[Hallicrafters] Matching Speaker Impedances in R-46 etc

[Mike Everette]

When replacing speakers, you may find that it's hard
to locate a 3.2 ohm unit.  8 ohms, and to a lesser
extent 4 ohms, became the "standard" by some time in
the 60s, it appears.

A while back I bought an R-46 on "the site" that
turned out to be a real problem.  The speaker had been
nuked!  Something had "kicked" it so hard, that not
only was the voice coil open, it had been blown loose
from its leads, and the cone was split from rim to
coil in three places.  The R-46 uses a 500 ohm to 3.2
ohm matching transformer; I actually wanted to use the
500 ohm feature.  Finding a 3.2 ohm 10 inch
replacement speaker was impossible; but I did find a
really nice, new-in-box 8-ohm 10-inch
musical-instrument speaker, for cheap as the dealer
was going out of business.

To maintain a better impedance match, I used a
6.3-volt center tapped filament transformer as an
autoformer.  The low-Z secondary of the 500-ohm
transformer was wired to the center tap and one
outside lead, and the speaker was connected across the
whole 6.3-volt secondary.  This worked much better
than connecting the 3.2-ohm winding directly to the
8-ohm speaker.

Triad Transformer Co. used to actually make an
autoformer for matching speaker impedances.  It was
one winding, with a full secondary of 500 ohms and
tapped at 4, 8, 16 and 32 ohms.  I have not seen one
of these in a long time, but they were once common.  A
lot of older hi-fi multielement speakers (Eico comes
to mind, and others) using crossover networks required
driving power at 16 or even 32 ohms, and when Japanese
stereos invaded our shores, most of them were only 4
or 8 ohm output.

As for matching 500 ohms to a low-Z voice coil when
you don't have an "original" transformer, you may find
that a 25-volt Line-to-voice coil transformer will
work better than a 70.7-volt unit.  I don't have the
schematic handy but if I remember right, the impedance
across the full secondary of a 25 is something like
1500 ohms.

73

Mike
WA4DLF



__________________________________________________
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around 
http://mail.yahoo.com