Using an isolation transformer between IC amp & guitar sustainer driver coil ?

[dedjazzgadgetz]

Hi everybody !

Like many guitar enthusiasts, I've built myself a DIY electro-magnetic guitar sustainer (-oh, that's sooo 2009... ! )

It's a TDA7052a 1W amp/32AWG 14 ohm-ish flat 2mm coil-over-neck-pickup combo, which works surprisingly well (although I still had to "modify" bridge & neck pick-up poles to improve response, i.e. swapping the poles around so they would "follow" my Strat's neck radius hence, driver coil being that much closer driving the strings (high B & E in particular). The mod doesn't seem to alter the pups' sound at all when sustainer is off...

What I would like to try out now is : totally isolate the sustainer coil from TDA driver amp/guitar "ground"... maybe using an isolation transformer ?

...yeah, I know: Why ? IDK.. it's just a hunch I have about maybe curing the "fizz" (although I'm using just the minimal power necessary to excite the strings, a residual signal is fed-back to the bridge pup, magnetically along the strings I think, thus altering the pup's sound -even more trebly- AND inducing a sort of "fizzy", light distortion which is annoying because I want a "clean guitar" feedback, not the metal-divebomb-thing... oh, well maybe just a little )

It would go like : driver-amp & guitar-gnd to primary/secondary to sustainer coil... (I think it would introduce a 240° (?) lag, which might be good... or not, IDK ! gotta try...)

So... would a transformer work to isolate guitar-ground from driver-coil AND still transfer driver-amp's power to it? If so, what would be a ballpark primary/secondary relationship & values (1 : ?... ? : 1... ohm values ? TDA amp puts out less than a watt) so I don't fry everything 1st try... Induction theory is sooo fuzzy to me...)

Well, thanks for reading!
See ya...

 

[dedjazzgadgetz]

At hand, I have a coupla small transformers :

-1 ohm primary/secondary 32 ohms CT
-1 ohm primary/secondary 62 ohms CT
-165 ohms primary/secondary individual 51 ohms & 49 ohms (Hammond 161C16)
-3k primary/secondary individual 63 ohms & 55 ohms

Any of those sound good ?

Thanks !

(BTW : the transformer needs to be smallest possible to fit inside the small guitar pot cavity, alongside my tiny driver-amp PCB)

 

[Mr RB]

Do you have a 'scope to look at the waveforms? If you are overdriving the little amp used for the coil, you might be feeding the coil with a square wave which will definitely add a "fizzy" sound. Ideally the coil should be driven with a sinewave.

 

[dedjazzgadgetz]

Hi there, Roman !

Well, the classic test in this case is susbtituting a speaker at the output of the amp (although it's an 8 ohms): the guitar sound out of it is pretty much clean, no artefacts of induction or distortion... (unfortunately, no scope)

When using the sustainer, the other (neck & middle) pick-ups must not only be de-selected, they have to be un-grounded, completely taken out of the circuit, both ends; otherwise, acting as antennas, induction from driver-coil happens (classic high-pitched squealing, not the desired acoustic-like, sweet feedback sound) if only de-selected...

...hence the hunch about driver-coil isolation from guitar-ground using a transformer. I *think* (more like -hope- ! ) that guitar strings pick-up (or attract) more of the driver mag. field by being grounded... does that make any sense ?

But the strings do have to be grounded through the metal bridge: if not, every finger contact yields 60 cycle hum...

 

[Mr RB]

I'm sorry I really don't know and have never done this myself.

However electrically the transduer coil you have added is already electrically isolated from the other pickup coil. Adding a transformer in there won't isolate it any more.

I think (guess) what is happening is that the closed loop operation has some phase shift that is corrupting the signal (because it is affecting itself). You really need a scope to tune it.

Some tuning might be possible using a capacitor in series with the transducer coil, you could try some different values and listen for possible improvements.

 

[dedjazzgadgetz]

Hi again Roman !

Well, the isolation I'm looking for is from guitar-ground. As things stand now, my sustainer coil is connected from IC amp output to guitar-ground.

I was thinking: a small transformer's primary connected to IC amp output & guitar-ground, inducing power to secondary connected to sustainer coil i.e. isolating the coil from interfering with ground (well in theory/hunch, that is ! , with a side-effect of phase-lag (?°) I guess, which might or might not be detrimental to the sustainer effect... only testing can tell for sure. There is a capacitor in the FET preamp section, so I'm thinking the transformer phase-lag might counter-act the capacitor phase-lead... or is it the other way around ?)

I have some small transformers (listed a coupla posts up) but I was hoping to get some ballpark figures of primary/secondary values to try out so I don't fry everything, again & again & again (getting to the coil is rather simple, but the pcb in the pot cavity is a ***** to get to, so I'd prefer testing outside the guitar).

Anyway, thanks sooo much for taking the time, Roman; very much appreciated !

Bye for now,
Joël

 

[Mr RB]

Hi again Roman !

Well, the isolation I'm looking for is from guitar-ground. As things stand now, my sustainer coil is connected from IC amp output to guitar-ground.
...

That's irrelevant. The sustainer coil produces a magnetic field, which moves the strings. Whether or not that coil is connected to amp ground which is connected to guitar ground does not matter, it's method of coupling the signal (from coil back to guitar string) is electrically isolated.

I'm sure the issue you are seeing is phase shift. Adding a transformer in the loop will change the phase shift and might make the issue better or worse. Likewise, adding a cap to the coil (as i suggested above) will also change the phase shift and might make it better or worse.

If others have said a transformer is a solution, it is not because of isolation it is because the transformer is affecting the phase shift.

 

[dedjazzgadgetz]

Hey, Roman !

Thanks for the reply; I appreciate the time you've taken warning me about failure/disappointment... but I will try with a transformer even though it doesn't make any sense, because I'm a stubborn s*%-of-a-$@#*h ! (and stubborness is one of the spices of moving forward, after all). Let's just hope I don't burn-out an IC (or ten ! ) or the coil ; but if it means I'm gaining knowledge (whatever the outcome), then it's all good... like you wrote earlier: "I'm sorry I really don't know and have never done this myself."

...meanwhile, if anyone would care to pitch-in with some ballpark figure of transformer prim./sec. values efficiently transfering power (that's the part I'm clueless about) from a 1Watt IC amp (TDA7052a, usually meant to drive an 8 Ohms speaker) to a 14 Ohm-ish sustainer coil (a 2mm 32AWG core-less inductor stuck flat/atop the neck pup), before I make the whole thing go up in flames !

Thanks again, Roman !

Bye for now,
Joël

(or maybe one of these small transformers I have might work:
-1 ohm primary/secondary 32 ohms CT ?
-1 ohm primary/secondary 62 ohms CT ?
-165 ohms primary/secondary individual 51 ohms & 49 ohms (Hammond 161C16) ?
-3k primary/secondary individual 63 ohms & 55 ohms ?)