-DeeT's "What Compressor" Page

original page: Jan 28, 2000
last update: August 19, 2008
(added photos, provided sharper schematic images)
The What Compressor is a super transparent vocal and instrument compressor that you can build.

"What Compressor"?

A few months ago, I was too cheap to purchase a good studio compressor, so I decided to try to build one instead. By now I have spent enough time to record an album and enough money to buy a few good compressors, but what a ride it has been! The result of my obsessive experimentation is the design featured here: a quiet, clean, transparent compressor that caresses delicate vocal and instrument tracks with a velvet touch.

On this web page you will find the complete story of the "What Compressor": initial experiments, later refinements, design trade-offs, and the all-important schematic, parts list and construction tips.

The Wife of Invention

Much has been written of "mother necessity", but the wife has to figure in there somewhere. My wife is a vocalist. (You can hear her sing here.) We'd been working on making a demo tape for her, and when it came time to lay down lead vocals, I blew the dust off my Alesis 3630 compressor.

For those who haven't seen the 3630, it has a whole lotta knobs. You can have any compression ratio, any attack and release times, soft knee or hard knee, rms or peak... and I think I've left a few out! Anyway we hooked up the compressor and started experimenting with it. Unfortunately, we just couldn't get it to sound good with her voice. I had previously had a similar experience trying to get a good sound playing my bass through it. The best sound was always in the "bypass" position :(

I assumed, as I had done with my earlier failed bass experiment, that I simply didn't know how to use compressors. After all, the Alesis does everything a compressor could possibly do. Sure, it's a low cost unit, but I assumed the difference between this and a fancier unit would be balanced ins and outs and lower noise. Feature-wise, the Alesis should stand up against anything. I mean look at all those knobs!!

The Inspiration

A few days later I was at my friend Scott's house. Scott also does music production so I mentioned my compressor troubles. Apparently he had had the same problem. He also owns an Alesis compressor, he said, but he wasn't sure where it was... maybe in a closet somewhere. Anyway, Scott invited me to try these impressive-looking green (or were they blue?) compressors labeled "Joe Meek". They didn't have as many knobs as the Alesis, but they had great big lighted VU meters with real movements, which more than made up for the paucity of controls. Scott connected a microphone and headphones and let me play.

The difference was striking and immediately apparent. The Joe Meek has four knobs, and no matter what you do, you get a sound that is useful for something. Even with large amounts of compression, the right setting of the knobs provides a transparency I wouldn't have believed possible. The Joe Meek can squish the heck out of a wave and you'd hardly know it.

Lucky for anyone who is enjoying this story, I'm not rich. I would have just bought a Joe Meek compressor and that would be that. [Uh, I don't mean to say that Joe Meek compressors are exceptionally expensive. They're really not.] Not wanting to shell out for a compressor and being utterly fascinated with the sound, I determined then and there to learn what was so special about the Joe Meek and attempt to imitate it. Little did I know I was embarking on a Journey that would take months and cost a good deal more than a Joe Meek compressor. Was it worth it? Absolutely!

The Joe Meek Difference

Danger: Curves Ahead

At the heart of the Joe Meek compressor is a photoresistor. Photoresistors were used in many of the best vintage compressors but much more popular today are VCAs (Voltage Controlled Amplifiers). A compressor's job is to reduce the output level in a manner that is proportional to the input level. Photoresistors do that job imperfectly (nobody was complaining in the early 1960s), while VCAs do the job with greater accuracy and control.

VCA GRAPH Here is a graph showing a typical compression curve, realizable with a VCA. The horizontal axis represents input signal level and the vertical axis represents output level. As you can see, the output matches the input exactly until a certain threshold is reached. Above the threshold, increases in input level still cause increases in output level, but a large change in the input causes only a small change in the output. This is the very definition of compression and a machine that can realize a curve this straight is an engineer's wet dream. Nowadays we have no shortage of happy engineers, and the Alesis compressor is more than capable of such performance.

PHOTORESISTOR GRAPH By contrast, here is a graph showing the typical compression curve realizable using a photoresistor. The output level tracks with the input at first, but somewhere near some ill-defined "threshold", the output gain tapers off. For a while there, the curve actually looks like the perfect compression curve, but then the compression softens up and eventually poops out. For very strong input signals, the output again changes the same amount as the input, just with the volume turned down a bit. When one is shooting for the mathematical perfection of the earlier graph, this one doesn't make engineers terribly proud. But it worked in a pinch in the old vacuum tube days.

