Friday, 3 March 2017

Audio Transformers

I ordered a Ground Loop Isolator from Ebay a few weeks ago for about £2 - a box with 2 audio transformers in it, usually for installing in a car between the line out of a car stereo and an amplifier. I got it for connecting between a radio and PC, in case there were problems with ground loops and other interference.

And here are the transformers inside
I have my Yaesu FT-7900 connected to a Edirol UA-25 USB audio interface using a 6 pin mini DIN to phono cable. One phono plug for discriminator audio (9k6 packet out) and another for normal (voice) audio. The first test I did was to try decoding some DMR using DSD+ software. This normally works OK using the 9k6 packet output.
DSD+ was trying to decode the DMR but there were so many errors it could not even get the correct colour code on a strong signal. Definitely the transformers causing the problem.
I checked their frequency response by looping the UA-25's output back to the input and playing some test tones through it, then comparing with a straight through cable.
The table below shows the insertion loss at each frequency compared to a straight through cable.
The -3dB point is somewhere between 50 and 100Hz, this would be OK for voice and data which used frequency shift keying only within the normal audio bandwidth of a radio, but for DMR (and probably other data modes using the same modulation), this is no good. It cuts off a bit high for music too, at least with a good amp and speakers.

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