I own two dynamic microphones (consumer grade ones with unbalanced outputs an 3.5mm plugs) both are 600 ohms.
the few pro or semi-pro mics I've handled were also labeled 600 ohms.
as long as the mutual inductance is above the impedance of the source and sink yes - a VHF balun won't make a good audio DI unit.
Bye. Jasen
It is a balance between minimising the load on the the mic and keeping noise under control. The optimum noise performance of the original descrete class A Neve mic pres was about 4.8K. A 2:1 transformer gives you 6dB of noise free gain and an input impedance of 1.2K plus all the other benefits of truly balanced floating inputs.
Ian
"Ian Bell"
** Utter nonsense.No such balancing act exists.
** Bollocks.** Even worse bollocks !!!
A 2:1 step up tranny provides no noise advantage * AT ALL * since it increases the source impedance as seen by the pre-amp by 4 times and so
*doubles* the noise voltage along with the signal voltage.Where the hell do folk get these WACKY ideas ???
** The input impedance is around 1200 ohms direct with most mic pres.Inserting a 2:1 step up REDUCES the load seen by the mic to 300 ohms, likely cutting its output voltage in half.
The net effect is a poorer signal to noise ratio.
....... Phil
Hello Jasen,
Excuse me for butting in, but there is a difference between the advertised impedance and the actual impedance. If you measure the AC microphone impedance, you will see it is usually lower that the advertised value and also varies across frequency. Further, unbalanced microphones are typically higher in impedance than balanced. 600 in the norm for unbalanced and 200 for balanced. I good rule of thumb used for years is the preamp should present a load of at least 5x the rated microphone impedance. This rule prevents frequency coloration of the signal due to the frequency dependence of the actual source impedance. Dynamic microphones have the most variation over frequency, and capacitor or electret microphones, the least.
A nit on your statement quoted above, I assume you meant 'magnetizing inductance' rather than 'mutual inductance'..
David
Hmm. If you go to the AKG or Sennheiser sites and check out the specs there most are around 150 ohms with a recommended input impedance of greater than 1k.
--
*Some days we are the flies; some days we are the windscreen.*
Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.
Check your source information, there might be errors in your statement. I'm talking about the thermal noise created in the resistive part of the dynamic microphone. This is the metohd used by most european broadcast companies to evaluate mic preamp performance. Best noise figure can be obtained if the power matching is correct: source and load impedances must be matched: Maximum power transfer from microphone to preamp, and reduction of the noise source impedance to 50% of the original. This theory is used in RF design too.
Btw, the best noise figure I've measured so far was from a TELEFUNKEN V-76 tube amplifier. 0.5dB this preamp has high turns ratio input transformer (about 1:50) feeding a pentode amplifier.
Not so nice terminology, but this time correct information. Correct impedance can be found from manufacturers websites and international standards. I checked my sources:
- MIC input source impedance from 0 to 200 ohms (N10, page 6)
- MIC input source impedance for noise measrurements: 200 ohms (N10, page 14)
- Nominal impedance of NEUMANN U-87: 200 ohms.
- Minimum load impedance for U-87 microphone: 1 k ohms
Sources:
- Elements of infrared technology, page 236
- N10 Nordic technical recommendation 3rd edition, oct 1983
Further reading:
- Mathematical noise modeling and analysis of some popular preamplifier circuit topologies. T.F. Darling, J.AES Vol 35, 1987
- Noise of sources, John Maxwell, National Semiconductor 1977
Matti
"Matti Adolfsen"
** Check the source of your's - f****it.It needs wiping immediately.
** So am I.But I actually know what I am talking about.
** Absolute BULLSHIT !! ** Absolute BULLSHIT !!** RF theory must not be applied to audio..
The world of audio uses "max voltage transfer" when linking sources to loads.
You are a moronic ass.
PISS OFF.
......... Phil
A typical modern console has around a 2k ohm impedance mic input.
'Rule of thumb' is to load with 10x source impedance.
Graham
A typical modern console has around a 2k ohm impedance mic input.
'Rule of thumb' is to load with 10x source impedance.
Graham
Please do your sums properly Don before making gaffes like that !
Graham
Loads of the inexpensive but half decent Asian mics are genuinely 600 ohms.
