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In electronics, 'Z' stands for impedance, which is measured in ohms. When the Low-Z switch is out, the input impedance of the Golden Age Pre-73 is 1200 ohms. When the switch is in, the input impedance is a significantly lower 300 ohms.
So what difference is this going to make?
Let's take as an example the classic Shure SM57 microphone. This has a specified output impedance of 310 ohms, which for convenience I'll round down to 300 ohms.
You can think of the signal chain like this...
The signal from the capsule of the microphone goes through one 300 ohm resistor to the output. The signal then sees another 300 ohm resistor in the preamp connected to ground. So the signal flows through 600 ohms to ground, and the preamp is connected at the halfway point.
What we have here is a simple voltage divider. Since the resistances are equal, the voltage of the signal is halved at the mid-point. So the preamp only receives half of the output voltage of the microphone. This is a drop of 6 decibels.
Now this is not entirely bad because other factors are in play that optimize the current the preamp receives, and in fact by having equal output and input impedance, the maximum amount of power is transferred, which should result in the best signal-to-noise performance.
Frequency response problem
However, there is a problem...
It is likely that the output impedance of the microphone will vary with frequency. Suppose for example that you had a microphone with a specified output impedance of 300 ohms, but in actual fact it was 300 ohms at 1 kHz and 900 ohms at 10 kHz. Where the drop in level at 1 kHz is 6 dB, the drop in level at 10 kHz is now 12 dB.
What we have therefore is a frequency response that drops by 6 dB from 1 kHz to 10 kHz. This mic will sound dull.
If you do the math with a preamp that has an input impedance of 1200 ohms you will find that the drop in level at all frequencies is less, and the difference in the drop between 1 kHz and 10 kHz is less too. In other words, the frequency response is flatter.
What you can expect from the Low-Z switch therefore is that when switched out, the frequency response is flatter than when switched in. Most likely you will hear less high frequency content when switched in, but the change will depend on the impedance characteristics of the individual model of microphone. If the preamplifier has been designed to take full advantage of the Low-Z mode, then there should also be a little less noise, but you'll have to listen quite hard to notice that.
In summary, the High-Z position will generally be the most accurate setting, but if you prefer the sound of the Low-Z button switched in, then use it. In the end, it all comes down to subjective taste.
By the way, some mics have a higher-than normal output impedance. For example the CAD Trion 7000 is rated at 940 ohms. You would expect the Low-Z switch to have more of an effect here. It is often thought that the best compromise for preamp input impedance is that it should be around five times the output impedance of the mic, so according to this principle you would need a preamp with an input impedance of around 5000 ohms for optimum results. However, pairing the Trion 7000 with the Pre-73 set to 1200 ohms might give you more of the 'darker' ribbon character. While this would probably not be the best choice as your only mic/preamp combination, if you already have more conventional mics available then it is always worth having another option in your palette of sounds.
Conversely, if you had a mic with an output impedance of just 60 ohms, then the Pre-73's Low-Z setting would be exactly right.
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