Audio matching xformer

James Bromley K7JEB k7jeb at COX.NET
Mon Oct 15 10:57:17 EDT 2007

Simple calculation:
P = (E)^2 / R
P = (70.7)^2 volts squared / 500 ohms
P = 5000 / 500
P= 10 watts
Use the 10-watt tap for a 500-ohm primary impedance


---- Bob Peters <rwpeters at SWBELL.NET> wrote: 
> What you are saying is partially true...In a true 70V Sound System there
> is a transformer on each speaker and depending on what kind of speaker
> the secondary is mostly 8 ohms. The Primary is normally 25V or 70V with
> taps from about 1/4 watt to 15 watts... Better quality transformers are
> from 5 to 30 watts...These are all Hi Impedance taps but normally a 600
> ohm input will give you a 8 ohm output... If I were you Barry I would
> use common and the 10 watt tap...My company manufactures these
> transformers...If you need a higher wattage yell at me and I will send
> you one...I actually have a couple of 100 Watt transformers...They are
> heavy  HI... There is a lot of info on 70V stuff at my company web
> site...
> For educational purpose only  HI...These are not amateur products...
> Bob W1PE
> Dallas
> -----Original Message-----
> From: boatanchors-bounces at
> [mailto:boatanchors-bounces at] On Behalf Of WA5CAB at
> Sent: Sunday, October 14, 2007 7:15 PM
> To: boatanchors at
> Subject: Re: [Boatanchors] Audio matching xformer
> Barry,
> The concept is that if you have (in this case) a 10 watt audio
> distribution 
> amplifier with 500/600 ohm output and from one to 16 remote
> loudspeakers, you 
> run parallel pairs of wires from the amp to each of the speakers (or you
> can 
> run a loop or buss - it doesn't matter).  In the speakers (which is
> where the 
> transformers are located) you connect the wires to common and to the tap
> correlating to the total number of speakers in the system.  For one, use
> the 10 watt 
> tap, for two, use the 5 watt tap.  Etc.  Local volume controls (if
> provided) 
> will be on the secondary side between the transformer and the speaker.

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