Funny impedance mismatch between magnetic and piezo?

[Triad]

Hello guys... I'd like to read your point of view about this issue I'm having.
I'm putting piezo sensors in the semi-hollow body of basses (passive) with an on/off switch. They work and add a nice "acoustic" warmth to the tone and allow the player to "drum" on the top of the instrument.
A strange thing I've noticed is that when the switch is ON, the tone of the magnetic changes too. It loses some high-end and a bit of output. An AlNiCo5 humbucker (that sounds very mid-heavy by itself) changes radically... loses much of the extra mids and a bit of high-mids.

As you all know, piezos are very high impedance devices. They should be buffered but I would still like to understand what goes on with this setup and find a way to make the different tone source live together without interactions (or at least not that big!). Any thought?

[David King]

Nothing "funny" about this. You can build a passive resister network to help bridge the gap. A megOhm or two between the piezo and the magnetic outputs should help preserve the mag tone.

[Triad]

Thanx! But why does this thing happen? It's a very strong interaction. Should I use a simple 1M resistor to have the magnetic pickup tone back?
In case it helps... the magnetic is a humbucker made with AlNiCo5 magnets, wired in series. Connected to volume and tone pots with a 0.47uf resistor. The piezo has no controls, it's wired to the on/off switch and then positive and negative reach the magnetic pickup wires on the output jack.

[MarkusBass]

So you just wired piezo in parallel to magnetic pickups - this won't work. Piezo pickup has very high iptput impedance and can be also modeled as a capacitor (few nano-Farads). You need a buffer. And most probably for magnetic pickups you also need a buffer. Otherwise, it will be quite difficult to adjust/mix signals from two sources. Take a look at this: Powerchip (OEM) - Fishman Transducers, Inc.
You have to know that piezo pickup alone also won't work if the input impedance of the amp you use is too low. You have to be careful with piezo.

Mark

[Triad]

Hello! Yes, I know. I'd only be curious to find out what happens on this setup and how I can get back the tone of the magnetic pickup when I use both. I don't need the piezo to work at 100%... if it's not buffered it only affects the highs but it's ok for me, I won't use it soloed.

[MarkusBass]

You are connecting a capacitor (piezo) to a magnetic pickup (inductor) and the sound changes. You are trying to save some money but this will result in poor sound, or difficulties to adjust the sound. You have to make a decission which way to go. I would at least build a simple FET buffer. Fishman's buffer/mixer is one of the best I've seen.

Mark

[Triad]

Well... I have a Bartolini TC3 here but didn't know to use it on this bass. It's not a matter of price and I must admit the bass doesn't sound "bad" with the piezo on... it sounds different, maybe "better" according to my tastes. David King's solution could be fine but I'll need more details.
The tone knob works perfectly, though. I'm just curious to find out what happens and how I can eventually fix it. If it's possible.
I plan to make the next pickups Low Z with a booster... but a piezo would always need a buffer. It would be a good choice if I wanted the "full" sound of the piezo to use it soloed but it's not what I need (luckily, I must add).
The strange thing is that the piezo isn't affecting the highs but a pretty wide portion of high-mids. And not in a bad way, but it's clearly audible.

[Mike Sulzer]

But why does this thing happen? I

The piezo is a capacitor. The magnetic pickup and cable capacitance are a resonant circuit. Put a capacitor across a resonant circuit and you lower the resonant frequency. If you just put a resistor in series with the piezo, its low and high frequencies get shorted. The pickup inductance is a low impedance at low frequencies, the cable capacitance is a low impedance at high frequencies. Use buffers and add the outputs. If the magnetic pickup resonance is where you want it with the cable capacitance, you will need a capacitor equivalent to the cable capacitance across the pickup at the input to its buffer to preserve its sound.

[Triad]

Thanx, this confirms my initial idea... even if it's really strange how the tone changes. It's not like a normal low-pass filter.
Anyway, the solution presented by David King is close to something I've seen used on Strats... I could try! Any other idea of how I can try to fix this without using a buffer? Even if the piezo loses the lows... not a problem since the magnetic is always ON.

[Mike Sulzer]

A megohm in series with the mag does not leave its tone unchanged.

Thanx, this confirms my initial idea... even if it's really strange how the tone changes. It's not like a normal low-pass filter.
Anyway, the solution presented by David King is close to something I've seen used on Strats... I could try! Any other idea of how I can try to fix this without using a buffer? Even if the piezo loses the lows... not a problem since the magnetic is always ON.

[Triad]

I've seen a similar solution on the wiring diagram for a Strat... but can't remember what it was for. And of course can't remember the value of the cap.

[David Schwab]

Nothing "funny" about this. You can build a passive resister network to help bridge the gap. A megOhm or two between the piezo and the magnetic outputs should help preserve the mag tone.

Are you sure about that? If you run a piezo through a resistor it will likely remove most of the low frequencies. That's how they remove the lows on piezo tweeters.

[David King]

David that's very true but I was trying to preserve the tone of the magnetic pickup in as simple a passive way I could think of. You can't have everything all the time (until you are willing to drop a 9V into the mix).

[J M Fahey]

What's the part of you *need* a buffer you don't understand?

[Triad]

I don't understand... do I need a buffer? LOL. Sorry, I know that making a piezo work in a passive circuit isn't the best solution but as I said, I only wanted it to affect the sound of the magnetic, dont' need it to work at 100%.
I do have a buffer but it's not made for magnetic/piezo... and I think that buffering the piezo alone could cause troubles.
Now that I remember, the resistor thing was used on a wiring to make sure the volume knob didn't cut the highs out on a passive piezo setup (for a cigar-box guitar, maybe?).