[ARC5] 70.7V Line Transformer for audio matching

[Richard Knoppow]

    Lets start with some fundamentals:
  The voltage ratio of a transformer is exactly the same as its 
turns ratio. If one winding has twice the turns of another it 
will have twice the voltage on it. Same for an autotransformer, 
voltage on the full winding will be the ratio of the turns at the 
tap to the end.
    Impedance is the square of the turns ratio. If the 
transformer has a 2 to 1 turns ratio it will have a 4 to 1 
impedance ratio.
    For the example of a filament transformer; assume it is a 
120V primary to a 6V secondary (rounding off here). The ratio is 
120/6 or 20 times. The impedance ratio will therefore be 20^2 or 
400. Hence it will be 400 ohms to 1 ohm.  A 12.6 V filament 
transformer (rounding again) will be 120/13 = about 9.25. The 
square is about 85 so 600 ohms on the 120 Volt side will 
transform to about 7 ohms on the filament side, about right to 
match many speakers to communications receivers with 600 ohm 
outputs.  The various other characteristics of the transformer 
may not be ideal for high fidelity but that is mostly not 
important and the transformer will work quite well.
    The constant voltage or 70 volt system is meant to assign 
power to a number of speakers in a PA system. Each speaker is 
connected to a secondary of some impedance depending on the 
design. The primary of the transformer is the one with variable 
voltage, sometimes marked as impedance. Its purpose is to 
deliberately introduce a mis-match to limit the power going to 
the speaker. For instance, in some 70volt systems the speaker 
side will be perhaps 200 ohms while the amplifier side may be 
variable over a range of from 200 to 2000 ohms, the higher the 
impedance the less power the speaker absorbs from the amplifier. 
With careful design the loudness of each speaker may be made 
about the same. That accomodates different size speakers and 
different areas of distribution. Such a system can become quite 
elaborate.
    Since the 70 volt transformer is marked for power ratios and 
possibly for impedance ratios it can be used for impedance 
matching. Remember, the impedance ratio is the square of the 
turns ratio and the voltage ratio while it is also the ratio of 
the power ratio between the input and output. Generally, for 
impedance matching of the sort asked about here the transformer 
is used exactly as it is for PA system use except that the ratio 
chosen is whatever matches the speaker to the line best.
     Keep in mind that its is the turns ratio that is important. 
Transformers will work over a quite wide range of impedances 
although there may be limits on the high and low frequency ends 
when use very far from the design range. So, a transformer 
designed to match 50 ohms to 600 ohms will also match 10 ohms to 
120 ohms with very little loss and also from perhaps 200 ohms to 
2400 ohms with little loss.
    Something like a filament transformer will usually work quite 
well at audio frequencies. The low response will be OK, after all 
its designed for 60Hz, the high end may be limited due to 
parasitic capacitances, etc, but, in the range of a 
communications receiver, perhaps up to 5Khz, it will do just fine.

On 1/7/2019 6:33 PM, Meir Ben-Dror WF2U wrote:
> I concur! That’s what I always did with the line transformers. 
> Hook up the speaker to the secondary according to the speaker 
> impedance. I connect a scope to the speaker terminals. Change 
> taps until the amplitude is the largest. All set!
> 
> 73, Meir WF2U
> 
> Landrum, SC
>
-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL