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DubHertz 26th September 2012 03:23 PM

Yamaha TRB preamp low output - HELP
Hi I have a Yamaha TRB5-II and I find the output of the preamp to be quite low compared to my other bass guitars, even a passive Yamaha BBN5 with the same pickups in it. What is the easiest way to get a bit more juice out of it?

Should I change /remove R55 and R56 - what is the purpose of these two resistors. or should i increase the gain at the final buffer stage by changing the values of R52/R53. if my calculations are correct the gain is 0.46 at this point.

Markus2006 26th September 2012 05:03 PM

R55/56 do not change the gain of the preamp, they change the characteristic of the potentiometer. The gain can be changed by replacing R53 (22k) to e.g. 47k (in order to decrease the negative feedback of IC1-2). Let me know whether it helped - my friend also wants to change the gain of this preamp.


phase_accurate 26th September 2012 07:56 PM

That's a beautiful bass. I was once interested in its 6-stringed brother. Because there was no chance to try one where I live - and before spending that much on a bass I'd rather like to try it in advance - I went for a cheap Korean one instead.

Are VR7 and VR8 trimpots to optimise the humbucking effect or can they be operated from the outside in order to change from humbucker to single-coil at will ?



DubHertz 27th September 2012 04:20 PM

Thanks Markus, I will try that at the weekend and get back to you. Can you explain how the treble control works, I dont understand why it has a dual ganged pot. I guess thats why there is always some treble left when you fully cut the treble pot.

phase_accurate - VR7 and VR8 are trimpots located on the preamp pcb, the pickups have a single coil and a dummy coil to create a humbuking arrangement, the trimpots allow you set a balance between the two buffer stages until you get the least amount of hum. It is a great instrument, a bit heavy but lovely to play.

phase_accurate 27th September 2012 08:02 PM


the pickups have a single coil and a dummy coil to create a humbuking arrangement,
Cool ! I like the single-coil sound but do also prefer it without hum. Therefore my ERB has stacked humbuckers. The dummy coil idea is also nice but it may be difficult to design and dimension for a one-off pickup.



DubHertz 1st October 2012 11:50 AM

Ok, so I carried out the mod on Saturday, this was acheived using Markus's suggestion of changing R53 to from 22k to 47k. it made a big difference, I now have the volume level turned down to halfway, before the resistor change I had the volume turned up fully.

The is no distortion or clipping at all it just sounds a lot louder and clearer. here is a picture just incase anyone else needs to do this mod

dalmatyn 12th July 2015 02:34 PM

Hi. I'm going to change the opamps in the preamp. My favorite is the OPA2604. It will definitely be a swap for the TL062. I wonder if even the OP200. After a low power consumption, this opamp does not impress parameters. What do you think?
I would be very grateful for permission to use the scheme in a better resolution. Greetings

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