Please explain LA-2 limit compress switch

[industrialarts]

Hi

I have been staring at the schematic and can come up with no explanation why it changes between limit / compress. I don't understand how that small resistor value changes the amount of compression in relation to the peak reduction control. Is it because it prevents the signal from being total shunted when the switch is in limit mode? Does the resistance of the T4 get that low when the photo element is driven hard?

Thanks

Mark Springer
Vintasonic

 

[ruffrecords]

I have spent a long time studying this and I think I know the answer. In the compress position (switch closed), the sidechain signal signal is derived directly across the opto. This means that the compressor is a feedback type. This in turn means the ratio is determined almost entirely by the characteristics of the opto itself.

(If the sidechain signal was taken from the input it would be a feed forward compressor and the ratio would also be determined by the gain of the sidechain)).

In the limit position, (switch open) the sidechain signal is taken from the junction of the two resistors. This means it is partly feedback and partly feedforward so the ratio depends to some extent on the opto but also on the gain of the sidechain. I have never been able to work out the math for this but it did a whole bunch of tests with a similar set up which are detailed in the attached document.

Cheers

Ian

 

[industrialarts]

I have spent a long time studying this and I think I know the answer. In the compress position (switch closed), the sidechain signal signal is derived directly across the opto. This means that the compressor is a feedback type. This in turn means the ratio is determined almost entirely by the characteristics of the opto itself.

(If the sidechain signal was taken from the input it would be a feed forward compressor and the ratio would also be determined by the gain of the sidechain)).

In the limit position, (switch open) the sidechain signal is taken from the junction of the two resistors. This means it is partly feedback and partly feedforward so the ratio depends to some extent on the opto but also on the gain of the sidechain. I have never been able to work out the math for this but it did a whole bunch of tests with a similar set up which are detailed in the attached document.

Cheers

Ian

Hi Ian

Thanks so much for your reply, I was actually looking at your circuit for the Poorman's Tube compressor as well. As I posted elsewhere, I am turning an Altec 601 into a dual mic-pre compressor (going slowly but going well). The PDF you attached couldn't have come at a better time, I am just starting to get the compressor/ side chain working. The info on stereo compression is gold

I'll post the schematic when I get it functioning, hopefully it'll be interesting enough so that others may comment and tweak my work. After all, I'm just a technician, so all that math stuff is above my paygrade.

Thanks again

Mark Springer
Vintasonic

 

[Hansblooey Ribbon]

Olde Threade. You guys familiar with Jurgen Haible's Tube Opto Compressor design? RIP Jurgen.

https://www.dragonflyalley.com/synth/jurgenHaiblePages/opto_compressor/jh_opto_comp.html​

 

[shabtek]

thats a remnant of the golden years of the web