Zoom A3 Input Impedance Mystery

[guitaniac]

According to the Zoom A3 manual, the input impedance of both the mic input and the pickup input is 1 Mohm. However, they perform very differently when I run a high impedance passive piezo pickup to them. (The mic input has a compound jack which will accept instrument cables as well as mic cables.)

When I run my passive pickups (a Baggs Hex in-saddle pickup and a PUTW I/O UST) to the mic input, I get the tinny sounding result which I expect when running to a 1 Mohm input impedance. (With the Hex, I'd even lose some bass running to Mama Bear's 4.7 Mohm input impedance.)

When I run to the pickup input, however, it handles both of those pickups beautifully - without any apparent bass rolloff. Go figure!

I was also surprised that one of the A3's models works well with the I/O UST. That particular pickup has never worked well with Mama Bear modeling or Aura sound imaging. I presume that's because the I/O is uncommonly top responsive (for a UST) and its signal's guitar-specific character tends to clash with the model's character.

The Hex pickup (in a nylon stringer) also takes well to the A3's nylon model, but that wasn't a surprise. Its string-oriented signal has always made a good platform for modeling. (It sounded super with Mama Bear's target guitar #9, a target guitar which often gave me feedback problems with other guitar/pickup rigs. Go figure.)


In any event, I'm kicking myself for not experimenting with taking my passive pickups directly to the A3 sooner. I just presumed (from past experience with other preamps) that a 1 Mohm input impedance would cause significant bass rolloff. I can only conclude that either the A3 manual is incorrect, or that the mic input's 1 Mohm input impedance is very different in nature from the pickup input's 1 Mohm input impedance.

[jonfields45]

I think you have the physics a bit wrong...

A piezo pickup can be modeled as a voltage source (your signal) in series with a capacitor. That is also a high pass filter whose cutoff frequency is proportional to 1/RC. To get that sufficiently low for good bass response you need a larger R, or input impedance, to load the pickup. 1M ohm is generally sufficient. In theory, but not in practice as tested by Doug Young, if the input impedance gets sufficiently high, the pickup's capacitance acts as a low pass filter (your signal shorts through the pickup's self capacitance instead of going to your preamp/amp).

No modern microphone preamp will be 1M ohm impedance because that defeats some of the goodness of a low impedance balanced signal source (maximum source to load energy transfer happens when the two impedances are equal and most noise sources are high impedance which get attenuated into a low impedance load). My guess is the spec has a typo for that number and it is in reality in the 1-10K ohm range which explains why you are getting the tinny (high pass filter cut off well into the frequency range of a guitar) pickup response plugged into the mic input.

As side note, a system that matches the source and load impedance may be lowest noise, but in the case of most passive guitar pickups, will sound terrible because of the filter you get from the pickup's reactance (series inductance for magnetic pickups and series capacitance for piezo pickups) has a cutoff frequency in the active range of a guitar. So you sacrifice some SNR to get a signal you actually like and generally 1M ohm is the input impedance desired for both types of pickup.

[guitaniac]

Thanks for the comments, Jon.

I was indeed surprised that the mic input impedance was stated to be so high. It must be at least 10Kohms, however, as I have no problem with running an active pickup system (via instrument cable) to that input. Its either that, or they somehow have different input impedances, depending on what is being run (1/4" plug or XLR plug) to the compound jack.

No, I don't understand the physics of it beyond Rick Turner's rule of thumb that the input impedance of the preamp needs to be at least 10 times the impedance of the pickup at some nominal frequency (1000Hz, I'd guess). I understand that capacitive reactance changes with frequency and that the lower the frequency, the higher the capacitive reactance and the total pickup impedance. That factor accounts for the bass rolloff if the input impedance isn't high enough.

In any event, I know from past experience that a 1 Mohm input impedance (as with the AG Stomp which I've used) or even a 2.2 Mohm input impedance (as with the BBE Acoustimax which I've used) usually causes a bass rolloff with the two passive piezo pickups which I have. Beyond that, there has to be a reason why the Baggs PADI (which was designed for use with my Baggs Hex and the other passive Baggs pickups) has a 10 Mohm input impedance.

The lesson I've learned with this episode is to always try it and observe the results, no matter what the stated input impedance is and how unlikely I think it is that it will work.