Terry wrote:

What about impedance matching?
Not familar with S-38 but my S-53 originally had a similar arrangement;
namely pin jacks and a small slide switch to switch from speaker to
head-phones.
That's now converted to a plug in quarter inch jack at rear of my S-53;
speaker disconnects when head-phones plugged in. But both speaker and phones
use the same output winding of the audio output transformer in the plate of
the S-53s 6K6 output tube.**
So if the S-38 is similar it suggests that the audio output is 'low
impedance'.
If someone used high impedance phones one might expect poor audio quality?
Not that high fidelity is normally a requirement when using headphones on a
shortwave 'communications type" receiver!
Or were those 'vintage' type phones that have been mentioned, in foregoing,
low/medium impedance anyway?
One reason I ask is that I have old German headphones in which, I think this
is correct, each earpiece is 2000 ohms; total 4000, probably originally for
a crystal set? I do know that you can hold one lead in hand and touch
almost anything metal with the other lead and get electrical noise!
Sensitive eh?

Impedances: Plate = say 5000 ohms. Speaker = 4 ohms. Phones = 600 ohms.
Therefo
5000/ 4 = 1250 transformer turns ratio = sq root of 1250 = 35
5000/600 = 8.4 transformer turns ratio = sq root of 8.4 = 2.9



Actually, you don't want an impedance match here. If you matched the
impedance and put 1/2 watt or so of audio into the headphones, you
would probably burst your eardrums! Most headphones will produce
adequate audio with just a few milliwatts of power.

73, Roger


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