Passive, Variable, Compressor / Limiter

[petercfoy]

Hi,

I'm trying to find a passive ( no batteries), compressor/Limiter schematic that I can build. I also want to be able to manually vary the threshold of when the compression starts, and the depth of the compression.

Has anybody seen something like this around, or would be able to help me work out my schematics?

I have found a few public-domain schematics that I'm working on to combine into one machine, but my electronics knowledge is limited...

Thanks in advance,
Peter.

[Ardric]

Have you seen this one?

http://sound.westhost.com/project45.htm

[brett]

Hi
That project suffers from loss of treble as compression goes up (no problem for a bass, of course).  Increasing the 33k resistor to 100k might solve the problem, but I've never tried it.
For semi-passive compression, you can run the signal past a couple of 12AL5 tube diodes.  They conduct in a smoothly non-linear way.  I'm fairly sure there's a schematic somewhere on the web
cheers

[petercfoy]

Thanks for these repsonses so far, I saw the Project 45 one and have been chatting to Rod about it. I'm thinking of combining it with another one I found, which is a signal powered dynamic compressor. I got the schematic, and it's based on 4x AA119 diodes and 2x 22u capacitors, taking a small amount of the input signal to power the compression and then the capacitors boosting the post signal again.

My wants are to be able to manually set the threshold at which compression kicks in, and also manually set the amount of compression of the signal. Hence looking at combining these two, or just adapting my other find by cutting a variable resistance into the beginning of the circuit and having a variable capacitance further on down .... haven't tried it yet, and am scared that it will be distorted and lossy beyond belief.... Can I attach tif files on here so I can show you the schematic? - Pete.

[Jay Doyle]

Is there a reason it needs to be passive? I'm afraid that you might be asking too much out of a passive circuit...

[petercfoy]

No, there's not a 100% need for it to be passive, I just don't want to be at the mercy of 'forgotten fresh batteries' etc... I'm trying to build my ideal circuit. If I could get this circuit schematic over to you that I found which has a semi-perfect signal powered compressor on it, then you can see that it is possible.... is you let me have your email, I'll forward it to you? Peter.

[Meanderthal]

That project suffers from loss of treble as compression goes up (no problem for a bass, of course).

Umm, I play bass, and I can honestly tell you that a muddy compressor is a useless compressor. There's a reason the good quality bass cabs have that horn(tweeter) stuck in there... unless your idea of a good bass sound is Jack Bruce Cream.

[Ardric]

Rather than change out the 33K for a 100K on the project 45, I'd try putting a buffered pedal of some sort in front of it.  If it's driven from a low impedence source, it won't be as much of a problem.  A bass with active pickups would probably be ok too.

But it needs that buffer to work right, and when it does work right it'll still have lots of minor problems.  It's hard to make a compressor or limiter that has a useful attack and decay time but doesn't distort the signal in some way.  Most stompbox compressors distort way too much for non-instrument use like vocals in a studio, and they're active circuits.  I think a passive compressor can be really cool and all, but I don't know how you could make it as clean or flexable as an active circuit.

[Noplasticrobots]

unless your idea of a good bass sound is Jack Bruce Cream.

What about Jack Bruce on Apostrophe? THAT'S a bass sound.