Is my LT44 transformer suitable for audio (de)coupling?

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

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.

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"

It needs wiping immediately.

But I actually know what I am talking about.

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

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.

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.

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.

-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.

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

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

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

Graham

multiple

just

They're not

Yes that is very similar. The great thing about using these discretes as front ends is that the knee frequency for 1/f noise is way lower than the transistors in the average op-amp. So not only don't they hiss - they don't rumble either. Three of them in parallel is about right for a mic at about 150 to 200 ohms.

in the

But what about the current noise? You need to multiply that by the source impedance to add in its effect. I've looked at a few FET input op amps with amazingly low voltage noise, and this always more than makes up the difference.

source

input

Yup, you really do have to read specs with a cynical eye these days.

Heh heh. But I'll guarantee which one sounds better...

Last time I played with this idea I found it very sensitive to RF interference.