Re: Guitar pickup output impedance

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).

The first graph shows the responses with no bleed circuit at all.
This is a good indicator of why a lot of folks want one in the first
place. That resonance peak (just a little over 2 kHz in this case)
is what gives a guitar with passive pickups/circuit its "character"
when the tone control's wide open, but notice that even with the
guitar at 3/4 of full volume, the peak disappears.

Next graph shows what happens with just the capacitor. First the
peak moves to higher frequency (which is why a larger cap value is
better if you're not going to use resistors), but more importantly
notice how quickly the low frequencies fall off as you lower the
volume.

Next shows the cap and series resistor. What the resistor
essentially does is keep the peak from moving so much as you turn
down the volume but you still have the issue with the bass
frequencies.

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.

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.

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.


.

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