Piezo and magnetic together - tone knob problem

[Andy GG]

Hi everybody, new poster long time reader here.

It is about my electro acoustic bass recently upgraded with magnetic pick up as in addition to its original piezo one.
⚠ Problem - tone pot on magnetic doesn't work (no change in tone when turned in any direction).

1. Piezo.
Bass has original preamp with EQ for its piezo, standard thing:

⚠Note: It works fine when alone as well as after adding magnetic to the same jack output.

2. Magnetic.
I had this idea to put some spare P pickups into the sound hole. Managed to put it there:

Added two pots: 1 vol 1 tone:

Wired them to standard Fender diagram and connected to that stereo jack output to which piezo has been attached, but tone didn't work. Tried all other wiring ideas I found online or in my other basses, but no joy.. This is current wiring:

(Red caps: 0.047uf, 0.047uf, 0.022uf. Green cap 0.0022uf) ⚠Note: the same result when green cap is NOT connected.

⚠Note: Sound is bright and clear no matter how you turned that tone knob.
So I replaced pot, replaced cap, replaced wires, tried different caps (single 0.047, two same value, two mixed value or 3 as above), no joy..

⚠Note: All that time volume pot (for magnet) and piezo system have been working fine. I can mix their levels no problem, eq piezo no problem.

3. Output jack.
Output wires from magnet and from piezo are connected to a stereo jack originally supplied with guitar:

⚠Note: Pots for magnet and preamp for piezo are not on the diagram but they are used indeed.

4. Conclusion.
As far as I know all has been soldered correctly, because when testing magnet direct to amp (through separate jack) it darkens the tone nicely as expected (even with 3 different caps and that green one as shown before). However when connected to a jack along with piezo - tone pot stops working and sound doesn't change, tone is not lowered anymore.

⚠ It looks like that piezo system somehow doesn't allow my magnet tone pot to dump the high frequencies down to the ground..

Why? How?
I'm sure someone has heard similar story before and solution was found.
Electric gurus please help.
I can't sleep anymore.

 

[RobbieK]

The problem is that you are trying to mix two signals that have very different impedances. A quick fix would be to try and increase the output impedance of the preamp by putting a resister inline with its output. But really, this will take some experimentation, and will always be problematic.

You need to buy or make a simple buffer for the magnetic pickup then sum their signals with a pair of resistors.

I would also respectfully recommend practising and improving your soldering technique. There's tonnes of YT videos on this. I honestly don't mean to sound like a jerk here, but it's a fundamental part of modding guitars and you will enjoy this great hobby far more as a result.

 

[Andy GG]

...mix two signals that have very different impedances.

Sounds like a problem.
I believed that having preamp for piezo onboard would make it work and take care of any mismatch that I read about ("you can't just mix mag and piezo passively", "piezo will need preamp to work" etc).

My logic was simple: if piezo's preamp prepares signal to have correct characteristics (such as impedance) for an amp's input (while magnet has them already), and assuming that amp's input won't accept just any signal but only a signal with proper impedance - then well, if I see that my amp indeed accept output from both pickups (tested separately) it means that both pickups must have the same impedance, isn't it?

Was I just being naive or maybe my preamp "EQ 7545R" doesn't do its work correctly?

 

[RobbieK]

Yeah impedance matching can be confusing for sure. A piezo pickup really needs to be buffered by a preamp with a very high input impedance to work best. And that's what your onboard pre is doing. But the preamp's output impedance is very low. Much lower than the (unbuffered) passive magnetic pickup. That's why you are having trouble blending these two signals.

If you tried to blend the signal direct from the piezo passively with the magnetic pickup, you would still have an impedance mismatch, but in this case it would be the magnetic pickup that is much lower impedance than the piezo.

So yes, a piezo needs a preamp even just to work properly on its own. And for blending a piezo and a passive magnetic, you need a preamp that's designed for this particular blending job. This is basically a tiny mixer with two inputs, and at least one of these inputs will have a very high (over 5 megohm) input impedance for the piezo. Weirdly, magnetic pickups are mostly designed to sound best with a bit of loading. (In a typical passive guitar, the volume and tone pots, and to a certain extent the cable, will do this.) So the other channel should have say, a 200K input impedance or the magnetic pickup will sound sort of hifi and strange. I'm pretty sure there'd be an off-the-shelf twin buffer for piezo and mag blending available these days. Try Bartolini, EMG etc. You would run this before your existing preamp with the piezo and mag pickups going in directly, then the output of this goes to the EQ 7545R.

As I mentioned, another approach would be to keep your separate p bass style vol and tone pots, but buffer this signal then sum the two low impedance signals with a couple of resistors. These are normally called "summing resistors" and you see this approach in many many schematics. A couple of 10K-22K resistors would work well for this.

I'm sure you could find a simple buffer on ebay or amazon, or you could have a go at making one. A simple opamp buffer like this, will work well.

Check out this page for more info.

 

[walterw]

Sounds like a problem.
I believed that having preamp for piezo onboard would make it work and take care of any mismatch that I read about ("you can't just mix mag and piezo passively", "piezo will need preamp to work" etc).

right, but that preamp means the piezo is now "active", and you can't mix active and passive signals together either (the active is all you'd hear).

the fix is an actual buffer made to do this specific mixing job, like the fishman powerchip or certain bartolini pres. they work by making both the piezo and the magnetic "active" and then mixing them together like two channels on a mixing board.

 

[Andy GG]

View attachment 3053840
Check out this page for more info.

..the fix is an actual buffer made to do this specific mixing job..

Thank you gentlemen for clarification.
I'm leaning forward building one. If it is only a few components then why not giving it a go?

 

[Andy GG]

I couldn't wrap my head around building a buffer just yet, I'm more of a shy passive beginner than an active maniac.
But!

I have found a solution

With all the bits of information i added 1+1 and went to see if there's any crosstalk between those 2 pickup systems.
And there it was.

I've put two 1k resistors between output jack and each of signal wires.

100 ohm felt too little and 10k seemed unnecessarily too big.

Now both pickups live peacefully together

 

[Warpeg]

Many years ago, I experimented with a similar project. Instead of an ABG, in was a Warwick Corvette that I added piezo bridge sections to. My thread may provide some help: Warwick, EMG, Piezo...will it work?