In-cavity 'exciter' fed by output jack signal - possible?

[mb1685]

I have a Stratocaster style partscaster and I am planning on the following setup:

-Master magnetic pickups volume pot (250k) (push/pull to select the neck pickup, allowing the non-standard bridge + neck pickup selection when the blade switch is in the bridge position)

-Master magnetic pickups tone 6-way rotary switch (with different capacitor and parallel resistor values)

-Standard 5-way blade switch for magnetic pickup selection

-Master piezo transducer volume pot (for passive piezo transducer adhesive mounted against the wood inside the cavity as in this video)

-Active buffer prior to output jack (Creation Audio Labs Redeemer)


Now for the really wacky part…

I’ve experimented before with using a small toy ‘exciter’ type transducer and placing it against the headstock or back of the body to get low volume feedback (being fed the monitoring signal from my audio interface). It worked surprisingly well and I recently came across some posts here where people had done similar experiments with good results.

The piezo transducer video linked above has given me the potential idea of attaching a second (SPL-emitting rather than microphone) transducer in the cavity along with a tiny power amp that will accept 9V to drive it (maybe something like this although that may be too large), and feeding this chain the guitar circuit's signal just prior to the output jack. I had the idea of using a push/pull pot for the master piezo volume pot mentioned above and wiring it so that the push/pull is connected to a 9V battery connector and can cut the power (which is fed to the tiny power amp) when I don’t want the guitar in ‘feedback mode’.

Note that I want the pot itself to control the volume level of the piezo transducer (which is entirely separate from the exciter transducer for vibration/feedback — I want both piezo tone blending and the ability to induce vibration/feedback with the other exciter as two distinct unrelated things) — I just want the push/pull to additionally function like an on/off switch for the power amp that drives the exciter transducer.

Is all of this possible? I guess the main questions that are coming to mind are:

-Is it possible to ‘tap’ the signal before the output jack so that it can go to the jack but another path can simultaneously go to the tiny power amp?

-If the above is possible, does this cause significant signal loss? The Redeemer does have a +6dB mode so maybe that could help a little bit. I do understand that it may be difficult to effectively even drive the exciter with the tiny voltages of a guitar signal without the use of a preamp and with only 9V available to the power amp.

-Is there any reason the push/pull pot for the master piezo volume couldn’t control volume for the piezo transducer while the push/pull opens and closes the circuit for the 9V battery driving the power amp?

Any help or insight would be greatly appreciated! Luckily my Strat has a swimming pool rout so hopefully I'll have enough room for all this stuff. 

[newey]

Someone else will need to weigh in on your other questions, but I'll try to answer this one:

Feb 9, 2021 21:27:10 GMT -5 mb1685 said:

-Is there any reason the push/pull pot for the master piezo volume couldn’t control volume for the piezo transducer while the push/pull opens and closes the circuit for the 9V battery driving the power amp?

That can be done, as I see it, but it isn't just a matter of cutting the battery. You would want one pole of the P/P switch to disconnect the amp's power, while the second pole takes the amp's circuit board in/out of the circuit

[ashcatlt]

Feb 9, 2021 21:27:10 GMT -5 mb1685 said:

-Is it possible to ‘tap’ the signal before the output jack...?

Yep. Straight wire split should work fine.

-If the above is possible, does this cause significant signal loss?

As long as the parallel total of the two input impedances is at least 10 times the output impedance of the buffer, you'll have no noticeable loss. The impedance of most things (pedals, amps, DIs, etc) that we connect a guitar to is usually pretty huge, and most amp circuits are likely to be plenty big enough, too, so it should just work.

The amp you've linked is just waaaayyy more than you need, honestly. I've driven these things from a simple headphone amp. It was a little weak, but by applying a good amount of distortion before that amp, I was able to get some pretty good feedback out of it. So maybe something a bit bigger than the cheapest headphone amp you can get (or a simple opamp gain circuit into a headphone amp), but you definitely don't need 2 x 100W.

