[TenTec] Heil headphones

[Dr. Gerald N. Johnson]

On Tue, 2008-01-15 at 20:36 -0600, Bob McGraw - K4TAX wrote:
> Do not confuse source impedance with load impedance.  Typically the load 
> specified  Z is 5 to 10 times the source impedance.  Thus a 3 ohm source 
> impedance is adequate to drive a 30 ohm load impedance.  To correctly drive 
> a 600 ohm load the source Z should be about 60 ohms otherwise inadequate 
> voltage will be developed should a lower source Z be available.  At the same 
> time with a 60 ohm source, connecting a 8 to 16 ohm load would likely cause 
> excessive current and could likely damage to the output circuit.

While this is nice for driving a low power load, it has been shown that
with a fixed source impedance, the load receives maximum power when the
load Z is the same as the source Z. That's only true when the variable
is the load Z.

In the case of modern solid state audio output devices, often they are
rated to drive a certain minimum load Z which is limited by their
current capability, and to provide better transient response of the
magnetic transducer (speaker or headphone) the source Z is significantly
smaller than the rated minimum load Z. But there's nothing in that to
say the load Z should 10 times the source Z. 
> 
> Also as discussed, another factor has to do with sensitivity of the 
> headphones vs. sound pressure level produced.  Some simply require more 
> power to produce the required SPL.
> 
> In my case currently with the Omni VII  the headphone suggested load is 8 to 
> 16 ohms but higher Z phones can be used without damage to the transceiver 
> but audio output to the phones  may be reduced.
> 
> All in all, I believe it is a hardware issue.  A small transformer might be 
> though to work but frequency response will suffer on both ends and still the 
> circuit may not have adequate power to drive phones with low sensitivity.
> 
> 
> 73
> Bob K4TAX
> 
73, Jerry, K0CQ