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<blockquote data-quote="Geoff Doane" data-source="post: 83160" data-attributes="member: 1155">Re: AC line loss causes humI ran into what might be a similar problem with some Focusrite compressors a number of years ago.&nbsp; It seems that these rather high end processors would hum, seemingly at random.&nbsp; Testing with a Variac revealed the problem.&nbsp; Everything was fine at normal (115V) line voltage, but when it dropped down below 110V, the hum appeared on the outputs, even with no input connected.&nbsp; It turned out that the power supply rails were falling out of regulation, and ripple would appear on them at lower line voltages.&nbsp; My preferred solution would be to replace the the power transformer with one with a higher secondary voltage, but Focusrite&#039;s answer was to simply lower the power supply rails from 15V to 12, giving 3V of regulation headroom.&nbsp; Lower the rails affects the maximum output capability of the device, but no one has ever noticed that there is less headroom, and hum hasn&#039;t been a problem since.&nbsp; Something similar might be going on in the OP&#039;s situation.GTD</blockquote>

[QUOTE="Geoff Doane, post: 83160, member: 1155"] Re: AC line loss causes hum I ran into what might be a similar problem with some Focusrite compressors a number of years ago. It seems that these rather high end processors would hum, seemingly at random. Testing with a Variac revealed the problem. Everything was fine at normal (115V) line voltage, but when it dropped down below 110V, the hum appeared on the outputs, even with no input connected. It turned out that the power supply rails were falling out of regulation, and ripple would appear on them at lower line voltages. My preferred solution would be to replace the the power transformer with one with a higher secondary voltage, but Focusrite's answer was to simply lower the power supply rails from 15V to 12, giving 3V of regulation headroom. Lower the rails affects the maximum output capability of the device, but no one has ever noticed that there is less headroom, and hum hasn't been a problem since. Something similar might be going on in the OP's situation. GTD [/QUOTE]

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