Introduction
The SX202 is a now discontinued but still very popular microphone preamp made by Symetrix. It is a 1U, half rack width, two channel, transformerless mike preamp based on the now obsolete and hard to find SSM 2015 mike amp IC. The stock SX202 is a fundamentally good sounding amplifier of the "clean and uncolored" variety. Unmodified, it is quite well respected by most engineers who have used one. However, the circuit implementation and component choices of the stock design significantly reduce the performance that the SSM 2015 is capable of.
These web pages outline several methods for removing these performance roadblocks, and will allow a skilled modifier to meaningfully improve the subjective and objective performance of this amplifier. The end result will be an amplifier whose distortion products are very low, largely benign, and essentially uniform over the audible frequency range. Subjectively, this makes for an amplifier that is not bright, but not dull, and also detailed, but not over-hyped. Essentially, you end up with an even better example of a "clean and uncolored" mike amp, a bit closer to what I think the SX202 designers were originally aiming for.
Of course, these modifications require some electronic re-work skill, will completely void your warranty, will greatly diminish the resale value if you botch them up, and will change the way the SX202 sounds, so keep that in mind before you act on what's written here. If you feel you're not equipped to do these mods, take your SX202 and these instructions to a competent technician, and they'll be happy to do the mods for you.
I will be adding more photos and distortion plots sometime soon, but for now, you can at least read about the mods, understand their purpose, and hopefully make a good choice as to which mods you will perform, and what to expect.
One issue to keep in mind is that there are essentially two different flavors of the SX202, each using different PC board layouts and component choices. So, the instructions, photos, and component designations may vary depending on which unit you have. The older units with the rev. C circuit board have 6 output jacks on the rear of the unit, and the newer units with the rev. G circuit board have only 3 output jacks. All of the photos are of a rev. C unit, unless otherwise stated.
Having cleared that up, let's dig in!
The four main issues that can be addressed in the SX202, in decreasing order of importance, are:
• SSM 2015 compensation capacitors
• Coupling between the SSM 2015 and the output stage
• Power supply bypassing and the internal grounding structure
I wanted to further improve the SX202, optimizing it for the way I work, and have chosen to also address the following issues. Whether you agree or not, it's probably good to at least read about these mods, decide whether they make sense for you, or investigate other related design changes that could better suit your requirements.
Finally, I present a list of !
Good luck with these modifications and enjoy your new preamp!