How to test an audio transformer

Thread Starter

Dsnead

Joined Apr 16, 2020
12
I’ve been reading books & online, trying to learn more about transformers. I’d like to do some breadboard tests and build some circuits. I bought a Radio Shack audio output transformer, 273-1380. I believe it is 1000:8 ohms outside & 500:8 ohms center tapped.

Is this picture a legitimate way to test it?

Or do I also need a dummy load in the circuit?

There’s not a lot of documentation on this transformer to verify my tests. And I’m getting some good results and some really odd results that I don’t understand yet. I'll take some screenshots of the results in an excel table and reply.

Thanks!
 

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Audioguru again

Joined Oct 21, 2019
2,048
Audio output transformers have not been used or needed for at least 64 years. What do you need it for?
Radio shack does not say and probably does not know its power rating.
It does not pass low audio frequencies and does not pass high audio frequencies so it is no good for audio.
It might make a good but expensive paper weight!
 

Reloadron

Joined Jan 15, 2015
5,667
Several options. You can just measure the DC Resistance which really won't tell you much, it won't tell you the actual impedance which is what is of interest. If you have the ability you can apply an AC voltage to the primary and measure the AC voltage on the secondary side. A good place to start is get the turns ratio so apply a low AC voltage like maybe 3 volts to the primary and measure the secondary. If I apply 3 volts and get out 1 volt I have a 3:1 turns ratio. Then apply 1 volt to the secondary and if the primary has 3 volts that pretty much covers the in/out and gives you the turns ratio. With the turns ratio known you square it to get the impedance ratio (if I remember correctly) so a 3:1 turns ratio is a 9:1 impedance ratio. Again look this up because I am not sure. Been a long time. The idea is to match any pair of impedance so a for example 8 Ohm speaker to a 72 Ohm source impedance.

I have an old Korean War era radio receiver designed originally for headphones but will drive a small 8 Ohm PM speaker. I used a small 120 VAC to 6.3 VAC step down transformer driving the 120 V primary side with my little 8 Ohm speaker on the secondary side. Been working that way for about 30 years. :)

Ron
 

wayneh

Joined Sep 9, 2010
16,400
Audio output transformers have not been used or needed for at least 64 years. What do you need it for?
My house had an intercom system installed when it was built 30 years ago. The OEM speakers are 60ohm, if I recall. They charge an absurd price for replacements, so I replaced it with a generic 8 ohm plus an impedance-matching transformer. Niche use, I suppose.

That itty bitty one from RS is only for hobbyist applications like an electronics starter kit.
 

Thread Starter

Dsnead

Joined Apr 16, 2020
12
Audio output transformers have not been used or needed for at least 64 years. What do you need it for?
Radio shack does not say and probably does not know its power rating.
It does not pass low audio frequencies and does not pass high audio frequencies so it is no good for audio.
It might make a good but expensive paper weight!
Mostly for general knowledge at this point, but I've had various projects throughout the last 10 yrs that I wish I had known more about audio electronics.
I have also read that they're not necessary anymore with new electronics (due to bridging think?), but I'm not sure where/when to draw the line. I just don't want to break expensive equipment.
 

Thread Starter

Dsnead

Joined Apr 16, 2020
12
Several options. You can just measure the DC Resistance which really won't tell you much, it won't tell you the actual impedance which is what is of interest. If you have the ability you can apply an AC voltage to the primary and measure the AC voltage on the secondary side. A good place to start is get the turns ratio so apply a low AC voltage like maybe 3 volts to the primary and measure the secondary. If I apply 3 volts and get out 1 volt I have a 3:1 turns ratio. Then apply 1 volt to the secondary and if the primary has 3 volts that pretty much covers the in/out and gives you the turns ratio. With the turns ratio known you square it to get the impedance ratio (if I remember correctly) so a 3:1 turns ratio is a 9:1 impedance ratio. Again look this up because I am not sure. Been a long time. The idea is to match any pair of impedance so a for example 8 Ohm speaker to a 72 Ohm source impedance.

I have an old Korean War era radio receiver designed originally for headphones but will drive a small 8 Ohm PM speaker. I used a small 120 VAC to 6.3 VAC step down transformer driving the 120 V primary side with my little 8 Ohm speaker on the secondary side. Been working that way for about 30 years. :)

Ron
Thanks Ron, I'd be interested in seeing that rig. Sounds pretty cool!
I'll post the math I used from the signal generator and oscope...see if you all think it's right.
 

Thread Starter

Dsnead

Joined Apr 16, 2020
12
My house had an intercom system installed when it was built 30 years ago. The OEM speakers are 60ohm, if I recall. They charge an absurd price for replacements, so I replaced it with a generic 8 ohm plus an impedance-matching transformer. Niche use, I suppose.

That itty bitty one from RS is only for hobbyist applications like an electronics starter kit.
What sort of electronics kit would it be useful in?
 

Audioguru again

Joined Oct 21, 2019
2,048
I have also read that output transformers are not necessary anymore with new electronics (due to bridging think?), but I'm not sure where/when to draw the line. I just don't want to break expensive equipment.
Nope. Output transformers were used 64 years ago because they were needed to match the high output impedance of vacuum tubes to the low impedance of a speaker. To produce power, a vacuum tube used a wasteful heater and a very high voltage with a low current. Transistors use a low voltage with a high current therefore are already a very low impedance.
An amplifier with bridging is actually two amplifiers, one for each wire of a speaker which effectively doubles the output voltage swing which almost quadruples the output power.
 

Thread Starter

Dsnead

Joined Apr 16, 2020
12
Nope. Output transformers were used 64 years ago because they were needed to match the high output impedance of vacuum tubes to the low impedance of a speaker. To produce power, a vacuum tube used a wasteful heater and a very high voltage with a low current. Transistors use a low voltage with a high current therefore are already a very low impedance.
An amplifier with bridging is actually two amplifiers, one for each wire of a speaker which effectively doubles the output voltage swing which almost quadruples the output power.
Thanks Audioguru!
 

wayneh

Joined Sep 9, 2010
16,400
What sort of electronics kit would it be useful in?
Good question, probably none of the modern ones, for the reason offered by @Audioguru again. I remember my first kit had one but that's a long time ago. I had a microphone with one built-in, and I know I've seen audio transformers in radios and other audio equipment. But transformers are relatively expensive and there's almost no reason to use one now. You can buy a handful of transistors for even ICs for what a transformer costs.
 

Thread Starter

Dsnead

Joined Apr 16, 2020
12
Here's what I got when I put a voltage through the transformer and recorded the secondary voltage on the scope. The top table is from the outer taps (1000:8 I believe). The math seems to work out on this table.

The bottom table is from the center tap which I thought was 500:8. But when I did the math, I get 240:8 or 500:16. Does this check with you?

Thanks again!
 

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wayneh

Joined Sep 9, 2010
16,400
Here's what I got when I put a voltage through the transformer and recorded the secondary voltage on the scope. The top table is from the outer taps (1000:8 I believe). The math seems to work out on this table.

The bottom table is from the center tap which I thought was 500:8. But when I did the math, I get 240:8 or 500:16. Does this check with you?

Thanks again!
Have you watched this? I didn't get thru it but it might have info for you.
 
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