Relationship between number of turns/inductance/AC Resistance

[achase4u]

Merry Christmas and happy holidays Gentlemen!

I'm not a science buff, this is clear. So I use my ears for testing, and numbers for consistency.

However, there comes a problem when switching spools of wire. For instance, one cant simply say 5.6k on one spool of formvar is the same sound as 5.6k on another, all things being equal. I remember going from REA's formvar to Elektrisola's skinny formvar, and I noticed how much brighter it sounded when winding the same. So I got my Extech LCR meter, and the big difference was inductance, much lower on the brighter sounding wire.

So how does one adjust for each spool? Simply the same number of turns per recipe? For instance, I used the older REA PE for my original later 50's style A5 Tele pickups. The sound I liked ended up being about 6.3k or so, and 3.0H with the plate on. This was at approx 8800 turns.

Now when I wind 8800 turns, same magnets etc, it comes out to more like 6.9k, 3.0H. Obviously, turns and inductance are the same, but ACR is quite different. This must mean the treble is cut more on the 6.9k, right? So how do you find the proper balance? Do you have to back of the number of turns and compromise? I cant simply go back to 6.3k, because that is a much much lower inductance, and that sounds different too.

I really hate re-adjusting recipe's with my ears each time I get a new spool of wire...

[FunkyKikuchiyo]

I really hate re-adjusting recipe's with my ears each time I get a new spool of wire...

Join the club! Every time I change one thing, I am completely rethinking everything else. If I look at it as a learning experience then it is extremely exciting and reminds me why I love music technology. If I'm obsessing over perfecting a design, I want to hit my head on something.

I can't give a perfect scientific answer, but here's what I got from what you wrote... do you mean DC resistance? Those numbers sound to me like DCR and not ACR - I've honestly never bothered reading ACR on a pickup. I could be wrong, but I thought I'd clarify. I've had a turn counter since I've started winding (huge life saver), and consequently have found absolutely no use for DCR except in making sure my solder joints on the leads burned through the insulation. Short of going crazy with some serious high tech equipment, the only useful number I've found is the inductance because it seems to have a correlation with the resonant peak. Understanding how resonant peaks work in an instrument, inductor, or anything else for that matter, can help me understand what it is I'm hearing. If it is in the audible range, it will provide the "voice" of the pickup... wood will have resonant peak(s) too, but in different ways. That is why instruments have hot spots and dead spots. If the peak isn't in the audible range, then your ears can play some weird tricks on you. I've had that problem a few times - measuring the inductance gave me some insight. Typically the higher the inductance the lower the resonant peak, but I don't think there are any constants or coefficients to determine exactly what inductance will provide what resonant peak.

Somewhere online there is an excel spreadsheet with inductance measurements of a myriad of pickups. I've had the fortune of playing almost all of the pickups on that list, and in looking at the figures, the inductance was the only one that seemed to correlate to what my ears were telling me.

[Mike Sulzer]

However, there comes a problem when switching spools of wire. For instance, one cant simply say 5.6k on one spool of formvar is the same sound as 5.6k on another, all things being equal.

It takes fewer turns of thinner wire to get a given resistance. You will also get a lower output and inductance. The resonance frequency of the whole circuit moves up and this usually results in a brighter sound.

Counting turns is better.

[achase4u]

Thanks guys - well it seems the Extech LCR measures AC R not DC - I think Possum told me it was more relevant in guitar pickups?

The way I've been looking at it is inductance mostly. The most I could get pickups to sound the same with different wire was to match the inductance - though they still had something different about them.

So I go by number of turns first, then make sure inductance is about the same...

[Mike Sulzer]

The way I've been looking at it is inductance mostly. The most I could get pickups to sound the same with different wire was to match the inductance - though they still had something different about them.

Yes, different size wire even when wound to give the same inductance would be expected to sound somewhat different because the resistance is different. This means that the resonant peak does not have the same height or width. But in practice the difference might not be too large.

[achase4u]

So basically. if any one aspect is different, itll sound somewhat different. You cant just adjust and get the same thing again, because that specific sound only happens at that particular resistance and inductance... I guess going by inductance gives me the best shot though...

[Mike Sulzer]

So basically. if any one aspect is different, itll sound somewhat different. You cant just adjust and get the same thing again, because that specific sound only happens at that particular resistance and inductance... I guess going by inductance gives me the best shot though...