-Is there any reason the push/pull pot for the master piezo volume couldn’t control volume for the piezo transducer while the push/pull opens and closes the circuit for the 9V battery driving the power amp?

newey answered this, but I'm not sure it's even that big of a deal to disconnect the amp. I mean, if you've got the pole, you might as well, but it shouldn't be necessary. It's not actually in the signal path, and if the input impedance is big enough to not be an issue when it's on, then it won't be an issue when it's off either.

The one thing you're going to want to be careful about is the physical placement of the driver as well as the routing of the wires on the driver side of the power amp. If they get close to about any other signal carrying wires, the pickups, or even sometimes the bridge (which might also implicate ground wires and shielding) the signal will couple through and then at best you'll hear a sort of distorted copy of your signal mixed in but it's very likely to create something that sounds (and actually kind of is) a heck of a lot like the feedback you get from microphonic pickups. Shielding all those wires themselves could help, but keeping them as far from everything else as possible is probably still a good idea.

[mb1685]

Thank you both so much! This forum is awesome. 

Not too long after I posted I realized how utterly ridiculous that power amp would be, especially since I just checked and my Dayton Audio exciter is rated for 3W, haha. That power amp probably wouldn't fit anyway! I decided that this one looks much better and just ordered it along with a 3.7V 2300 mAh Lipo battery (which seems like potentially the best tradeoff between capacity and physical size -- I don't want to have to unscrew the pickguard and yank the battery out for recharging every week). This power amp looks short enough that I might be able to just put a tone knob on its potentiometer and use that to switch on/off the power and control the level of output (since if this even works, it would be great to fine tune things on the fly). I should mention that when I asked about cutting power via a push/pull switch, my concern was is simply preserving battery life rather than any kind of tonal effect on the guitar circuit.

I'm a stickler about not adding any mini-switches to the pickguard and keeping the classic look, so I think I'm going to get a 5-way superswitch and trade the position 2 (bridge+middle) for bridge+neck. So the new configuration will be:

-Master volume knob
--Push/pull takes the piezo transducer in and out of the output circuit (all or nothing -- which is fine since I only want a subtle blend anyway, and from what I understand the output will be fairly weak being passive)

-Master tone rotary switch

-Master power amp level to control output level to the exciter speaker

-5 way superswitch

I was thinking that both from a space and EMI perspective it might be best to put both the piezo transducer and the exciter against the wood in the tremolo cavity and hopefully I'll have enough room to run required wires through the drilled channel for the grounding wire to the trem claw (if there's not, expanding that channel doesn't sound TOO hard -- and if it looks sloppy I don't really mind since it's hidden). I'm excited to get started!

[ashcatlt]

I would strongly suggest you get the things and actually try out different placements in some temporary way before you start removing wood and making permanent decisions. The signal leakage I've talked about is pretty touchy. I wouldn't be surprised if even having it near the trem springs caused squealing and unwanted noise. Can't promise it would, but not sure you can know until you get it in there and crank it up. I still think a 5W+5W stereo amp is a bit overkill, but it'll work. If you wanted to get cute, you could run into one side, then put the output of that into the input of the other side, maybe with some parallel diodes for clipping, and get some real shaking going on.

Here's a different thing that just occurred to me: When you have a guitar in front of an actual amp, and the the speakers are shaking the guitar, and the guitar is feeding back, you can move the guitar around and get it to feed back on different notes, break to harmonics, and fun stuff like that. With the driver stuck in one place on the guitar, none of that happens. It'll pretty much just pick a note and feedback on that until you stop the string or fret a different note or whatever. When I have used mine, I actually end up going into the computer and then back out, and I often will put a randomly modulated all-pass filter on the output being sent to the driver in order to simulate that thing of waving the guitar around in the air, and it works pretty well. The magnetic sustainer things (ebow, fernandez, etc) very often have a "harmonic mode" which is actually just flipping the polarity between the pickup and the driver. You might at least consider something along these lines to give you a bit more control over the action. A continuously adjustable passive all-pass filter is not exactly easy, but we might be able to come up with some sort of filter that could get you down the road a ways.

