microphone impedance matching

[nostar]

Im using some old small tube amps with XLR microphones, using this inline xformer (CP8201):

http://www.audio-technica.com/cms/electronics/f4b735b4cabc30ea/index.html

I took it apart and, sure enough, theres just a tiny xformer inside. I would like to purchase the equivalent bare transformers (or something better if there is any reason to believe that there is a potential of sound quality loss using this xformer) so that I can use a remote mounted XLR socket, and the addition of a phantom power circuit I designed. Can someone explain the math for determining the specs for a transformer, and explain what it is electronically that happens if I connect a low impedance mic to a high impedance mic input? To help in this discussion, the input of the amp I am using is simply a 1/4" TS jack connected to the grid on one of the triodes of a 12AD7 tube, parallel to a 500Kohm input resistor to ground. For determining the transformer specs, the specs on the data page for that CP8201 pretty much says it all (250ohm in, 50 KOhm out, 20-20KHz range).

I look forward to any insight 🙂

 

[lance_the_boil]

Hi,

I can't answer your exact question but......

In my experience most microphones have considerably lower impedances than the mixers they are plugged into.

Mixers have a high impedance as that gives the greatest tolerance for different impedances of source. In low power (voltage circuits) electronics the efficiency that is lost due to not matching impedances does not matter.

In fact as almost all mics are designed with this in mind there will be a danger that if you try to acheive lower impedance at the input then you may cause some sound anomolies with the microphone at lower frequencies (where more energy is present compared to higher frequencies).

I hope this helps with what you are trying to acheive.

Lance.

 

[Nigel Goodwin]

Hi,

I can't answer your exact question but......

In my experience most microphones have considerably lower impedances than the mixers they are plugged into.

Usually five times or more! - matching it loses half the signal.

Mixers have a high impedance as that gives the greatest tolerance for different impedances of source. In low power (voltage circuits) electronics the efficiency that is lost due to not matching impedances does not matter.

I would go further than that - with matched impedances the best possible efficiency is 50%, because the unmatched preamp is active (rather than passive) you use voltage transfer and not power transfer, this makes the 'transfer' much more than 50% efficient - but as you're supplying power to make it work, the actual electrical efficiency will be less.

For the situation with a mike transformer I would suggest the mike should probably match the input of the transformer, as it's a passive device, so power transfer is a good idea. But the output of the transformer to the grid of the valve won't be matched, because it's active and you're looking for voltage transfer.

 

[Wavebourn]

We don't amplify just a voltage, we amplify a POWER. If impedances are matched maximum power is transferred; if loaded on lower impedance we transfer more current, but loose a voltage, if loaded on higher impedance we transfer more voltage, but loose current, again we need more of a power amplification to compensate gain loss. The less power amplification we need, the less noises and distortions we get (theoretically, because design of amplifier with the same power amplification specs may be different).

Also, microphone manufecturers expect certain load inpedance to obtain specified curves of a frequency responses.

When mixer consoles have very universal inputs audio engineers prefer to pay money for off-board microphone preamplifiers...

 

[jan.didden]

We don't amplify just a voltage, we amplify a POWER. If impedances are matched maximum power is transferred; if loaded on lower impedance we transfer more current, but loose a voltage, if loaded on higher impedance we transfer more voltage, but loose current, again we need more of a power amplification to compensate gain loss. The less power amplification we need, the less noises and distortions we get (theoretically, because design of amplifier with the same power amplification specs may be different).[snip]

I think that if you do some real-world number examples you'd be surprised.
Would you put an 8 ohms resistor in series with your power amp output to 'match' the speaker for max power transfer?

Jan Didden