Right, but it might not be such a big deal. For example, if a pickup were a bit bright because its resistance is a bit lower, one could put a resistor in series with the pickup to make up the difference. This would not be exactly the same thing, but it would be very close.

[Steve Conner]

Typically the higher the inductance the lower the resonant peak, but I don't think there are any constants or coefficients to determine exactly what inductance will provide what resonant peak.

Correct. The self-resonant frequency depends on the inductance, but also on the self-capacitance of the coil, which is another independent variable, an extra piece of information.

Self-capacitance depends on turn count, wire gauge, kind of insulation material and winding scatter.

There's no reason why you can't measure the resonant frequency. Duncan list the DC resistance, inductance and resonant frequency of all of their pickup models, and those are the three main degrees of freedom of pickup design, if you like.

[John Carlsen]

Hey guys. This may seem like a total n00b question but how do you measure inductance? Is there a way to do it on the same multimeter that i check dc resistance? Also, how does one go about testing the ACTUAL output (power) of a pickup? Thanks!

[FunkyKikuchiyo]

Hey guys. This may seem like a total n00b question but how do you measure inductance? Is there a way to do it on the same multimeter that i check dc resistance? Also, how does one go about testing the ACTUAL output (power) of a pickup? Thanks!

The way I did it... bug an engineer friend to loan you a really nice LCR meter and don't give it back for a long time.

You need a different meter which will almost definitely cost more than your multimeter. LCR meter... L = inductance, C = capacitance, R = AC resistance. There is one that is the "message board standard" here, an Extech something or other... I'm yet to use one and no one got me one for Christmas. The ones I have used have been okay, but you want to make sure you can control what frequency at which you measure inductance, or else the data you collect won't be all that useful; at least I didn't find them useful. Without my notes in front of me, I think I made my measurements at 1 kHz. In my experience the cheap ones don't let you do this. They are wonderful little units... the more you learn about electricity and sound, the more uses you'll find for the thing. If you're super nerdy you can go through your box of capacitors and see how close the tolerances REALLY are!

Wow, I'm a dork...

On the output... there is another thread that is currently a flame war and I don't want to move it over here. I will say personally that I very rarely see someone collect data on "output" that I find to be genuinely empirical because you have to control many things such as pickup height, string gauge and pick attack.

Steve... how do you measure resonant peak? I've seen abstract data and different results from the same pickups, and the different data comes from different methods. It seems just hold it to a little tone generator would be the most effective thing to do, right?

[Steve Conner]

There are two methods of measuring resonant peak. I've used both, though admittedly on other resonant circuits, since I build amps, not pickups.

One way is to sit with a signal generator, a scope and a spreadsheet. You inject the signal from the generator into the pickup, either using a drive coil of a few turns, or a large resistor straight into the electrical side. Then you measure the output with your scope at a bunch of frequencies, type the results into the spreadsheet, and plot a graph of it. This is the method that's been used since the 50s.

If you're in a hurry you can just twiddle the generator until you find the frequency that gives the biggest signal, this is the resonant frequency, assuming that the pickup has one.

The other new-school way works the same, but you drive the pickup with white noise and connect the output to a FFT analyzer with averaging. It draws the whole response in seconds. The required software is available for free, but interfacing the pickup to your computer soundcard is a little trickier. The details of interfacing probably account for the differences in results between different people. Maybe some day I'll make an adaptor box which about 2 people will buy.

Either way you get a graph which is basically the standard 2nd order transfer function, and you can read off resonant frequency and Q from it.

[FunkyKikuchiyo]

That's how it was suggested to me earlier Steve... Thanks! I want to say that I have read a few other ways of doing it that made much less sense to me. Maybe that is what was throwing me off.

I can't retrace my steps, but my resonant frequency data has often been misleading. For example, I can find readings on a p-bass pickup that will give it a sky high resonant peak, but the inductance is still relatively high. Does that mean that someone is reading some secondary harmonic of what I might subjectively interpret as the resonant peak?

[peskywinnets]

The way I did it, was to pay some Chinese bloke on Ebay a tenner (£10) & he sent me one - at that price it's a no brainer....plus I get to contribute to global warming (anything that warms up the UK is a bonus in my books)