Get your tuts out for the lads....(re stacked humbuckers)

[Mike Sulzer]

I was just explaining to StarryNight why you still need to cancel hum even when the impedance is low.

...My main push is to cancel magnetic hum...

[StarryNight]

I was just explaining to StarryNight why you still need to cancel hum even when the impedance is low.

yes, my mistake. I was assuming impedance had an effect on magnetic hum, but rather it's the relationship between the two coils that determines the amount of magnetic hum/phase

[earthtonesaudio]

I'm interested in this. How much are you amplifying, and how bad is the hiss from the active components?

[peskywinnets]

I'm interested in this. How much are you amplifying, and how bad is the hiss from the active components?

I'm running the pickup's +ve & -ve signals into a TL074 arranged as a differential amplifier The gain is about 3x ...the preamp's hiss is discernible vs a standard humbucker into my PC's RME soundcard (but not objectionably)

A David Schwab suggested, I'm probably better going with thinner gauge wire (& more windings) to get the raw pickup signal boosted up & away from the TL074's noise floor. To this end I'll be experimenting with about 2,500 turns of 0.05mm wire (towards establishing what the bare minimum amount of turns I can get away with for a good SN ratio). Once I have that to hand, I'll come back to to 'signal coil upper' & 'dummy coil' lower arrangement.

Got to say though - the extra bandwidth such a low impedance pickup provides is a revelation - it's like that whacky moment immediately after getting your ears syringed (assuming you've had your ears syringed!) - the top end get's whacked up beyond all belief! (but it's not shrill - it's top end in a smooth crystal glass like quality)

[RedHouse]

....the preamp's hiss is discernible...

If you're into some DIY work, try the matched pair implemenation on page-2 of this PDF file.

It's very low noise, you should be able to just snipp the front end from this schematic and pop it in front of your existing circuit.

Here is another version which also works well.

(I've built both BTW)

[David Schwab]

Got to say though - the extra bandwidth such a low impedance pickup provides is a revelation - it's like that whacky moment immediately after getting your ears syringed (assuming you've had your ears syringed!) - the top end get's whacked up beyond all belief! (but it's not shrill - it's top end in a smooth crystal glass like quality)

Is is beautiful. My first pickup was about 500 turns of 42 on each bobbin of a P bass pickup, and then run into a Barcus Berry preamp.

It was a wonderful tone. The top end was smooth and bright and the mids were very flat. It went into an 8-string bass that was heavy on the mids.

[peskywinnets]

Ok update - the way I had my earlier circuit wired did not buck hum (I established this after purposely placing a small wall wart next to the pickup - humsville!!). I must have got the maths trace on my scope setup wrong.

So I'm doing some more testing right now. I've now rejigged my preamp to be like this...



In other words...a straightforward signal addition of the lower (dummy coil) to the upper (signal coil).

My first test shows that when combining the two signals, the overall output signal is *very* weak - almost certainly because the lower (dummy) coil is picking up too much string signal too - and then cancelling the main string signal out when summed. I guess I have three options...

1. Magnetic shielding (to stop the upper coil magnetic field permeating down into the lower dummy coil & therefore inducing an antiphase signal ...it's this antiphase signal cancels out the main signal when summed. Truthfully, magnetic shiedding sounds like a whole new (big) area that I'm not sure I can be bothered to venture into!

2. Go all traditional & place a magnet of opposite polarity in the lower (reverse wound) dummy coil - at least then when I then sum the dummy coil signal & upper coil signal, I'll get some signal addition going on.

3. Place more vertical distance between the two coils (though not really an option - this needs to be a body surface mount pickup & I only have 9mm of working height to play with in the first place.

I've just CAD'ed/Cut another rough bobbin (10mm wide, 9mm high in total) & wound it (very) quickly!! This time with 700 turns of 0.1mm wire on the upper, and 500 turns on the lower....



I'll knock up some pole pieces & give it a run out soon.

[earthtonesaudio]

If you have an active preamp and "hum balance" control, do you really need the dummy coil to be the same dimensions as the signal coil? Seems like as long as the axis of the coils are parallel, it doesn't matter how big, or where it is. Just adjust gains to suit.

