Found a Compellor in local ads and then this older post on modding it, so I had to try. When two more mono Compellors showed up… ugh… you know… these are fun for toying around!
Stereo Compellor side-chained with gate signals from a Korg SQ-1:
So, what’s the Compellor idea?
Unlike other dynamic processors, the Compellor can seriously push your signal levels without audible pumping. You have some really slow going levelling action and a limiter on the output side. That’s, as far as I know, is the main reason is was/is popular in broadcasting since it can manage dynamic changes quite elegantly without flutter or distortion. Now, that’s also a reason why you cannot hear it working much.
While its “inaudible” action is cool for some applications (like broadcast-typeset and forget stuff), it kind of limits creative uses. Sure, you could get specialised dynamic processors for that, but you can also mod a Compellor easily and have it both ways.
What makes the Compellor shifting tons of decibels in such an understated manner are mainly two factors: the first is a really slow attack time and the other is a quite long hold phase of dynamic processing action. While the final limiter prevents any incoming signal overshooting desired limits, long attack helps avoiding a pumping compression onset, while hold prevents the signal suddenly sagging off. All of this is excellently described in the service manual by Aphex.
Listen to this drumloops in the following clip here. First attack time is normal, then drastically reduced. The hold time is is also reduced throughout the entire recording so you can hear the effect of attack time better:
In the following clip, on the other hand, attack time is already lowered, and I changed the hold time to a lower value:
In both cases you can hear the compressor working after I engage the modifications.
Why mod this at all? Reduced times can be cool for creative uses! I’m sure there are more serious and mixing-wise sensible reasons, but hey, it’s like with genders, folks – variety, diversity and creativity rule.
A well respected musician and a guy called Richard taking a nap… probably after making excessive use of Aphex Compellors.
Some Modifications
That’s a mono Compellor (Model 301, AX). When you pull off the front lid and unscrew the potentiometer holders, you can flip the top PCB up (there’s hinges on the back, cool).
It’s green!
Attack Time Switch: this is described in the service manual and can be moderately changed in the mono version by a rotary switch that allows for two settings (faster “speech” and slower “music”). R222 at the leveller sensing circuit is 5.7M – you can lower this to 2M or 1M (I did the latter), either with a switch or fixed. An older mod suggested simply wiring a 1m potentiometer across R222, but I don’t like the results too much since anything under 200k sound meh…
Variable Attack Time: this is what I do on my mono version since there is vacant panel space. Simply replace R222 with a 200k-330k resistor and a 2m linear potentiometer (connect wiper and pin3) in series. This way you can dial a more specific attack time suitable to your processing material.
Sidechain mod: this is not active in all settings (a proper side chain would directly affect the VCA control), but since you can continuously blend between compression and levelling on the compellor, I like this version quite well since you can easily alter between how much side chaining affects the VCA. Actually, drop the diode in the picture – simply use a 200k-470k resistor. Any pulse of positive voltage engages compression, i.e. ducks your main signal, and (that’s why not using the diode makes sense) any voltage close to ground lifts your VCA level. (Audio demo right at the very top of the page.)
Hold time switch: the dynamic verification gate shown this pic practically checks how much the comp/leveller VCA is pushing. If the signal into the VCA falls below the action threshold, the DVG goes: “Yeah, come back in a while and we see if you’re still below threshold” vibe. You can speed this sluggishness up by reducing the value of C209. An old mod suggested deactivating the DVG entirely by pulling U202, but II think you can do better, say, by switching between capaitor values: 1nf for really fast, 220nf for moderately fast and the stock 1uf (NB the schematic *mistakenly* indicates .33uf) for slow. I keep a fixed value of 220nf in my stereo version.
Variable hold time: If oyu have the place for a potentiometer (I’m using this on my mono version), then simply wire a 2M linear potentiometer between both terminals of C209. Wire the pot as a variable resistor (i.e. connect wiper and pin 3) – the more you turn this potentiometer clockwise, the faster C209 discharges and, ergo, hold time decreases.
“No-hold” Switch: I have this on my 320 since there is not much space for potentiometers. Simply wire the anode of C209 to ground via a dual on/off switch.
Places for attack and hold mods are super easy to find.
Fusing leveller and compressor sensing: this only makes sense if the DVG hold time is *not* zero. It is also a bit experimental. The result is basically that you reduce the attack/release values in compression mode, which can bring in some dynamic variations. Simply wire 100ohm resistor to TP “F” (output of the levelling detector) and the right hand terminal of R206 (part of the compressor detector).
Sounds like this:
Hold-as-Gate Switch: this fun mod makes your limiter silence the processed material as long as the hold phase of the DVG is active. That means, anything going over the processor’s threshold actiavtes a period of time where processed audio material passes the length of which is set by DVG hold. This is only really useful, of course, when you have the vairable hold time modification in place.
Sounds like this:
What you do is simply wire the right hand terminal of R206 (where it’s connected to Q201) to the right hand terminal of R142 (connected to R141) via a 330k resistor and an on/off switch.
Deactivate the limiter: Aphex suggest in the service notes that if you get clicks and pops on short comp attack times, you can deactivate the internal limiter. I like mine active but hey, having options is cool. While Aphex suggest to pull pins 6-7 of U102, I find an easier (and switchable!) way is to pull U102, PIN5 (+IN) to ground.
What this might be good for in creative terms, is that you can smash the hyped up signal coming out of the Compellor into another limiter for even more juice. Aphex themselves recommended using their own Dominator Multiband Dynamic Processor for that use case (as they would).