Simple compressor?

[C Bradley]

I'm looking for a schematic for a simple compressor. How complex are they? Can they be built with opamps? I want something simple and small and good sounding. Also, how do they do what they do?

Thanks,

Chris B

[Mark Hammer]

The Orange Squeezer and the Dynacomp are both relatively easy builds and sound great.  Both use relatively easily obtained parts although neither could be built with a short visit to a Radio Shack.  If you live anywhere near a reasonable industrial electronics place, you would be able to find all the parts you need atthe counter.  Alternatively, places like Small Bear, Mouser, Jameco, Digikey, etc., would have everything you need by mail.  Both circuits permit a wide array of parts substitutions as well, which is nice.

Most compressors approach the task of maintaining a constant level by providing a large fixed boost and turning down the signal when it gets louder.  If the reduction tracks the signal well enough, then something close to a constant level is achieved at the output.  All compressors use what is referred to as a "sidechain" which detects the current signal level and provides a voltages (or current) proportional to the signal level.  That voltage/current, in turn, controls a part of the circuit which varies the signal level or stage gain.  

The degree to which the control circuitry is "driven" by this proportional voltage/current determines how much it adjusts gain/level, and so the amount of compression.  because many circuits will reduce something more in response to peaks detected by the sidechain, it is common for "more" compression to produce a quieter output level (i.e., less volume).  Because of this, some compressors will have a "gain recovery" stage after the compression stage which allows you to boost the signal to compensate for this signal loss.

In the case of the Dynacomp, the actual gain itself is reduced when a larger signal occurs, whereas in the OS the boost remains the same but the input signal is reduced.  Both will work fine, although I think I prefer gain reduction rather than input attenuation.  The Dynacomp increases output level as compression is increased, whereas the OS does not.

ANY device that uses a sidechain will have a different feel, depending on the propertties of the sidechain.  More particularly, if it is slow to respond to sudden changes (attack time) it may permit sudden peaks to come through before it clamps down.  If it responds to changes quickly, such initial peaks may not make it through unaltered, but at the same time there may be a slight "warble" or ripple to the sound as the note dies away and the sidechain continues responding to very brief fluctuations in signal level.  Conversely, if the sidechain takes longer readjust or fade out (decay time) following its initial adjustment to transients, it may be slow to respond to new information arriving quickly, but also will provide a much smoother decay.  Because not every sidechain is identical, many will find they prefer this compressor over that one for heir particular instrument or objectives or style.  Neither is objectively "better" just better suited to what the musician is trying to do, given the quirks of the sidechain.  Ideally, one opts for a sidechain with variable controls to be able achieve different feels, though this is rare in stompbox-type compressors.

Because of the way in which sidechains respond, plus the relative energy-level of  higher and lower frequency content, it is very common for larger amounts of compression to "dull" the sound.  I haven't worked with the OS but the Dynacomp becomes more midrangey and less sparkley as you increase compression.  Some designs offset this by boosting the treble either before or after the compression circuitry, although the ideal is to split the signal into two or more bands and compress them separately so that the treble is not sucked away the moment a low-frequency transient occurs.

If you go to the Electronic Musician magazine website, they have an archived article there from 2001, which provided a very nice comprehensive overview of compressors and limiters including many models considered to be the finest examples at that time.

[Peter Snowberg]

For small, simple, and good sounding compressor (not to mention classic sounding too), I would suggest building and Orange Squeezer.

Operation in a nutshell utilizes three basic parts; an attenuator, an amplifier, and an envelope generator. Compressor topologies differ a little, but for the OS circuit- First the signal goes through an attenuator stage made from a JFET. A control voltage causes more of less of the signal coming in to be shunted to ground. The second stage is an amplifier which does a coulpe things. It buffers the attenuator stage so you can drive the next effect nicely, but it also boosts the signal for the envelope generator. The final part is the envelope generator. This is simply a capacitor which is charged by the audio through a diode, and a resistor to bleed the charge off (the release value). This generates the control voltage for the attenuator.

As you play, sound gets amplified by the amp stage and the envelope generator produces a control voltage that roughly follows the amplitude envelope of the signal. Play harder and the CV goes up, chewing more of the input in the attenuator stage. Play softer and less CV causes less to be attenuated.

If you drive the envelope generator with a hot signal, you will have fast attack. You could slow it if desired (for example) by placing a resistor in-line with the charging diode. The resistor across the envelope capacitor determines the release time.

Good luck!

-Peter

[Arn C.]

I never tried the compressors you guys have talked about.  I made and like the AMZ SSM2165 compressor.  Real easy to buid and low parts count and sounds great!

parts:  1 IC
          1 Voltage Regulator
          1 resistor
          2 pots
          7 caps

Arn C.

[Peter Snowberg]

I have been wondering for a while about building a unit with two SSM2166s in series (with different settings) and an NE5532. Thanks for the quality report on the 2165! If I only had the time.... maybe I'll have to make some time.

Chris, A basic SSM2165 unit sure sounds worth a build. You can also use it's big brother the 2166 which give you some more adjustability. Analog will send you samples of either (or both) right from their web site.

Arn, how would you rate the feeling of the envelope generator? I know that's a strange subjective measure, maybe a better way to ask is, How transparent is the act of compressing? Thanks!

-Peter

[Mark Hammer]

I can answer that.  I made one out of the 2166.

It can provide some serious compression, but set for the less than serious, it sounds pretty unartificial.  More compression = more volume in this instance.  The 2166 also has built in downward expansion, which you can preset or install a variable threshold control.  I went for the control.  A little odd having something that can get you a hot steady signal then just cut out when it dips below threshold, but if you set the compression and expansion thresholds right it feels pretty transparent.  I installed a switchable time constant cap for different attack/decay time but I probably selected a poor pair of values since the difference with the additional parallel cap is too subtle to be of use.

I wouldn't rank it as my favourite compressor - for transparency and low noise I still like the EPFM one - but is has some nice features and sounds decent, plus it provides some serious gain if you want, and is a ridiculously easy build.