Hexaphonic Pickup Project

[big_teee]

You can do it like EMG does. Differential connection to a standard op amp, in this case an LM4250 (disregard what the schematic says).

[ATTACH=CONFIG]23717[/ATTACH][ATTACH=CONFIG]23717[/ATTACH]

I guess L1, & L2 is the pickup?
And, what would be a good DCR Range for L1, and L2?
Thanks,
T

[Mike Sulzer]

But why would you want to?

What the circuit does right:

It cancels magnetic hum well. The two coils put out hum signals with the same polarity which then subtract because they are connected to inputs with opposite polarity. The fact that the two coils are loaded differently does not affect this since essentially no hum current needs to flow throught the 30K resistor to the inverting input.

What it does wrong:

It loads the two coils differently at high frequencies, causing a drop in the highs from one of the coils.
The signals of the two coils have opposite polarity, and the subtraction of the diff amp makes them the same so that they should add. The voltage on the summing junction (negative input) is determined by the voltage from the coil connected to the positive input and so this is increases the current through the 30K resistor, causing a large loading effect when the impedance of the other coil gets high as it does at high frequencies. If they use medium impedance coils, this would not be too much of a problem. How many turns do they use?

You can do it like EMG does. Differential connection to a standard op amp, i

[rjb]

But why would you want to?

More basic objection:
Why use two inductors per string when one will do?

[rjb]

The slow student raises his hand...

For those who haven't followed all the external links - cycfi is using pre-wound inductors from Chipsen, and replacing the soft ferrite cores with neo magnets.

Silly questions:
1. Is Chipsen providing two different parts, or are you wiring half of them "backwards"?
2. To save you a manufacturing step, could Chipsen build the parts with magnetized hard ferrite cores? Would that work for your application?

[Mike Sulzer]

How else do you cancel the magnetic hum?

More basic objection:
Why use two inductors per string when one will do?

[Mike Sulzer]

For those who haven't followed all the external links - cycfi is using pre-wound inductors from Chipsen, and replacing the soft ferrite cores with neo magnets.

Silly questions:
1. Is Chipsen providing two different parts, or are you wiring half of them "backwards"?
2. To save you a manufacturing step, could Chipsen build the parts with magnetized hard ferrite cores? Would that work for your application?

Or, why not just add a neo magnet while keeping the ferrite core to get a bit more signal?

[rjb]

How else do you cancel the magnetic hum?

By adding the signals from two adjacent strings.
I guess I'm not the only one who was sleeping in class....

[rjb]

Or, why not just add a neo magnet while keeping the ferrite core to get a bit more signal?

Because you'd no longer have "possibly the world's smallest active pickup".

Edit:
Where would you add it?
It's a PC-mount inductor.
No room underneath.
Put a neo on top, and you've got too much string pull.

[Mike Sulzer]

By adding the signals from two adjacent strings.
I guess I'm not the only one who was sleeping in class....

Oh grow up. Adding signals across strings works fine for a bridge pickup when adding the separate outputs together. It does not work on a neck pickup because there is a gap in the response with string bending when you move into the gap between poles. It also does not work if you are using a string output alone. What is there to cancel with?

Also it works on a bass: wider spacing and not much bending.

Now go back to sleep.

[Mike Sulzer]

Because you'd no longer have "possibly the world's smallest active pickup".

Edit:
Where would you add it?
It's a PC-mount inductor.
No room underneath.
Put a neo on top, and you've got too much string pull.

Baloney. A thin small neo works fine on top. The main problem is getting it to stay there. Also there is nothing preventing anyone from designing a pc card compatible with a slightly larger neo on the bottom of the ferrite. It might take something really technically difficult, like a hole in the card.

No need to urge you back to sleep. You were never awake.

[rjb]

Adding signals across strings works fine for a bridge pickup when adding the separate outputs together. It does not work on a neck pickup because there is a gap in the response with string bending when you move into the gap between poles.

I think this has already been covered (or, at least, implied) in class.
Presumably, the hex pickup would be used near the bridge to minimize string cross talk.

It also does not work if you are using a string output alone. What is there to cancel with?

I was going to ask teacher about that.
In the assigned reading (you did read the assigned text, didn't you?), he wrote that noise is cancelled when the signals are added; I think that should read if the signals are added.
In the case of a hex fuzz, for example, I would think you'd want to "de-hum" each string before going to the effect.

Now go back to sleep.

Zzzzzzzzzzzzzz.

[rjb]

A thin small neo works fine on top. The main problem is getting it to stay there.

Possibly a trivial problem; still a problem.

Also there is nothing preventing anyone from designing a pc card compatible with a slightly larger neo on the bottom of the ferrite. It might take something really technically difficult, like a hole in the card.

To quote a wise man, "But why would you want to?"
Looks to me like you're adding unnecessary (key word "unnecessary") bulk and complexity for the questionable advantage of "a bit more signal".
Referencing the graph in post 48, looks to me like the prototype is already a few dB hotter than a Strat pickup.

No need to urge you back to sleep. You were never awake.

Yawn, did you say something?

PS-
Mike, I apologize for the "sleeping in class" crack. I meant to insert a smiley but forgot.
And fwiw, I agree with your assessment of the EMG ckt.

[cycfi]

For those who haven't followed all the external links - cycfi is using pre-wound inductors from Chipsen, and replacing the soft ferrite cores with neo magnets.

Silly questions:
1. Is Chipsen providing two different parts, or are you wiring half of them "backwards"?
2. To save you a manufacturing step, could Chipsen build the parts with magnetized hard ferrite cores? Would that work for your application?

We asked Chipsen to coil half clockwise and half counterclockwise. We also asked them to replace the core with neos. We get the magnets elsewhere and send them to Chipsen. They don't know anything about magnets and would not care about them (it's not their business). It took a *lot* of convincing to even get this far. The tradeoff, of course, is the minimum order quantity which I am willing to shoulder.

I wouldn't mess with the coil's design further, unless there's a significant reason to do so. IMO, they are in perfect shape right now. It's easy to underestimate the effort needed to bring something into production.

[cycfi]

You can do it like EMG does. Differential connection to a standard op amp, in this case an LM4250 (disregard what the schematic says).

Interesting. LM4250 is ultra low power (50uA), but quite noisy at 40 nV/√Hz. That would amount to a lot of hiss when the gain is cranked up in subsequent stages. I took care not to go beyond 8 nV/√Hz.

[rjb]

We asked Chipsen to coil half clockwise and half counterclockwise. We also asked them to replace the core with neos. We get the magnets elsewhere and send them to Chipsen.

Kewel. I had misunderstood one of your linked discussions, and thought you were having to physically pull out the original ferrite cores and replace them with neos. Didn't realize Chipsen was providing coils with neo cores. Way to go!

PS:
Sorry for messing up the classroom, uh, thread, with a food fight.