[Boatanchors] Constant-voltage audio transformers

[J. Forster]

Yes, I understand the theory.

The only 70.7 V one available from RS is a unit with a minimum power tap
of 5 Watts, or 1000 Ohms. That is identical with the little audio output
one I already have.

So, if I have to shop for a transformer, I might as wello look for a
little tube output transformer, which is a better match.

Actually, I have been offered somethng pretty close to what I need, so am
all set.

Best,

-John

==================


> 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
>
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