Help! Help! speaker demaged!

[senderj]

I demaged my pair of 18 years old Tannoy speakers last night. I think the bass coil was demaged but the tweeter is still working.

The story: I completed my DIY GG preamp (my first project) on Wednesday night and it sounded great with my 18 years old Harman/Kardon amp (use the power amp part) and the Tannoy. I leave it on (but others off) for the whole day-time of Thursday to burn in. Then yesterday I added an LED at the front for power on indicator. As advised, I checked the GG's power supply to the amplifier part and any DC or AC at the output and everything was ok. I connected it to the HK, switched them on in the order: CD player, GG, HK. But when switched on the HK, the Tannoy poped and I could heard the wrapping of the coil cracking. Now the bass stratched out and freezed in that position.

The question: Since both L and R speakers were demaged at the same time. I don't think it is the speaker problem. Afterward, I check the output of the HK for DC or AC and got nothing. So I don't know whether it is the HK or the GG caused the demages. The Tannoy is very old and is time to be replaced. But, without knowing the cause I dare not to connect a new pair. Can somebody teach me how to find out whether the remaining equipment are ok or not?

[I_Forgot]

Since the tweeters did not cook, it seems that some DC went through the speakers. The tweeters would be protected from DC by the capacitors in the crossover.

It is possible but an unlikely coincidence that your amp just happened to die and take the speakers with it at the same time you put your new preamp into the system. The most likely scenario I can think of is that your preamp is DC coupled and has a DC offset problem.

If the amplifier also is DC coupled, it will amplify the DC from the preamp and do what it did to your speakers.

Take the preamp out of the system. Put a load on the amplifier (preferrably a resistor rated for whatever power the amp can deliver, but a couple watts will do if you can't get a big one). Turn everything on, with no music playing) and measure the voltage across the speaker terminals. Be sure to check for DC and AC. Now play some music and check it again.

Now check the preamp by itself- power it up and measure the DC and AC voltages at the output (no music).

If everything looks OK, put the preamp back into the system and recheck the amplifier output. If there's little or no DC, play some music and try measuring AC voltage at the amplifier output.

If you have DC coupled either the preamp or power amp, I suggest adding a coupling cap. They are normally used to prevent problems like you have experienced. People complain about the "sound" of the coupling cap, but what sounds worse, a coupling cap, or blown speakers?

I_F

[senderj]

Thanks for the reply.

I have used ordinary multi-meter to check, but without resistor, the output of both amp, but there were neither DC nor AC. Will adding a resistor make a difference?

[senderj]

I have checked the output of the HR power amp for DC. WIth a 10 Ohm resister, it is 0.1V reversed polarity. WIthout the resister, it is 0.15V. One thing observed: if I leave it on, the voltage slowly dropped to 0.4V in 1.5 minutes. Is this normal?

The GG preamp does not have any DC in the output.

[testlab]

Sounds like the amp has a bad interstage coupling cap or the filter cap(s) on one of the supply rails is going away. Which HK amp is it?

[senderj]

It's a PM665 integrated amp. I use the power amp part for the GG. Any adivse for me for repairing it?

[dnsey]

The Tannoy is very old and is time to be replaced. But, without knowing the cause I dare not to connect a new pair.

The older the better as far as Tannoys are concerned! Once you've determined the fault, get your speakers reconed (or send them to me to dispose of them ).
In the meantime, find some throwaway speakers (literally - anything will do) for testing this and future projects before risking your good speakers.