Garbage Compactor - Super simple compressor circuit

[StephenGiles]

I've looked hard but I don't see a circuit, only a picture.

[Gurner]

hi guys: found this...386 compressor..nice n simple too..

http://www.aronnelson.com/gallery/main.php/v/BinOfBrett/Simple+MOSFET+compressor_small.jpg.html

There's still no series resistance in line with the FET (which is replacing the LDR in the above example)... it's gonna need the guitar volume to be rolled off to have much impact. The win with the above circuit, is that any signal voltage lost by putting series resistance in situ, can be counteracted by the LM386 gain 9ie the output from the above is taken from the LM386 output vs the one proposed in the posts immediately above  which doesn't)

[markeebee]

I built Brett's Aussiemart a little while back, which is very similar to his mosfet comp with a couple of tweaks, including a resistor in series with the mosfet:


http://imageshack.us/photo/my-images/682/aussiemartmosfetcompres.jpg/

(Not sure if image embedding is working at the mo)

Originally I thought I would just use it as an extra input stage for a mu doubler, but I liked the sound of the Aussiemart so much I made the doubler switchable and now I use the compressor part a lot.   The compression is slightly vague, but it does give a nice bright level boost.  It makes my Tele sound more like a Tele.

So definitely worth adding a series resistor IMHO.

[Gurner]

So definitely worth adding a series resistor IMHO.

I see a surprising amount of posts here where posters clearly think that using a variable resistor (or a component acting as one ....eg LDR, FET etc), acting on its own will affect a signal voltage ...it won't - a variable resistor acting on its own (ie in isolation with no other components) affects only the signal current ....but most circuits that folks deal with on here, a predominantly signal voltage centric.

By adding in a series resistor, it then forms a potential divider chain which thereby uses the changing signal current (resulting from the change in variable resistance) to correspondingly change the signal voltage.

Re the aussiemart ...using an LM386 in this way is not optimum IMHO ...I can see why folks are drawn to the circuit (low component count, simple build), but with just a couple of minor tweaks, an opamp could be used  instead ....much higher input impedance, less noise, lower current draw blah blah....



(note: the above schematic is not mine, it's just one I saved a while ago...can't remember the source - some on line electronics mag), it's not specific to guitar & it'll need a bit of tweaking here and there...eg for a start, change that 1k (R1) resistor to ground at the front, up the biasing resistors (R3,R4) in value etc....you'd need to make R8 variable (release) & a variable resistor between Q1 collector & C4 (attack)

[arawn]

Only one problem that is an automatic gain control similar to cable tv usage which does work differently than a compressor. foremost it has a broader frequency spectrum

[Gurner]

Only one problem that is an automatic gain control similar to cable tv usage which does work differently than a compressor. foremost it has a broader frequency spectrum

If you look at the actual circuit...'the moving parts' aspect is almost identical to the aussiemart ahem 'compressor', it simply uses an opamp (LM358) vs the aussiemart's audio/power amp (LM386) & uses a tranny in the peak detect feedback cct (vs a diode in the aussiemart) ....it's working principle is pretty much the same. (btw, I wasn't advocating the cct I posted as a 'compressor', but if someone was going to build the aussiemart, IMHO basing one on the variant I posted is better for the reasons I mentioned in my last post - you'd just need to add in a variable resistor for release (R8), a variable resistor in Q1s Collector for attack etc)

I'd say that what's being discussed by the OP is not a traditional compressor anyway (with all the controls one normally allows ...threshold, ratio etc). An AGC will boost the low signals & attenuate the high signals ....if you look at the aussiemart (which is what I was referencing), that's essentially what it's doing too (it's using the gain of the LM386 to boost & the fet to attenuate)...as is the cct above.

As an aside the differences between what we'd call a compressor and an AGC circuit are mute ......most simple diystompbox 'compressor' circuits are nothing more than AGC circuits......they're reigning in the large signals, boosting the low fading signal...towards levelling out the dynamic range, which allows you to get the guitar higher in the mix. (the controls that I think you're alluding to, thereby making it more what most here would more deem 'a compressor' simply alters how fast the AGC reacts)

[arawn]

as long as it's in the right bandwidth, cool!! I'm used to seeing cable tv agc's which are setup to work in a much higher bandwidth, like 5- 100 mhz sometimes even as high as 1 gigahertz. Not the 20hz to 20 khz we ideally are messing with.

[Jneely88]

Any audio of this around?

[Hatredman]

I've looked hard but I don't see a circuit, only a picture.

There, I fixed it!