Impedance, Witchcraft, and other such Tomfoolery.


“Impedance is witchcraft”
a phrase I’ve repeated often.

Ok so for a while I’ve struggled with getting accurate measurements of this with my spectrum analyzer.  The output is a buffered signal, and every time I tried to measure the differences in the impedance settings I just came up with this.


Screenshot 2018-02-07 07.00.45


Small differences in overall output volume, and a slight roll off in the really low frequencies.   I know there has to be more going on here,  But how do I measure it?  Helix wants to see a pickup here, and my spectrum analyzer only puts out a buffered signal.  Well I got an idea from watching a video interview of Tor from TC Electronic.  He was saying that the way that toneprint actually works is that the pickup in your guitar actually picks up the magnetic field fluctuations from the magnets in your phones speaker.   This got me to thinking of how I could replicate that.  The best I could come up with was this. Its two pickups taped together.  The first one is connected to the output of my spectrum analyzer, and functions as the speaker coil, and the other picks up the signal and goes into the guitar input of helix.



I can now switch the input impedance in Helix and see changes.  While I realize I can’t accurately determine exactly how many dBu difference there is in each signal because this is far from a calibrated setup.  I can measure the differences which is really what I’m after.  So after testing this here is what I came up with, and my ears back up the data.  Which is about as scientific as I can get with this experiment.

Screenshot 2018-02-07 07.15.53

The solid blue line at 5dBu is the test tone, and ignore the small spike at 60hz as I know this to be an anomaly within my test equipment.

This is much different data than we saw with the buffered signal.  The purple line at the bottom is the 10k Ohm setting and the yellow one at the top is the 1M ohm setting.  Auto setting just changes the impedance based on what the first block in your patch wants to see.   For example a fuzz face wants 10k Ohms, so helix switches it to that when you have a fuzz first in your chain.  It is important to note that it doesn’t matter if the fuzz is on or off.  Helix still switches the impedance.

On a more interesting note in addition to the 6dBu …ish average (it changes depending on frequency) difference, we can also see a pronounced high frequency roll off centered at around 2khz as we decrease impedance from 1M Ohm down to 10K Ohm.

Im just glad I was finally able to measure this and it verifies what my ears have been telling me.


Thats all for today .


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