Over at analogvibes I see an interesting project.
You can buy a chassis there for the LA-2A compressor.
It is always nice to have a professional chassis to build your work around.
A tough question: would it be worth it to build one?
It will be pretty expensive.
Are there better, newer designs out there?
I mean it was designed befor the '60s, that's old.
Maybe some better designs came along?
I am not a professional and with bass I hear a lot of variaton in the envelope as I play.
So a limiter or compressor would be nice.
The LA-2A has the performance to live up to its hype.
The key to its effectiveness is the unique electroluminescent element that modulates the audio signal. It is one of those things that is now "obsolete" but has never been matched in what it could do. If you want one, I have one but it is not exactly cheap.
TUT2 shows schematics of Altec all-tube compressors and a simple one I devised. They are all relatively easy to make and do not use any rare or expensive parts. The LA-2A schemo is in there, too.
With bass, you can get some compression pretty easily from a dual triode just by cascading gain stages and adjusting levels to suit your instrument and your playing style. More stages make it easier, but you can do a lot with a little.
The LA-2A uses all standard components except for the modulation device.
The transformerless compressor uses two dual-triode tubes where one is wired as diodes. You could use a dedicated rectifier tube as in the Altec schematic, but then you have to source it.
It looks pretty complicated until you follow it through from the input at the left. Really, it is just the left half of the circuit that comprises the compressor, the right half is simply a line amplifier and meter.
The signal from the input feeds a voltage divider comprised of R7 (68k), R6 (1k-2k7) and R1 (1M Gain pot). The output side of the electroluminescent device is wired in parallel with the Gain pot. The side-chain signal is tapped after R6 and is fed to a parallel 12AX7 (V3), which in turn feeds a 6AQ5-EL84 (V4) that drives the input of the EL element through a capacitor. The 12AX7 also drives the meter in one of the selectable monitoring positions.
In the voltage divider, R7 can be shorted out so there is a harder compression effect. Otherwise, the function is as a softer limiter. In the side chain, the Lim Response control shunts a cap that effects the attack of the compression.
The right-hand side of the circuit is the line amp comprised of cascaded 12AX7 stages (V1) feeding a Whyte cascode follower (V4, 12BH7). There is feedback from the follower output back to the cathode of the input stage, setting a fixed gain of about 66x, or 36dB. No doubt the input and output transformers and this line amp add character to the sound which many sound engineers and artists enjoy.
The power supply is a simple CTed plate winding with solid-state diodes feeding a CRC filter. There is tap off of this to a tube zener which provides a reference voltage for the EL to work against in one of the meter switch positions that would indicate the amount of compression rather than the output signal level.
01-27-2025, 09:33 AM (This post was last modified: 01-27-2025, 09:36 AM by Strelok.)
Hi Kevin,
Thanks a lot, man!
The other schematic was a bit muddy but this one is very clear!
What did you do to it?
I see that two obsolete pieces are requiered: the T4A and the T4A/B.
The T4B I have seen on a site, they made it like the original.
There is no stand-off for the heaters, I read about heaters with a CT, is no stand-off necessary then? Hum is balanced or so?
A special feature of the Altec 436/8 is that there are no controls.
In your design there is a control at the voltage divider, did the Atec folks not want that?
Aha, a double diode it is!
Which one could I use? The 6AL5 is long retired like you said.
I have been looking for "double diode tube" but all I get is radio museums haha.
The transformers in the LA-02A circuit are nothing special inasmuch as you can use standard available modern devices to build the circuit. The input TX is 600:600 if you go vintage with values, but can just as easily be 10k:10k.
Since the use of 600-ohm lines came about from telecom design, it was already unnecessary for broadcast and PA applications, as there is no need in the latter situations to transmit power; we only need to transmit voltage, hence the suitability for line level 10k transformers.
The Whyte cascode follower at the output allows the use of a 1:1 output transformer. The vintage unit might have been rated for 600-ohms, but we can use 10k here, as well.
Some of the power amps that were current when the LA-2A came out needed quite high line levels compared to modern PAs. So, it was not uncommon for a line amplifier to have significant gain as here, 36dB instead of maybe 8-20dB.
Regarding the transformerless compressor from TUT2, I used a dual triode as a dual rectifier simply to be able to build the circuit quickly with available parts. Every electronic tube evolved from a diode
Thanks again for the info.
A PA-system... that explains why there are no controls on the Altec.
Just keep the input within a certain range and that's it.
So I can use a 6G2 in your device?
Or did you use a double tride without connecting the grid?
I think it would be very cool with yoyr SpectrumPreamp.
Yeah that is a nice way to show that it all started with diodes
The LA-2A kit at analogvibes would set me back 2000 dollars, but would it really be that expensive?
Chassis 400, T4A/B maybe 300, transforners... oh that is already 1000.
$400 for a chassis - maybe if you want it to be pretty and look exactly like the LA-2A. You can still make it pretty for a lot less, using a chassis from Aliexpress - easily cut the cost in half or more. You can also get very nice meters there, too, and the large Alps stepped pots.
Hammond's 140UEX has performance to match any Jensen or Lundahl. It is 10k:10k with no CTs, which makes the winding simpler and it has the best performance in the entire series. It can also handle gigantic signals. I would suggest using one for the output and probably another for the input.
You can go simpler with the input and output and be transformerless, especially considering these are not technically part of the "compression" circuit.
The tricky expensive piece is the EL cell. I have one if you want to email me for a price. Sit down when you receive the reply
Like I said above, everything other than the EL element is bog standard.
For the transformerless compressor, just use a dual triode for both ssections and try it. It is not that hard of an effect, so you may not want to invest too much into it. But, yes, you can sub a real dual-diode.
Regarding finding a clear schematic: I simply did a search and opened a few in separate tabs. Then I went into each and zoomed in, saving the best one and deleting the others. It's a standard search technique and applies to anything else, even youtube, music, etc (none of the latter do I do).
Since the transformerless compressor circuit uses two dual-triodes AND you will be using tube sockets, you are free to try any of the 12A_7 family in either position.
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