Chilton C30 Compressor Questions

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etheory

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Mar 21, 2011
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Hi all!

This is purely an academic exercise, but I'm sure that someone should be able to help me out here.

With reference to the schematic available here for the Chilton C30 Compressor: https://www.gearslutz.com/board/attachments/geekslutz-forum/348919d1370939176-chilton-c25-compressor-schematic-chilton-c30-compressor-sch.jpg

Does anyone know what the actual value of resistor R59 is? It looks like it's either 4.7K or 8.7K but it's extremely unclear.
What about R50? It looks like 33K but it could also plausibly by 33R or something else entirely.

Also the portion of the side-chain starting around Z5b is completely nonsensical and I think there must be an issue with the schematic. Without negative feedback, Z5b becomes a comparator, and as a comparator, it is really only on or off. If this is the case, the attack portion of the side-chain is like a sledge-hammer, which simply cannot be the case in the real device.

Can anyone with an actual device please clarify what the actual device looks like in this area?

My guess is that some feedback resistors around Z5b were missed from the schematic.

Thanks!

Sometimes when I see something like this, I just have to understand it, which normally means creating a simulation for the entire circuit, but also often ends up with me finding curious issues that I'd really like to resolve just so I know how things really work.

Any help would be appreciated, but especially from someone with the actual compressor. Even better would be a clearer copy of the schematic, or any corrections from people who have worked on these devices.

 
etheory said:
Does anyone know what the actual value of resistor R59 is? It looks like it's either 4.7K or 8.7K but it's extremely unclear.
I would vote for 4.7k, since a) 8.7k is not a standard value and b) the more-or-less logarithmic progression seems more correct.

What about R50? It looks like 33K but it could also plausibly by 33R or something else entirely.
Definitely 33k. 33R would not make sense here.

Also the portion of the side-chain starting around Z5b is completely nonsensical and I think there must be an issue with the schematic. Without negative feedback, Z5b becomes a comparator, and as a comparator, it is really only on or off. If this is the case, the attack portion of the side-chain is like a sledge-hammer, which simply cannot be the case in the real device.
You're probably right. I would think the connection that goes to the ratio pot should also go to the inverting input.

My guess is that some feedback resistors around Z5b were missed from the schematic.
Probably just a connection on the schemo.
 
abbey road d enfer said:
I would vote for 4.7k, since a) 8.7k is not a standard value and b) the more-or-less logarithmic progression seems more correct.
Yep that's what I ended up thinking it had to be also.

abbey road d enfer said:
You're probably right. I would think the connection that goes to the ratio pot should also go to the inverting input.
Probably just a connection on the schemo.
The problem with that suggestion is that is would mean the opamp negative input would then be forced to be the same as the positive input, so it would be held at 12V (the circuit's "virtual ground" point), which means the vca would never compress.

I tried in a simulation to put a 10K resistor in this position (top of the ratio pot to the negative opamp input), and the issue in that case was that the attack portion of the circuit doesn't seem to "attack", it's basically instant, which means either this compressor is INSANELY fast to attack, or something is still wrong.

abbey road d enfer said:
Definitely 33k. 33R would not make sense here.
Agreed, that's what I decided on also.

Thanks so much for the thoughts, really appreciate it.

I'd really love it if someone with the actual unit could quickly check that opamp section for the real arrangement, it'd be really interesting to know.
 
etheory said:
The problem with that suggestion is that is would mean the opamp negative input would then be forced to be the same as the positive input, so it would be held at 12V (the circuit's "virtual ground" point), which means the vca would never compress.
Think it over.  This stage is an inverter. The output reacts to currents hitting its inverting input. You're correct in assuming teh inverting input is stuck at 12V. It does not mean that the output cannot move.

I tried in a simulation to put a 10K resistor in this position (top of the ratio pot to the negative opamp input), and the issue in that case was that the attack portion of the circuit doesn't seem to "attack", it's basically instant, which means either this compressor is INSANELY fast to attack, or something is still wrong.
If you follow my advice, you will see that the capcitors C17/C18 actually limit the speed.
Agreed, that's what I decided on also.
 
abbey road d enfer said:
Think it over.  This stage is an inverter. The output reacts to currents hitting its inverting input. You're correct in assuming teh inverting input is stuck at 12V. It does not mean that the output cannot move.

It does without a resistor though.
i.e. connecting the top of the ratio pot to the negative input of the opamp means the top of the ratio pot would also be held by the opamp at 12V and hence no compression. Even if the output of the opamp itself does move.

I'm sure this is what you meant though.

I'd still love to know what the real device has there, but totally agree it must be something like the 10K feedback resistor. I'll try upping the capacitor in this attack portion in the sim to see the attack slew limiting in action but with the 10nF and 100nF the attack is blindingly fast and probably sub 1mS. So I guess this would just be a very very fast limiter.

Thanks again for the thoughts I really appreciate it. It's quite a neat simple little circuit.
 
etheory said:
It does without a resistor though.
i.e. connecting the top of the ratio pot to the negative input of the opamp means the top of the ratio pot would also be held by the opamp at 12V and hence no compression. Even if the output of the opamp itself does move.

I'm sure this is what you meant though.

I'd still love to know what the real device has there, but totally agree it must be something like the 10K feedback resistor. I'll try upping the capacitor in this attack portion in the sim to see the attack slew limiting in action but with the 10nF and 100nF the attack is blindingly fast and probably sub 1mS. So I guess this would just be a very very fast limiter.

Thanks again for the thoughts I really appreciate it. It's quite a neat simple little circuit.
You're correct. I believe there's a ,mistake i the schemo. This type of non-linear circuit was quite popular in the 70's and 80's (actually it's the basis of many PPM drivers). I believe the FB network goes to the inverting input and the output of the opamp goes to the ratio pot. That's what makes the most sense for me.
 

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