Phrazemaster

Capacitor Compressor?
« on: July 29, 2019, 05:40:55 PM »
Forgive the probable stupidity of this idea - it's probably been done - but can a compressor be made using capacitors and resistors only? I know that resistors change the time constant of the capacitor, so would that provide a way to smooth out peaks...?

Please no flames - I've been around here awhile but I'm a total noobie when it comes to electronics.

Thx,

Mike
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Re: Capacitor Compressor?
« Reply #1 on: July 30, 2019, 02:12:05 AM »
No,  a compressor is a nonlinear circuit, and resistors and capacitors are essentially linear components - no combination of them will make enough nonlinearity to perform noticeable compression.

Capacitors "smoothing out" signals is another way to describe a filter,  a circuit that has different gains for signals of different frequencies. However, given one frequency, there's no way to make a filter that treats large signals differently than small signals - that's what a nonlinear circuit could do, not a linear circuit like a filter.

ruffrecords

Re: Capacitor Compressor?
« Reply #2 on: July 30, 2019, 05:54:04 AM »
There is a class of compressors often used in amateur radio that simply gently clips the signal; its a bit like over driving a guitar amp. You can make a version of this using a couple of back to back diodes and a resistor or two.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

iprovlek

Re: Capacitor Compressor?
« Reply #3 on: July 30, 2019, 07:16:24 AM »
http://www.261.gr/limiters.html

Here is some simple schematics for limiters and compressor with diodes and resistors!

gyraf

Re: Capacitor Compressor?
« Reply #4 on: July 30, 2019, 07:16:53 AM »
For a capacitative compressor, you would have to be able to electrically control the value of a capacitor.

The only parts that i know of that can do this are varicap- or tuning diodes that change capacitance slightly as function of an applied voltage.

But the range of these is sub-nanofarad, and quite nonlinear, so probably most suitable at modulated RF frequencies

Maybe something like a RF mic circuit (see discussion in the /mic subforum) could make it possible to AM modulate audio and thus compress? I'm pretty sure that it has not been done before..

Jakob E.
..note to self: don't let Harman run your company..

JohnRoberts

Re: Capacitor Compressor?
« Reply #5 on: July 30, 2019, 09:54:30 AM »
For a capacitative compressor, you would have to be able to electrically control the value of a capacitor.

The only parts that i know of that can do this are varicap- or tuning diodes that change capacitance slightly as function of an applied voltage.

But the range of these is sub-nanofarad, and quite nonlinear, so probably most suitable at modulated RF frequencies

Maybe something like a RF mic circuit (see discussion in the /mic subforum) could make it possible to AM modulate audio and thus compress? I'm pretty sure that it has not been done before..

Jakob E.
Yes, but even that is arguably an active device using voltage control.
=======
Mike, perhaps start by reading about how resistors and capacitors behave. 

You could also search for "Capacitor compressors" , and might find a machine that squashes capacitors. (bad joke, actually you will find motor start capacitor discussions).

As you already observed passive Rs and Cs can create frequency dependent transfer functions, but not level dependent compression, which generally requires active (semi-conductor) circuitry.

JR
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Phrazemaster

Re: Capacitor Compressor?
« Reply #6 on: July 30, 2019, 12:28:08 PM »
Thanks everyone.

I do understand cap/resistor basics. I was just thinking how since caps smooth peaks...that’s kind of what a compressor does. But I see it’s quite a bit more complicated than that.

The frequency dependent aspects of caps is what makes it untenable I can see...

Thx
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***************
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** * Kablooie!

abbey road d enfer

Re: Capacitor Compressor?
« Reply #7 on: August 01, 2019, 05:33:59 PM »
Just to add in. Capacitors "smooth the peaks" or filter the ripple by discharging during the troughs not by limiting the peaks
Actually they do, in conjunction with a resistor in a RC low-pass filter.

Quote
so in a certain sense they add energy
No they don't.

Quote
as a filter they are high pass not low pass.
A filter involves at least two elements, e.g. a cap and a resistor. Connected R-series/C-shunt they constitute a low-pass filter. Connected C-series/R-shunt they constitute a high-pass filter.
Please refrain from trying to explain things you don't master enough.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Pip

Re: Capacitor Compressor?
« Reply #8 on: August 05, 2019, 11:17:46 AM »
Actually they do, in conjunction with a resistor in a RC low-pass filter.
 No they don't.
 A filter involves at least two elements, e.g. a cap and a resistor. Connected R-series/C-shunt they constitute a low-pass filter. Connected C-series/R-shunt they constitute a high-pass filter.
Please refrain from trying to explain things you don't master enough.

Original Post Removed.

Sorry if I led people astray.

You are right I am wrong. It was a simplification is all and overly so. I will refrain from such contributing in the future.

In an attempt to make it right!

http://www.learningaboutelectronics.com/Articles/High-pass-filter.php

https://www.electronics-tutorials.ws/filter/filter_2.html
« Last Edit: August 05, 2019, 11:31:27 PM by Pip »
Pip
New York City
http://geosonixlab.com

Re: Capacitor Compressor?
« Reply #9 on: August 12, 2019, 04:39:38 AM »
Here is what I thought of as a control element that you could somehow call 'capacitive' in that the capacitance of the MOS is varied in order to change the signal gain.

It will inherently low pass the control voltage so would only give you slow attack/release times, as you can see it'd have a very limited control range of just over 3 dB and signal would have to very small to avoid distortion.

All in all I'd say this is a poor choice for gain control. Aside from its obvious drawbacks I just find that designing signal paths in a capacitive regime is annoying and doing it intentionally seems masochistic. Sometimes you inherit a capacitive regime (typically from a sensor: mic, piezo etc.) and the first thing you do is typically to buffer/"read" and then complete the signal path in a stiff bridging regime (ie. driving high Z loads from low Z sources).

Thanks for the thread, always good to think about various approaches. I'd love to hear if someone has a trick that would make this approach work well.


 

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