Graham
** How hysterical.
Dopey Drawers Pearce's worst gaffes ARE his erroneous sums.
Plus his INSANE insistence that RF and audio are the same !!!
.......... Phil
Thank you! I did make a gaffe. The actual figure for the Neve noise figure is about 3dB. That is still unforgivably poor for high end kit
- it is in fact no better than my little Behringer. Ten years ago I was designing satellite receivers working up at 12GHz. The noise figure I was working to was 0.3dB.
The last audio preamp I made had a noise figure of about 0.5dB, because I was willing to use multiple parallel discrete transistors for the input circuitry. Making it any better than this would have been possible, but unwarranted because unlike the satellite receiver, it wasn't pointing at a cold sky, but a warm microphone.
d-- Pearce Consulting http://www.pearce.uk.com
"Don Pearce" Poopie Bear
** A *f****ng stupid* one.Like hundreds of others and another one, right now !
** The published curve shows it is typically less than 2 dB. ** It is SFA additional noise in practice.PLUS none at all when a typical condenser mic is used, as is the norm.
** Who gives a rat's f*ck ?Go stick you irrelevant & erroneous RF s**te up you f****ng ARSE !
You are NOTHING but a trouble making, posturing, pommy
ARROGANT PIG - Don Pearce.
** Bet the ASININE FUCKWIT matched the source and load impedances.Then miscalculated the **REAL** noise figure.
ROTFLMAO !!!
......... Phil
Thanks for that. I did have a quick glance at some specs for modern mixers but those I looked at seemed shy of quoting the mic input impedance.
Yup.
--
*To be intoxicated is to feel sophisticated, but not be able to say it.
Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.
No problem, we all goof up from time to time. ;-)
Indeed
Care to name which ones you were using ?
Back in the days when I was at Neve, the then V series ( Mks 1 and 2 ) consoles ( and just about everything else except the digital console ) had a mic pre using a step up transformer and a 5534. The quoted noise for that was a rather poor
-126dBu and it didn't actually measure any better either IIRC ! I was somewhat surprised to say the least.
The recent mic pres I've done ( quite economy types ) manage about -128.5 - as long as you factor in the extra little bit to account for the true noise equivalent bandwidth of the measurement set : -3dB @ 22kHz 4th order is about 23kHz NEB.
Graham
ohms.
Yes - I have a box of old MAT-01s from PMI. They are strictly reserved for such projects. I don't know if they are still available.
( a-126dBu
surprised to
The 5534 is not bad, but I wouldn't say it is the quietest way of doing things. I had to make a very small preamp (just one op amp) for a high impedance (50k) microphone. I searched for ages for quiet op amp before I realised that an OP27 is optimized pretty well perfectly at this impedance, with an excess noise of only about 1dB. Amazing!
long
I really wish noise was expressed as a noise figure, rather than a level. That way it wouldn't matter what impedance you were using, you would simply have a figure of merit that told you how much worse the pre was than theoretically perfect.
d-- Pearce Consulting http://www.pearce.uk.com
Actually, as a follow-up, following some discussion in r.a.p , my most recent mic pre featured a modest increase in input Z to 2.5k. I note from the links posted here that Rupert has gone as far as going to 10k now, which did surprise me a little. I'd like to spend some time listening to the effect of loading on various mics actually.
Graham
"Poopie Fuckwit Bear"
** Wank, wank ,wank ,wank, wank ,wank ..... ** The simple reason was given - you blind as a bat ass.The mic input on that unit doubles as a balanced line input.
....... Phil
ohms.
I recall the beast.
Somewhere I think I have some of those similar Nat Semi parts that featured multiple devices on-die. Forget the part number now. Oh no - I *was* right - the LM394 - just checked in case. Though that would be an IC but the M just means monolithic. They're not even insanely expensive now !
Also took a look at some esoteric fet data a while back. Noise somewhere down in the
500pV/sqrt Hz region. Interfet is the company.consoles (
using a
-126dBu
surprised to
Indeed not. They could have used something from AD or PMI and instantly improved the noise figure.
They're good op-amps. Never had the budget to design them into anything though. :-(
as long
Hmmm, I wonder how that would go down with those who 'cheat' by using a 150 ohm source instead of 200. I note that Mackie ( I think ) is now quoting noise with the input
*shorted* too.Graham
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