I just picked up one of these pretty cheap on ebay and it uses a JRC NJU4069UBD, NE571N, and JRC4558SD (SIP, presumably driving the tone stack). Soundwise it's definitely reminiscent of the Hot Tubes kinda overdrive, kinda aggressive with a tight bottom end and the dual band active eq allows for a pretty good range of tones but gets pretty noisy with too much treble as one would expect. The compressor circuit definitely needs some tweaking. I'm gonna have to learn up on the NE571. When the gain is high the compressor pumps up the noise before reducing it again as the sustain decays. Not too bad but very noticeable and a bit unusual. I'm gonna see if I can get a schematic from Yamaha. For the price ~$30 it's a pretty decent overdrive.