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problem with two transformers

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whiz115

Member
Recently i did an order for two audio transformers 5Kohm - 8ohm...
today i received my order and when i measured DC resistance i discovered
that both of them are 400ohm - 1,5ohm :eek:

since i have no experience with audio transformers... can someone tell
me if what i measure is ok? does the impedance will be as i ordered it to be?!

thanks!
 
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k7elp60

Active Member
Audio transformers are rated in impedance. The 5k and the 8Ω are the impedance. The resistance measured with an ohm meter is called the resistance of the windings. I would say they are typical values.
 

Willbe

New Member
The impedance is usually checked at 400 Hz or 1kHz; not at DC.
 

MrAl

Well-Known Member
Most Helpful Member
Hi there,


Because many output devices like speakers have an impedance (Z) of
8 ohms or 4 ohms you will find some transformers rated in terms of
it's primary and secondary impedance when connected in the circuit.
Yours is rated the way it would work if you connected it to a speaker
that was 8 ohms, but you could use it with 4 or 16 ohms with either
more loss (4 ohms) or less loss (16 ohms) in efficiency.

In other words, when you connect the "8 ohm" winding to an 8 ohm
speaker, the ac impedance of the primary (5k winding) is 5000 ohms.
The transformer has to be driven with an ac voltage however, and
the test frequency is usually 1kHz for audio work.

You can test your transformers a bit better by calculating the
turns ratio and then injecting an audio test frequency into the
primary and measuring the output. Since they spec the primary
as 5000 and the secondary as 8, 5000/8 equals 625, and the
square root of 625 is 25, so your turns ratio is 25. This means that
when you input 1vac (ac only) into the primary (5k side) you will get 1/25
volts which is 0.04vac out of the secondary (8 ohm side). This will
tell you if the transformer is working properly.

Since you noted that the primary dc resistance (ohmmeter) was
400 ohms and the secondary dc resistance was 1.5 ohms, there
will be some losses. With a 4 ohm load there will be more losses than
with an 8 ohm load, and less loss with a 16 ohm load.

The turns ratio (in this case 25) allows your circuit to have to only
drive an impedance of 5000 ohms instead of 8 ohms directly, and
driving a load of 5000 ohms is sometimes easier than trying to drive
8 ohms directly so the transformer helps in some circuits to match
the output of the circuit to the speaker.

As a side note, you can make an interesting little boost converter
by driving the 8 ohm side with a small voltage. The output volts
will be about 25 times higher. Wont be too efficient though, and
with small transformers you cant put too much voltage into the 8
ohm side either, but it's still interesting.

Another advantage to using a transformer (besides impedance matching)
is that it offers some degree of isolation from the circuit. This
is a good idea when using it for driving headphones so you dont get
a shock from the headphones if the circuit runs off of the line voltage.

One last note...
If you connect it to a 4 ohm load instead of 8 ohms, then the primary
impedance will be 2500 ohms instead of 5000. If you connect it to
a 16 ohm load instead of 8 ohms then the primary impedance will
be 10000 ohms instead of 5000.
 
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