But let's put numbers aside for a moment. Do our ears do mathematics?? Look at both curves again and ask yourself which one would probably sound better to your ears. It's only natural that smoother transitions will be less intrusive than sharp ones. If your motivations for compressing are mathematical in nature (say, if you're implementing a compander-based noise reduction system or providing precise limiting to protect against overloading an amplifier), then you're far better off with what's behind curtain #1. But if you just want to listen to a compressed sound and enjoy it, don't make math, make music.

The Envelope, Please

The other feature that sets the Joe Meek compressor apart is its unique behavior with respect to time. All compressors contain what is called an "envelope generator" to control the speed at which the gain is reduced and allowed to return to normal. Most compressors provide knobs controlling the attack and release times, so the user can adjust the timings for specific purposes. Generally you want a fast attack, so loud sounds don't escape uncompressed, and a slow decay, so the listener won't hear objectionable "pumping" as the volume is turned up and down Barry Gibb style.

example envelope Here is an example of an envelope generator's behavior. The top line shows a brief pulse in the input signal. This could be caused by someone yelling "boo!" into a microphone, then remaining quiet for a few seconds. The bottom line shows the resulting envelope. It rises with the input signal, but not immediately. While the input stays loud, the envelope signal remains high. Then comes the release time. The input signal is gone, but it takes quite a while for the envelope to recover. This has the beneficial effect that when the average input signal level remains the same the volume isn't constantly fluctuating up and down. The compressor thus "rides the peaks" in the input signal, without "riding the wave" and bouncing around a lot. (Electronics enthusiasts probably already recognize that an integrator is at work here. Good catch!)

The Joe Meek compressor is special in that it has two (countem, two!) different release times. Brief impulses in the input cause a quick release, but the loss of a sustained input level causes a slower release. To understand the need for dual release times, let's think for a moment about what our ears do every day.

Let's say we are sitting quietly, just listening to the gentle whirring of our hard disk and computer fan. Ahhh, isn't that nice? Now let's say you slam the space bar unusually hard: bang! Did you notice what just happened? Your ears had to deal with a louder sound, so they sort of "turned down the volume" inside your head. After the brief noise was gone, you could hear your hard disk whirring again, but it took your ears a small amount of time to readjust to the quieter environment. Try it!

Now let's say you're listening to that hard disk again, when your modem suddenly answers. "Beeeep...phwchchchchchchchchk!" Doggone it! I thought you'd shut that thing off. Anyway, your ears had no trouble getting comfortable with the louder sound coming from the modem, and after the modem quits, they adjust back. But did you notice? Because the modem sound was around longer, it took your ears just a little longer to recover from that than from the brief sound earlier.

So what does all this mean? It means that inside your brain is a Joe Meek compressor! Well, not really... but your ears really do "turn the volume up and down" in response to the sounds around you, and they do take different lengths of time to recover from loud sounds, depending on the duration of the louder sound. Now here's the key: if the compressor does to the sound what your brain would have done anyway, you tend not to notice it. That's the magic behind Joe Meek's marvelous transparency: it's distorting the heck out of the wave, in just the same way your brain does all the time. So, it doesn't sound mangled, even though it most certainly is. Cool trick, eh?

dual release envelope This is what the dual release times look like. On the top is the input signal level. A brief burst is followed by a brief silence, then comes a longer loud sound, and silence again. On the bottom is the envelope, employing Joe Meek's (and your ears') dual release times. The brief impulse causes the attack, followed by a decay that is slow, but not terribly slow. The longer sound induces the exact same rate of attack, but the decay is much more gradual.

The "Joe Cheep" Design

Once I had worked out the above and felt I was on to the essence of the Joe Meek sound, I decided to publish my little hack. It is still available.

Hack alert! Hack alert! The files listed in the next paragraph are not the What Compressor but an earlier effort. If you're looking for the schematics for the What Compressor, please proceed to the bottom of this page. To build your very own "Joe Cheep" compressor, here are the schematic, circuit description, and parts list and construction hints.

Beyond Joe Cheep

The story should have ended there, perhaps, but my wife is a really good singer, and the Joe Cheep just wasn't cutting it. My curiosity wouldn't leave me alone, so I got back to work.

