Yamaha bass preamp circuit opinions? Overly complex for the stated "+3db" gain switch function.

[Prostheta]

Hi everyone. Long time lurker, but nothing of value to add to the conversations that I've followed over the years. Certainly not one just to say, "nice mod dood".

So I have both a 2002-3 Yamaha RBX 774 and 775 active basses with identical preamps. Aside from the issues of these being known to come from the factory with the switch miswired and non-functional, they are pretty simple from the outside. A 9v preamp that is placed post pickup selector switch, with onboard master vol/tone pots and a switch for that +3db boost. No mention of tone shaping (which I think is what the circuit may be doing) but the complexity implies there is more happening than is presented.

Yamaha even provided the PCB layout and schematic in their service manual, which is exemplary.
https://www.yamaha.com/yamahavgn/Doc...X774_775_E.pdf

My knowledge of electronics falls a little short here. What I do understand is that IC1a (let's call the left half that) is providing a negative feedback gain of -((R2+R3)/R4) or 0,46x to a little over unity? What I am stuck on is the network beyond C3 and whether IC1b is bringing that gain back up to human scale, and whether there is any tone shaping going on. This is a little above my pay grade. Obviously the part around D1 manages reverse supply conditions and supply buffering including the hanging half supply line.

[Dave H]

IC1a is a non-inverting amplifier. Boost gain is (R2+R3+R4)/R4. Non boost gain is (R2+R4)/R4. A difference of 3dB.
I think IC1b is a notch filter.

[loudthud]

The gain equation for a non-inverting opamp would be 1+ ((R2+R3)/R4) . The second opamp is the active version of the Baxandall tone control but has fixed resistors instead of pots. It also provides gain at treble and bass frequencies. It needs drive from a low impedance source and that's what the first opamp stage provides.

Here's a version with pots that includes a Midrange control:

[Prostheta]

Thanks gents. Oddly enough this week I was idly thinking about Baxandall tone circuits, and how I should really have spent time studying them instead of sleeping through those key lessons in the 90s at college. Perhaps my unconscious part understands Baxandall equalisers where my conscious does not. Possibly reasoning as to why I should have slept through the exam also?

That this circuit incorporates an equalisation stage tracks with the other models in the Yamaha RBX range of that period, and how they had a "modern" scooped sound, or in this case dialled in exaggerated high/lows. The other preamps in this range have EQ controls onboard which I presume are the same circuit, but transposing R11/R12 and R5/R6 with a potentiometer of some similar combined value. It isn't impossible to consider making a new PCB for this purpose using dual concentric pots with transplanted wafers for the differing values....

This definitely explains why the basses have the characteristics that they do. Excellent. Have a great weekend guys!

[J M Fahey]

hat.

As Loudthud said, that is Baxandall active tone control, with Bass and Treble pots replaced by fixed resistors and set to "9" so always boosting.

Yes, if available, proper value concentric pots can help you.

[Prostheta]

hat.

As Loudthud said, that is Baxandall active tone control, with Bass and Treble pots replaced by fixed resistors and set to "9" so always boosting.

Yes, if available, proper value concentric pots can help you.

Thanks. At this stage I have more been concerned with what that stage of the circuit was doing, as I suspected it was shaping the tone somewhat. As to exactly what it was doing wasn't clear, however this is now abundantly so. I don't know if I want to go adding in pots to adjust this, as the bass only has reasonable spacing for two pots. It would however be interesting to breadboard this out and change a few key components in that EQ. The boost switch isn't that useful to my mind, so this could potentially be repurposed. The circuit has potential for simple yet profound modifications. I appreciate your input.

[J M Fahey]

I don't know if I want to go adding in pots to adjust this, as the bass only has reasonable spacing for two pots.

2 concentric pots take same space as a single one.

Only problem is that they are hard to find, expensive, require special knobs, but no extra drilling or real estate.

This is one example:

https://www.rockinger.com/en/parts/e...c-pot-2-x-500k

Search around.

EDIT: a single 50k+50k Concentric pot makes your Bass and Treble separately adjustable, can fit in current Tone pot hole, no big deal losing it, active tone controls are way more useful, responsive and powerful than that old relic.

Guess crude old style treble cut only tone control was added so Musicians were not TOO pissed off with modern Technology

PS: I searched for "concentric potentiometer 50k"

https://www.google.com/search?client...&bih=635&dpr=1

[Prostheta]

2 concentric pots take same space as a single one.

Only problem is that they are hard to find, expensive, require special knobs, but no extra drilling or real estate.

This is one example:

https://www.rockinger.com/en/parts/e...c-pot-2-x-500k

Search around.

EDIT: a single 50k+50k Concentric pot makes your Bass and Treble separately adjustable, can fit in current Tone pot hole, no big deal losing it, active tone controls are way more useful, responsive and powerful than that old relic.

Guess crude old style treble cut only tone control was added so Musicians were not TOO pissed off with modern Technology

PS: I searched for "concentric potentiometer 50k"

https://www.google.com/search?client...&bih=635&dpr=1

Thanks! At this stage I am not thinking of modifying the circuit, and was just trying to get towards understanding its function. The buffer stage with variable gain was very simple, and I make a variety of similar circuits for various instruments already. I was simply failing to identify the Baxandall stage, mostly because I studied digital logic, PLCs, etc. rather than analogue. The first bass I bought back in 2003 had a faulty circuit, which I swapped out and made passive. Either way it was underwhelming. I recently bought a 4-string version from the same period that is working 100%, and was curious as to what the preamp is (or was) supposed to be doing. I knew it was some sort of tone shaping circuit, and in its standard state it's pretty good out of the box. I may re-use the design for other instruments in the future now I have a basis of understanding from which to work. I'm familiar with concentric pots, etc. however the bass(es) in question has/have recesses around the lower bevel which limit the diameter of the lower part of the knob stack, so it's not really crossing my mind at this stage. I'm somewhat of an old hand at the instrument design and build end of things in the wood, high amounts of CNC, etc. What I have not done is to revisit my studies to understand various filter topologies, which I can't kick myself enough for!

If I were to incorporate some variable tone shaping, I would likely do so with a rotary switch after testing say, six combinations of EQ out on breadboard. I've never been one for encouraging fiddling around on the instrument too much as it's a player distraction. One of my favourite instruments was the Aria Pro II SB-1000 with its six-way variable LPF with mid bump and single pickup. You get what you get, and those six voices deliver six very specific playable sounds. Doing so here might allow me to figure a variety of EQs from the default modern through to the opposite with mid-range emphasis. At the very least, this would allow me to keep controls at a minimum, maintain the original voicing and external appearance. The only question remaining is how desperate I am to keep the passive tone pot, exactly as you state.