"Variable" Impedance DI box KK pickups?

[James May]

Originally Posted by jonfields45

The time constant that might be interesting in this hunt for a low pass filter is the input impedance of the preamp, resistive, times the total capacitance of the PUP.

I agree this is the interesting time constant. Not to belabor the point any more than this, but I believe this time constant can only result in a highpass filter response (bass rolloff), since the pickup capacitance is IN SERIES with the signal source, and the preamp's resistive impedance is IN SHUNT. My post #18 explains this (I hope!).

[Cuki79]

Originally Posted by James May

I don't have samples handy to measure, but I suspect, based on their behavior, that they are similar to the Fishman AG UST, likely 300-500pF. The coaxial cable used in some pickups is even less at about 1nF per meter.

Thanks James,

good to know.

Cuki

[jonfields45]

Originally Posted by James May

I agree this is the interesting time constant. Not to belabor the point any more than this, but I believe this time constant can only result in a highpass filter response (bass rolloff), since the pickup capacitance is IN SERIES with the signal source, and the preamp's resistive impedance is IN SHUNT. My post #18 explains this (I hope!).

I think your post was a good simplified lumped parameter intellectual exercise, but I've experienced that low pass filter first hand and think a more detailed model might show that the capacitance of the heads becomes a significant ground return at higher frequencies when the load impedance approaches parity in the pass band. I also understand your argument that the capacitance of the parallel heads is canceled since they are all modulated in-phase with the same signal.

But the bottom line is neither of us did any more than wave our hands at the problem (no simulation and no detailed model) and the manufacturers of these pickups are not inclined to be all sharing a common hallucination which I did encounter with a K&K Mini into a Mackie mixer that had a 10K/10M input switch; 10K was high pass, and 10M was low pass until I put this into the mixer before the guitar cable:

https://drive.google.com/open?id=0B_...2xJQnA1Y0VZdFk
https://drive.google.com/open?id=0B_...ll0WnlldTBBblk

I used this set up for several years and was never able to successfully delete the 1M shunt resistor combined with the 10M input selection.

And at the risk of sounding a bit brash, if hand waving was all that was needed for good design, ToneDexter would not hum into a headphone when not grounded. Try a Zoom AC-2, A2, or A3 with headphones, for example. Any model that cannot reproduced observed behavior is wrong.