[R-390] Subject: Re: R-390 Audio Transformer

[2002tii]

Dan wrote:

>For years I've been using a small filament transformer (117 Volts to
>12.6 V 1 Amp) as a 600 to 8 Ohm transformer with my 390 and 390A.
>Cheap, it was maybe $5 new, and it works fine, has a tested -3dB 20 c/s
>to 20 kc/s response.

I have posted on this in the past.  Pretty much any small 6/8/10/12 V 
power transformer will be rated for much more power than a boatanchor 
puts out, so its low frequency performance as a speaker transformer 
will extend much lower than the intended 60/50 Hz power frequency.  I 
have measured quite a number of diverse power transformers for this 
use, and I find that the high frequency performance is equally good 
-- the first self-resonant frequency is usually well over 20 
kHz.  For reference, this would be very good performance for a 
high-fidelity tube amplifier output transformer, albeit at much lower 
full-power loading (made possible by the operation at 
lower-than-rated power and because the internal transformer in the 
390 has already stepped the impedance down to 600 ohms -- the lower 
the primary-secondary impedance ratio of a transformer, the better 
its performance can be at both ends of its bandwidth).

Simply put, the vast majority of small 120 V to 6/8/10/12 V 
transformers, used to match radios with 600 ohm outputs to 
low-impedance speakers, will be far better than the audio from any boatanchor.

Finally, as others have noted, the precise ratio is not 
critical.  Using a somewhat lower-voltage secondary winding may sound 
better -- it will present a lower impedance to the speaker (thus, 
giving better damping), and a higher impedance to the radio (thus, an 
easier load to drive).  Unlike most RF circuits, speaker-level audio 
circuits are not typically operated with matched impedances.

Best regards,

Don


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