[ARC5] Tip of the day:

[Roy Morgan]

On May 25, 2015, at 4:38 PM, J Mcvey via ARC5 <arc5 at mailman.qth.net> wrote:

> ...I found that a small "70 volt" line transformer used for long runs on Public address amplifiers does a pretty close match with the high impedance output of the ARC5/SCR274 receivers. The 625 mw tap works fine.
> more info here:http://www.crownaudio.com/media/pdf/amps/138905-1_10-05_constant_voltage.pdf 

JH,

Thanks for the link to that guide.  I assume that it was written by the engineering folks at the makers of Crown amplifiers, which are widely used and respected in industrial, scientific and engineering applications world wide.  My quick reading of the guide leads me to believe that the authors really do know what they are talking about.

There have been published a number of tables showing the primary input impedances of line transformers.  I have included a table of values below to get folks started without finding the extensive details elsewhere. Note these points:

- The impedance values assume that the secondary of the transformer is loaded with the indicated speaker voice coil impedance - generally 4, or 8, and sometimes 16 ohms.
- If the load impedance is half or double the rated value, changes need to be made to the power handling ability of the unit, and the frequency response will also change
- The *actual* impedance of a speaker can and does vary from perhaps half or less to three times the nominal value, so efforts at precision here will be in vain.
- The quality and power handing capacity of audio line transformers varies quite widely.  Examples on hand here include:
   - Some transistor output transformers claimed to match 600 ohm radio outputs to speakers.  Very small.  Likely able to barely manage a half or one watt.
   - Midland Model no 25-600 5 watt, 25 volt - likely a modest performer
   - Realistic (Radio Shack) Cat No. 23-1031, 10 watt, 70 volt, secondary taps of 4, 8, and 16 ohms.  Likely a medium performer, especially at modest power levels.
   - A Peerless (Altec Lansing) 70 volt transformer rated at 30 watts - likely a superb performer.  At the moment I can’t locate either the transformer or the Peerless data sheet that tells about the series of about 4 such transformers they produced.  Rating on these transformers seems to be +/- 1 db from 60 to 15,000 cps at rated power with insertion loss about 0.5 db.

The Hammond company makes a 600 ohm to voice coil transformer (I can’t find it on the AES site just now) that is likely a very fine performer.  It is their model 119DA, is rated at 12 watts, and sells for “less than $40.00”.    It is touted as the right thing for use with the R-390 series of radios, which by the way have a rated audio output of one half watt (or thereabouts).


This article:
http://www.radiomuseum.org/forum/audio_transformer_substitutes.html
Is the source for the list of impedances below.  
It also includes information about using these transformers as output transformers in both single ended and push-pull configuration.
______________
More on using 70 volt transformers to match impedances. Here is a list of
watts taps verses impedance;

.31watts        14000 ohms
.625w           7000  ohms
1.25 watts      4000  ohms
2.5 w           2000  ohms
5 watts         1000  ohms
10  w           500   ohms
12 watts        360   ohms
24 watts        180   ohms
40 watts        110   ohms

Here is the chart for 25 volt speaker transformers:
.31 watts       1680  ohms
.625 watts      840   ohms
1.25 watts      440   ohms
2.5  watts      220   ohms
5    watts      120   ohms
________________

On a related note, I recently measured the input impedance of a military LS-3 speaker with original matching transformer and speaker.  It was very close to the expected 8000 ohms. This speaker was used with the BC-348 and other military radios of the era that typically had output levels on the order of 1 to 5 watts.  The transformer in some of these speakers is marked 1 watt, but is the size of the 10 watt unit I mention above.

Roy

Roy Morgan
k1lky68 at gmail.com
K1LKY Since 1958