How Do You Identify Input Transformer Types?

[Vikki]

Picked up an oldish solid state Audix PA amp at the junkers today, there are 3 metal can input transformers all with the number3129 printed on the top of them. Looks like the input wires are orange and white and there are 4 more wires red, green yellow blue. Iwas hoping i could make a DI box as on BO's site, do you work out the ratio's using the resistance of the windings or does the 3129 number mean something?
Vikki(uk) :grin:

 

[Bjorn Zetterlund]

Do you know the make of the transformers? Sowters tend to be 4 digits...if they are, give 'em a ring; they are usually friendly and helpful.

...Or you can send one to CJ, and he will get the ole hacksaw out for ya!

Bjorn

 

[toby]

i'd get the meter out and work it out from the impedance. if you want to make a di box the unbalanced end shoukd be around 10k at a rough estimate. the balance end should be about 600 with a ct ( so it will meter 300 to both other pins). dunno what the extra wire is. could be a CT on the 10k, or there could be two separate 300 ohm primaries?


toby

 

[Vikki]

Thanks
I'll have a go with the meter and see what i can figure out.
Vikki(uk) :grin:

 

[mcs]

You can't measure impedance with a multimeter!

What you will measure is DC resistance, which has nothing do do with impedance. You can't measure the impedance at all in fact. You can check the turns ratio and then calculate the impedance ratio.

Connect a signal generator to one of the coils and pass a low-level signal though the transformer (just some mV). Then measure the voltage on the primary and secondary, and then you will have the voltage ratio, which is the same as the turns ratio. If you masure 10mV on one side and 40mV on the other, then the voltage ratio is 1:4. The impedance ratio is then 4^2 = 16.

You will need a good meter for these measurements. It has to be sensitive, and it has to work at the measurement frequency. Not all meter will measure correctly at 1kHz. If you don't have a good mV meter you can use a scope instead.

Best regards,

Mikkel C. Simonsen

 

[NewYorkDave]

An ohmmeter is almost useless for this purpose. It will only tell you the DC resistance of the windings.

Inject a 1kHz sine into the primary and read the voltage across the secondary winding. The difference between the input and output voltages is the turns ratio of the transformer. The impedance ratio is the square of the turns ratio.

Since these are mic input transformers, we'll assume that the primary winding was intended to be terminated by a 150-200 ohm source impedance. So, take your measured turns ratio, square it, then multiply that number by 150 ohms--and that will give you the secondary impedance. So, you'll have a "150 : X" transformer.

When selecting a transformer for a particular application, there are many more important parameters besides turns ratio. But at least this quick measurement will tell you if these xfmrs bear further consideration for the circuit you have in mind.

 

[NewYorkDave]

Ha! Looks like we both posted essentially the same message at the same time :green:

 

[clintrubber]

An ohmmeter is almost useless for this purpose. It will only tell you the DC resistance of the windings.

FWIW, we had the thread at the old place about whether or not measuring TXs with DC could harm them. If I remember it correctly it wasn't that harmfull unless the lower Ohmage-ranges on a multimeter were used, but the advice is popping up here & there (like the Amplimo-website - warning against it).

Problem here is that for TXs like you mention, the low ranges (say 200 Ohms) are likely to be used.

I guess it's not always that clear if a TX has been bitten too much by DC,
but as it seems the people around here don't have had much problems in this respect.

Bye,

Peter