[ARC5] 70.7V Line Transformer for audio matching

[Roy Morgan]

Ah HAH!  A practical approach, bravo!

The basic idea of the wattage indications on a line matching transformer is that the chosen tap will deliver (about) that much power to a load equal to the load (speaker voice coil) tap in use, ASSUMING, that the line (in this case 70 volts) is running at 70 volts.  The line voltage varies with the program material being fed to the line (voice, music, fire alarm, whatever).

Here from my notes are the impedances at the various line input taps for the 70 and 25 volt line transformers:

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

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

PS: I have an original LS-3 speaker meant for use with the BC-348 and it measures very close to 8,000 ohms impedance.

PS2: The actual audio impedance of almost any speaker varies quite a bit across the audio range.  The GR manual for their Y-Z Bridge shows a plot clearly illustrating this for a table radio speaker.  (The GR YZ bridge is meant for such measurements and does a superb job of it.)

PS3: a small 6.3 volt filament transformer would do well to drive the 8 ohm head set.  Impedance ratio is the square of the voltage ratio:
(120/6.3)^2 = 400. So I the primary would be 4000 ohms, the secondary load would need to be 10 ohms.  Close enough!
(I think I have this right - it’s been a long time since I worked any algebra.)

PS4: losses in normal line matching transformers may be 3 db or more.  In high quality units such as UTC or Western Electric, the loss would be much less

Roy

> On Jan 7, 2019, at 6:38 PM, Bob kb8tq <kb8tq at n1k.org> wrote:
> 
> Hi
> Nothing is going to break from a mismatch at audio. One “traditional” approach
> is to switch taps until you find the one that is loudest or “sounds best”. 
> Bob
> 
>> On Jan 7, 2019, at 6:31 PM, Mark K3MSB <mark.k3msb at gmail.com> wrote:
>> 
>> Hello all.
>> I need to match the 4K audio output of my BC-348-R receiver to a standard 8 ohm head set. 


Roy Morgan
K1LKY since 1958
k1lky68 at gmail.com