Student looking for help understanding how the LA-2A works

[sammorris]

Hi all,

This is my first post, so hopefully I'm posting this in the correct area and not breaking any rules...

I'm doing a research on the LA-2A compressor. As a Sound Tech student, I'm used to looking at things mainly from an engineering rather than electronics perspective. I don't have much experience with schematics so I'm a bit stuck on a few details. I've also seen the simplified schematic (attached) but still don't fully understand it.

I know that the LA-2A compressor uses a photo resistor for attenuation (as the bottom leg of a voltage divider) and EL panel to drive this. Would somebody be able to help explain where the signal driving the EL panel (sidechain) is derived from, and how/if this is affected when it's switched between limit/comp?

I know that limit mode shorts R7, but I can't explain what effect this has.

I'm also trying to establish why its a "feedback" topology as they describe it in the manual. It seems that the sidechain is taken either from the middle of the voltage divider? Does that mean any attenuation also affects the signal being send into the sidechain?

From the manuals, I've found one block diagram suggesting it's a feedforward topology (left - from 1176 current reissue manual) and another that suggests it's feedback, but limit mode also sends some of the input to the sidechain (from the 2-LA-2 manual, but notice it says its a diagram of an LA-2A).

I'd really appreciate any insight here, even if it's just pointing me to another source.

Many thanks!

Sam

 

[Voyager10]

Welcome! I think you're in the right place - no sensible engineering questions are refused.

Firstly, the two block diagrams do a rather inaccurate job of showing where the sidechain path is taken from. In the actual schematic, it's taken from the middle of the gain reduction cell. Secondly, a more 'original' schematic (posted here: Urei) shows the comp/limit switch as "open" for limiting and "closed" for compression. That makes a lot more sense, as follows.

With the switch closed, the 'to sidechain' signal is exactly the same as the 'to makeup gain' signal, which is (with a bit of amplification) the same as the output signal. So the sidechain is being driven from a version of the output signal, and it's therefore a feedback design.

With the switch open, the sidechain signal is a mix (a simple linear combination) of the 'to makeup gain' (i.e. output) signal and the input signal. So it's neither 100% feedback or 100% feed-forward in this mode.

But it's fairly easy to see that, if no other controls are touched, you get more compression when the switch is opened: the sidechain input signal is higher than it would be when the switch is closed, so there is more light and the photo resistor has lower resistance. In theory, this might lead to 'negative' compression, where an increase in input level gives an actual lower output level, but we can assume the R6/R7 values are tuned to match the opto-cell characteristics, so it ends up as limiting, or at least a high compression ratio.

 

[sammorris]

Hi @Voyager10 and thanks so much for the explanation.

With the switch closed, the 'to sidechain' signal is exactly the same as the 'to makeup gain' signal, which is (with a bit of amplification) the same as the output signal. So the sidechain is being driven from a version of the output signal, and it's therefore a feedback design.

So just to check I've got it all now...

In compress, as the photo resistor's resistance drops, the levels to both the sidechain and make up circuits are reduced equally? So the sidechain = make up gain (and therefore output signal) making it a feedback topology.

But in limit, a small amount of input signal is also fed into the sidechain, where it is combined with make-up gain (output) signal. Having the effect of causing more light to be emitted and an overall higher level of attenuation?

Thanks again for the help!

 

[sammorris]

This schematic from the original '65 LA-2A seems to have the switch that bypasses R7 labelled the other way round.

Having a read of this thread has me a bit confused again, as @ruffrecords refers to closed switch as "limit", therefore R7 bypassed, so I wonder if he's referring to the Teletronix schematic.

@Voyager10 already attached UREI's schematic, which states "limit" mode has the switch open, so with R7 in the circuit.

Could there possibly be a mistake on one of the schematics? Or is this purely a design change? The two different schematics are attached for easy reference.

 

[ruffrecords]

In both those schematics it is comp with the switch closed. If I said otherwise in the past I apologise.

Cheers

Ian

 

[CJ]

Welcome sammorris! Half the server of groupdiy is full of enough la2a to keep you busy til 2025.

Thus guy had some in depth analysis on various topics concerning that compressor.>


https://groupdiy.com/search/232384/?q=La2a&c[users]=PRR&o=relevance

 

[CJ]

Oh, and this monster thread>

https://groupdiy.com/threads/all-things-la2a-related.3278/
And this might help>

https://groupdiy.com/search/232391/?q=T4b&o=relevance