Measuring audio isolation transformers, some results

1

Hello everyone,

I’m investigating isolated line-level output and input at the moment. It seems possible for a decent frequency range, provided the levels are quite low, around 0.3Vrms (-10dBV) level. Here are the specs. I am aiming at.

Line Out:

  • Source impedance around 100 to 600 Ohms
  • To load impedance 10k Ohms or more
  • Sends 2Vrms level but could be as low as 0.3Vrms

Line In:

  • Input impedance 10k or more
  • From source impedance around 100 to 600 Ohms
  • Accepts 2Vrms level but could be as low as 0.3Vrms

I am curious if anyone has suggestions for a decent isolation transformer I could measure?

Here is the frequency response of a few transformers.

ATX Transformer Frequency Response
ATX Transformer Frequency Response800×600 5.31 KB

The 100-001 is something like these cheap transformers built around EI-14 cores:

image
image140×210 18.8 KB

and the other options are larger and more expensive.

Low-frequency response and THD is always an issue with such small devices for two reasons:

  1. large inductance needed to avoid low-frequency roll off and this is just plain hard in a small form factor, and,
  2. voltage-time (V-t) product of the signal is larger at lower frequencies, meaning that the core can more easily saturate.

The THD shows an interesting behaviour. For very low input levels (less than -16dB) THD is around 1% and this improves as the signal level increases. But below 100Hz pretty much any signal level cases greater than 1% THD.

ATX-100-001 THD with input
ATX-100-001 THD with input1537×964 15.5 KB

I suppose I am abusing the transformer a bit. It is designed for 600 Ohm : 600 Ohm and I am using it terminated with 10k Ohms. Although this might improve the frequency response because the current in the secondary will be less. Although it might be a bit more complicated than that :thinking:

2

Might be a bad transformer for passing a high quality audio signal.
But these small transformers can come in handy if you are trying to do
wire in some metering circuits (vu buffer etc) to an output and want to keep that circuit isolated.

I always wondered if there would be some cheap mains isolation transformers that would be suitable for audio? They should be good for low frequencies at least…

3

Hello Dan, Jensen audio transformers may be a better choice for low distortion and high bandwidth. Line Input | Jensen Transformers
Although they are more expensive and used in professional studio equipment. Another high quality provider is Lundahl: Line input - Lundahl Transformers

4

hello,

I use Radial - TWIN ISO Line Isolator
You can find some time this unit in 2nd hand market around 1/2 less brand new stuff
with 2 Jensen inside is less expensive than buy part separately