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HD500 Input Z


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I took my HD500 into the lab to measure it's input impedance to see if it was "real" or just an simulated/emulated effect. Summary: it is a real resistor.


Details: I used an Audio Precision APx525 to generate a 100mV RMS signal at 100 Hz (because 1kHz caused too much cross-coupling at the higher impedance settings of 1M and 3.5M) with a 20 ohm output impedance (which is negligible). I switch in various series resistors until the signal out of the HD500 into the APx525 (either XLR out, 1/4" out, or SPDIF, it made no difference) dropped 6dB. Only the first FX block was installed in the HD500- the Studio EQ set to its default settings. The results are below:


HD500 Input Z selected / Input Z measured

22k / 22.21k

32k / 32.03k

70k / 69.8k

90k / 90.1k

136k / 136.5k

230k / 232k

1M / 999k

3.5M / 4.57M (noisier measurement than the others)


So what?


Well, the HD500 input Z interacts with humbucker and single-coil pickups in an identical way that amps and FX boxes do. Humbuckers generally like (or require, depending on your point of view) higher impedances than single-coil pickups. Also, if you're driving the HD500 from a source w/o pickups, such as another FX box or a wireless receiver, then input Z selection should have no impact on your tone (as long as the box can drive the selected input Z). Generally, lower input Z offers lower noise but is harder to drive (prone to amplitude loss or more distortion).

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Would be interesting to see if front-ending the input with a solid pre-amp would generally improve the tone dynamics

achieved at the output. Unfortunately, amongst the gear that I have, a dedicated pre-amp isn't one of them.

Could be possible tapping into the pre-amp of any guitar amplifier tho, if it has an FX Loop, that is.


Good post. ;)

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