Rockman schematic - question w/attachment

[casey73]

This is my first attempt at posting a schematic here. It's actually a photo of a schematic. Hope it works. I extracted this from a schematic of the original SR&D Rockman, found on the U.S. Patent site years ago. I redrew it to include only the distortion circuit, removed the op amp buffer in front and added simple TS style buffers in front and back. All values were taken from the patent document, and two resistor values were missing, as noted with a "?" on my drawing. Having built several tubes screamer clones, it all makes sense to me except for the FET/diode arrangement in the first op amp stage. I plan on breadboarding this circuit sometime this week, but would like to know what that portion of the circuit is/does. It appears as some kind of feedback loop clipping arrangement, but I'd like to get a better explanation from those with more design experience.

[Steve Conner]

I think the MOSFET thingie is some sort of attempt at adding a level-dependent DC shift to the signal. That brings out even harmonics when the following stages are heavily overdriven, which should make it sound more tube-like, as explained in the famous Russell O. Hamm paper on tube distortion.

That's the only explanation I can think of, since the 22M resistor is too big for it to have any waveshaping or clipping effect directly. Unless that 22M were a misprint (accidental or deliberate)

PS. I used to have the rack-mount Rockman with its MIDI and 100 blinking lights, and I seem to remember the overdrive sounding fairly crappy. I used it with an old solid-state PA amp and a 4x12". I remember there were LEDs inside near the input jack that flickered with the drive. When I think about it, I probably should have kept it, I bet it would rock with a stereo tube power amp.

[casey73]

Rockman schematic

Thanks for the feedback. Haven't got around to breadboarding it yet. I suspect you may be right about the schematic containing intentional errors. And, some part numbers had not corresponding values in the patent. I remember reading somewhere that this is an effort to thwart those who want to duplicate the circuit. I bought the original Rockman when it first came out and later owned several Rockman modules. I remember seeing LEDs in the sustainor preamp and I didn't seen anything like that in the patent schematic. I've long since parted with them via Ebay, as I've simplified everything. I used to run my original Rockman through a 20 watt PA amp and it sounded much better than through any standard guitar amp.

I breadboarded the Mockman circuit from runoffgroove, and it is similar to the Rockman tone, but is still lacking. I like to Rockman style distortion and want to come up with a circuit that come close to emulating it with a smaller parts count. There are other factors like pre and post distortion EQ and compression that factor into the tone of the originals.

[David Schwab]

PS. I used to have the rack-mount Rockman with its MIDI and 100 blinking lights, and I seem to remember the overdrive sounding fairly crappy.

I don't know about the rackmount version, but listen to those ZZ Top songs from the 80's, like Sharp Dressed Man. That's all Rockman! When they asked Billy how her got that tone in GP, he said he didn't get the tone, it was Tom Schulz! He said he used the Rockman direct on all those records. I guess it's more or less the Boston tone too.

Having said that, I remember trying out a Bass Rockman, thinking it was... OK... and when I handed it back to the salesguy, he said "you're the first person to try one of those and not buy it". He seemed shocked! I told him all my floor pedals get the same tones.

[Steve Conner]

No way! I thought the guitar tone on "Sharp Dressed Man" was great, and you're telling me it's made by a box of op-amps? I guess my failure with the Rockman was just down to not knowing anything about how to play guitar. Or maybe it didn't suit my tastes at the time, I was playing mostly metal.