Rickenbacker pickups

[Les]

I'm not sure how much would be needed. You know, just running the test signal through a differentiator would prob allow an ordinary power amp to be used. Very little power is involved here....

for a 500 turn coil say 1cm deep

i=Bl/munought*n = .005T *.01/(1.26e-6*500) = 79 ma.

Diameter does not matter for the field, but does for inductance.

That's for .005 Tesla...50 Gauss

Not much, huh? It's just that driving with the brakes on by using thevenin equivalent of a current source needs higher voltages than most amps can do.

A cheap power op amp like the LM12 set up as a transconductance amp would work too. The coil would be in the feedback loop. Old servo drive amps were made this way. In fact one of my cnc machines has 5.6KVA of transconductance amplifiers in it.

But a tester coil would need a power of just a few watts. And that is mostly imaginary power because it is a reactance. Only the DCR would consume any real power. I^2*r tiny....

Oh, by the way....check out the SPICE pickup response from Greg's numbers:

http://www.rickresource.com/forum/vi...&sd=a&start=15

[Les]

Ok guys I checked a couple things on the tester.

Considering a simple air coil of about 10,000 turns a well coupled 50 gauss field gives around 125 millivolts out. So we're ok on that assumption I think.

The best bet is to build a transconductance amplifier for the tester. Can you guys build fairly simple amps with power ics? If so this is the way to go.

For an easier solution I SPICED up a low power op amp before a normal power amp that acts as a frequency limited differentiator. With this it forms an all pass for coil current. Current output is flat within +/- 1 db from 40 hz
to 15 kHz or so. There is some phase shift with this, so phase data would not be accurate unless it was software corrected. The differentiator has to be built up too, but it's just half a dozen parts that cost only a few dollars.
It could be run by a lab power supply or 2 9V batteries. If you make a mistake with wiring the effects are much less dramatic than a power amp!

With this and about a 2" diameter coil of 500 turns would complete the physical section. The coil DC resistance needs to be about 4 ohms. That means fairly fat wire...something like #32. I'll let you guys figure that out....you're the coil winders! If there are problems we can change the coil size.

The rest of the system would be a simple audio speaker test program that can do MLS/pseudo random test signals. You would then get an amplitude and phase response graph.

So....current source or regular amp with a differetiator?

As for my higain to toaster converter.....it's designed. Ordering up the parts now.

[salvarsan]

Hey, Les...


Whoa.

50 gauss was what I saw on a 2-D magneto-static simulation.
Got no bucks for a $$$$ 3-D magnetodynamic simulator.

Let's work backwards and infer the delta-gauss from the pickup output.

It's a given that you can bang on 6 strings and generate a 50mV signal
from 5000 winds of #42 around some Alnico rods that subject the strings
to a static ~150 gauss at 3/16" from the rods.

If we can infer the delta-Webers/sec, then can we infer the delta-Gauss?

-drh

[Les]

I thought the 50 gauss was a given. I'm not sure I can deduce anything
from the static gauss and N dphi/dt.

One easy way to confirm here is to take just any old air coil and run a single tone through it from any old amp. Just make sure the DCR of the coil is at least a few ohms so as not to short the amp. Record the current through it. (oscope or ac voltmeter with a shunt is fine) Something will come out of the pickup. Record that voltage.
We need to know the turns on the coil. That's enough information to find out ecactly what is needed. can you do that?

[salvarsan]

I thought the 50 gauss was a given. I'm not sure I can deduce anythingfrom the static gauss and N dphi/dt.

One easy way to confirm here is to take just any old air coil and run a single tone through it from any old amp.
...
can you do that?

Ayup. Ripped the voice coil out of a crummy 2" tweeter,
got an executable of sigGen built. Will let you know...

-drh

[Possum]

....

this is all kinda old hat here. Here's the classic work on this stuff:
http://buildyourguitar.com/resources/lemme/

[salvarsan]

this is all kinda old hat here. Here's the classic work on this stuff:
http://buildyourguitar.com/resources/lemme/

What part of using MLS signals and deconvolution to pulse response and spectral distribution did you not understand?

hint: perhaps they aren't the same thing as Lemme's overpriced resonant freq tester.

-drh

[Joe Gwinn]

What part of using MLS signals and deconvolution to pulse response and spectral distribution did you not understand?

Umm, cool it. The fact is that probably 90% of the people on this forum have not a clue what the above sentence means, or how it differs from Lemme's approach, and are none the worse for it. So a better approach is to write a tutorial.

[salvarsan]

Tutorials are, not merely a good idea, but excellent in practice.
I recommend them to everyone, and would ask any who are able
to help in writing them, perhaps even to locate them at www.pickupedia.info.

