Yamaha bass preamp RBX-370A

"Listen to me and I'll tell you no lie
Too fast to live, too young to die*
I bought a new machine today and say ...
It take your breath away"  - Chris Spedding.

 
RBX-370A

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:
The green line is the output. The input stage is greatly influenced by the reactance of the pickups, which give you a huge frequency spike at around 10KHz, like a waa-waa pedal. Which is probably why people put the volume control before the preamp, to stop the treble boost. Since I don't know the actual reactance of the pickups, I'll adjust the treble-cutting C15 400p capacitor by ear when it is installed, simulations are only a guideline.

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
*I'm ancient enough to remember watching this on t' telly.

Well, when I say 'telly', t'wer only cardboard box, but t'wer telly to us.

1 comment:

Unknown said...

Hey Frank I just bought a yamaha bass (I believe it is an rb750) and the preamp didn't work. While searching for info I came across your post here. While mine isn't exactly the same it is really close to the one you show here. The circuit board traces are damn near identical but the components are labled differently. Anyway, you seem to be well versed in electronics would you have any info, tips, or magic you could throw my way to assist me in troubleshooting this? Thanks man, take care.