[TenTec] 565 low headphone levels

[Jim Brown]

On 3/12/2013 7:21 AM, Roy wrote:
> The Orion has an effective headphone output Z of about 600 ohms. So 
> with 8 ohm cans, nearly all the audio power is dissipated in the 
> series, matching resistors.

I haven't been following this thread, but in this sentence I see the 
seeds of serious misunderstanding about impedance and audio.

Modern audio systems are VOLTAGE matched systems, with output stages 
that are constant voltage sources and input stages that draw very little 
current.  Good engineering practice for audio can be summarized as follows:

Audio OUTPUT stages have an actual output impedance about 100 times 
lower than the LOAD they are intended to drive. For example, the output 
Z of a speaker power amp that drives 8 ohm loudspeakers is usually less 
than 0.1 ohm.

Likewise, the output impedance of a typical line level output stage is 
on the order of 100-200 ohms, and it drives input stages that have in 
input impedance of 10K - 50 K ohms.  The 100 ohm output impedance is 
usually a resistor of about that value added in series with the output 
chip to isolate the output stage from the capacitance of a long length 
of audio cable. That capacitance could otherwise cause the output stage 
to oscillate.

Headphone outputs are a special exception for two reasons. First, 
headphones don't take much power to drive loud enough for us to hear, 
and second, we are regularly inserting and removing a plug, which can 
short the output and blow the output stage.  To protect the output 
stage, a low value resistor (typically less than 100 ohms) is added in 
series with the output stage, so the output impedance is essentially 
equal to the value of that resistor.

The impedance of a set of headphones defines how much current (and thus 
the power) that will be delivered to the headphones, and the SENSITIVITY 
of the headphones defines how much voltage (or power) must be delivered 
to the headphones to reach a defined loudness.

A resistor should NEVER be added in series with an existing audio output 
stage to "match" impedances. 600 ohm circuits have not been used in 
audio for at least 50 years.  The only folks who think they're "proper" 
are old farts like us who might have worked in broadcasting 60 years 
ago, during the last days of tube audio gear, when that was good practice.

73, Jim K9YC