[Boatanchors] Constant-voltage audio transformers

[Dave Maples]

All: After reading the request for a transformer to couple a BC-221 meter to
low-Z headphones, I thought it might be helpful to some folks to read some
information about 70.7-volt audio transformers.  These devices are actually
very handy for solving some of these kinds of problems.  They are also
cheap, which is always attractive.

Why are they called "70.7-volt transformers"?  It's because they are
designed such that if the primary is fed with 70.7 VAC from an audio
amplifier, the transformer and associated speaker will draw a specific
amount of power from that audio amplifier.  The amount of power that will be
drawn is basically only a function of the wattage tap selected on the
transformer.

This arrangement makes it very, very easy for a single audio amplifier to
drive a large number of speakers at different power levels while ensuring
that the amplifier is properly loaded (which, as all of you know, is
especially important with vacuum-tube amplifiers).  All the installer has to
do is to add up the wattages drawn by each speaker in parallel, and make
sure that the total wattage matches the amplifier power rating.

How does that help us?  Well, the primary of the transformer may actually be
a very high-impedance load, depending on the wattage tap chosen.  That's
what we are looking for.  Unlike the audio technician, though, we are going
to have to calculate what we want.  That's really easy, though.

1. If you recall, P=(E*E)/R, where P= power (watts), E=RMS volts, and
R=resistance in ohms.

2. By multiplying both sides by (R/P), we find that R=(E*E)/P.

3. Since E = 70.7 VAC, E*E=70.7*70.7=5000

4. R, therefore, will equal 5000 divided by the wattage tap you choose.
Choosing the 1-watt tap (assuming you have the proper impedance on the
secondary) will give you a 5000-ohm load.  Choosing the 2-watt tap will give
you 2500 ohms.  Choosing the 8-watt tap will give you 625 ohms, which is
just about perfect for an R-390 or similar.


You can also find 25-volt speaker transformers, just as you can find 70.7
volt speaker transformers.  The same principles apply, except that in (3)
above the value of E*E is 625, not 5000.  This means that the 1-watt tap on
a 25-volt transformer will present a 625-ohm load to a source, which again
is a pretty close match for an R-390 or similar.

>>>The point is that you can apply either to your situation, and you can do
so KNOWING what load you are presenting to your source.  You don't have to
guess.<<<


The Quam TBL-25 and TBL-70 are great little transformers for this purpose,
and are quite cheap.



One other thing to remember: These transformers are NOT designed for
constant DC current to flow through them.  They are AC-only devices.


I hope this is helpful.

Regards,
Dave WB4FUR