# 70.7V line transformer

Discussion in 'General Electronics Discussion' started by Y2KEDDIE, Mar 20, 2013.

1. ### Y2KEDDIE

259
15
Sep 23, 2012
Ok, I understand how to match a 8 ohm speaker to an output tube. I match the plate resistance to the primary. I can calculate turns ratio and thus calculate impedance.

What I can't grasp is how do you match a 70.7 volt line to speaker transformer. It must have a primary impedance. How would one design a output stage using such a transformer?

2. ### john monks

693
2
Mar 9, 2012
The 70.7 volts refers to a standard driving source for speakers in RMS.
So, for example, if you want to drive a 100 ohm speaker with a 70.7 volt line you the formula

Z = (70.7^2) / 100 = 50 ohms.

The output stage would be designed as a constant voltage source not exceeding 70.7 volts RMS.

This may help
http://www.crownaudio.com/media/pdf/amps/138905-1_10-05_constant_voltage.pdf

Last edited: Mar 21, 2013
3. ### Y2KEDDIE

259
15
Sep 23, 2012
The Crown link was helpful. I've never seen circuitry for a tube amplifier using 70.7 line. I can see it would be fairly easy to build a solid state output that has realitvely low output impedance. I'm thinking this could be accomplished without a tranformer on the amplifier itself.

With a 100 watt tube amplifier there must have to be a transformer matching the tube plate resistance to what ever speaker transformer connected to a speaker.

4. ### duke37

5,364
772
Jan 9, 2011
In the UK, the PA system uses 100V line, perhaps this is peak and the same as 70.7V line.

Valve amplifiers can produce a low output impedance with negative feedback so that the loading can be varied as required.

The speakers are coupled to the line with transformers. These have multiple primary taps to adjust the power and multiple secondary taps to match the speaker impedance.

693
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Mar 9, 2012
6. ### davennModerator

13,837
1,952
Sep 5, 2009
no they are different

coming from PA installation work, many of our amplifiers would have multiple outputs

8 Ohms, 50V, 70V or 100V for feeding different styles of lines
we mainly used the 100V feed but some times if there was a fault on the 100V speaker line that lowered its resistance, we would drive it with the 70V output to keep the system running till we could find the fault.

for example a normal load ( non fault condition) of 100W, a 70V line would be 47 Ohms and a 100V line would be 100 Ohms

Dave

Last edited: Mar 21, 2013
7. ### Y2KEDDIE

259
15
Sep 23, 2012
If I understand: Suppose it is a 100 watt tube amplifier. It's output transformer matches the Push pull output to 70,7 line. (10000 : 50 ohms Z for 100 watt load). Now if I want to power a 10 Watt 8 ohm speaker a 500 : 8 ohm matching transformer would be needed.
Z= 70.7 sq'd / 10 watts = 500 ohms.

Is this correct?

8. ### john monks

693
2
Mar 9, 2012
Your math looks totally correct to me.