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

True - but this would just effectively reduce the impedance of the circuit.

I'm not going to bother with you anymore. You're an ignorant rude prat.

I in the past have been using their 600 ohm 1:1 transformer to make such a device, with much success. I still want to measure it. Buying two transformers, a box, and wiring, is much more complicated than purchashing this premade "Ground Isolator" I do have reservations of having the unit attenuate the signal voltage.

greg

"Dave Plowman (News)"

How about not bothering SEB too ?

So says another know nothing, pommy wanker.

....... Phil

There are no doubt some losses.

Primary DCR = 136 ohms Secondary DCR = 175 ohms

Turns ratio probably follows this pattern, IOW a slight step up perhaps to minimize apparent loss.

I would guess its working impedance at around 3 K ohms. This is the range where the resonance is nicely damped and the signal handling ability allows for good dynamic range.

sound the two resistores in series are parallel with the input to me

Bye. Jasen

The resistors are in series with each other, that combination is in parallel with the input. Because the resistors are 6.2k, the combination applies 12.4k as the maximum possible input impedance of a microphone preamp.

Correct.

But being a total of approx 13k will have little effect across 150 ohms.

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.

"Don Pearce"

Thermal noise in a 20kHz BW and 150 ohms

= 0.22 uV or -130.9 dBu.

Get you calcs right - d*****ad.

( You forgot to allow for the 0.775 volts bit. )

BTW

This is what Rupert has to say on the matter:

NOTE

his comments re 1000 to 1200 ohms input load being "normal".

.......... Phil

"Matti Adolfsen"

The best signal to noise ratio is obtained when the mic impedance is MUCH less that the load impedance. This allows the mic to generate the greatest signal voltage at the input while noise is a function of actual mic impedance in the audio band.

The vast majority of *dynamic* mics are of 250 ohms or less impedance.

Condenser mics generate their own noise - far more than the vast majority of mic pres do - so it rarely matters what impedance they are, far as noise performance goes.

........ Phil

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?