What is DC offset and why did I blow my speakers

Pbb: I agree with what you are saying. By selecting a table that is very near the top of the hill in terms of isolation from feedback and using a vacuum platter to "suck" the record flat with the platter, i've minimized the potential for either problem to be much more than an after-thought in my systems. With the expenses incurred in doing this, buying a decent record cleaning machine(s) and taking the time to "prep" each LP, it is surely a time and money intensive investment. When one is willing to do such things though, the results can be VERY good. Quite honestly, i was able to obtain surprisingly good results using a far less costly TT / Tonearm / Cartridge set-up and far less labor intensive "record preparation" a few years back. The results were good enough to make me want to invest the amount of time and money that i finally did though. Sean
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In laymans terms, what they're saying is happening is that your BAT phono preamp is just letting straight DC current go through the circuit and into the speakers... just like when you hook a battery directly up to a wire connected to both of it's terminals, (try it sometime if you haven't done this) what they're proposing is the straight DC current just fried the voice coils...

It's possible that a tube is arcing or something, possibly damaged from vibration or shock during shipping, and a trip back to the manufacturer might provide the easiest way to correct the situation... aside from something happening with a tube which would affect the output voltage, other factors involved could be a leaking capacitor, the resistence of the IC's that you're using with the unit, and a couple of other theories like the subsonic or ultrasonic oscillation (feedback) theories that have been proposed in the threads which would show up on a scope while the music was playing, but otherwise wouldn't be heard at all...

with a DC offset problem, there would not be any excessive speaker excursion necessarily, as what would be happening is when music is being played, along with the source signal some straight DC current would be passed into the speaker as well, so when no music is playing the speakers would be fine... however, when music was playing the speakers would start to heat up... a DC offset problem, or voltage leakage, results in the speakers heating up and the voice coils melting....

if you've got speakers that have brass plugs in the center, like the SEAS Scanspeaks, you can actually touch the center cone and feel whether it heats up at all while the music is playing... but from what you're describing, it does sound like the symptoms you would get if the voice coils were getting fried... as the voice coils overheat, they begin melting the wax they're embedded in and they begin losing their perfectly round and symetrical shape... as the wax continues to melt, the coils start to separate from the wax which holds them in place, and when the wax has bubbled and melted enough, the speaker ceases to function... so at first you notice the sound "drop off" as the wax starts melting, and then... no sound... just a blown speaker... best thing would be to have the manufacturer check the unit and hope they can find the problem...

If you do want to experiment with the unit to check for speaker excursion, or to feel if it's heating up because of a DC voltage leak being sent into the voice coils, just limit the playing time to around 30 seconds or so... this is enough time to make whatever checks you may want to... but it's not long enough for the wax to melt should it be a problem with DC voltage being sent directly into the speakers...

Oh the joys of turntables! Sean,nobody talks about warps and feedback these days, just of liquidity. The analog turntable: when everything is right, the planets align and the moon is holding water, it sounds better than anything elso on God's green earth. For all the other days CD and SACD is not bad.

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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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.

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?

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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