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Re: Shotgun mic Vs on-board mics - optimum noise impedance

Subject: Re: Shotgun mic Vs on-board mics - optimum noise impedance
From: madl74
Date: Sat Jul 19, 2014 4:09 am ((PDT))
> well it was not private, but I agree with you 100 per cent. matching for =
power transfer went out with human powered telephones.


No matter, I was going to reply to the list anyway to try to clear up the
confusion about _noise_ impedance matching.

It is not nonsense. If you look back on the conversation, I am talking abou=
optimum noise impedance matching. In particular, I recommended a practical=

"pile of bedclothes" noise test as a check. Forget the theory, do the test.=

Does a low impedance mic work well into a high impedance input?

You cannot assume that an input which has feedback components and is
optimised for low noise at 20K will also give minimum noise when mismatched=

to 200ohms. It may do, but you cannot make the assumption that a "consumer"=

product is designed for minimum noise from 200 ohms to 20K.

A recorder with built in mics will most likely have preamps designed for
minimum noise to match the built-in mics. Most likely it will give similar=

good results with external mics of the same impedance, but the only way to=

find out the preamp noise figure for a low impedance mic is by a practical=


Actually there is a crude test which could give a clue to the optimum noise
impedance and that is to compare the closed circuit input noise with the
open circuit input noise.

I agree that impedance matching on short cable runs is not important in
terms of power transfer, with the exception of high impedance sources and
inputs where the capacitive component of the cable impedance becomes
prominent and the HF is affected.

Just to complete the discussion, long cable runs can affect the frequency
response of the source impedance, the cable impedance, and the input
impedance don't all match. Depending on the mismatch, this can be expressed=

as reflections at the points of mismatch causing variations in frequency
response. This becomes important with cable runs comparable with the
wavelength of light and I tested my total of 800 metres of mic cables which=

were OK at 200 ohms nominal impedances.

My frequent advice in all cases of discussing theory is to experiment ad
listen to the results.

David Brinicombe

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