Driving a headphone amp off a phantom power mic line

[Piglit]

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

 

[Adrian Tuddenham]

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

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.

 

[Phil Allison]

"Adrian Tuddenham"

Piglit


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.

** 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 !

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.

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




......... Phil