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What is DC offset and why did I blow my speakers

So I have a Krell integrated and typically play CD's with out problem. I added a turntable and a BAT tubed phono stage and it sounds great for about five minutes, then my sound starts to drop off and two minutes later I've got two blown woofers. I talk to a few people who say that I probably had a DC offset problem and to check the DC voltage at the output capcitors on the phono stage.

In laymen's terms (not an expert here), can someone describe to me what might have happened and what a DC offset is, how it is corrected, etc. I don't want to go through another set of speakers.

And what else might have caused something like this. I replaced the speakers and back to just the CD now without any problems. I really want to get this phono back up and running so what should I do next.

Thanks in advance!
jspownell
sean
6,229 posts
 
My thoughts are that the phono stage either has a DC offset problem and / or you were pumping a ton of subsonic info into a vented speaker. This can cause excessive woofer excursion, resulting in their demise. If you're speakers are vented and you want to play vinyl, i would HIGHLY suggest using a subsonic filter. Record warps can produce GOBS of energy, especially if the arm / cartridge are not properly set-up / matched. Other than that, you may have to send your phono section back to the manufacturer for an inspection.

Outside of the problems at hand, it sure seems like BAT gear is mentioned in more than a few threads of this nature. It makes me wonder how good QA is over there ?? Sean
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ivanj
640 posts
 
How close is the turntable to the speakers? Probaly have it in between the speakers? What kind of TT and cartridge is it? What kind of speakers are they?
jspownell OP
4 posts
 
The turntable was in between the speakers with approximately three feet on both sides. They were good quality bookshelf speakers running from an Aries with a Clear Audio Sigma Wood.
sean
6,229 posts
 
As mentioned, you were either suffering from acoustic feedback ( speakers too close to a turntable that is vibrating with the beat of the music ), the records were producing tons of low bass due to record warps or the phono section is defective.

If you want to see if it was acoustic feedback, you would have to have some speakers with functioning woofers hooked up. You would have to select the phono section and crank the volume WAY, WAY up without playing anything. If you can do this and you don't hear a loud low frequency howl, your turntable is stable and not suffering from acoustic feedback in a severe manner. It is possible for acoustic feedback to set in once music is playing, but this is a "quick & dirty" test.

As far as record warps go, you can watch a record go round and look at the woofers. If the record has a warp to it, you will see the woofer jump when it comes to that section of the record. The only cure for this is to not play that record, use a "high pass filter" or find a way to flatten the record out.

As far as the phono section passing DC, that is something that the manufacturer will have to correct. There are ways to fix this, but it would be best for them to correct the problem rather than you have to resort to performing a band aid that may reduce the performance of the system. Sean
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