[newey]

Feb 12, 2021 14:46:19 GMT -5 mb1685 said:

I'll have enough room to run required wires through the drilled channel for the grounding wire to the trem claw (if there's not, expanding that channel doesn't sound TOO hard -- and if it looks sloppy I don't really mind since it's hidden)

On every Strat-ish body I've seen, that channel is about enough for the one grounding wire and not 2 or more. To enlarge it won't be easy, either- I've considered doing that as well, and gave up pretty early on. You can't just drill it out unless you have a very small right-angle drillhead. The other option involves a tiny rat-tail file and a lot of handwork . . .

[blademaster2]

I am fascinated by this, and I eagerly hope to hear how your experimental results work out.

I hope you can avoid all electronic feedback and simply excite the body with this vibration. That will be awesome.

Very cool stuff.

[mb1685]

Feb 12, 2021 22:27:24 GMT -5 ashcatlt said:

I would strongly suggest you get the things and actually try out different placements in some temporary way before you start removing wood and making permanent decisions. The signal leakage I've talked about is pretty touchy. I wouldn't be surprised if even having it near the trem springs caused squealing and unwanted noise. Can't promise it would, but not sure you can know until you get it in there and crank it up. I still think a 5W+5W stereo amp is a bit overkill, but it'll work. If you wanted to get cute, you could run into one side, then put the output of that into the input of the other side, maybe with some parallel diodes for clipping, and get some real shaking going on.

Here's a different thing that just occurred to me: When you have a guitar in front of an actual amp, and the the speakers are shaking the guitar, and the guitar is feeding back, you can move the guitar around and get it to feed back on different notes, break to harmonics, and fun stuff like that. With the driver stuck in one place on the guitar, none of that happens. It'll pretty much just pick a note and feedback on that until you stop the string or fret a different note or whatever. When I have used mine, I actually end up going into the computer and then back out, and I often will put a randomly modulated all-pass filter on the output being sent to the driver in order to simulate that thing of waving the guitar around in the air, and it works pretty well. The magnetic sustainer things (ebow, fernandez, etc) very often have a "harmonic mode" which is actually just flipping the polarity between the pickup and the driver. You might at least consider something along these lines to give you a bit more control over the action. A continuously adjustable passive all-pass filter is not exactly easy, but we might be able to come up with some sort of filter that could get you down the road a ways.

Great points! I'll definitely test things out in temporary/reversible ways until I (hopefully) get something I'm satisfied with.

The idea of the daisy-chaining one output into the other input and maybe using diodes is genius! When I experimented with an (externally placed) exciter with my audio interface I indeed found myself needing to use a quite heavily compressed amp simulator on the monitoring signal to easily get a lot of harmonic excitement going on, so the extra gain and diode clipping could do wonders for squashing the dynamic range a bit and sending a healthy RMS level to the driver. I'll definitely give that a shot if the bare bones wiring isn't doing what I'd like. I'm curious if I may end up needing to add a basic high pass filter to the circuit though -- I had to do some EQing to the monitor signal to get rid of the tendency to howl, like a semi-hollow body might.

Great call on the largely static nature of the interaction between the guitar and the exciter because of the fixed positioning. A circuit that modulates polarity sounds completely over my novice head right now but I'm definitely enticed by the idea of bugging you guys about it later.   I actually found that with the external exciter + audio interface experiment I wasn't incredibly bothered by that -- probably because I'm not really looking to get melodic Ebow-like sustaining feedback lines but rather 'happy accident' type feedback that maybe creeps in every now and then on a couple of chords throughout a take. With the audio interface I was able to dial in a sweet spot where I'd get just a subtle amount of the fundamental (so essentially extra sustain) the majority of the time and a random higher harmonic every once in a while, and that was a lot of fun. I'm hoping I'll be able to achieve the same here.

