SR-71 Burnt Crossover Resistor
I recently built 3 Zaph Audio SR-71 speakers with the upgraded crossover components and pre-built cabinets. After breaking them in for a couple of days I decided to crank 'em up for a while. After listening to them for a few minutes I started smelling something burning, the unmistakable smell of letting the smoke out of an electronic component. I quickly turned the volume down and started checking for damage. Luckily I didn't mount the crossovers inside the cabinets and it was readily apparent the 4 ohm resistors labeled R0 on the schematic in series with the tweeters had fried, charring the PCB and actually burning the piece of wood shelving I'd mounted the crossovers on. The amplifier connected to the speakers is capable of an honest 200 Watts per channel and I admit I was playing them very loud. Even at this volume level the speakers sounded very nice if lacking a bit in the low bass until the resistors fried and their tweeters quit working. I located six 20 watt, 8 ohm non-inductive power resistors at the local electronics emporium and wired two in parallel on each crossover replacing the original resistors. They seem to sound fine though I need to measure the response of each speaker to ascertain whether any additional damage was done. The paralleled resistors barely get warm to the touch. Any ideas where the most likely points of additional damage may lie, assuming there is any? Anyway, I'm happy with the SR-71's but anyone wanting to listen to them at ear splitting volume levels may want to beef up resistor R0 and maybe avoid the problems I had. Hat's off to John Krutke for a fine design.
Running full range, my estimate of which component to fail first under high power was the woofer. Then the tweeter, and very rarely, the crossover components.
That said, what component fails first is highly dependent on the musical content. A more continuous signal at just the right frequency could indeed fry a resistor first rather that the much more common dynamic peak causing a mechanical driver failure. With any design it doesn't hurt to double up on crossover components for power handling.
Something else to be aware of: While the resistor is definitely shot, it may have been caused by another component failure in the system. Replace the resistor first and see if the system sounds normal, but don't be surprised if something doesn't sound right. Then begin your troubleshooting to find the real failure point.
Also be aware that 200 watts is more than double what is really appropriate for the SR71, so you might want to exercise caution with your volume control.
Being a NOOB at all this, I was wondering what would be the best way to start diagnosis. The preamp amplifier combination I bought came with a microphone to be used for automatic equalization. Could this be used with measurement software to measure frequency response to detect any anomalies? I have no means to measure the inductors or capacitors. One thing I've noticed after repairing the crossovers is that the speakers sound rather harsh peaking in the treble range. Sibilants especially seem to be too pronounced. If it's not against the forum rules I could post the brand of the preamp amplifier combination I purchased if that would be of any help. Also any recommendation for free measurement software would be appreciated. Please understand I realize the failure was entirely my fault and I am not trying to put the blame on anyone else but me. Thank you for replying to my post.
Everyone has to start somewhere. Feel free to post your equipment and link to it if you like, we encourage it. Hard to talk about a stereo system without a complete view.
We even document it in the forum rules.