Re: Guitar pickup output impedance

On Sat, 22 Aug 2009 09:48:49 -0400, "RichL" <rpleavitt@xxxxxxxxx>
wrote:

RS <RS@xxxxxxxxxxx> wrote:
On Fri, 21 Aug 2009 19:24:35 -0400, "RichL" <rpleavitt@xxxxxxxxx>
wrote:

Oh WTF, it's Friday, hectic week at work, but I had a little time on
my hands, so I made a PDF of the response graphs I made
corresponding to the bleed circuit and uploaded them:

http://www.geocities.com/rpleavitt/Bleed_circuit_graphs.pdf

Nice work! Do you remember what values you used for pickup
inductance/resistance and cable capacitance? This was a model of your
P92's, right?

Thanks! Yep, the Fralin P92s. Fortunately I saved the file in which I
desribed what I did. Pickup: resistance = 8.52 k, inductance = 7.41 H,
capacitance = 150 pf. Cable capacitance = 480 pf (30 pf/foot, 16 feet).

Very good. If I get back to writing simulation stuff, the treble
bleed network may be at the top of the list. I'll try those values as
a starting point.

The resistor also subdues the high-end peak/boost, which is why I've
used it. Your curve corresponding to lowest volume setting has a very
pronounced peak. The -20db curve peaks sharply too, with surprisingly
little boost of frequencies above that peak. Not sure I follow why
that would be.

Me either, off the top of my head. It's been almost 3 years since I did
the calculations. I'd have to think about it some more.

When I've used those circuits, I hear more highs above the resonant
peak. That's one of my problems with that. I was considering some
multi-stage thing to filter them more than the series resistor does.
Tough to do much with passive, lossy circuitry though.

Finally, cap + series resistor + parallel resistor. The "Q" of the
peak gets larger as you turn the volume down but the peak frequency
pretty much stays put, and more importantly, the bass frequencies
are falling off at roughly the same rate as the other frequencies
with decreasing volume.

The parallel resistor is effectively changing the taper of the pot,
which I never cared for, but I suppose your 500k value is pretty
conservative.

Yes, you're right, it does change the effective pot taper but as you say
with 500k it's not drastic and to me it's a price worth paying to get
other desirable features.

The taper on most Fender pots results in around 1/3 or 1/4 resistance
at 1/2 rotation, so I imagine that you could enhance that further by
going with a 1/10 taper and lowering the resistor value even further.
That is, if it's doing what you want.

So no the curves don't exactly retain the same shape but to the ears
it's doing the job I intended.

I just went back over my notes from the resistor/capacitor
substitution box experiments, and I did end up with larger cap values
than I remembered. Closer to the values you derived from your
simulation (560p + 82k, I believe). But those values now sound too
shrill to me.

So maybe this is the inspiration I needed to revisit that and write
the modeling program. Cool to see that your graphs landed so close. I
didn't expect that.

I'd like to fuss with this stuff a little bit more. At the time, I was
sort of under a time crunch, I had to get the final circuit to the
luthier fairly quickly so I didn't have the luxury of investigating a
lot of other possibilities. But now I'm thinking of modding the circuit
(and getting new pickups) for a Highway One Strat that I have, and I
really don't like the stock sounds from it. I figure if I'm going to
swap out pups I might as well take a more careful look at the circuit,
not only treble bleed, but also some sort of mid-range control. So I
hope to be digging back into this in the not-so-distant future.

That may merit a more automated approach to modelling. How are you
going to do the mid control...inductors, I presume?

.

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