Another thing I'm curious to see with this experiment: my rotary tone knob setup is sort of an experiment in 'pickup modeling' based on my own measurements of my pickups and some dabbling in LTspice using Antigua's measurement database. Along with the buffer (which should effectively 'lock in' capacitance and resistance loading relationships within the circuit), I'll have 6 different capacitance + parallel resistance configurations available on the rotary to model the electrical response of some pickups I've chosen. Since the (exciter's) amp input will be fed the signal just before the output jack, I'm hoping that the changes in response could perhaps influence the interaction between the guitar and the exciter. E.g. in rotary position 6 a given note (at the same spot on the fretboard) tends to feed back at the second harmonic, in position 3 it tends to go for the third harmonic, in position 1 it tends to go for the fundamental, etc. If that ends up being the case I think that would make it even more inspiring to let the different tones influence my playing in different ways. But I'm probably just daydreaming at this point. 

[mb1685]

Feb 14, 2021 21:08:35 GMT -5 newey said:

Feb 12, 2021 14:46:19 GMT -5 mb1685 said:

I'll have enough room to run required wires through the drilled channel for the grounding wire to the trem claw (if there's not, expanding that channel doesn't sound TOO hard -- and if it looks sloppy I don't really mind since it's hidden)

On every Strat-ish body I've seen, that channel is about enough for the one grounding wire and not 2 or more. To enlarge it won't be easy, either- I've considered doing that as well, and gave up pretty early on. You can't just drill it out unless you have a very small right-angle drillhead. The other option involves a tiny rat-tail file and a lot of handwork . . .

Ah, that's a shame! That does sound like a tremendous pain. Hopefully I'll have enough room under the pickups then. If I don't have enough room for the exciter and the piezo transducer then I'll likely ditch the piezo, it's more of a novelty idea for me anyway.

[mb1685]

Feb 14, 2021 23:43:45 GMT -5 blademaster2 said:

I am fascinated by this, and I eagerly hope to hear how your experimental results work out.

I hope you can avoid all electronic feedback and simply excite the body with this vibration. That will be awesome.

Very cool stuff.

Glad to hear it! I'm anticipating that it won't work at all how I want it to because that's just generally how my ideas tend to go (whether it's guitars or anything else). 

I'll certainly check back in with my results, but it will probably be quite some time. My young kids and my job have been keeping me really busy, but eventually I hope to carve out some time to start this experiment. This winter storm down in Texas isn't helping either!

[blademaster2]

Well, you will be able to achieve what I have not been if you *do* get to try this. My family and day job have kept getting in the way just like yours.

I have wanted to try an embedded exciter for years on a solid body electric, just to see what it would do when the body wood gets the equivalent stimulus of a loud amplifier in the room - while not needing to risk my hearing and police visits. 

Good luck with it, and with the Texas cold weather. Here in Canada our homes are built for it but yours are not.

[newey]

mb1685-

I know that what you are doing is not exactly like a sustainer circuit, but it seems to present some similar features/issues. So I thought that the DIY sustainer thread by the late member 4real might be of interest to you, and might have some ideas re: placement of components, etc.:

guitarnuts2.proboards.com/thread/799/diy-sustainer-blend-phaso-caster

Now, a bit of history here, for those who weren't around back when 4real graced us with his postings:

---

When I first saw this thread by mb1685, my first thought was "Uh-oh, here we go again!". For some reason, the topic of sustainers seems to make otherwise sane guitarists go off the rails. 4real came to us, at the invitation of JohnH, after he was basically chased off the Project Guitar forum where he had a long-running thread on building a DIY sustainer circuit. The original PG thread ran to something like 75 pages or so, and after a while, 4real was mercilessly trolled by a couple of members (one in particular) who claimed that he was faking his results, that his sustainer could not possibly work as advertised, or that he was hiding some secret info needed to make it operate properly, he wasn't fully forthcoming with his information.. Eventually, PG locked the thread due to all the abuse.

The trolls then hounded 4real mercilessly all over the web, flooding his email, posting accusatory nonsense in any guitar website he visited. When he came here, I promised him better policing, and in fact I had to lock the thread above due to abuse by the trolls, who followed him here as well.

By the time he came here, 4real was over and done with sustainers, and had moved onto other projects, but the "sustainer trolls" still would not leave him alone.

So, even all these years later, I keep a close eye on anything that remotely resembles any kind of feedback or sustain circuit, anticipating the return of the trolls.

