[R-390] Headphone recommendation

[Charles Steinmetz]

Roger wrote:

>Pick your self up a small (5watt) line voltage transformer for
>speakers. Radio Shack use to stock them.
>
>These will transform the 600 ohm local from the
>receiver to 8 ohms in todays head phones. The line out of the
>receiver is a 1/2 watt. Some what less at the front panel head phone jack.
>
>You can put the transformer on the back of the receiver or make up a small
>patch box on a cord with another jack in the patch box that matched the modern
>head sets of today.

You can also use a small 120v to 12v power transformer.  I've tested 
a great many of these, and they all have excellent fidelity at the 
1/2 watt power level a 390 generates (way better than the radio itself).

A 120v to 13.85v transformer would match the 600 ohm output to 8 ohms 
-- perfect if what you are driving is an 8 ohm resistor.  A 120v to 
12v transformer matches the 600 ohm output to 6 ohms, excellent for a 
real-world 8 ohm speaker (an "8 ohm" speaker has peaks and valleys in 
its impedance curve, and generally drops as low as 3 or 4 ohms at 
some frequencies -- so matching to 6 ohms instead of 8 ohms makes 
life a little easier for the output tube than a "perfect" 8 ohm match 
would).  Note that loading the radio output with an impedance greater 
than 600 ohms does not harm the radio, with the possible exception of 
leaving the output open circuit and hard clipping the amplifier for 
hours on end.  (But I have not seen even that cause problems in a 390/390A.)

If you need more level (but be careful! old hams don't say, "Huh? 
could you repeat that?" for no reason), you may be able to use a 
transformer with a higher-voltage secondary, depending on the 
impedance of your phones.  Few "low impedance" headphones are 
actually as low as 8 ohms (though some are).  Most are in the 300 ohm 
range (per ear, so about 150 ohms when you parallel the two 
sides).  If you absolutely need the extra level, you could use a 
transformer with a secondary voltage of, say, 36v, which would make 
the 600 ohm radio output "look like" 54 ohms to the headphones.  But 
only do this if you absolutely need the extra level.

To get an approximation of the impedance of moving-coil headphones, 
measure their DC resistance with an ohmmeter.  Because they are an 
inductive load, the audio-frequency impedance will be higher than the 
DC resistance you measure, not lower.  True 8 ohm speakers and phones 
generally measure 6 ohms and below (per ear).  "300 ohm" phones 
generally measure from 200-280 ohms (per ear).  As a rule of thumb, 
any phone that measures 100 ohms or above (per ear) could safely be 
used with a 36v transformer (both ears in parallel).  But again, only 
do this if you absolutely need the extra level.  And if your phones 
measure less than 100 ohms (per ear), stick with a 12v secondary.

[Note that measuring the DC resistance will not give an approximation 
of the audio-frequency impedance of electrostatic headphones.]

If you use a power transformer with a 240v primary, double all of the 
voltages given above.

Best regards,

Charles