That 2180 VCA frequency response

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fontazuma

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Nov 25, 2014
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Hi,

I’m looking to calculate the frequency response for this VCA in the application circuit for a That VCA.

High pass = 1/(2 * Pi * 20k * 10u)
= 0.79 Hz

Low pass = 1/(2* Pi * 20k * 22p)
= 361.7kHz

Is this correct?

I have a DBX 166XL that has a similar design. I want to swap out the non polarized electrolytic with a 1uF film cap instead , which would give me 7.9Hz high pass.

Anybody using a smaller value under 10uF?

Thanks
 

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abbey road d enfer

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fontazuma said:
Hi,

I’m looking to calculate the frequency response for this VCA in the application circuit for a That VCA.

High pass = 1/(2 * Pi * 20k * 10u)
= 0.79 Hz

Low pass = 1/(2* Pi * 20k * 22p)
= 361.7kHz

Is this correct?
Yes.

I want to swap out the non polarized electrolytic with a 1uF film cap instead , which would give me 7.9Hz high pass.

Anybody using a smaller value under 10uF?
No. I'd rather use a higher value electrolytic. I use 100uF in that position, so there is so little voltage across the capacitor distortion is unmeasurable.
Using a 1uF film there is bound to develop voltage across it where noticeable distortion could occur.
A 10uF film cap, as suggested by radardoug, would be a sensible option, but the stray capacitance may be a subject of concern.
 

fontazuma

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radardoug said:
Why not just buy a 10 uF film cap? They are available and small.

I have a few 1uF wima film caps. I guess I could just throw it in for now.

10uF for the next order. Im avoiding electrolytics for this cap for personal preference. The stock had a 22uF non polar electrolytic cap there.

 

abbey road d enfer

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fontazuma said:
Im avoiding electrolytics for this cap for personal preference.
Personal preference has nothing to do with science. I challenge anybody to distinguish a film cap from an electrolytic in this position, as long as they are of a correct value and good quality of course.

[/quote] The stock had a 22uF non polar electrolytic cap there.
[/quote]Which is a pretty decent choice.
 

fontazuma

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From my personal experience in building and modding electronics audio gear, my ears tell me more often than not that electrolytics seem to bring out more sibilance and none with film caps. Not science, just my personal preference. So I don't do that test anymore and go straight for film caps in the capacitor coupling position.

My dbx 166xl is being modded for clean low distortion as I can with my parts bin, mainly for controlling dynamics for snare drum during tracking. Perhaps for vocals too if it sounds good after the mods.

Thanks,

Leo

 

plimousse

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abbey road d enfer said:
Personal preference has nothing to do with science. I challenge anybody to distinguish a film cap from an electrolytic in this position, as long as they are of a correct value and good quality of course.

Right !!!
 

jensenmann

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Some years ago I refurbished a bunch of dbx160X and XTs. They had 1uF MMK caps in that particular spot and suffered clearly audible (and measurable) from lowend loss. After replacing them with 10uF MKS all was good. Before I never liked the X or XT on bass guitar or kick drum. After the mod they were quite usable for these instruments.

One could argue that 1dB down at 20Hz isn´t much of a deal breaker, but IMHO it´s not about the frequency range but the phase shift. The improved 160X/XTs not only had a more solid lowend, they had a clearer and more defined lowmid range. Since the transfer function cannot be the reason for that it must be the phase shift which gets more linear at the lowend with the increased coupling cap.
 

Bo Deadly

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fontazuma said:
Thanks for that. I’ll live with the 1uF for awhile until my next parts order of 10uF.
I would live with that forever. 1u into 20k is 8Hz. In fact, because THD drifts up a bit with input level, I think one could make the case for using 220n here to low cut below 40Hz because reducing LF increases dynamic range and thus reduces input level. Although presumably LF is already cut by the time it reaches the VCA so whatever.
 
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