I have a Counterpoint solid state power amplifier which I have been using since a year with my B&W 805 matrix speakers. This morning I changed my preamp to a Schiit Valhalla 2 Tube preamp and everything played well as usual. When I turned on my system this evening, both speakers were whining like a burglar alarm and I immediately switched off the power amp to be safe. There was a burning smell from the power amp for sure like some thing went wrong and the power amp has a 6 Amp 3Ag fuse for each channel next to the banana terminals. Upon observing, one fuse of the left channel had blown.
Tried to borrow a new power amp from a friend, only to find out that the LF/Mid drivers of both speakers are not working. The tweeters are working fine. Is it even possible for an amp which has a fuse at the output, still blow the speakers?
The speaker LF crossover has only 2 parts, a 2mH coil and a 6uF capacitor.
Tried to borrow a new power amp from a friend, only to find out that the LF/Mid drivers of both speakers are not working. The tweeters are working fine. Is it even possible for an amp which has a fuse at the output, still blow the speakers?
The speaker LF crossover has only 2 parts, a 2mH coil and a 6uF capacitor.
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The Counterpoint certainly could have blown the speakers. The six amp speaker fuse would not have done much
to protect them. It is also possible that defective or loose interconnect cables could cause the noise you heard.
Since this problem happened the same day as the new preamp was connected, that should be under suspicion as well.
to protect them. It is also possible that defective or loose interconnect cables could cause the noise you heard.
Since this problem happened the same day as the new preamp was connected, that should be under suspicion as well.
Oh, quite easily. The fuse is sized to protect the amp, not the speaker. I'd invest in some very cheap speakers or even just drivers to use before hooking that pre/amp combination up again. You may have run into preamp/amp oscillation issues or DC or some combination. A decent multimeter may also be useful if it has good AC response.
You will also need to use it to ensure you have continuity across L1, no continuity across C1 and see what's goin on with the driver itself. Assume nothing before you start buying replacements. 😉
As someone that killed the LF driver in an inexpensive pair of speakers once, is it the capacitors inline with the tweeter that protects it from DC or is there another reason they survive?
The fuse is unlikely to protect the amp even - fuses prevent short-circuits from starting fires, that's their principal purpose, to melt and break the circuit before the wiring burns.
When investigating a known fault it may be more useful to replace fuses by series resistors that limit the current to a level safe for the circuitry, especially with class B amps where the quiescent current draw is much less than the peak current. That and using no load or a selection of dummy loads again to keep the current level low and avoid cascade failures from the original fault while you are diagnosing. Do calculate for such a series resistor the worst-case power dissipation and ensure it won't burn up too!
When investigating a known fault it may be more useful to replace fuses by series resistors that limit the current to a level safe for the circuitry, especially with class B amps where the quiescent current draw is much less than the peak current. That and using no load or a selection of dummy loads again to keep the current level low and avoid cascade failures from the original fault while you are diagnosing. Do calculate for such a series resistor the worst-case power dissipation and ensure it won't burn up too!
Yes, the series tweeter capacitor(s) protects the tweeter from DC at the amplifier output.
I should mention that DC alone doesn't explain the audible whine. That sounds more like some sort of oscillation issue, and because it happened with a new preamp it's more likely to be a preamp/amp oscillation.
Sometimes the oscillations are ultrasonic and the first symptoms you are aware of is the smell of a burning tweeter coil.
Sometimes the oscillations are ultrasonic and the first symptoms you are aware of is the smell of a burning tweeter coil.
Counterpoint amps usually have lateral mosfet output devices and can oscillate suddenly when there's a ground fault at the input stage. It would have been a combination of input offset and oscillation. The direct coupled input stage would cause the DC offset without a ground reference connection at the amp input. The neg feedback loop on the inverting side of the input differential pair can provoke a phase shift significant enough to induce oscillation of the entire amp circuit.
At a minimum, the zobel resistor close to the amp output stage will have smoked from the oscillation, as the inductor paralled with it would have posed a high series resistance at higher frequencies, making the resistor the effective path of current. Most of the time, lateral fets will have exceeded their SOAR from oscillating, taking the mosfet out with it, especially when no series drain resistors are used at the output stage. I believe they used Hitachi 2SJ/2SK TO3 output devices in these amps. You'll have to replace these in pairs or groups when repairing the amp to guarantee reliable operation in the future.
At a minimum, the zobel resistor close to the amp output stage will have smoked from the oscillation, as the inductor paralled with it would have posed a high series resistance at higher frequencies, making the resistor the effective path of current. Most of the time, lateral fets will have exceeded their SOAR from oscillating, taking the mosfet out with it, especially when no series drain resistors are used at the output stage. I believe they used Hitachi 2SJ/2SK TO3 output devices in these amps. You'll have to replace these in pairs or groups when repairing the amp to guarantee reliable operation in the future.
How do I check my tube preamp if it's output is having DC which in turn has gone into the power amp and blown the speaker ? Using a Multimeter set to DC voltage and testing the output RCA of the preamp? What reading is unsafe ?
Yes, but is your power amplifier direct coupled? That's more likely not the case. Perhaps your preamp produces a thump. This may not be as easy to see without a scope.
Thump while start up and shutdown ? But I always switch on the preamp first and switch it off the last.
Is it even possible that DC from the Tube preamp has gone into the power amp and damaged it? Also, since I switch on the power amp in the end, the whining sound started the very same second I pushed the power on button. It took me about 3 secs to react and turn it off. Then waited for about a minute and switched it on again to check if it remains. And it did.
Is it even possible that DC from the Tube preamp has gone into the power amp and damaged it? Also, since I switch on the power amp in the end, the whining sound started the very same second I pushed the power on button. It took me about 3 secs to react and turn it off. Then waited for about a minute and switched it on again to check if it remains. And it did.
A multimeter will work just fine. Anything over a few millivolts is bad. If your amp is DC coupled this will multiply the DC by 20x or so, so probably good to be below 10 mV. Also, leave your preamp on for a while and recheck.
To be complete, a simple multimeter will check for DC, but it won't help you quantify how badly turn on thumps are or how/when an amp goes into oscillation. The ideal way to test for DC and turn on thmps is with a scope and a representative load resistor which matches your amp's input impedance. You put your meter and / or oscilloscope across that load resistor.
I once had a cable pull as I moved some cables naturally with everything on. Dumb move heard a squeal took my midrange out in about a second. So yes it can happen.
Rob 🙂
Rob 🙂
Just took out the drivers from the cabinet and measured the DC resistance. It shows 0.2 ohms. And the drivers have become hard. They don't compress when pushed with the hand. Looks like the voice coil is cooked