TV/VCR/Stereo Troubleshooting/Headphone Update


QUESTION: The headphones arrived, alas the seller didn't put any packaging in the box so I had to fix them, both speaker assemblies had become detached from the shells. Luckily easily fixed with some glue.
I tried them first with a cheap hybrid tube amp I bought from Ebay a few years ago. It had a valve first stage then Class A MOSFET output.
It always got very hot, despite me adding extra heat-sinks.
Tried the headphones with that first and the heat-sinks stayed cool.
Tried it with the other amp and that also stayed cool.
The sound is good, warm lower frequencies, as the speakers are the old type with paper cones. However the high frequencies are very good indeed.



ANSWER: Thanks for the report.

The "cool" performance of the amps indicates the headphones don't consume as much power as maybe thought, meaning, the impedance is much higher than the mfr rating.

Do, you know how to measure impedance?  I'd be curious to know what their actual impedance is.

You can measure the impedance with an ohmmeter, but that merely tells what the dc resistance is. To measure more precisely you'd need audio oscillator and a resistor in series so you can get the voltage and current.

In any event, good luck with the new headphones!


PS: You mentioned the warm low frequencies.  What do you think of the electret high end response.  Electret tweeters always sound a little brittle to me.

---------- FOLLOW-UP ----------

QUESTION: Thanks for your help. I do have an Ohm meter, but alas no oscilloscope.
Now I have had them playing for a while they are sounding better.
With a Class A amp the excess power is converted to heat, so maybe the headphones are consuming power which would otherwise be converted to heat? Or doesn't it work like that? The amp outputs 1 watt but consumes around 30 watts, I have a power consumption meter.
Many thanks for your help again.



It depends on the circuitry.  In class C, B or AB the dissipation of heat in the amp output stage is very low under zero output to the load. It is also very low at full output. It maximizes around 1/3 power output.

In class A the output stages are producing heat all the time but it diminishes some when delivery to the load increases. But, most audio class A are not true class A but a modified version which slides with power bias settings in the Amp. But theoretically the power dissipation in the amp is maximum at zero output and as the volume is turned up the delivery to the load  decreases the power dissipation in the output stage.  

But, another but, if deliver to the load goes up to a high level the heat is dissipated in the headphones and they will get warm or hot to the touch!

So, it's a complicated matter.

Getting back to measuring the impedance: If the headphones have no internal transformer or reactive elements the dc resistance would nominally be just a little less than the ac impedance.  Therefore, if the impedance rating were 8 ohms, the dc resistance would be expected to be around 6 to 7 ohms.

Hope this is not confusing.  

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