Circuits involve active electronics and a dummy coil.

[AlDiMeowla]

Maybe PM me, or upload and link to an external imagehosting site?

[Mike Sulzer]

Hello and sorry for necrobumping the thread. I am very intrigued by this FET-dummy coil buffering, that Mike was talking about. But it seems the schematic has disappeared. Would anyone of you happen to have a copy of it to post again? Thanks

I am having trouble getting it to attach.

[AlDiMeowla]

Hello and sorry for necrobumping the thread. I am very intrigued by this FET-dummy coil buffering, that Mike was talking about. But it seems the schematic has disappeared. Would anyone of you happen to have a copy of it to post again? Thanks

[Mike Sulzer]

Sure, you can do that with FETs. But remember what we are trying to do. The idea is to put the voltage from the dummy in series with the pickup coil without the dummy's inductance adding to that of the pickup coil. Therefore we measure the dummy's voltage: a very high impedance is used for this so that you do not load the dummy and thus alter the relative levels of the different hum harmonics. The follower lowers the impedance while "following" the voltage so you can insert it in series with the "ground" lead of the pickup with minimal effect on the sound of the pickup from the adjustment pot..

Yup just a follower is obviously the best way if you use matching pickup and dummy.
In my case I'm trying to tame a Tele, so bridge pickup and neck pickup are way different.

As a middle ground between the pickups for the dummy I'll use a Strat single, hence why I look to add a bit of boost, to match the bridge noise level. neck obviously will need just passive pot adjustment.
I thought about using local feedback inverting amp with a single jfet like in tubes:
Designing Single-Stage Inverting Feedback Amplifiers
But I can't find schematics of such an amp using jfets on the web... jfets are not able to do that?


Thanks for the replay after all those years

[Amit]

Yup just a follower is obviously the best way if you use matching pickup and dummy.
In my case I'm trying to tame a Tele, so bridge pickup and neck pickup are way different.

As a middle ground between the pickups for the dummy I'll use a Strat single, hence why I look to add a bit of boost, to match the bridge noise level. neck obviously will need just passive pot adjustment.
I thought about using local feedback inverting amp with a single jfet like in tubes:
Designing Single-Stage Inverting Feedback Amplifiers
But I can't find schematics of such an amp using jfets on the web... jfets are not able to do that?


Thanks for the replay after all those years

[Steve A.]

I've now made a hum cancelling coil for a strat-like guitar (my G&L S-500). It works great and it's pretty easy to do.
Details are at https://sites.google.com/site/string...for-sc-pickups

I just checked out uneumann's link and found it to be interesting although it deals strictly with passive dummy coils...

I've attached a PDF capture of the page for future reference...

saf.dc.pdf

Steve A.

[Mike Sulzer]

I prefer the source follower in this application because of the low impedance looking back into it. (If you use the circuit for two pickups this provides isolation.) But the circuit with gain should work as you say. Also be careful with noise (hiss). If you use fewer turns on the dummy and amplify up, you might notice hiss.

I like the low current because battery life is long, and it works, but there is nothing wrong with changing the circuit for higher current with traditional biasing if you like.

Att. Mike Sulzer.
Old thread but looking for answers :-)

I know my way around tubes but not much about transistors/fets etc.
Anyway I breadboard your dummy coil buffer circuit, testing several J201 i'm getting at the output (top of 27k resister) between 0.55v to 0.85v (Vdd is 8.6v).
Is that ok?
This means current drain is like 30nA...
I'm not getting any voltage reading on the gate... i guess its in nano-volts... shouldn't the gate be biased to some level like mid Vdd?

* post edit: guess i haven't built anything in a while... gate is at 0V while source is at 0.55-0.85V so its fine for mV noise signal :-)
I was just thrown off by googling jfet followers schematics and seeing them all being biased by a divider to mid Vdd.

Btw, if i'll use a gain stage (common source, which inverts the signal) before the buffer, with the dummy at same wind direction as the pickups, does it mean I'll also get some level of electrical noise cancellation?

[Amit]

Att. Mike Sulzer.
Old thread but looking for answers :-)

I know my way around tubes but not much about transistors/fets etc.
Anyway I breadboard your dummy coil buffer circuit, testing several J201 i'm getting at the output (top of 27k resister) between 0.55v to 0.85v (Vdd is 8.6v).
Is that ok?
This means current drain is like 30nA...
I'm not getting any voltage reading on the gate... i guess its in nano-volts... shouldn't the gate be biased to some level like mid Vdd?

* post edit: guess i haven't built anything in a while... gate is at 0V while source is at 0.55-0.85V so its fine for mV noise signal :-)
I was just thrown off by googling jfet followers schematics and seeing them all being biased by a divider to mid Vdd.

Btw, if i'll use a gain stage (common source, which inverts the signal) before the buffer, with the dummy at same wind direction as the pickups, does it mean I'll also get some level of electrical noise cancellation?

