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I understand that it is not uncommon to use a "Y" cable to plug two similar dynamic mics into a single input in certain situations (e.g. individually miked rack toms). What happens if two dynamic microphones with dissimilar impedances are connected in this way? Does the impedance mismatch affect the relative sensitivities of the two mics?

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    "I understand that it is not uncommon to use a "Y" cable to plug two similar dynamic mics into a single input in certain situations (e.g. individually miked rack toms)." I'm not sure where you got that understanding from, but I've never even heard of anyone doing this in about 30 years of recording and engineering. Commented Jul 1 at 19:17

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I don't think using a y cable to combine 2 mics is that common, I'm in agreement with Todd Willcox's comment that I've never actually seen that in many years recording and engineering.

But, if the y cable is correctly wired to combine the 2 mics, so their ground, +ve and -ve (ie the signal, and the flipped signal) are all connected accordingly then it shouldn't pose much of a problem. Mic inputs are designed to have a fairly high input impedance (relatively).

The ideal state (for signal transfer) is a low impedance source (the mic) feeding a high impedance input (the mic pre), that way the most voltage is dropped in the circuitry you want the signal to be passed to. This is not the case when you want to use electrical energy as some kind of power, where you want the impedances to be matched, but this is not relevant with transferring a signal to an amplification stage.

You need to know what output impedance your mic has, but most recording mics these days have a low output impedance, 50Ω - 1000Ω. Going into a mixer with a similar or roughly higher input impedance, say 1kΩ - 10kΩ, will be fine for one mic. Combining 2 mics, well, it would depend heavily on the circuitry of the mic in question but at most the impedances would sum, even in that case they would likely transfer fine to a mic pre stage. They may even combine to represent a lower impedance than either mic has individually, which would slightly improve transfer.

Generally, bad sound from impedance mismatch starts to become noticeable when the impedances are out by a few order of magnitudes from ideal, so I wouldn't worry too much, just try it and see if it sounds odd.

The range a mic pre is expected to deal with will probably not be challenged with two normal low impedance mics. If you are using piezo etc, well thats a whole different area where more knowledge of the circuitry would be needed to predict what would happen..

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By using a Y cable, you are placing the microphones in parallel, in terms of an electric circuit. This means that if two like microphones were connected, the amount of current flowing through both microphones would be halved (equal amount of current through each microphone). The overall circuit resistance would also be halved. Now resistance and impedance are not entirely the same, depending on the active and/passive components used in thr microphones. You certainly don’t want to try this for microphones that use phantom power, without ensuring you are providing enough power. Dissimilar microphones would complicate this. Assuming you are using the same passive microphones, the sensitivity of the microphones with less current throughput will affect signal gain. Over driving the circuit may cause signal quality loss (distortion) and damage components. Reduce the gain to zero or power the mixer off before connecting them in to the Y cable. Do the same before unplugging them. Some mixing desks will cope quite well because they are designed to deal with a much broader impedance range, and microphone arrays. The overall impedance will likely not change linearly when using different types of microphones. As the components will react differently and alter the current flow accordingly. Even if the mixing desk can handle the different impedances, it doesn’t mean your microphones will perform as well in parallel.

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