Too fast to live, too young to die*
I bought a new machine today and say ...
It take your breath away" - Chris Spedding.
I was going to buy a Flying V, like Chris Spedding's, but the only one on Craigslist was a 7-string, so I thought "why not an 8-string?", and ended up buying a bass.
When I was a punk, I had the cheapest P-bass in the world, and I put a Yamaha sticker on the headstock. Now I am totally legit.
Anyhoo, it is active, and I found the schematic for the preamp from the RBX760A II SERVICE MANUAL, which has the same PCB:
It is a bit noisy with the treble up, so I'll replace the electrolytic capacitors, replace the high value resistors with metal film and put in a TL072 opamp for lower noise and shorter battery life. If it is still noisy, I'll redesign the circuit to my own 3-transistor version:
My design uses a mosfet for a volume boost, followed by 2 BJTs to give an inverted signal for the -ve feedback loop which involves the tone pots. This should be low noise, low current drain, and $15 will give you high quality components. So this should be quieter and have more battery life than a fancy $300 onboard preamp.
It uses much the same tone control feedback loop as the Yamaha, and has a pretty flat frequency response:
Also, there was a high frequency noise which went away when you touched the strings. So, I took the pickups out, and the pickup cavity had been painted with conductive paint by an incontinent chimpanzee. So I repainted it with "Wire glue" carbon paint from eBay, and the noise is gone.
The Steinberger circuit is much the same as the Yamaha:
Back to Chris Spedding:
|Chris Spedding in the middle
Well, when I say 'telly', t'wer only cardboard box wi' hole in t'middle, but t'wer telly to us.