Re: Line Level Isolation Transformers
I finally got around to testing the box for crosstalk and external hum pickup.
Using the Audio Precision test set, 1kHz crosstalk from CH B into CH A is about -120 dB. That figure was measured with +4 dBu on the B channel, and it improves by a couple dB if the level is increased to +20 dBu, indicating to me that much of that signal is noise, not 1 kHz tone. At the extremes of the audio spectrum, 20 Hz and 20 kHz, the crosstalk increases to -110 dB, so still pretty good. Usually, both inputs are terminated by the generator source impedance (probably about 30Ω), which should keep things quieter on the output under test. Even unplugging the A input to the box from the generator only degraded the crosstalk by about 2 dB (at 1 kHz), so less significant than I might have guessed.
As for noise, the measured noise on each channel (22 Hz to 22 kHz band limited) was about -110 dBu. If there's a standard for external magnetic fields, I don't know what it is, so I just used a random wall wart on the end of an extension cord that I could orient in different positions to try to find the worst possible case. The worst I could find induced enough noise in one transformer to measure -53 dBu at the output (inputs terminated by the test set). This was with the top of the wall wart pressed against the side of the ISO box enclosure. At 6 inches away, I could read -80 dBu in certain orientations, and even a foot away, I could still read something above the noise floor, if I turned the wall wart in the right direction.
So, it turns out that these transformers are pretty good at containing their own magnetic fields, but not very good at rejecting external fields.
I've actually experienced hum pickup into DIs when they got close to wall warts powering keyboards, although I'm not sure about the shielding properties of those DI transformers. I haven't used these ones enough to run into any problems yet.
GTD
I finally got around to testing the box for crosstalk and external hum pickup.
Using the Audio Precision test set, 1kHz crosstalk from CH B into CH A is about -120 dB. That figure was measured with +4 dBu on the B channel, and it improves by a couple dB if the level is increased to +20 dBu, indicating to me that much of that signal is noise, not 1 kHz tone. At the extremes of the audio spectrum, 20 Hz and 20 kHz, the crosstalk increases to -110 dB, so still pretty good. Usually, both inputs are terminated by the generator source impedance (probably about 30Ω), which should keep things quieter on the output under test. Even unplugging the A input to the box from the generator only degraded the crosstalk by about 2 dB (at 1 kHz), so less significant than I might have guessed.
As for noise, the measured noise on each channel (22 Hz to 22 kHz band limited) was about -110 dBu. If there's a standard for external magnetic fields, I don't know what it is, so I just used a random wall wart on the end of an extension cord that I could orient in different positions to try to find the worst possible case. The worst I could find induced enough noise in one transformer to measure -53 dBu at the output (inputs terminated by the test set). This was with the top of the wall wart pressed against the side of the ISO box enclosure. At 6 inches away, I could read -80 dBu in certain orientations, and even a foot away, I could still read something above the noise floor, if I turned the wall wart in the right direction.
So, it turns out that these transformers are pretty good at containing their own magnetic fields, but not very good at rejecting external fields.
I've actually experienced hum pickup into DIs when they got close to wall warts powering keyboards, although I'm not sure about the shielding properties of those DI transformers. I haven't used these ones enough to run into any problems yet.
GTD
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