# Variations in PA Transformer response

Discussion in 'Electronic Design' started by Martin Stall, Feb 14, 2011.

1. ### Martin StallGuest

Can anyone help me figure this out?

I have two standard PA 100V output line matching transformers, of
different size from different suppliers. Each has an 8 ohm primary.

Transformer #1 is rated at 5W
Transformer #2 is rated at 60W, and is obviously larger in size.

Transformer #2 has much lower internal resistance on both windings,
and a lower turns ratio. How can this produce the same "100V" as
transformer #1?

Second question. Why does the samller transformer have a far better
low frequency response, even though there is less steel?

Thanks for any insights along these lines.

Martin Stall

2. ### Phil AllisonGuest

"Martin Stall"
** I think you are confusing the terminology.

The 100 volt winding is most likely the primary.

** The 5W tranny should have a 16:1 turns ratio - so 100 volts comes down to
6.3 volts.

The 60W tranny should have a 4.5:1 turns ratio - so 100 volts comes down
to 22 volts. .

** You need to say why you think there is a difference.

The maker's specs ? You tested them somehow ?

...... Phil

It looks as though the larger transformer has been wound with fewer
turns-per-volt of thicker wire so as to give higher overall efficiency
at the expense of L.F. response. The turns ratio of the smaller
transformer has probably been adjusted to compensate for the losses on

The amount of steel per watt is the important factor, not the total
amount of steel.

4. ### Don KlipsteinGuest

The second transformer takes higher input voltage, due to higher input
power with the same nominal input impedance.

5. ### Uwe HercksenGuest

Hello,

if you take two transformers, one for 100 V to 10 V, rated at 5 W and
the other also for 100 V to 10 V, but rated for 60 W, both should have
the same turns ratio.

But you are looking at something different, the output voltage for both
transformers is the same, 100 V. But the input voltage must be
different, because the first should deliver 5 W into 8 Ohm, the second
60 W into 8 Ohm. For 60 W, a higher voltage is necessary than for 5 W.
It is correct that the turns ratio is lower for the second one.

Lets calculate, P = U^2/R
U = sqrt(P*R) U1 = sqrt(5*8) = 6.32 V U2 = sqrt(60*8) = 21.9 V

The second transformer uses thick wires for higher current, therefore
the windings have less resistance.

Bye

6. ### Uwe HercksenGuest

Hello,

Don was right, because he and Martin defined the 8 Ohm matched side as
input and the 100 V side as output of the transformer when the
transformer is connected to the amplifier. If the transformer is
connected to a speaker, the 100 V side is input and 8 Ohm side is output.

Bye

7. ### Uwe HercksenGuest

Hello,

but you should accept that it is possible to define the 100 V winding as
primary as you do, but it is also possible to define it as secondary and
the other winding as primary.
If you don't like my postings, don't read it. But if I go away or not is
my decision, not yours.

8. ### Phil AllisonGuest

"ChrisQ"

** Commonly, the primary has taps labelled in watts ( 0.5, 1, 2, 5, 10
etc) at both 70 and 100 volts while the secondary has various load
impedances like 4, 8 & 16.

Means the same tranny can be used for many different jobs.

Since modern 100 volt line amps range up to 1000W per channel power
atings - one can supply several thousand speakers at modest ( ie background
music) levels.

.... Phil