Let’s have a look at this Symetrix 501 “Blueface" PCB (the “Greyface" is pretty the same, I have one). I didn’t bother to make an overlay image since the schematic is available and it’s a one-sided board.
The other side. Looks impressive! (It’s a joke, actually, the board is a pure crap).
The biggest problem of the Symetrix 501 is the PSU. The axial caps, which are rare, expensive and not so good overall, can be easily replaced with some radial low profile ones.
Two Panasonic FC caps in this case.
Two diodes should be installed for the voltage regulator protection. There are many ways to do it, I found this is the best for me. BTW it’s a beautiful, beautiful board, isn’t it?
The ridiculously small caps at the PSU outputs must be replaced. I used two 560 uF Pana FR caps. It’s overkill, but it’s just what I had then. As you can see, I replaced the voltage regulators as well, just in case. I didn’t have radiators in my shop so I’d install them later, just small ones, not so much heat is there.
The output caps are 100 uF, which is tremendous overkill.
The TDK B32524Q3106K what I had at the moment fitted well. This is an MKT cap (not so good but should be better than an electrolytic anyway), I believe WIMA can offer twice of its capacity in the same package with the MKS-4, but for the standard line-level impedance 10uF should be enough. The shunting with a small capacitor should be considered, but I skipped that part then. I have to open it later for the VCA upgrade, anyway.
The coupling cap before the VCA was replaced with the WIMA MKS-2 10 uF and shunted with the WIMA MKP-2 0.033 uF on the reverse side of the board. I forgot to take a photo. I replaced every other electrolytic cap with the Pana FR regarding the measured offset.
Every op-amp in the signal path and the side-chain/detector/etc. was replaced with the 8-pin socket. Basically, the input and output op-amps make a significant difference and this circuit can tolerate lots of options, but the other ones definitely affect how the compressor works, therefore there is a room for an upgrade and some fine-tuning.
I didn’t upgrade the VCA since I don’t have them.
It was a noisy shitty bastard. Lack of “image”, piercing, distorted and thin highs, blurred transients. It’s all gone with all the changes and good modern op-amps. The noise is lower at whopping 20+ dB now. I like the character of the compression on the drums, it sounds fun at extreme levels. And the most important part: it doesn’t sound like a DBX.
Despite all of this, the phase response at the low frequencies is still bad. It was about 20 degrees at 20 Hz with the old components, it’s exactly the same (graphs are absolutely identical) with the new ones, so the smaller capacitors at the output are not a problem. And yes, I checked the old ones.
I will replace the VCA and solve this mystery then since this compressor doesn’t have bypass at all. No, really. No bypass. So maybe it’ll eliminate the problem.
In the end, I don’t consider the Symetrix 501 as a good platform for an upgrade. The unit’s price is absolutely ridiculous, people want about 200 bucks for this crappy distorted shitbox. But if you own one already or can obtain it for an adequate price, like 30-50 bucks, it may be a fun project with a fun result. Otherwise, there are lots of better options.