Three people can't do it alone.

-drh

[Possum]

....

I didn't actually read this thread, just noticed the same old thing being discussed again....

what I meant was this subject has been done before here. I've used a computer MLS frequency analyzer and used Lemme's drive coil method before. The drive coil method is the best way to go, my drive coil I think is about 5 ohms, it needs to be real low or you get skewed results. Ultimately what I ended up was alot of pretty little charts that are pretty much useless. The majority of guitar pickups out there all produce very similar charts. I've also used a pulse software used to test speakers and same thing. Top that off there is stuff you just can't measure and most of it happens in the higher frequencies and isn't discernible on charts, only hearing tests work there.

What I was hoping for and never found was something like an LCR meter that has a 10K test signal that would read AC resistance and inductance in that range but Joe says it all falls apart in testing pickups in those ranges. What I want to see is NUMBERS in higher frequencies, charts are useless. Even if something like that existed, for making pickups there is stuff you can't measure, period. How do you measure "vocal" type tones on a meter? How do you measure quack?

I don't regret having messed with this stuff, it was fun but I don't use it for anything, very rarely I will though. Its unlikely I would build something as I am not a tech and don't read schematics and don't have time for that stuff really. The LCR meter and Joe have taught me enough that I know how to manipulate coils in a pickup to move them toward what I want, so the LCR meter is my primary tool but again, its what comes out the amp at a gig thats the final word :-)

[black_labb]

I didn't actually read this thread, just noticed the same old thing being discussed again....

what I meant was this subject has been done before here. I've used a computer MLS frequency analyzer and used Lemme's drive coil method before. The drive coil method is the best way to go, my drive coil I think is about 5 ohms, it needs to be real low or you get skewed results. Ultimately what I ended up was alot of pretty little charts that are pretty much useless. The majority of guitar pickups out there all produce very similar charts. I've also used a pulse software used to test speakers and same thing. Top that off there is stuff you just can't measure and most of it happens in the higher frequencies and isn't discernible on charts, only hearing tests work there.

What I was hoping for and never found was something like an LCR meter that has a 10K test signal that would read AC resistance and inductance in that range but Joe says it all falls apart in testing pickups in those ranges. What I want to see is NUMBERS in higher frequencies, charts are useless. Even if something like that existed, for making pickups there is stuff you can't measure, period. How do you measure "vocal" type tones on a meter? How do you measure quack?

I don't regret having messed with this stuff, it was fun but I don't use it for anything, very rarely I will though. Its unlikely I would build something as I am not a tech and don't read schematics and don't have time for that stuff really. The LCR meter and Joe have taught me enough that I know how to manipulate coils in a pickup to move them toward what I want, so the LCR meter is my primary tool but again, its what comes out the amp at a gig thats the final word :-)

did you try graphing the differences between the pickups? i think that would get some more meaningful results.

[Possum]

..

Thats exactly what I did, if you mean did I take numbers readings off different points in the charts, no I didn't do that. I only saw noticeable differences by comparing radically different types of pickups like a low wind strat pickup against a humbucker, then you can see whats happening. but when it gets into high frequencies they tend to track the same up there. The only real use I got out of this stuff was when I was working on some blade pickups for tele neck. I had about 6 prototypes and listening to them didn't help because a couple of them sounded nearly identical. Using the charts I could see one had a slight bit more midrange, you really have to overly one chart on top of another or you'll never see the differences. I would love to have some rock solid method of seeing exactly what happens in higher frequences using specific test frequencies up there, but I never found anything that was affordable. Anyway, I'm not alone in my opinion of this stuff, Kinman, is a real technical guy, says on his website he tried all this stuff and abandoned it eventually for the same reasons I did.

[salvarsan]

Simply, I don't think we are paying attention to the same things.

For example, the differences between strat-style alnico 5 and alnico 8 coils
are pretty obvious when you plot output at different pulse drive levels.

You can infer relative bass response from droop in square wave response.

I won't argue whether or not that is useful to _you_, but I like being able to
see subtle things in a pickup without having to mount it and bang out a few
Townshends worth of chords.

(Is that a measurement unit? A Townshend?)

-drh

[John_H]

1 Townshend equals 250 Framptons

[Possum]

....

Framptons aren't being printed anymore thank god.
I understand, if you are looking at radically different ingredients like A5 and A8. But what I was more interested in was say adding 500 more winds of wire. You almost can't even see that difference on a chart. But you can hear it.

These guys did some good work on guitar pickups, taking geek-dom to new levels. Guess if you have school money buying your toys you can get pretty crazy with this stuff:
http://online.physics.uiuc.edu/cours...p_results.html