[yeldarb]

Frank, I own one of your 4i pre-amps with the tranny upgrade, driving a small Sonographe amp which is not too shabby.  I just had a small self powered subwoofer, with a Class D or switching power supply, hooked to my preamp with a 10 foot interconnect.  Electrical line noise apparently causes a feedback from the subwoofer amp back to the preamp through the interconnect.  This noise then goes back out to the main amp and is heard in the main speakers, as a distortion very similar to a really bad ground.  Floating the power ground on either the main amp or sub makes no difference.  I had a similar problem with some switching supply mono amps.  Any line noise made a  "burp" in one amp that went back to the pre and out to the other channel and to the sub (different one), in that case.

Now when I phone this problem in the techs think that I am nuts.  But isn't this possible?  I don't think that there are any blocking caps in the preamp that would stop the trash from getting back in the loop.

[craig223]

You may need to get a transformer to put between the preamp and the sub.  I had similar troubles with a satellite receiver.  Once it was connected to the preamp, a hum was introduced.  My transformer is a Rolls and does the trick.  Others on this site have had good luck with Jensen transformers.  Do a search on the site for more info.  Good luck!

[avahifi]

I suspect that high level switching garbage from the digital amplifier is getting both into the AC power lines and into the signal ground between the preamp and that subwoofer.  I would be thinking about doing two things; first make sure the digital amp powered subwoofer AC supply is on a separate line from the rest of the system and as far away as possible.  Second, if all else fails, you may need to transformer couple the signal line between the preamp and the subwoofer amp as craig223 suggests.  Make sure both the signal and ground side are isolated.  The output of the preamp is AC (capacitor) coupled, but that won't help with the switching noise you are getting.

Frank Van Alstine

[yeldarb]

Thanks, Frank.  I found a solution.  I returned the sub to the maker and ordered another of what I already had (now on another system) because I had no problems of any sort with its A/B traditional sub amp.  The problem sub sure was pretty, though.

I think it is ridiculous that a manufacturer would recommend that you buy a line conditioner / filter because their product amplifies and passes on noise.  This digital switching stuff apparently isn't what I want to deal with.

[yeldarb]

New sub came in yesterday (Partsexpress Titanic 10" kit).  Put it together, hooked it up and, NO NOISE!  This is a pretty nice little sub to 30 Hz, if you haven't heard of them, though it won't win ANY beauty show.  Now why is it that this one makes no weird noises and the digital ones do?   

[avahifi]

Because digital amplifiers are not ready for prime time yet.  They radiate garbage.

Frank