'Low performance' transformer tests

[Dr.EM]

I have a pair of LT44 audio transformers. These are about the cheapest audio transformers you can buy and you wouldn't expect much performance from them. I remember however using them to break a ground loop, expecting all the bass to dissapear, but being pleasantly surprised with thier sound. With this in mind, I decided I would run a quick test to see how they actually perform.

LT44 AUDIO TRANSFORMER & chr(42) & RoHS & chr(42) &

Using HOLMimpulse, frequency response and distortion is measured at approximately 0.1mW and 0.5mW primary coil power levels. The 20k side is driven from an op-amp with 150Ohm series resistance and the 2x1k outputs are used in series and loaded by 47kOhm. The response without any transformer is shown in blue and this represents my soundcard, the TL072 op-amp (for gain and output buffer) and any cabling, cap coupling etc.:



All responses are raw unsmoothed, the wigglyness at the top is a mystery and occurs even with no circuitry at all; soundcard artifact presumably. More detail on the frequency response can be seen here:



I am impressed with how low this response extends and its overall flatness. I had (perhaps wrongly?) assumed that small/cheap audio transformers do not couple bass frequencies properly. I am now thinking that perhaps, at low power levels at least, the downside is simply increased distortion in low frequencies. Nonetheless, these small and very cheap transformers don't exceed -40dB (0.01%) THD at 25Hz with low power input, which is fairly decent.

Just thought some of you may find this interesting! I would show you a photo of the test setup but my camera is reporting a 'card error'.

[Dr.EM]

Correction - should be 1% THD at 25Hz as represented by -40dB. Good for this frequency but it doesn't lower too much until much higher frequencies so perhaps it isn't so great. Still impressed by frequency response and distortion may be acceptable depending on harmonic content.

[simon7000]

Yes those are typical results! That is why so many musicians' direct boxes contain very cheap transformers. If they lost low frequencies even they would notice. Distortion of course is well masked by the amplifiers they use!

Nice work!