Hello,
Earlier today I fried a driver in one of my speakers. However, I do NOT believe it was because of my power amp. I have a 2 channel ps audio that was modified and upgraded 2 years ago and was bench tested professionally to be in great shape.
Just wondering what kind of tests I can do on my own at home with a multimeter to see if there are potential problems with my amp.
Things like DC offset.. bias... etc.
Any help would be greatly appreciated, strangely enough it is hard to find anything doing a google search on how to bench test your own home power amp (GOOGLE brings up crap for CAR STEREO!! BLAH)
Thanks for your time
Steve
Earlier today I fried a driver in one of my speakers. However, I do NOT believe it was because of my power amp. I have a 2 channel ps audio that was modified and upgraded 2 years ago and was bench tested professionally to be in great shape.
Just wondering what kind of tests I can do on my own at home with a multimeter to see if there are potential problems with my amp.
Things like DC offset.. bias... etc.
Any help would be greatly appreciated, strangely enough it is hard to find anything doing a google search on how to bench test your own home power amp (GOOGLE brings up crap for CAR STEREO!! BLAH)
Thanks for your time
Steve
Yes. Idle disconnected output DC level (<.2v)
DC Current through output transistor emitter resistors at idle (20-40 ma per TO3 transistor pair, less on TO220 output transistors). I=V/R. This current may change at various heat sink temperatures depending on previous watt history. Use an alligator clip lead on the meter minus probe at least, voltage >24 across your heart from hand to hand can stop it. Keep one hand in your pocket when working on power amps with rail over 24 VDC. Wear no jewelry, 3 VDC can burn your finger off through a ring.
If you want, buy resistors of the proper wattage and ohms to load test the amp. I use 225 watt 10 ohms resistors with slide tap at 8 ohms to test my 35 w/ch tube amp and 60 w/ch transistor amp . I use two 5 ohm 200 W resistors in series on each side to test a 400 w/ch amp at 8 ohms. I have another pair I haven't soldered up in parallel to test the amp at its 650 W/ch @ 4 ohm rating. An AC meter that works at music frequencies is necessary to test watts out into a resistor. I use a Simpson 266 VOM with a 20 VAC and 200 VAC scales. You can't buy these, they are "obsolete". You are permitted to buy DVM's that only work properly at 60 Hz and 50 Hz. Or you can buy a $500 scope, or a $40 broken one, with $50 each 10x probes.
If you solder a cheap surplus 4 ohm speaker in series with a properly rated PTC "fuse" or high wattage 1 ohm resistor, and two 4700 uf capacitors minus to minus, across one of the 5 ohm resistors, you can listen for distortions at all watt levels. I use a surplus FM radio for a signal source, repaired with a fixed resistor in place of the unavailable slide volume pot. I buy surplus speakers at charity resale shops. Car radio speakers are usually 4 ohm speakers. The back to back capacitors cause small IM distortions on high frequency sources like top octave piano and brush percussion or bells. But the keep the speaker from ripping the suspension if the amp jumps suddenly into DC output. I traced various causes of DC on speaker for over a year on my PV-1.3k, ending up with a bad solder joint on an op amp socket, probably from the factory 14 years ago.
If your amp has serious DC on speaker problems, a light bulb in series with the AC input, or a 1500 watt room heater in the case of >1.2 kw amps, can prevent output transistors from blowing up while you are finding all the bad parts. I had about 100 such parts, many found power off with a DVM ohms scale. One has to disconnect the speaker DC detection safety circuit while one is finding and repairing these problems, then reconnect it at the end of the repair.
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Make a dummy load for testing rather than speakers. Get yourself some aluminium clad power resistors rated 4.7 ohms (4 ohms is better if you can get them). Make them up on a heatsink so you have a bank of resistors you can connect in either 4 ohm or 8 ohm configuration. Connect that to your amplifier.
You will need to cool them of course. A heatsink with fan cooling, or even throw them in a bucket of oil (messy).
You will need to cool them of course. A heatsink with fan cooling, or even throw them in a bucket of oil (messy).
Or just use some plastic laquer on the connections and then put them in a bucket of water. Have used 4X10W resistors (1Ohm each) this way for years on amps up to 200W with no trouble. Not messy at all.
May I mention I made the connection on a simple 6mm plywood nicely made som it covers the bucket and acts as connectionboard for the speaker cables. Perhaps I should take a pic of it and place in here?
KISS is easy when You know how to 🙂
picture
Actually yes a picture would be helpful, that is a great idea!
Thanks for all your advice
Actually yes a picture would be helpful, that is a great idea!
Thanks for all your advice
Actually yes TANDBERG, a picture would be helpful... very clever idea indeed!
Thanks very much for everyone's advice... if you think of any thing else just let me know
Thanks very much for everyone's advice... if you think of any thing else just let me know
I bought some of these tapped 10 ohm 225 W resistors in 1989 for about $8 each. They are $32 now at newark.com. Newark also has 4 ohm 225 W resistors, set minimum and maximum 4-10 ohms, and set watt minimum for 200, and see what is cheap an in stock in the US. Don't buy direct ship (from UK) which is $25 shipping extra per line. I got similar non-tapped 5 ohm 200 W resistors at apexelectronic.com for $5 each last year. He has minimum $10 shipping and won't put heavy (like transformers) and fragile (like resistors) components in the same box.
I cut the end of a PCAT case with an auto body grinder, then cut tabs in it with a die grinder, use safety glasses. I bent the tabs up and stuck them in the end of the resistor to hold the resistor up off the coffee table wood. Voila, 225 W 8 ohm resistor pair, no oil or water required. It takes a 130 W pistol soldering iron to put 14 ga wire on the tabs. Use a file or grinder to remove the sharp edges off your sheet metal before bringing it in from the garage.
I cut the end of a PCAT case with an auto body grinder, then cut tabs in it with a die grinder, use safety glasses. I bent the tabs up and stuck them in the end of the resistor to hold the resistor up off the coffee table wood. Voila, 225 W 8 ohm resistor pair, no oil or water required. It takes a 130 W pistol soldering iron to put 14 ga wire on the tabs. Use a file or grinder to remove the sharp edges off your sheet metal before bringing it in from the garage.
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Yes, or use the resistance wire coil from old, open style electric jug elements. Trim the resistance wire element to exact length or fold and rewind on the ceramic former to parallel lengths for ~ 4 and 8R loads.
A glass or high temp. plastic container for > 0.5 litres of water is likely fine as a heatsink for short-term load resting on small-medium amplifiers.
It would be a good idea to scope check the outputs with real speaker loads.
An amp may be stable with a resistive load but with such a reactive load may break into oscillation / parasitic oscillation (ie requires signal).
Especially as a driver has burned out.
Pete
An amp may be stable with a resistive load but with such a reactive load may break into oscillation / parasitic oscillation (ie requires signal).
Especially as a driver has burned out.
Pete
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