Unexpected piezo preamp blend effect.

[Tony Done]

Hi folks this is my first post to this group.

Very briefly, I've been playing for about 55 years, mostly acoustic fingerpicking, but in the past couple of decades I've gt interested in electric guitars and guitar electronics. - I'm a compulsive tinkerer.

Anyway back to the title topic. A while ago a had a go at combined a magnetic and piezo pickup in an acoustic guitar. After a fair bit of mucking about I ended up with the magnetic pickup in series with the output of the piezo preamp, a Maton AP5. The included a phase switch and volume control as a push-pull pot on the magnetic. I can now get any blend of magnetic and piezo output, from all-magnetic to all-piezo. It works much better than I expected, and in a surprising way. Even with the battery of the preamp pulled, the magnetic pickups works at full volume, that is as if the preamp had no resistance. Likewise, even when the volume is turned right down on the preamp, the magnetic pickup comes through full and strong. Is this normal, or is there something odd about this preamp? There doesn't seem to much adverse effect of the preamp on the magnetic signal.

Also, IIRC, the magnetic volume pot is an A250K, and it doesn't really act as a volume control, it only takes a tiny amount travel to go from full-on to full-off. Is there any easy solution for this? make a low-velue pot?

Thanks.

[Steve A.]

Can you add a series/parallel switch, and/or individual toggles to bypass the magnetic and piezo pickups? Series linkages can get complicated...

Steve Ahola

[Tony Done]

Yeah, I could use switching options, but the blend would be a nice feature if I could get it right. I might just take the exerimental approach. Parallel might work, but I probably tried it. Since the preamp doesn't seem to have any resistance, it might not have been possible to get the magnetic into the blend.

[J M Fahey]

Please draw a schematic of your setup, also state what pickup you are using.

In general, I *guess* you wired both in series, and your blend pot is set with wiper to center point between both, one end to ground and the other end to passive pickup "hot", so one potentiometer half is across the piezo preamp, other half across the magnetic pickup.

So in halfway position both produce sound, all the way to the right you short the passive, all the way to the left you short the active.

That will work reasonably well if you use 2 passives, but as is, you will NOT have a smooth blend action, active impedance (thanks to its preamp) is WAY lower than the passive one.

It can be solved, but we need the actual schematic, my crystal ball is running out of batteries.

[Tony Done]

Please draw a schematic of your setup, also state what pickup you are using.

In general, I *guess* you wired both in series, and your blend pot is set with wiper to center point between both, one end to ground and the other end to passive pickup "hot", so one potentiometer half is across the piezo preamp, other half across the magnetic pickup.

So in halfway position both produce sound, all the way to the right you short the passive, all the way to the left you short the active.

That will work reasonably well if you use 2 passives, but as is, you will NOT have a smooth blend action, active impedance (thanks to its preamp) is WAY lower than the passive one.

It can be solved, but we need the actual schematic, my crystal ball is running out of batteries.

OK thanks, but I'll have to get back to you when I've had a look at the wiring on the pot, I didn't draw a schematic, and it's been a while.

The magnetic is just a strat SC, modified for acoustic strings, the resistance will be about 5.6K ohm, the preamp is a Maton AP5 and the piezo is an inexpensive-looking UST. What I can tell you from memory is that the magnetic, with its volume and phase control, is wired in series with the hot output of the preamp.

What I'm really curious about is why the preamp is behaving as if it doesn't have any resistance wrt to the magnetic, even when the battery is pulled. It is just counter-intuitive to me.

[Tony Done]

Ok, this is a pic of the blend control:



The pickup, via a shielded two-wire cable, goes to the phase switch and thence to the hot and wiper of an A250K pot, the pot is also earthed to the shielding at the switch. The whole thing is just a 0 to 250K resistance in parallel with the pickup. This is then wired in series with the output of the preamp, so it goes piezo>>Preamp>> magnetic>> socket, plus Preamp earth>> socket and magnetic shielding>>socket.