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Driving a headphone amp off a phantom power mic line

Trying to devise a way of efficiently driving a phone amplifier off a

48 volt phantom powered mic line. The acitve lines (2) are current limited typically by 6k8 resistors, and allowing for an additional 1k on each leg to bridge into the amp, that gives around 12ma short circuit, 6ma at 24 volts - the point at which maximum transfer power is available. I would like more current and less voltage (Nat Semi etc have some nice efficient little class D amps that work on 5 volts that are ideal for driving headphones). Hence I need a DC/DC converter. The problem is that all the buck regulators in the TI, Nat Semi offerings loose efficiency drastically below 100ma or so, thus defeating the purpose of the exercise. None of the switchcap regs work at 24 volts. Any other suitable devices out there ?. M
Reply to
Piglit
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Generate the headphone signal at higher voltage and lower current, then use a small audio transformer to step the impedance down. It will give you isolation too.

Unless the headphone amp is running in Class A, you are going to have terrible problems preventing the variable loading on the power rails from causing trouble to the mic circuit. You might have to finish up using a battery instead.

--
~ Adrian Tuddenham ~
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www.poppyrecords.co.uk
Reply to
Adrian Tuddenham

"Adrian Tuddenham"

** Excellent suggestion.

A common audio op-amp ( ie NE5534) may be used from a + 24 volt ( zener stabilised) supply and drive a cap coupled transformer matched to the headphone load - typical headphones have near constant impedance across the audio band.

An electro cap across the +24 volts will allow current peaks over 12 mA with a 6 mA average value - no problem for most op-amps.

The optimum load presented to the op-amp is about 10 mA at 10 volts = 1000 ohms.

Power max = about 50mW = damn loud !

** I dearly hope no poor mic is expected to work on the same input !!!!!!

........ Phil

Reply to
Phil Allison

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