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.


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.

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