Al Klase - N3FRQ - 23 OCT 2017
|This is the
methodology I've developed to evaluate headphones and
headphone-transformer combinations for crystalset use. I'm
leveraging some work doen some years ago by the late Ben Tongue.
My test set is based on an Hewlette Packard 353A Patch Panel. These were originally used with an audio oscillator and AC voltmeter to mahe measurements on telephone circuits. There are a lot of these kicking around, hamfests etc.,and they are generally pretty inexpensive.
I stripped out all the original components except for the precision 600 ohm attenuator, added Ben's "FILVORA" and built the following circuit.
I also included an 8:600 ohm transformer, but the radio I'm using for a source delivers enough voltage not to need it.
|I generally use
an AM receiver tuned to our local news-talk station, that has nicely
leveled audio, for my audio source. Impedance is
unimportant. You oly ned to deliver 0.775 VRMS as indicated by
the VU meter.
Due to the 4 dB pad and the fixed 26 dB loss of the FILVORA, the attenuator dials, that have been offset 30 dB, indicating the power delivered to the load in dBm regardless of the load impedance.
|What can I expect?
A lot depends on the sensitivity of your ears. I'm down at least5-10 dB from 20 years ago.
For younger ears:
Check transformer losses.
One might actually measure the sound pressure level coming out of the phones. Add -3 dB if you only measure one phone at a time.
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