Tying piezo w/preamp into active (EMG) system

[GregP]

Although blended piezo and magnetic systems seem to have entered a fairly mainstream space right now, I can't seem to find many diagrams to achieve what I'm after. Here's the idea:

On the magnetic side, I have an EMG 89 and 81TW, Both are dual mode (quasi-coil-split). I have 2 25K push/pull pots for mode switching ("coil splitting") duty, plus an extra "normal" 25k pot that was originally meant for tone. The switch is a Gibson-style 3-way. For the magnetic system, the plan is to use the push/pull as 1V, 1T and ditch the normal pot. So, a master volume and a master tone that I'd probably rarely use, and a push/pull dedicated to each EMG.

On the piezo side, I haven't actually procured anything yet preamp-wise, but I was thinking of grabbing the smallest acoustic preamp I can find, and finding some space for it, maybe with the panel facing outwards on the back of the guitar. We'll see if I can find one that fits; otherwise I might be back to square one.

Assuming the components work out, the piezo elements "tie" together and feed into the preamp, which has its own output.

I don't know if the impedances of the magnetic and piezo systems are going to be an exact match because they don't seem to list the impedance of these cheap acoustic preamps... but I'm hoping that since both systems are active, at the output stage there's going to be a match.... maybe that's too much to hope for?

Assuming they are a match, this means that each respective system essentially has its own master volume. While a blend pot could be interesting, I'm fine with just two separate volume controls to create my "blend". That being the case, what is the right stage to tie these things together? Both of them just feed the output jack?

Thanks for any input you might have!

 

[curtisa]

If the manufacturers aren't willing to provide detail on the piezo preamps you're probably limited to just buying one and trying it out. It's possible that the two can be mixed by simply joining the two outputs together at the output jack, but with one signal source being of unknown lineage you won't know until you try it.

You could probably ask the question of the bigger manufacturers to see if they have an answer. Fishman, LR Baggs or Graphtech might be able to tell you straight away if they have something that would work.

[GregP]

Thanks Curtis.

I think I might have either been reading incorrectly or just not understanding a spec page (and I don't have a link; I just filed it away and closed the tab). Further research SEEMS to indicate that pretty much any preamp (whether on an active pickup or in a piezo preamp) is concerned with input impedance, but then matches it to "line level". If this is true, then tying them together at the output jack should theoretically work.

Probably the big companies would be able to help me out, but I feel bad asking for advice when in reality I'm probably just going to buy some generic $15 preamp.

Thanks for the reply! If anyone can confirm/support the concept that the EMG output is "line level" and an acoustic preamp (active, of course) is also "line level", that would be great. Otherwise I guess I'll just move ahead with the experiment.

Cheers,
Greg

 

[curtisa]

40 minutes ago, GregP said:

Further research SEEMS to indicate that pretty much any preamp (whether on an active pickup or in a piezo preamp) is concerned with input impedance, but then matches it to "line level

That's only part of the full story. The ability of two outputs to be mixed by simply tying them together will also be a function of (at least) the relative output levels of the two signals and the output impedance of the two signal sources. From memory an EMG has an inherent output impedance of 10K. The piezo pickup preamp is a black box, and unless the manufacturer publishes this information, the output impedance and nominal signal strength is unknown.

There is no universal standard specifying output levels of pickups or piezo preamps. You play harder - you'll get more output. You have hotter pickups - you'll get more output. You put the pickup closer to the strings - you'll get more output. The design engineer of the piezo preamp figured you'd need extra grunt because he was feeling generous one morning while slaving over the PCB layout and schematic - you'll get more output.

There's no way of easy way of evaluating outputs of EMGs vs random acoustic preamps on paper. They may be closely matched or they may not. They may mix well or they may not. Unless you can get some advice directly from the manufacturers, or you're willing to try rolling your own piezo preamp, you may be stuck with either trial and error, or putting the call out to anyone who may have tried doing this before and can make a recommendation on what works.

FWIW, I vaguelly recall that the Graphtech Ghost preamp allowed you to trim the relative outputs of the mag and piezo pickups before mixing. I'm not sure if other companies offer this kind of circuitry.

[GregP]

Thanks for the follow-up, Curtis. Certainly tells me I need to get my ducks better in a row.

It's funny; some of the black box preamps show input impedance but not output. Not sure why.

Will look into the Ghost system.

[curtisa]

Here's some avenues to explore:

LR Baggs Control-X preamp allows you to trim the piezo against the mag.

Graphtech Acousti-phonic preamp also allows trimming of the piezo pickups.

Both options place the mixing of the two pickup systems on board the preamp, rather than at the output jack.

[GregP]

Unless I'm going crazy, this is a flawed diagram:

 

http://www.graphtech.com/docs/default-document-library/powering-active-pickups.pdf?sfvrsn=2

 

It seems to have things mislabeled and/or missing. For example, the QSW (quick switch) shows the piezo and mag systems entering, but not what is leaving. But this is picked up later going into the output jack as "SW" (not QSW) I believe.

This being the case, I believe that what they are doing is NOT putting active mags directly into the Acousti-Phonic system, but rather using a switch to either alternate or sum directly before the output. Electronically, going directly to the output would be the same as the middle position of this switch.

[curtisa]

I don't believe there is anything wrong with the diagram. Don't confuse the fact that the switch has wires labelled "mag" and "piezo" which may not actually carry these signals. It looks more like the mag and active signals are being shunted/shorted to ground via the switch when moved to the associated position, thus killing the unwanted signal source rather than selecting the wanted signal. We also have no idea what kind of electronic magic is being performed on the Acousti-phonic PCB that may allow for the switching/blending of the two signals.

The block diagram shown at the bottom of the page illustrates what goes on in the PCB at a fairly basic level. The piezo signals get fed to a dedicated buffer and EQ section prior to being fed to a piezo volume pot. The active pickup(s) go direct to the mag pickup volume pot. The output of the piezo and mag volume pots then get blended together before being passed to the output jack.. The "St/Mono DETECT" widget changes the output signal routing based on whether a stereo or mono lead is plugged in, determining whether the output contains a mix of piezo and mag through the one cable, or the piezo and mag signals are sent out through left and right independently. The Quick Switch just does its thing when connected to the two "QSW" terminals as shown. How this is achieved is not known (and presumably we, as mere mortals who just want to rock, don't need to know), but it should work as expected.

[GregP]

Holy Jumpin' Doodle... I totally missed that the wires from the top half match up to pins on the bottom half (the Acousti-Phonic). It seems obvious as soon as you spot it, but I had been seeing them as separate things and was thinking, "where are these wires coming from and going to, out of the blue?"

Thanks for your continued help, Curtis!