[R-390] 600 ohm audio

[Jim Whartenby]

If anyone is having trouble finding audio line transformers, I have two dozen NOS boxed transformers available.  
Rated for both 25 and 70 volt lines at 10 watts, the primary winding has 8 taps and the secondary is set for an 8 ohm speaker.  
Will transform the following impedances, as printed on the box:62.51252505001000200040005000
$7.50 each plus shipping from 71901Jim

Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy

-----Original Message-----
From: Thomas Chirhart <k4ncgva at gmail.com>
To: Tom Frobase <tfrobase at gmail.com>
Cc: R390A <r-390 at mailman.qth.net>; Tisha Hayes <tisha.hayes at gmail.com>
Sent: Sat, Feb 19, 2022 4:05 pm
Subject: Re: [R-390] 600 ohm audio

I have a few of these:
A-8102’s
A-8103’s
24572 70.7v voice coils..
73 Tom K4NCG 


Sent from my iPhone

> On Feb 19, 2022, at 11:17 AM, Tom Frobase <tfrobase at gmail.com> wrote:
> 
> Just as a follow up, I needed a transformer for a unit I recently sold.  I
> have a box of 25v transformers which were popular before 70v audio
> distribution.  I found the 1 watt tap was exactly 600 ohms into an 8 ohm
> load.  They should be very common on EBAY. … Tom, N3LLL
> 
>> On Sat, Feb 19, 2022 at 9:43 AM Tisha Hayes <tisha.hayes at gmail.com> wrote:
>> 
>> (Sigh) late to the conversation but I have a few things to add;
>> 
>> If you are going to browse through ebay for 600 ohm to 8 ohm transformers
>> they can be a little pricey; People who have them know that they are
>> sort-of-rare and charge accordingly. But... people who are selling off the
>> old 70 volt field coil transformers do not know the other uses for the
>> transformer as an impedance converter from 600 to 8 ohms; You can get those
>> pretty cheaply.
>> 
>> If you look at old speakers there were quite a few that were originally
>> designed for 600 ohm sources. Some of the Hallicrafters speakers supported
>> a 600 ohm input and if you took the speaker apart you would find a little
>> transformer in there.
>> 
>> You can also get transformers that are multi-tap and that allows you to
>> match up to 4 ohm speakers as well. Just checking quickly I see that
>> radiodaze has the 119DA that would allow you to connect up a 4 or 8 ohm
>> speaker just by using a different tap.
>> 
>> Another alternative would be to make your own little audio amplifier that
>> can take in a high-impedance input (600 ohms, whatever) and drive an output
>> to 4 or 8 ohms. You could even do it with an op-amp or a simple transistor
>> based amplifier because those devices don't really care about the input
>> impedance that much and are capable of driving low impedance outputs (just
>> not at a very high level), so it would be good for low impedance headphones
>> or ear-buds.
>> 
>> If you want, you can even find 600 ohm headphones, but nowadays those are
>> usually high-end studio headphones. I have the Sennheiser HD600 that I do
>> use for radio listening but mostly I use them for high fidelity music
>> listening. Those are pricey ($300 headphones) and might cost more than your
>> radio. Another place I have found impedance adapters are the ones used for
>> David-Clark aviation (pilots) headsets.
>> 
>> Another route I took was to get a Bogen distribution amplifier; This is a
>> monsterous, 50 watt audio amplifier that you would use for a building and
>> had a 600 ohm input that could drive a variety of output impedances. If I
>> ever have a south american dictator surrounded in an embassy and I want to
>> drive them crazy/ out of the embassy I would use that to play Barney the
>> Purple Dinosaur songs non-stop for a few weeks. (or anything from
>> Nickelback).
>> 
>> *Ms. Tisha Hayes*
>> 
>> *AA4HA*
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