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Author Topic: LTSPICE - pickup blend pots
Digital-
Larry

Posts: 192
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Post LTSPICE - pickup blend pots
on: August 16, 2012, 16:49
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Getting back to my P-Rails! I've finally tracked down at Seymour's web site, SOME useful information, BUT NOT ALL, about these pickups.

https://www.seymourduncan.com/products/dimensionpages/p_rails_specs.shtml

Bridge P-90 coil has 10.2 k ohms DC resistance, and a resonant frequency of 3.1 kHz.

Bridge Rail coil has 8.56 k ohms DC resistance, and a resonant frequency of 6.0 kHz.

Now they didn't give the inductance explicitly, so I tweaked the inductance until it gave a resonant peak at about the right place in my circuit. I've asked Duncan support to tell me what the inductance is for the P-Rails (they do list this info for other pickups). I'll update this post if and when they tell me.

Here's the schematic and response of the P-90 coil on its own, with a 500k volume and 100 pF cable capacitance (I left the tone control off).

Image

Peak is just above 2 kHz.

Image

Now, here's the schematic and response of the rail coil on its own. The values are a bit off, as the resonant peak is not in the right place, but the main point of this exercise is that one coil's resonance is "high" and the other coil's resonance is "low". If they happen to be the same, then this doesn't turn out to be a very interesting exercise!

Image

Peak is about 5 kHz.

Image

If you just short these together (parallel humbucker), you get a single strong peak located between the peaks of the two pickups on their own. The peak is closer to the peak of the coil that has lower DC resistance.

Image

Peak is just below 4 kHz.

Image

If, instead, you add a blend pot between them, then you get a range of responses. At one end, it's "more like" the P-90 coil on its own, and at the other end, it's "more like" the Rail coil on its own. However, with the blend pot in place, one coil affects the other even when the pot is all the way to one end! So a blend pot is for sure going to affect your tone and make your passive pickups less distinctive than they are when not connected this way.

In between, it is nothing like just switching the pickups together. One peak goes down while the other one comes up. When the pot is in the middle, you get two peaks. The peaks are also not as high and defined as they are without the blend pot.

Image

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If you had active pickups, that would be a completely different story. Here I have added a generic op-amp component configured as a non-inverting unity gain buffer on the output of each pickup before the signal hits the blend knob.

Image

Now you can see that at each end, the distinctive peak of the individual pickups is preserved. In the middle you get one peak going down while the other one goes up and a notch between them.

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Uploaded with ImageShack.us

My conclusion? A pickup blend knob as described would work best with active pickups.

DavidRaven-
Moon

Posts: 8
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Post Re: LTSPICE - pickup blend pots
on: March 22, 2013, 22:05
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Quote from Digital Larry on August 16, 2012, 16:49However, with the blend pot in place, one coil affects the other even when the pot is all the way to one end! So a blend pot is for sure going to affect your tone and make your passive pickups less distinctive than they are when not connected this way.

How where you modeling a blend pot? What taper was your blend pot?

On an M/N taper blend pot, when the pot is in the center, you have just a few ohms resistance between the pickup's hot side and the output, and almost the full resistance of the pot's value to ground. So it is like having a volume pot on "10". The other pickup is fully shunted to ground, as if it's volume was on zero.

The only way you would still have some signal from the other pickup is if you lifted the grounds on the blend pot.

Here's a 500K Bourns blend pot as I measured it with the wiper parked at the center detent.

Image

DavidRaven-
Moon

Posts: 8
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Post Re: LTSPICE - pickup blend pots
on: March 22, 2013, 22:11
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Oh and that P-Rail is a lousy sounding pickup. It doesn't do either thing very well, and together in series it's mud.

Digital-
Larry

Posts: 192
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Post Re: LTSPICE - pickup blend pots
on: June 13, 2013, 21:53
Quote

To answer your question #1, I modeled a blend pot in what is probably a stupid way. Therefore my conclusion must be, never have Digital Larry rewire your guitar!

Here's a thread from another forum talking about how to get the best results from an M/N blend pot.

https://www.talkbass.com/forum/f38/blend-pot-tapers-624841/

And I never did put the P-Rails in my Ibanez. I had fun thinking about it and Spicing all over the place but that's about it.

DavidRaven-
Moon

Posts: 8
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Post Re: LTSPICE - pickup blend pots
on: November 8, 2014, 15:51
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I'm in that thread posting the same info I just did here. 😉

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