[R-390] R-390 600 ohm audio solution

[Charles P. Steinmetz]

pete wrote:

>A 10-12.6 volt filament or control transformer can be used as a 
>perfectly acceptable output transformer.  You only need a 1-amp 
>secondary rating to give you a 10 watt output rating.  You may not 
>get response to up 20,000 Hz but it's bass response is good down to 
>60 Hz. I've used one for years.

I've measured lots of filament transformers used in this manner, and 
they very often exhibit a high frequency -3dB point well above 20 
kHz.  If you choose one rated at 5 VA or more, the low frequency -3dB 
point at 2 W will be less than 20 Hz and the distortion will be low, as well.

Note that all of this holds as long as the transformer is used with 
the radio's existing output transformer, to step from the radio's 600 
ohm output to a 4 or 8 ohm speaker (with no DC current in the 
filament transformer primary).  A filament transformer will not work 
well as a plate-to-voice-coil transformer (with DC current in the 
primary winding).

Best regards,

Charles