She Sells Sea Shells

Compressors have a tendency to exaggerate sibilance. This is because the amount of energy in an "S" sound is substantially less than the energy in voiced sounds. When the "S" sound arrives, the compressor detects the drop in incoming signal level and turns up the gain and preSto! You've got a sibilance problem.

The traditional solution is something called a de-esser. The most common type of de-esser is little more than a special compressor that is particularly sensitive to sibilance frequencies. When the "S" sound comes along, the de-esser turns down the volume, and the sibilance problem is solved.

I didn't find this solution very satisfying and found another way to look at this issue.

The Highs Have It

Anyone who has worked with sound reinforcement or recording knows that bass frequencies use much more power than the higher frequencies. Yet, when we buy speakers or a stereo, we want "flat" (equal) response to all frequencies. What is the explanation for this apparent paradox?

The answer is that our ears are not equally sensitive to all frequencies. It takes a powerful bass signal in order for us to consider it "loud", whereas a comparatively small amount of power at a high frequency can send us running for the exits. So, a mix that sounds even to our ears must have lots of bass frequencies and fewer high frequencies.

If a compressor could be made to agree with our perception of volume, to regard levels as equal that we hear as equal, that might solve a lot of problems. For one thing, there would be no need for a "de-esser", because the "S" sounds just as loud to us as the rest of the word. The compressor would simply "do it right the first time" and not turn up the volume on the "S" sounds. Another benefit is that the tendency of compressors to "pump with the bass" would be mitigated, since the compressor wouldn't consider the bass to be any louder than we think it is.

Okay, so I knew what I had to do, but what is the curve? What is happening that causes our ears to be more sensitive to high frequencies? I'm no ear expert, but here's the line of reasoning that led me to the curve I'm using. I assume, first, that the apparent loudness of a sound is proportional to the amount of energy transfered to the ear drum. Physics says that an ear drum wiggling 1mm at 100Hz has half the energy as it does wiggling the same distance at 200 Hz. If my initial assumption is correct and I can ignore other factors, that means the ear's sensitivity to low frequencies drops off at 3dB per octave.

With this goal in mind, I developed a filter to tailor the compressor's response to my idea of the ear's characteristics. The addition of this filter rocketed the sound from "Joe Cheep" to "Whoa, Deep!", a delicious, sensual experience. We started recording with this new sound, and we were very happy with it. But there was one improvement still to come.

The "April Resistor"

One day April was mixing her vocals and she told me I wasn't going to believe this, but when she plays back a mix of both dry and compressed vocals, it sounds better than either compressed or dry, at any level. I assumed she was confusing some other effect, but I found I could hear it, too! A little online research revealed that the practice of mixing dry and compressed vocals is known, if a bit obscure. It restores lost spectral content, at the cost of limiting the maximum amount of gain reduction possible.

I decided that a "mix" knob on a compressor could be a hard sell and set out to find a "magic ratio" of compressed to dry that sounds transparent, yet still permits a decent amount of compression. I settled on a maximum compression amount of 14 dB, which isn't super high, but it's higher than is usually practical for most vocal and instrument tracks, and the improvement in the sound was undeniable.

Can I Hear It?

Here are some low bitrate mp3 files featuring my wife's vocals before and after 12 dB (for pop vocals, a lot) of compression. After hearing the compressed track, the uncompressed one sounds thin. Nuances on the uncompressed track are easily lost in a mix.

Can I See It?

Finally I've uploaded a few pictures.

How About Specs?

We have an audio analyzer where I work, so I can oblige.

Compression modes: Stereo or Dual mono, true bypass
Dynamic Range: >120dB
THD @ 1KHz +4dBu: <0.05% for all knob settings
Maximum Gain Reduction: 14dB
Stereo Tracking Error: <1dB
Compression Ratio: Variable, Max. Approx. 1.5:1

How Can I Get Me One Of Those?

If you'd like to build your very own "What Compressor", here is all the information.

I've also built ten prototypes so others could help me evaluate the design, but I probably don't need to be buried with all ten when I die. They're somewhat ugly (diecast box) but super clean electrically and mechanically, with all surface mount construction. Some of these prototypes are spoken for, but others are not. If you think you might like to buy a "What Compressor" prototype (subject to availability of course), send me an email.

Go [back] to -DeeT's Hacks Page.

David B. Thomas (dt@dt.prohosting.com)