Stupid question about SSL sidechain filter

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Nickos

Well-known member
Joined
May 11, 2006
Messages
83
Location
Italy
Hi all!!
I would like to put a sidechiain filter on my ssl clone
I've seen this on a post:



existing 47k summing resistors        bypass switch         back to PCB
                                                             /
                                                       o--/  o--o
                                                       |          |
--------------/\/\/\---------------o         |          |
                                           |-------o---| |---o------------->
--------------/\/\/\---------------o
                                      capacitor of choice




Now.... how to calculate the value of the capacitor?  ::)
Thanks!
 
Thank you for your reply.
What should I use for R in this case?
If I want to filter below 60hz, for example, what value should I use?

Thank you very much,
Nickos
 
Nickos said:
Thank you for your reply.
What should I use for R in this case?
If I want to filter below 60hz, for example, what value should I use?

Thank you very much,
Nickos
You probably won't change the 47K resistors. This value is used for constant power summing with the 33K feedback resistor in the sidechain VCA following opamp for unity gain (47K/SQR(2)=~33K). Without additional buffers as in the other published sidechain filters, I'd probably replace the 22uF after this opamp with a 220nF for a 72Hz HPF and switch this 22uF in parallel to this cap back in circuit for filter bypass. -3dB cutoff for this example value is 1/(2 x PI x 20K||20K x 220 x 10^-9)=72Hz. YMMV.
 
Harpo said:
You probably won't change the 47K resistors. This value is used for constant power summing with the 33K feedback resistor in the sidechain VCA following opamp for unity gain (47K/SQR(2)=~33K). Without additional buffers as in the other published sidechain filters, I'd probably replace the 22uF after this opamp with a 220nF for a 72Hz HPF and switch this 22uF in parallel to this cap back in circuit for filter bypass. -3dB cutoff for this example value is 1/(2 x PI x 20K||20K x 220 x 10^-9)=72Hz. YMMV.

Are you referring to the 22uF cap after pin 7 of the TL074?

Don't you want to filter the sidechain signal before the sidechain VCA, and not after it?  What am I missing?
Why would this be better than the OP's diagram?
 
regularjohn said:
Are you referring to the 22uF cap after pin 7 of the TL074?
Yes. Lots of ways to shave a cat.
Don't you want to filter the sidechain signal before the sidechain VCA, and not after it?  What am I missing?
Why would this be better than the OP's diagram?
Debatable, YMMV. As long as it is filtered before the rectifying and timing stage, it should give the same response.
If I didn't misread the OPs post, he wanted to change the 47K summing resistors to whatever value.
It is not better, just different and keeping both 47K resistors, the switched series cap from his diagram is odd value 112nF or 2*56nF in parallel for his wanted 60Hz rolloff.
 
I don't quite understand your equation you posted.  What are the two parallel lines between the two 20K values?
I'm trying to calculate for a ~200Hz filter.
 
regularjohn said:
I don't quite understand your equation you posted.  What are the two parallel lines between the two 20K values?
I'm trying to calculate for a ~200Hz filter.
Hi Dustin. These 2* 20K resistors are in parallel, both connecting to (virtual) ground, so this hpf is set by 22uF with resulting 10K shunt for a rolloff at 0.72Hz.
For your 200Hz rolloff, this 22uF cap would be substituted by a 79.5775nF cap (~47nF||33nF).
In the OPs spot after the 47K summing resistors this cap would be (1/(2*pi())/(47k||47k*200Hz)=3.38628E-8F=~33nF.
Build both for a 200Hz hpf with -12dB/oct slope.
 
Aha!  Thanks so much!  I didn't realize that || meant parallel, of course looking at it now I feel kinda like an idiot not having figured that out, hehe  :)
That's why my calculations were coming out all f'd up!
 
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