Interleaved windings?

[peskywinnets]

We all know that mics can be very quiet, especially if they cost some bucks. Why not just just plug a low impedance (150 Ohm) humbucker into a mic preamp or sound desk mic input via balanced XLR and call it good? (I'm lazy around PCB creation.)

That would do it, but it's just I'm of the curious ilk - not to mention unfeasibly tight!

BTW....I didn't buy any special wire for my bifilar wind...I just fed two 'feeds' of wire onto my bobbin while winding - once you've finished, solder the end wire of one of the ''coils' to the start wire of the other ....voila, a centre tapped coil.

If you've not tried it....certainly worth it for the revelation!

[Mike Sulzer]

And having dabbled with several low impedance pickups with opamps operating unbalanced...I'm happy to report that a bifilar/instrumentation config certainly slams all other low impedance combos wrt s/n ratio.

You seem to be implying that the amplifier noise is lower. As Joe pointed out, it is not. Also as he wrote, you do not need the center tap to get the common mode rejection.

[peskywinnets]

No I'm not implying at all ....from what I'm seeing, it is lower (way lower) vs a traditional two wire low impedance pickup fed into an standard non inverting opamp arrangement. ( & for the same base components)

With a balanced three wire feed into an instrumentation arrangement, any noise sitting on the 'common' leg (be it true ground, or a virtual ground say at 1/2 VCC)...is erhm, common & therefore not amplified. It may well you can get get some common mode rejection going on without the centre tap,....dunno I've not tried it out (but I will, just to see what's what) all I'm commenting on what I'm experiencing ...certainly not wanting to get involved in "My technical prowess & theory is bigger than yours' situation (which is dreary at beast).

Perhaps I ought to have got a solicitor's written letter first & then said "hey guys, if you have a less ideal ground plane or a slightly noisy virtual earth in your preamp input stage, then you really ought to check out a bifilar wound pickup into an instrumentation amp - I'm finding that it's a lot quieter, even though I'm told it can't be" ....but that wouldn't have been quite as convenient or as snazzy.

[David King]

I checked Ebay and a 2.5 Lb spool of Litz 8/44 runs $200. The OD is .009" which is same as 32 AWG single .0088".

[Joe Gwinn]

People are not saying that the balanced input from a low impedance coil cannot work, they are just pointing out the details. Specifically, center tap probably not required, and one must also use a low noise amp, because once you knock down the induced noise, the opamp noise will come through loud and clear.

Joe Gwinn

[peskywinnets]

People are not saying that the balanced input from a low impedance coil cannot work, they are just pointing out the details. Specifically, center tap probably not required, and one must also use a low noise amp, because once you knock down the induced noise, the opamp noise will come through loud and clear.

Joe Gwinn

Hi Joe.....no the bit I take umbrage with is someone directly saying I'm wrong.

I never said you need a centre tapped coil to take advantage of common more rejection...I'm saying that I used one in conjuntion with an instrumntation amplifier arrangement....and the lack of noise on the circuit's output is very noteworthy. For that bit, (which is the significant bit of info), I'm not wrong.

Perhaps I should have reinforced my point by outlining that I use PWM variant LDO regulators (becuase they're very efficient wrt battery power) ...but these LDOs are somewhat noisy. If you bias a high gain traditional opamp arrangement at around 1/2 VCC with the voltage derived from one of those & you're gonna get a signal swamped in noise (*especially* a signal derived from low limpedance pickup - eg one with just 200 windinws)

By using the noisy regulator as the reference voltage to bias the opamp through each 'coil leg' via the centre tap, thje regulator noise becomes 'common' & is therefore much reduced.

I'm all for learning new stuff, so if someone can link me to a traditional opamp amp setup (eg non instrumentation variant) utilizing a 'virtual ground' biasing - and which gives equally good 'reference voltage' noise rejection.....then, aye, that'd be grand.

[Steve Conner]

I agree that the center tap is pointless. It doesn't do any harm but it's not needed either.

Just get yourself a Vcc/2 reference, and hook one resistor from it to each input of your instrumentation amp, then dangle the coil between the two inputs. Same rejection of reference noise, because the noise is still common mode. No center tap needed.

Put another way: It's the instrumentation amp that does the noise-rejecting magic, not the centre tap on the coil.

[peskywinnets]

Nicely put ....I realise it's the instrumentation amp that doing the noise rejection & not the centre tap.

Just a couple of points....

Doesn't biasing in such a manner with a dedicated resistor to 1/2 VCC on the +ve/-ve legs of the opamp add in thermionic noise of it's own? (and as such, won't this thermionic noise be unique to each of the resistors & therefore not 'common'?).

A bifilar wound coil (in principle) could possibly give better tolerance vs a resistor (even 1% tolerance resistors). And what you want for the absolute best noise rejection is the closest match (like for like) on each on the instrumentation ampifier input legs.

Not being confrontational here...generally just curious. I'll actually give this 'resistor/no CT' a try tonight & see if there's a similar result wrt noise levels on the output of the amp.

[Steve Conner]

Doesn't biasing in such a manner with a dedicated resistor to 1/2 VCC on the +ve/-ve legs of the opamp add in thermionic noise of it's own? (and as such, won't this thermionic noise be unique to each of the resistors & therefore not 'common'?).

I guess so. But, I think the impedance of the coil shorts out the differential-mode component of the noise. (The resistance for use in the Johnson noise equation is the coil DC resistance in parallel with the bias resistors: if the coil resistance is much smaller than the bias resistors, it'll dominate.)

And of course the instrumentation amp rejects the common-mode component of the Johnson noise, the same way as it rejects reference noise.

One possible problem is, the resistor arrangement may be noisier at high frequencies. The inductance of the pickup coil will make it less effective at shorting the noise as frequency goes up. At a high enough frequency, the noise resistance will be the resistance of the bias resistors alone.

But then, with a center tap and no bias resistors, what's the noise resistance at high frequencies? I don't even know how to calculate it

[CRU JONES]

Centertapping a coil is different than using 10k resisters for three reasons that I can think of! Besides, if a man wants to center tap a coil, let him!
Yo Pesky! hey man, Those guys that want you to use resisters, they don't even wind coils! You shouldn't let them bother you!

Peace,
Cru