[peskywinnets]

If you have an active preamp and "hum balance" control, do you really need the dummy coil to be the same dimensions as the signal coil?

No I don't - I was just being lazy wrt making a smaller specific 'noise' coil bobbin - I pretty much dupilcated the bobbin in CAD for the bottom (dummy) bobbin as for the top bobbin.

That said, I think I'll likely end up with a design where the lower (dummy coil) bobbin is pretty much the same overall physical size as the top bobbin....but I'll just use less winding space on it....ie using the rest of the bobbin as 'dead space' between the top (signal) and lower (dummy coil) for magnetic separation.

I spent a fair bit of time on this last night, but didn't make much progress becuase I got sidetracked with significant problems with component failure! (somehow I managed to take a regulator out & an opamp out!!)

[Steve Conner]

Why have you put those 22nF caps across the pickup? That'll kill your high end.

And, your instrumentation amp is messed up. You have the hum in differential mode and the wanted signal in common mode. So you're only amplifying the hum, while the signal gets unity gain. You should go back to the normal instrumentation amp circuit and swap the connections to one of the coils instead.

A simple diff amp with one op-amp and four resistors would probably work just as well.

[Mike Sulzer]

Probably not, the impedance of the pickup is low.

Why have you put those 22nF caps across the pickup? That'll kill your high end.

[peskywinnets]

Why have you put those 22nF caps across the pickup? That'll kill your high end.

And, your instrumentation amp is messed up. You have the hum in differential mode and the wanted signal in common mode. So you're only amplifying the hum, while the signal gets unity gain. You should go back to the normal instrumentation amp circuit and swap the connections to one of the coils instead.

A simple diff amp with one op-amp and four resistors would probably work just as well.

The caps are there to suppress radio frequencies - (without them, I was actually rx'ing some London radio stations! The other night I could hear "Return to Sender" by Elvis on my speakers!) so yes, they kill the top end - but radio frequency top end! (not even your dog would discern the difference between them being there & being removed! Believe me on this one - I did A/B tests - & with them in situ, there's still more top end than glass being shattered in a metal walled room!)

Obviously, R4 & C9 form a low pass filter filter frequencies under 64KHz to pass ....btw, C4 & C8 are actually 5nf, not 22nf - the cct was just a quick 'lash up' so people could get a general vibe for what I'm doing - while I'm on, for the sake of simplicity assume pin 10 goes to ground not VCC/2 (but in reality I'm using a virtual ground - so the coil centre tap goes to VCC/2 & so does pin 10)

Re your comments on the circuit. Just to recap what my pickup config is into that circuit...

+ve signal = the upper 'signal' coil (positive polarity with pole pieces/magnet)

-ve signal = the lower coil ....'noise signature' (the lower coil is reverse wound - negative polarity...no pole pieces or magnet)

Therefore by summing the two signals (via IC1c), the negative noise is added to the positive noise - it therefore cancels. However, there is presently some cancellation of the (wanted) string signal too, but that's becuase I haven't implemented one of the 3 options mentioned in my last post.

Re a simpler circuit - true, but when I'm dicking about, using three opamps gives a little more flexibility on the fly.

I'm all ears wrt other circuit configs/options though....

[Dave Kerr]

Are you concerned with the footprint of the pickup at all, or just its height? Having a wider core in the bottom would allow more noise signal from the dummy per turn and perhaps sense less of the musical signal as well.

[peskywinnets]

Alas, it's the footprint that I'm focused on - I don't have a lot of guitar real estate space, so need to keep within the confines of about 10mm wide & 9mm high.

I think my best solution is to put a magnet in the bottom coil too, but since it'll have to be of opposite polarity to the top coil, there's going to be some repulsion going on where the similar magnet poles meet(ish) in the middle.

What I need is a polepiece 9mm long, including two magnets on each end (& how to stick down two neo magnets onto a bit of 3mm dia steel where they want to repel one another!)

[Mike Sulzer]

Why do you want to put a magnet (as opposed to a permeable magnetic material) in the bottom coil? What does its polarity have to do with anything?

I think my best solution is to put a magnet in the bottom coil too, but since it'll have to be of opposite polarity to the top coil, there's going to be some repulsion going on where the similar magnet poles meet(ish) in the middle.