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

True - I was just trying to correct your 1.2k, which while hardly a typo was certainly a slip of the decimal point.

No - that's the input impedance of a Neve desk - one of the classic designs. Others too. More modern ones may be higher.

Never knew that - thanks. I wonder why?

The current Neve mic amp (5012) has a 10k input impedance.

One more thing - the Neve mic pre has a pretty poor noise performance. At -128dBu equivalent at the input, that is about 6dB above pure thermal noise. That is 4 or 5 dB more noise than they should be achieving.

In a professional microphone input you have in parallel:

- Phantom supply (2 x 6.2 k 0.5% resistors to +48V)

- switchable symmetric 20 dB pad (about 2 k input impedance to 200 ohms output)

- and the microphone amplifier

To get the best possible noise figure from a mic preamp, you must match the amp input impedance to the microphone. This is 600 ohms in most cases. but:

if you are going to split one microphone to several mixers (FOH, monitors, recording), there parallelled impedance of these mic preamps should be higher than 600 ohms. In practice, many preamps are designed to 2 k impedance, some monitor amps are as high as 4 k (the loss in noise figure does not matter much on a noisy stage anyway..)

Hope this helps

P.S. back to the original question: several consumer units are not happy when loaded with 600 ohms transformers. These output impedance of a CD player might vary between 30 ohms to several k ohms, usuall having serial capacitors in the output: this means high impedance at low frequencies. A 10 k transformer might have better chance not to cut the bass.

As far as the DC path is concerned, you are right. The two resistors are in parallel. But as seen by the microphone signal on the balanced pair, they are in series.

Once you are in a position to need a 20dB pad, the impedance is no longer important, because internal noise is no longer an issue.

Microphones tend to be somewhere around 100 ohms in most cases. And to get best noise performance, you don't match impedance. You make sure the impedance of the microphone gives the best balance between current noise and voltage noise from the amplifier. The amplifier will be designed to put this point somewhere around the 100 ohms you expect from a mic.

If you want to split one mic the best way, run it into a single preamp, then make the split after that, so that the best noise performance is maintained.

Transformers don't have "an impedance" as such. They transform the impedance of whatever they are connected to by the square of the turns ratio. So what impedance is presented as a result of putting in a transformer depends on what the impedance of the equipment is.

They had input balance to unbalance transformers. So you might as well get the best practical matching? I suspect later units with electronic balancing are more like your 10k or so.

Could you name a modern mic with an actual 600 ohm (balanced) output impedance?

Or indeed any?

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

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.

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

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.