DAC drops out when Furnace starts or stops. HELP

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Correction:

I said,

You never said if the furnace or hot water heater are gas or electric.
If electric my guess is they are fed by single phase 240Vac.
10-09-15: jea48

Every time my furnace or water heater (gas, power vented) kicks on or off my DAC drops out momentarily.
10-07-15: Jadedavid

Guess I missed that......
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Thanks to all for the suggestions and info. After listening again tonight I have had no drop outs with the 2nd digital cable in place.
However I think "Almarg" has hit on the root cause.
I have been constantly fighting a ground loop hum that I cannot cure.
I have tried cheater plugs on some/all pieces of equipment, plugging all the audio gear into one strip, isolation transformers ETC ETC.
I have come to the limits of my knowledge of finding the source of the imbalance.
Thanks for the ground/neutral info from "Cdrc".
I think it"s time to call in an electrician. Hopefully I can find one who understands this type of problem.

I think it"s time to call in an electrician. Hopefully I can find one who understands this type of problem.

Your average electrician will not solve a ground loop problem.

Here is a white paper for you to read.

I have tried cheater plugs on some/all pieces of equipment, plugging all the audio gear into one strip, isolation transformers ETC ETC.

What equipment in your system uses the AC power mains safety equipment ground? (Check all audio equipment with an IEC power connector for the safety equipment ground pin/blade. If only 2 blades the equipment ground is not used on the equipment.)

Are you sure you are hearing a hum or could it be a low level buzz?

It would help if you listed your equipment, ICs, speaker cables, and any after market power cords you are using.

By chance do you have a TV with CATV or a Dish connected to the system?
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FYI, I have found no correlation between length of USB cable (1m vs. 1.5m) and performance across brands. You are not assured superior quality/performance from a supposedly ideal 1.5m USB versus another brand 1m. It is also relatively easy to find a 2m USB which will outperform any given 1m or 1.5m alternate brand.

I have a few times found that 1m sounded better of the same brand than 1.5. So, you can say I'm not a believer in the idea that 1.5 is somehow magical for sound quality of USB.