[R-390] 500/600 ohm audio to 4/8 ohm speakers

[KR4HV]

First of all, Merry Christmas to all and a Healthy, Happy New Year in
2013!!!

 

I am posting a chart to help some in using line-to-voice coil transformers
with military receivers and other audio equipment and  realize this subject
has  been covered but may be of help to some with a more detailed chart of
some possibilities (and some that are not)  such as use on the BC-348 series

as well as all the Collins  R-xxx derivatives and other receivers. The
military surplus transformers are getting scarce and can be somewhat
expensive when found.

 

The line-to-voice coil transformers are available in many configurations of
primary voltage vs. wattage as well as single and multiple secondary
impedances for speakers.  They are also available with high end frequency
response specifications for the 

purists among us, although, most good PA versions will be completely
adequate for AM/SSB communications.  They are not do-all, end-all solutions
but work very well and are cost effective.  Some combinations will even work
for the very high impedances found on some of the BC series radios.

 

I have listed but two of the many manufacturers and models on my spreadsheet
below.  It's not a rocket science revelation but since I was fiddling around
and have done the work already it may be of help. 

At least one can see right away what may or may not be of help in a
particular situation.

 

I will send a Excel file of the spreadsheet to anyone who wants one and I
will put it on box.com if I can get logged on today.  Hopefully the copy &
past version will come through lined up correctly.

 

73,

 

Walt

KR4HV

 

 

	

	
PA Speaker Transformer  Watt Taps vs. Equivalent Impedance 

KR4HV 12/2012

				
	
(aka Line-to-voice coil transformers)

	

	

	
These work great for audio matching transformers for military & older radios
where speaker outputs are

						
	
500 ohms or 600 ohms.  They can provide better fidelity for SWLing than
military surplus and are less $ to boot. 

					
	
They can be reversed for other audio matching needs. Can provide a good
input match for external amplifiers

						
	
and even microphones in a pinch (use two back to back) if mounted in a
shielded case and kept away from power transformers in power supplies.

		
	

	
Charts show ratio combinations at a glance.  

	

	
Some have both 25v & 70.7v primaries and a 4 & 8ohm secondary.

	

	
I have included several wattage taps for educational purposes. All taps
options are not be available & vary per manufacturer/model#.

				
	

	
Check Models and pick the one that meets your requirement for primary &
secondary impedance and power rating.

				
	
They come with various wattage taps and frequency response specs.

									
	
It makes no difference  whether it is 25v or 70.7v if the impedances
required match the primary & secondary taps.

				
	

	

	
Example# 1: The Quam TCH70 is rated at 10W & 50 Hz- 20 kHz with 5 different
primary wattage taps & 8ohm secondary. 

				
	
Example# 2: The Philmore Brand #MT2570 sold at Fry's  is rated at 10W & has
both 25v & 70.7v primaries with .625w, 1.25w, 2.5w, 5w & 10w taps  & 8ohm
secondary. 

	

	
One should be able to select a transformer with a primary & secondary tap to
work well with military and some older radios.  There are line-to-voice coil
transformers with 

	
various combinations of primary power & secondary speaker impedance ratings
as well as frequency response specifications.

				
	

	
25V Line-to-Voice Coil Transformer

	

							
Primary taps will be marked C & taps for volts (if dual) and watts

			
	
Primary Effective Impedance

using

Watts Tap

	Secondary will be marked C, and taps for 4ohm, 8ohm, etc.

				
		
1250.00

		0.50

	

		
625.00

 

 

1.00

Z=V^2/W - Works Great for 600ohm and also works for 500ohm with small
voltage gain

			
		
600

 

 

1.04

W=V^2/Zreq.

									
		
500

 

 

1.25

Z=V^2/W - Works Great for 500ohm and also works for 600ohm with minimal loss

			
		
300

		2.08

	

		
312.50

		2.00

	

		
250

		2.50

	

		
156.25

		4.00

	

		
125.00

		5.00

	

		
104.17

		6.00

	

		
78.13

		8.00

	

		
62.50

		10.00

	

		
31.25

		20.00

	

		
25.00

		25.00

	

		
20.83

		30.00

	

		
12.50

		50.00

	

		
6.25

		100.00

	

	

	

	
70.7VLine-to-Voice Coil transformer

	

	

	
Primary Effective Impedance

using

Watts Tap

	

		
9996.98

		0.50

	

		
4998.49

		1.00

	

		
3998.79

		1.25

	

		
2499.25

		2.00

	

		
1999.40

		2.50

	

		
1666.16

		3.00

	

		
1249.62

		4.00

	

		
999.70

		5.00

	

		
833.08

		6.00

	

		
624.81

 

 

8.00

Z=V^2/W - Works Great for 600ohm and also works for 500ohm with small
voltage gain

			
		
600

 

 

8.33

W=V^2/Zreq.

									
		
499.85

 

 

10.00

Z=V^2/W - Works Great for 500ohm and also works for 600ohm with minimal
loss

			
		
500

 

 

10.00

W=V^2/Zreq.  Works Great for 500ohm and also works for 600ohm with minimal
loss

			
		
300

		16.66

	

		
250

		19.99

	

		
249.92

		20.00

	

		
199.94

		25.00

	

		
166.62

		30.00

	

		
99.97

		50.00

	

		
49.98

		100.00