LA2A feedforward/feedback design?

[Kingston]

Your La2a is broken.

If you've heard - and most importantly measured - some units that are classified as "high end" the LA2A is a lofi unit in comparison. The line amp stage is an odd design to say the least. There are better ways to get 60dB of gain with tubes with less distortion.

But then, if you improve the distortion aspects of the design and you no longer have an LA2A.

 

[Biasrocks]

If you've heard - and most importantly measured - some units that are classified as "high end" the LA2A is a lofi unit in comparison. The line amp stage is an odd design to say the least. There are better ways to get 60dB of gain with tubes with less distortion.

No doubt.

I rarely have the gain beyond the 10 o'clock position.

But then, if you improve the distortion aspects of the design and you no longer have an LA2A.

Exactly!

Mark

 

[DaveP]

I'm a bit late on this thread but I don't agree with the basic premise that there is audio feedback in the LA-2A.

A feedback circuit is like most Vari-mu's in that a signal is taken from the output, rectified and applied to the input.  You can never put a volume control in the middle of one of these because as you turned it down you would lose compression.  Feed forward is when a sample is taken from the input instead of the output like the LA-2A, the output can then be controlled separately.

The LA-2A takes a sample of the input and amplifies it to drive an EL panel attached to a specific LDR (which is the vital key to the whole performance). In fact it drives 2 units as the other one controls the meter in a bridge circuit.  The result of all this is that the input to the main amp is controlled.  You can't control the output separately on a feedback design except by pads.

Look up some old pre-war compressor designs and quite a few were feedforward back then, it was not until Western Electric and Presto came up with their classic feedback design that the other designs went out of fashion.

Incidently, you do not have to rely on one source for the EL/LDR, that can be successfully modified to a lightfast tube containing a green LED and the original Allied Signal LDR fitted to oppose it.  The drive amp has to be modified to use a 3W output TX to drive the LEDs.  I made one of these 4 years ago and have used it for every recording since.  The main advantage is that an LED lasts for at least 50,000 hours whereas EL panels have to be replaced more often because they wear out (get dark) after a while.  They were designed for 50/60Hz so 600Hz will wear out 10 x faster (i.e."wear" is frequency dependent) . This is due to the migration of Cu atoms within the EL phosphor.  LED's are just as fast as EL panels too, probably faster but it makes no difference because  the "rise time" of the LDR is what controls the dynamics of the compression.

best
DaveP

Head of R&D
Phosphor Technology
Stevenage
UK

 

[ruffrecords]

The LA-2A takes a sample of the input and amplifies it to drive an EL panel attached to a specific LDR (which is the vital key to the whole performance). In fact it drives 2 units as the other one controls the meter in a bridge circuit.  The result of all this is that the input to the main amp is controlled.  You can't control the output separately on a feedback design except by pads.

The LA2a takes a sample from the output of the gain control element.... and then does as you say. It is a feedback circuit but not at audio frequencies. Most compressor/limiters are like this.

Cheers

Ian

 

[DaveP]

Ian,

I take your point in that the sample will be decreased as the LDR goes down in resistance.

To make it independent it would have to be fed from the TX side of the series 68k.

best
DaveP

 

[ruffrecords]

Ian,

I take your point in that the sample will be decreased as the LDR goes down in resistance.

To make it independent it would have to be fed from the TX side of the series 68k.

best
DaveP

Correct. Then it would be feed forward.

Cheers

Ian