I think my preamp cooked my speakers HELP

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merickrc

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Joined 2009
#1
hooked up the preamp last night and it sounded distorted when i turned it up i looked over at my speakers and the woofer cone was flexed all the way out like the speaker was tring to turn it self inside out i went over to the speaker and it was smoking so i disconected the preamp hooked up my cd player to the amp then tried the speakers now the mids and highs are all crackely i think the speakers are shot what do you think let me know thanks merick
 
#4
merickrc said:
i think the speakers are shot what do you think let me know thanks merick
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At least the woofers and maybe some components in the crossover. The other drivers should have survived. Disconnect the woofers and check, if the crackling goes away. Maybe it is the protection cicruit reacting to the partly fused voicecoils.
 
#13
How long were you using the amp, before it fried your speakers?

The speakers seem to have fried because of too much DC going through them. Have you measured the DC offset at the amps speaker terminals? If so what is the reading?
 
#15
Your power amplifier seems to be working well with the CD player. So the DC must have come from the preamp.

The schematic in the link shows a capacitor before the last stage, so there should not be any DC coming through the signal path.
  • - Check, if both rails are connected and working. Measure the voltage at the op amp pins 4 and 8 for that.
    - Check, if there are shorts between the traces, especially from the supply voltage to the signal path. Disconnect the preamp for that and measure resistance from the output to each rail. If the resistance is zero, find the short.
    - Check, if the op amps are the right way round.
    - When you are sure that everything is as it should be, measure the preamp output. If there is a DC output at rails level, the op amp is probably fried and needs replacing. If it is lower, swap the op amps and see, if the situation improves with a different op amp in the place of U3.
    - Don't connect the preamp again, before you know for sure that it is working correctly. Check for DC at the output. Check, if the signal strength is correct.
    - Put a capacitor greater than or equal to 2,2 µF between the potentiometer and R1 in your power amplifier to avoid that the same thing happens again, when any fault in your sources leads to DC at the power amp input. And use Ci, if you have not already used it.
 
#17
AndrewT said:
the 3886 schematic shows no DC blocking cap at the input.
Dangerous for newbies.
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So the title of this thread s/b be my power amp smoked my speaker.
DC block is shown as "optional" on the chip amp schematic linked to by OP. Should be standard incircuit and optional to bypass if you know what risks that implies.
Doubtful if the smoking speaker incident would of happened if that DC cap had been inplace.
 
#18
infinia said:
DC block is shown as "optional" on the chip amp schematic linked to by OP.
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The DC block is not shown at all. The optional capacitor that is shown deals with the amp's own DC offset and guarantees that that is not amplified with the AC gain.

infinia said:
Should be standard incircuit and optional to bypass if you know what risks that implies.
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The seller, Peter Daniel, has given his opinion on that topic many times.

infinia said:
Doubtful if the smoking speaker incident would of happened if that DC cap had been inplace.
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It would doubtlessly not have happened.
 
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