[uneumann]

I've now made a hum cancelling coil for a strat-like guitar (my G&L S-500). It works great and it's pretty easy to do.
Details are at https://sites.google.com/site/string...for-sc-pickups

[uneumann]

Great article! I admire people who can explain things as well as this. I don't find your name anywhere on your site

one typo:
"....One is electromagnetic and this noise source is addressed by shielding. Again, there are lots of websites and articles about shielding. Ideally, a metal shield surrounds the guitar electronics, including the PUs, and is tied to the guitar cable ground...."

A conductive shield, such as foil, "shields" from the electric field, not the electromagnetic field.

By reading your article, I am quite certain that you know this and it is only typographic

Excellent job, I think you should sign your name to it!
Cheers,
Ethan

Thanks Ethan - good catch - EM is not my strength so I used the terminology loosely. I've made the fix - and actually caught another in the process. Thanks again.

My site has my gmail address (uneumann@gmail.com) if anyone wants to reach me. As for signing, it seems like email is a sig these days. ;-)

-Ulrich Neumann

[CRU JONES]

Great article! I admire people who can explain things as well as this. I don't find your name anywhere on your site

one typo:
"....One is electromagnetic and this noise source is addressed by shielding. Again, there are lots of websites and articles about shielding. Ideally, a metal shield surrounds the guitar electronics, including the PUs, and is tied to the guitar cable ground...."

A conductive shield, such as foil, "shields" from the electric field, not the electromagnetic field.

By reading your article, I am quite certain that you know this and it is only typographic

Excellent job, I think you should sign your name to it!
Cheers,
Ethan

[uneumann]

I'm new here, but recently did some work with a dummy coil on a Tele ... seems this is not a new idea to you all - but it was to me.
I have the whole story posted here with pictures, if you're interested.

https://sites.google.com/site/string...for-sc-pickups

[CRU JONES]

The Cap is actually a good idea. The bias network in series with the winding resistance creates a LR high pass... A few dB... But none the less, I like that you add it

The 2.7M are also wise! Good thinking. The 2.7M's are in parallel with your volume. Thermal noise will be shunt by the volume control. I'll check that link next time I jump on a computer. Thanks for sharing. I like your "hair brained" ideas

[CRU JONES]

That schematic makes the active circuit look simple. I'm glad to finally see that schematic though.

There are two choices: 1) make the dummy coil an insignificant load 2) make the the dummy coil an insignificant load. Active electronics is actually the least technical way to do this. IMO

[Steve A.]

I'm considering this for a two pickup RW/RP guitar, which necessitates invertering the coil for one of the pickups...

Nate:
IMO there is a simpler way to accomplish this without having to use electronic circuitry to invert the dummy coil- check out the wiring diagram for the Gibson Blueshawk:





The first picture is the official Gibson wiring diagram while the second one was what the curator of the BluesHawk website drew up himself with some corrections made after he was finally able to obtain the official schematic. (Gibson ought to post his drawing since it is much easier to understand!) He goes into a lot more detail in his very excellent single-purpose website, which is undoubtedly everything you wanted to know about one of Gibson's most interesting guitars!

www.blueshawk.info/resources

I believe that the dummy coil used in the Blueshawk is basically a P-90 coil without the magnets or any kind of metal load. The BH selector switch has just 3 positions, all of which are humcancelling, so you don't get to A/B the effect of the dummy coil (which from my experiments adds some compression and loses some of the brightness.) It seems like most people think that it is the RW/RP properties that create the humcancelling effect but I figured out on my own strictly by accident that it was the [B]reverse winding[/B] that was responsible for the humcancelling effect and that the reverse polarity that keeps the pair in phase. I guess that is common knowledge now among guitar techs but I was shocked when I discovered that since the Brosnac book (1st edition) was my bible and he did not consider the possibility of a humcancelling pair that was out-of-phase (see the chart on Page 35 1st ed. which has a 4 x 4 grid showing all 16 combinations of strat pickups).

In any case my interpretation of the drawing is that the two pickups are NW/RP with the windings of the neck pickup inverted in the middle switch position. The neck pickup is South up and the bridge pickup is North up, but all three positions of the selector switch are humcancelling (the dummy coil is bypassed in the middle position which is a series linkage.) It would be easy to add a switch (or a pole of a 4P/5T or 4P/6T switch) to bypass the dummy coil in the neck only or bridge only positions when humcancelling is not needed. (I think that Gibson left that option out to keep people from A/B-ing the effect of the dummy coil and discovering that it sounded better without it!) One more thing to note in the drawing- for the middle position of the switch the signal to the controls is shunted through a treble bleed RC network consisting of a 100k resistor and a 0.0047uF cap to brighten up the sound of the series linkage. You should be able to add a switch (or switch poles) to be able to get a parallel linkage in the middle position.

Good luck!

Steve Ahola


EDIT I had drawn up a wiring diagram inspired by the BluesHawk which had a RW/RP pair of P-90's wiring in series with a dummy coil in the middle. The middle position would have the two P-90's in series- I forget if I was able to get them in parallel as well. I'll have to see what I can come up with again. (My LP Jr Special is just begging for a dummy coil between the two P-90's under the pick guard.)