[blademaster2]

Warning understood, although the notion of exciting the body mechanically with the amplified signal is not the same as - nor trying to emulate - what a sustainer would do.


Maybe it could be used that way (inadvisably), but as I see it the aim is to add body resonance to the signal as if the system was turned up much louder than it is.

[mb1685]

Yikes! I sympathize with 4real and with the mods for all the work and diligence it required to keep the trolls at bay. I used to hang out on Harmony Central and the The Gear Page many years ago when I had more free time, so I know how ruthless the trolls can get.

Thanks for linking 4real's thread! It looks very interesting. Sad to hear that he's passed away. 

Honestly, this is a topic that probably will make me go insane.  But I'll refrain from doing so here so as to keep the drama to a minimum. Ever since experimenting with my audio interface and the externally mounted exciter, I've become convinced this is the missing component to bridge the gap between the experience of playing with a real amp vs. playing direct (along with readily available things like a quality audio interface, great amp sims and IRs, etc.), so I could see myself obsessing over it. I guess my busy life at home could be a blessing in disguise though since it wouldn't allow me to get to that point and necessitates it being a fun experiment instead.

[newey]

Feb 16, 2021 13:08:39 GMT -5 blademaster2 said:

the notion of exciting the body mechanically with the amplified signal is not the same as - nor trying to emulate - what a sustainer would do.

Understood that's it's not the same, but it seems like some of the issues with placement, noise etc. would be similar.

mb-

You can obsess away here, we are used to it! (and we are all equally guilty of it as well . . .)

[ashcatlt]

Feb 16, 2021 14:49:17 GMT -5 mb1685 said:

Ever since experimenting with my audio interface and the externally mounted exciter, I've become convinced this is the missing component to bridge the gap between the experience of playing with a real amp vs. playing direct...

I thought so, too. I actually have a couple just stuck on the end of longish cables and for a while I had one spot on my headphone amp dedicated to them so I could just plug them in, stick them on whatever guitar I was using, do a little routing in Reaper, and it it would start howling away. There a 22 minute long piece on one of my albums where I recorded each of my guitars feeding back through the whole piece. I didn’t have quite so many as I have now, but it still added up to a few hours, and I kind of had other stuff to do that evening. I’d go in, put a guitar on a stand, plug it in, stick the transducer on the headstock, hit record, and go back to what I was doing. They were all tuned so that open notes were in key and had a plugin allpass filter modulating slowly and randomly. There was a baby sleeping the next room for most of the time.

But there’s no way I could permanently add this to any of my guitars simply because I’d then want it on all of my guitars. In the studio, I switch between them all the time, and on stage I might choose one of a few depending on what I’m doing. Oh and also I refuse to put a battery in my guitar. One could theoretical phantom power it from the floor, but at that point why even put the amp in the guitar? Oh and the allpass thing which I kind of need for a lot of what I do. (I have had some results with modulating it via an expression pedal, so that I can “wave” the guitar around with my foot) Oh and frankly most of the time both in studio and live, I still prefer to actually just crank it up. But I’m “lucky” to be able to get away with that pretty much whenever I want. Oh, and sometimes it’s nice to have the rest of the band shake the guitar, too. Like, sometimes if the guitar has a choice between two notes to feedback on, the bass or some other instrument will kind of help it decide. Or maybe the guitar wasn’t quite about to feedback on its own, but the kick drum shook it just enough to get moving and then it escalated from there. Even in that long piece, I sent a mix of the band (there’s a whole “song” or three over all this feedback) to the transducer as well.

Not that I’m trying to talk you out of it. I’m sure you’ll get some great mileage out of it. It IS a good way to go, and generally does feel and act almost identical actual SPL. One might consider some way to access the input to the amplifier, but short of a second jack, I’m not sure how to accomplish that.

[blademaster2]

That is great to hear. I have long imagined trying this, but only as an experiment (like all of my guitar innovations, so it takes ages to hear the results) and have never yet done it.

Knowing that you have a result of something along those lines is encouraging, especially when you said you would have it in all of your guitars and that it replaces real SPL. Excellent.