Symetrix 501 compressor/limiter
There are maybe four or five at least eight different versions of this compressor out in the wild, and the schematic floating around is accurate only for the last revision. This one is actually the very first circuit revision (but the second cosmetic revision). One thing I’ve learned is that the faceplate color or power switch style basically tells you nothing about what’s inside.
The first circuit revision is recognizable by the “black box” side chain detector and the Allison Research-branded blue EGC-101 VCA (Allison Research later became Valley People Inc). This circuit appears in all the black-faceplate models and in the earliest grey-faceplate/toggle-switch versions. Later toggle-switch versions have a Valley People-branded green TA-101 VCA, but there are two different sidechain versions and two different output stages (with different output levels) among these units. Sometime during this period, the compressor knee got a little harder, and the limiter knee got a little softer. Part of the early ‘auto’ envelope circuit migrated over and is always-on in the later toggle versions. The first push-button versions have a Valley-branded blue EGC-101 (yes, they switched back) but retain the later sidechain and the later cross-coupled output stage. Later push-buttons have the EGC-101 circuit but have a DBX2150 on a daughterboard slotted in the EGC-101 pads (and some other components unpopulated). Subsequent push-buttons have a revised PCB for the 2150, and the output stage got revised AGAIN. The final version is the blue-faceplate version (black knobs), which is what’s shown on the schematic.
Good luck finding a pair of 501s that sound alike. Moreover, some early units invert the polarity of the sidechain inserts, meaning that two units from different eras may not be stereo-link compatible. All 501s are XLR pin-3-hot, rather than the usual pin-2-hot -- meaning that if you mix XLR and 1/4″ connections, the polarity will flip.
This 501 needed a full re-cap. I also bulked up the power supply and added some additional protection to the regulators (a very common modification to these units). I replaced the signal-carrying LF353 opamps with OPA1642s, which lowered the noise floor a few dB. The output capacitors in these units are polarized, and often end up reverse-biased depending on exactly which output stage the unit has and how much component drift has set in. Though the PCB has pads for a transformer, you can’t find one with the correct dimensions and pinout anymore (oh yeah, and different 501 versions used different transformer dimensions and pinouts). I replaced the output caps with good quality bi-polars in this unit.
Most 501s have electrolytic input caps, which aren’t really needed. You can safely short these out to DC-couple the inputs. Symetrix themselves did this on the last circuit revision.
On the black box sidechain, input impedance is 9.1k (10k input Z at the detector AC coupled to 100k at the jack). So I made a little patch cable with a 100nF series capacitance, creating a switchable sidechain highpass at 175Hz. This can be easily switched from the front panel. On later models with opamp sidechains, the sidechain input impedance is about 6.5k.
One thing to watch out for when working on these is the very fragile PCB traces. Solder carefully, and don’t overheat the traces. If you’re careful and use a temp-stable iron, re-work is quite easy.
