also use a feedback sidechain circuit, which all produce a much different response from the LA-2A.

The important question is what kind of compression are you looking for? How do you plan to use it? Maybe a "variable mu" design is more what you're looking for?

Cheers,

Michael

[karsten21]

Hey!

Thank you verry much for all the good input. I see, I have to search prodigy pro DIY for more information first. It´s clear, that the T4B unit has an essential effect to the "sound" of this compressor. But 100$ or more for an Lamp with a photo-cell seemed to me to expensive; I would not pay for mystic thinks...

At least I would like to build an true tube compressor. Not using OTA´s as controlled amplifier and a tube as outputdrive like C1 from TL Audio. It´s a funny idea, I know, but I realy would like to understand how it works and at the end it should be usable ( or better :-) )
So if a vari-mu is much cheaper: Where to get appropiate information about it?

Thanks a lot again for your patience... and so sorry for my bad english.

Karsten

[Michael Koster]

English is OK but what's an OTA?

No mysticism or magic involved; a proper T4b is just not that easy to make. The EL panel response of intensity vs. voltage and the color of the light are both important, as is the selection of individual CdS cells for proper release time. There are 2 of them in each T4b that need to be matched for gainreduction and metering. Remember, the replacement units are expected to sound exactly like the originals, so they should have good parametric consistency. They also are expected to be plug compatible and look like the originals, hence the octal base and metal can. And the quantity is low for any kind of production, so they're hand built in small batches.

It does sound like you want to start with something simple like an Altec 436, which uses a remote cutoff tube for gain control. You can find out about these on the Prodigy Pro DIY board as well.

Cheers,

Michael

[karsten21]

Hey michael,

OTA is an Operational Transconductance Amplifier like the good old CA080E or the better LM 13700 ( which is used at the TL Audio compressor ).

Yes you are right: I discount the complexity of an T4B,sorry for that.
I will start now with an simple type, thanks for the hint and:
I learned I have first to informe more in detail before a big glowing come to my eyes if I see these fnice schematics...

Best regards
Karsten