Introduction
A compressor is an effect which reduces the volume of a signal. See the block diagram below, the effect can be thought of as a variable-gain amplifier and a level detector with the level detector setting the amplifier gain.
In most guitar compressor effect pedals, the input signal A and the control signal B are the same, or the amplifier output is fed back into the level detector. A high amplitude input signal is compressed while lower amplitude signals are mostly unchanged. This effect is often used to emulate power amp sag in a tube amplifier.
Sidechain compression is less common in stomp boxes, but everywhere in music production. In this configuration, an external signal, or sidechain, is fed into the level detector. The sidechain volume controls the volume of the output signal.
There aren't many guitar pedals which feature sidechaining; in my search I've only found the Empress Effects Compressor and the Deep Space Pulsar that explicitly offer it. Otherwise, the best options are rack effects or DAW plugins.
If you're wiling to put in some work, and have a compressor pedal you don't mind cracking open, you can play with sidechain effects in an inexpensive stompbox.
Goals
At the end of this project I wanted a compressor with the following:
- Sidechain input
- Sidechain level control
- Ability to switch between sidechain and regular compression (unmodded function)
The Victim
Never having tried a compressor before, I got the TC Forcefield form their budget Smorgasbord of Tone line. TC was recently acquired by MUSIC Group, the same company that owns Behringer. All but one of the pedals in this series are reworked Behringer designs, which more often than not means they're reworked pedals from other well known brands. With a little bit of research, I traced back that the Forcefield was based on the Ibanez CP9 compressor, the schematic of which can be found on
tonepad.com.
Looking at the CP9 compressor, you can see that it's an OTA compressor like the MXR Dyna Comp.
electrosmash.com has a great analysis of this pedal which was used when designing my modification. If you're interested in this mod, or want a better explanation of how compressors work, please read that first.
The key piece in performing this mod is to separate the variable gain amplifier (referred to as OTA from here out) output from the level detector (referred to as envelope detector from here out). Once that connection is broken, you can plug your sidechain into the envelop detector to control the OTA.
The CP9/Forcefield has an excellent breakpoint, capacitor C7 is right between the OTA output and the envelop detector circuitry. Also, it's a through-hole part so very easy to remove.
Getting the sidechain input into the enclosure proved to be the most puzzling part of the mod. The 1/4" jacks are on the top of the enclosure with the DC input between them, so adding the sidechain input to the top wasn't an option without relocating the DC jack. The right and left sides of the enclosure are actually a separate piece from the enclosure face, which makes it difficult to wire anything to the left or right side. Also the enclosure is steel, so not easy to drill.
I noticed that both the input and output jacks of the effect were TRS jacks. Sadly, the output jack ring conductor is soldered directly to the ground plane and breaking this connection would be too difficult. The input jack ring conductor allows the battery to be switched out of the circuit when the input is unplugged, a pretty common arrangement. I decided that I probably wouldn't be using a battery in this effect, so cut the trace from the battery to the ring conductor. Now I could use the ring conductor of the input jack to get the sidechain signal into the pedal.
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| Ring conductor trace cut, jumper wire to boost/cut block |
This adds some complexity when cabling; if I want to use the sidechain then the input jack has to be connected to a TRS splitter with the ring connection as the sidechain input and the tip as the effect input.
Sidechain Input Conditioning
I wasn't sure what level of signals I'd use for sidechaining so I decided to add a boost/cut block to the sidechain input. The EHX
LPB-1 is a simple booster with fewer than 10 components, all of which I happened to have on hand. I was able to squeeze it onto a perfboard a little larger than a quarter, this sits sideways between a couple of the effect's potentiometer shafts. Some electrical tape prevents the LPB-1 board from shorting to the Forcefield PCBA. The C7 cap was moved onto this board so that breakout wires could be added between the OTA and envelope detector. The diagram below is a basic illustration of the mod. the only parts added to the original PCBA are the LPB-1 components, a switch, and some wire.
In order to switch between the normal compressor operation and the sidechain mode, a SPDT switch was added to the input of C7. Remember, the input to this cap is fed into the envelope detector. The switch pole is connected to the cap input, switch throw 1 is connected to the OTA output (normal operation) and switch throw 2 is connected to the LPB-1 output (sidechain mode).
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| Modded PCBA, cap C7 space is to top right of added potentiometer |
I tested the pedal by running a constant synth tone into the pedal input while running bass kicks into the sidechain. Whenever a kick hits, the synth briefly drops out of the mix. The response is still sensitive to the sustain and attack controls the pedal came with, and the mode switch lets me return the pedal to its normal configuration. I'll call it a success.
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| Pedal reassembled |
Future improvements:
- The LPB-1 is a guitar booster and as a result it doesn't have a very sensitive bass response. The sidechain effect is probably used most effectively to duck bass hits, so it'd be a good idea to change the caps of the boost/cut board to allow more bass through.
- Using a TRS splitter for the sidechain is a bit of a pain. Moving around the DC jack may be worth the convenience of having a dedicated 1/4" connector. A pigtail connector might work too, though that'd be pretty ugly and fragile.
- Loss of battery function is a bummer, it'd be great to get that back either by finding a way to use the ring conductor on the output jack, or adding a 1/4" input somewhere.
- The LPB-1 board is just kind of rattling around in the enclosure. This box isn't likely to leave my basement, but if it's going to last I'll need to work out some method to secure it to the PCBA or enclosure.
- The sidechain level control is way too close to the pedal's original level control. I underestimated just how massive the stock knobs were when I was drilling the enclosure. Swapping the level pot and sidechain switch would have been a good idea, since they're the same size I could probably just go ahead and do that...
Conclusion
This mod wasn't all that difficult to perform once I had a plan, the most time consuming part was probably wiring up the LPB-1 board. A similar modification could probably be done using compressor pedals with different designs; like optical or FET compression. The basic strategy of separating the variable gain amplifier output from the level detector should still hold, though it may be more difficult than it was here.
