WHAT COMPRESSOR David B. Thomas dt@dt.prohosting.com http://dt.prohosting.com/hacks/what.html Jan. 28, 2000 circuit description parts list construction hints testing and alignment procedure CIRCUIT DESCRIPTION See schematic sheet 1. The signal path consists of inverting buffer U1A, follower U5B, and inverting mixer/amp U5A. The compressor action happens at the voltage divider formed by V1 and R25. "April Resistor" R28 mixes in a small amount of dry signal to restore transients otherwise lost to the compression. This is the reason the maximum gain reduction is 14dB, and also part of the reason this compressor sounds so good. U1-B implements a 3dB per octave high pass filter to shape the compressor's sensitivity to different frequencies. R3, along with gain stage U2A, sets the amount of sidechain gain and thus determines the amount of compression. U2B and U3A form a full wave rectifier for the envelope generators. The first envelope generator is the peak detector consisting of active rectifier U3B, R10, R13 and C1. The second is the simple passive averaging circuit containing R14 and C2. Active rectifiers U4A and U4B select the highest voltage from either envelope generator at any given moment. This is how the dual release time is implemented. When S3 is engaged, the highest voltage from either envelope generator from either channel is selected, ensuring stereo tracking. Mixer U8A combines the composite envelope generator signal with a bias voltage, for driving the vactrol's led. The indicator led is also driven from the same signal. The PNP transistor is used for current gain, to drive the leds without loading the op amp. That closes the loop. The vactrol's led shines on the photoresistor, causing its resistance to drop, which lowers the amount of signal fed from input to output, which is what compressors do best! MISSING CONTROLS Originally I was going to offer the traditional controls like "Attack", "Release" and "Threshold", but I found that my design could either be a first-rate transparent compressor for vocals and instruments or a third-rate general purpose or effects compressor. I decided to stick with what it does well and fix those settings. The user need only dial in the amount of compression desired. Naturally some people like having lots of knobs, but one friend did tell me he can make his fancy dbx unit sound as good as mine, if he has lots of time to tweak! PARTS LIST C7,8 1000 uF, 50V electrolytic C30,46 100 pF C29,45 10 nF C5,43 1 nF C34,50 330 pF C31,47 33 nF C6,44 3.3 nF C32,48 470 pF C33,38,39,49,51,52 680 pF C9-28,53-58 .1 uF C2,40,36,37 10 uF 16V tantalum C1,3,35,41 1 uF 16V tantalum C4,42 33 uF 16V tantalum D17,18 1N4003 diode D1-9,12-14 1N914 diode D10,11,15,16 yellow led (must be yellow) JP1,2 jumpers (removed for alignment) Q1,2 2N4403 or equivalent PNP (2N3906 ok) R1,2,5,9,10,14-16,18,26,33-37,42,44,50,58-61 10K R13,41,43,65 1K R6-8,19,22-24,45 20K R32,57 221K R30,55 22.1K R28,51 27.4K R4,17 2.21K R40,64 464 ohms R12,53 46.4K R38,62 4.64K R21,49 562 ohms R11,47 5.49K R25,46 681 ohms R27,54 68.1K R20,31,48,56 6.81K R39,63 trim pot, 50K linear R3,29,52,66 10K audio taper pot S1,2 DPDT toggle switch S3 DPDT toggle switch (may use SPST) U2-4,6,8-10 TL062 op amp U1,5,7,11 MC33178 low noise op amp U12 7815 +15v voltage regulator U13 7915 -15v voltage regulator V1,2 Vactec VTL5C9 vactrols (matched) CONSTRUCTION HINTS The myriad .1 uF capacitors on sheet 3 are for bypassing each op amp power supply pin and each voltage regulator terminal. Locate each as close as possible to the pin being bypassed. On the schematic, all polarized capacitors are shown with the negative plate curved and the positive plate flat. Keep all leads as short as possible. This circuit can work in a breadboard but will perform better on a PCB, ideally a 2-sided board with a good ground plane. The MC33178 op amps can be substituted, but low noise op amps able to drive 600 ohms must be used. The TL062 op amps are used for less critical purposes and may be substituted more freely. If possible, select the two vactrols from a larger batch, for matching characteristics. The way I do it is to connect the whole batch (3 to 10) of vactrols in series (the led portions), then put a 100k resistor in series and connect the whole mess across a 12 or 15 volt DC supply. Then just go down the line measuring the resistance of each photoresistor, and select the two that are the closest to each other. The better the match, the better the tracking you'll get in stereo mode. The yellow leds really do have to be yellow. The led inside the vactrol is yellow, and yellow leds have different voltage drops than the other colors. My design requires all the leds to have approximately the same voltage drop. The small capacitors across the op amps are important if the recommended low noise op amps are used. Low noise op amps exhibit gain at radio frequencies and the last thing you need is 770 KOB AM coming in throughq your compressor! TESTING AND ALIGNMENT PROCEDURE Once the circuit is built up, apply 20VAC and quickly check the +/- 15 volt supplies, feeling for any heat on the regulators or other chips. If all is well, remove JP1 and JP2. For each channel, measure the resistance across the photoresistor and set the trim pot for 50K ohms. Then reinstall the jumpers. Connect with an audio source at line level and monitor the output. Ideally, use a microphone (with a preamp of course) and monitor with a headphone amp. If you have a studio mixer, you can use a TRS insert cable to patch the compressor into a microphone's channel. Enable compression on the channel you've connected (non-bypass). The compression is very subtle so at first you might think it isn't working. Do your first tests with the post gain knob all the way down, so you'll at least know for sure whether the vactrol is attenuating the output signal. Set the compression knob all the way up. The gain reduction led should light up with the incoming audio. You should notice at least that the output signal is louder with the compression knob turned down than it is with it turned up. Now with the compression knob all the way up and the gain knob all the way up, turn up the microphone enough so you can hear background noise in the room. When you operate the bypass switch, you should hear the noise level drop. Now speak at a normal level; it will sound loud in the phones. While speaking, flip the bypass switch back and forth. You probably won't hear much difference in the audio level as the switch is moved. That means the compressor is working: quiet sounds (room noise) are amplified but loud sounds (speaking voice) are not. Try the other channel and make sure it behaves the same way. Then try turning on the link switch for stereo operation. Input to one channel will not be output to the other channel, but the gain reduction leds will always light up the same, even if only one channel is receiving audio.