I am concerned about impedance matching of a microphone circuit. This is an electret mic.
Perusing standard XLR type audio cables, I don't see any impedances mentioned. This would be handy. Anyone know what this number might be?
I would assume the output of the mic and its associated circuit would match this at typical audio frequencies.
Thanks
Fritz
Perusing standard XLR type audio cables, I don't see any impedances mentioned. This would be handy. Anyone know what this number might be?
I would assume the output of the mic and its associated circuit would match this at typical audio frequencies.
Thanks
Fritz
Except for old fashioned long telephone cables, say between cities, cable impedance is somewhat irrelevant for the lengths encountered in audio engineering. That's why you will rarely see a specification for audio cable impedance.
A typical microphone cable will have a characteristic impedance of around 100Ω ±50% or so. DMX cable, which is very similar in construction to microphone cable, has a characteristic impedance of 110Ω. (DMX signal were origninally sent through microphone cables until a specific characteristic impedance specification was adopted.)
However at the propagation speed of electrical signals in cable which is around 60-70% of the speed of light, a single wavelength even as short as that for 20kHz will be hundreds of kilometres, and 20Hz a thousand times longer.
In pre-amplification telecommunications impedance matching was used to maximise power transfer from source to load. But once there is the ability to amplify the signal at the receiving end, power transfer is not as important as achieving the maximum signal to noise ratio. From memory occurs when the load impedance is at least 3 times the source impedance, but to allow fan-out (i.e. the ability to connect more than one load) the load impedance is more likely to be at least 10 times the source impedance if not higher.
Impedance matching the cable per se has no benefit in an active audio circuit, in fact isn't even possible if the source and load are different impedances.
I'm sure someone will jump in and pull me up if I have got part of this wrong.
A typical microphone cable will have a characteristic impedance of around 100Ω ±50% or so. DMX cable, which is very similar in construction to microphone cable, has a characteristic impedance of 110Ω. (DMX signal were origninally sent through microphone cables until a specific characteristic impedance specification was adopted.)
However at the propagation speed of electrical signals in cable which is around 60-70% of the speed of light, a single wavelength even as short as that for 20kHz will be hundreds of kilometres, and 20Hz a thousand times longer.
In pre-amplification telecommunications impedance matching was used to maximise power transfer from source to load. But once there is the ability to amplify the signal at the receiving end, power transfer is not as important as achieving the maximum signal to noise ratio. From memory occurs when the load impedance is at least 3 times the source impedance, but to allow fan-out (i.e. the ability to connect more than one load) the load impedance is more likely to be at least 10 times the source impedance if not higher.
Impedance matching the cable per se has no benefit in an active audio circuit, in fact isn't even possible if the source and load are different impedances.
I'm sure someone will jump in and pull me up if I have got part of this wrong.
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OK and thanks for all the good info. I note in Galu's spreadsheet the mics shows 2.2K--same as mine (MIC_POM-3535P-3-R)
John worked out the theory nicely and that makes sense. I will be interfacing my mic to an op-amp, then into and ADC. Eventually into a Zoom recorder as well---seems like impedance matching could be difficult.
cheers
fritz
John worked out the theory nicely and that makes sense. I will be interfacing my mic to an op-amp, then into and ADC. Eventually into a Zoom recorder as well---seems like impedance matching could be difficult.
cheers
fritz
I should have added that once the cable length approaches a wavelength of the signal, without impedance matching there will be reflections caused at the cable interface. This is why impedance matching is important at RF frequencies from a few MHz up, i.e. to prevent ghosting on baseband video signals for example.
In radio's the transmitter to aerial has to be SWR'ed "standing wave ratio" so there are minimum reflections.