Here is the relevant data

The setup for this experiment was a Shure SM57 dynamic microphone routed into a Magneto Audio Labs VariOhm, a variable load impedance device, which was then routed into a Focusrite Saffire Pro 14. Room EQ Wizard was used to collect the data.

The purpose of my experiment was to explore the effects of load impedance on a variety of microphones. To this end, I have also included the results of the experiment using an AKG C414 B-XLS as well.

According to impedance bridging principles, optimal voltage transfer is achieved when the load impedance is roughly ten times larger than the source impedance. The results of my experiment, however, have proven me otherwise. Ideally, the voltage of the microphones at the output of the signal chain will decrease at each reduction of load impedance, but instead we see a rise in voltage up to 600 Ohms, and then a significant reduction in voltage at impedances lower than 600.

Is my understanding of impedance bridging principles wrong or is the experiment flawed?

  • \$\begingroup\$ Please show the circuit you used to do the measurements. \$\endgroup\$
    – Andy aka
    Commented Aug 18, 2014 at 10:34
  • \$\begingroup\$ This is the best circuit I can give you at the moment. If you require a circuit diagram of the setup, I unfortunately will not be able to supply that as I do not possess circuit diagrams of any of the aforementioned equipment. \$\endgroup\$
    – Long Tran
    Commented Aug 18, 2014 at 10:56
  • \$\begingroup\$ Is any of this transformer coupled? \$\endgroup\$
    – user32885
    Commented Aug 18, 2014 at 11:14
  • \$\begingroup\$ I opened up the VariOhm and found two transformers, one transformer for three impedance values each. \$\endgroup\$
    – Long Tran
    Commented Aug 18, 2014 at 11:20

1 Answer 1


The general idea is that you get maximum voltage transfer from one stage to the next if the receiving stage doesn't load the source. Therefore it helps to have the source impedance << load impedance. This isn't true for maximum power transfer where you want the source impedance equal to the load impedance, but in audio you're generally looking for voltage amplification in the signal chain.

Your anomoly is because you are measuring at the output of the signal chain, which may have a varaiable impedance device inside, but there's also lots of other circuitry in the chain. Without knowing exactly what that circutry is doing and how it affects the signal you can't generalize about the effect of the impedance provided by the VariOhm.


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