Audio Isolation Transformers: what do these specs mean?

Thread Starter

cspwcspw

Joined Nov 8, 2016
49
I get some mains hum. Perhaps caused by an earth-loop between my laptop's external screen, and the AUX-in (low-level signals) on my guitar amplifier. So in the spirit of DIY, I bought some cheap "audio isolation transformers" (https://www.banggood.com/10pcs-1300-8-Ohm-Audio-Transformer-EE14-Transformateur-Audio-POS-Transformador-p-1094073.html)

The "details" of the product include:
Model: EE14
AC impedance : 1300:8
Frequency Type: High Frequency

EE14 seems to refer to the E shape and size of the ferrite core. (Any clarification or confirmation on this?)

Both primary and secondary have DC resistance of about 134 ohms on my DMM.

Any guesses on what the 1300:8 means? My thinking (possibly wrong) is that impedance cannot be lower than measured DC resistance, so I struggle to read "8 ohms" into this anywhere. And I doubt this is a winding ratio. Is 1300 a frequency? Some rough attempts to measure impedance seemed to suggest the two winding impedances are the same.

I have a few of them, so I could pull one apart and count windings or look at the shape of the ferrite core, or even put some DC or AC through the thing to see when it melts. But it seems nicer if I could first understand a little more from the "specs", or more generally, how one goes about "characterizing" a transformer that may have been salvaged from a halogen light adapter or a LED driver.

Thanks
Peter
 

MrChips

Joined Oct 2, 2009
19,924
These are not meant to be isolation transformers.
They are impedance matching transformers. They are intended to be used in the output stage of a class-A audio amplifier to match 1300Ω impedance to 8Ω impedance.
 

Thread Starter

cspwcspw

Joined Nov 8, 2016
49
These are not meant to be isolation transformers.
They are impedance matching transformers. They are intended to be used in the output stage of a class-A audio amplifier to match 1300Ω impedance to 8Ω impedance.
Aha! Thanks. From what I find at https://www.quora.com/What-is-impedance-matching-transformer it seems that the the "matching" factor depends on (N1/N2) squared. So maybe I'm expecting to find about a 13:1 winding ratio inside. And I assume the winding with more turns is the "primary" that connects to the amplifier output, and the fewer turns winding will go to the speaker?

Would this also be usable to match, say 2600 ohm impedance to 16 ohm speakers? Or do the specific input and output impedances depend on resistance in the windings, etc?

And if I used two of these back-to-back, would I likely get reasonable "audio isolation"?

Thanks
Peter
 

Thread Starter

cspwcspw

Joined Nov 8, 2016
49
I guess product details are in error. I verified that resistances on both windings are about the same. So I wired up the 5V 1Khz square wave signal on my scope to one winding, and scoped both input and output, as in pic below. Looks 1:1 in terms of input and output voltages. Tried experiment both ways (using the other winding for input, etc.) essentially the same picture. Not sure if this attenuated wave is telling me something fundamental about the inductance of the transformer, or about the impedance of the signal generator in the scope. But as it stands, this is not going to make for faithful audio reproduction! I suspect that this wants much higher than audio frequencies (specs did say High Frequency Type). I think it is probably intended for small buck voltage converters.
 

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