Axiom Home Page Forums General Discussion Technical Questions DIY amp + problems with 4 + 8 ohm speakers

Hi,

I've started accumulating my axiom collection slowly, choosing to start right from the top with a pair of m80 fronts. They're surrounded by cheaper speakers (Merak) center + surrounds, which are 8 ohm. I'm using a DIY amplifier kit using a tripath chip (similar to that in Sonic Impact T-amp), which should deliver 100W/each channel (40-50W RMS). I've been having constant power problems, melting out a power supply, and was wondering if this might have to do with having two different sets of resistances on my speakers (though likely crappy condo electrical wiring is to blame). The amp is very energy efficient, and isn't generating a lot of heat, so if something were the problem I'd imagine it would be surges (last time I connected the power supply it totally fried the Monster Powerbar it was connecting to, though it's Monster so I shouldn't be that surprised).I was just wondering if anyone else using a DIY amp/dedicated amp was having similar problems with power. Thanks!

Sounds like the amp to me. Perhaps it dislikes the 4 ohm load of the M80's. It's kind of impossible to speculate since it's a custom product, but I'll bet it's related more to the overall load than to the notion that you have speakers of different impedance on other channels.

My guess is that, yes, your amp cannot handle the 4 ohm load. From the little I know about DIY, there should be websites that tell you how to build an amp that can handle the 4 ohm load without melting, and without limiting power to a 4 ohm load.

A quick side note, Merak speakers were made by Axiom, but were discontinued a few years ago.

If you're melting power supplies in a DIY amp my first thought would be ultrasonic oscillations (no I'm not kidding). It's pretty easy to fix once you know what the problem is -- even a simple RC filter on the amp input will cut down the frequencies above audio enough.

There's a possibility that driving a lower impedence load might make the amp more prone to oscillate (although I don't remember why) but I doubt that mixed impedences have anything to do with it.

The symptoms of oscillation would be that one or more amp modules get VERY hot and either trip the protection circuitry or "let the smoke out".

It could just be something boring like a short, of course

Thanks for the suggestions. The amp is based on a similar chip as the T-amp, which can handle 4 or 8 ohm loads without any problem. In fact, I've been using my little T-amp in the meanwhile with my m80's, and it can handle the load without any problem. I hope this one doesn't blow out too, but it isn't running too hot. Since the technology is the same on both chips (switching class-D amps, though they like to call them class-T), I'm not sure why this would be a problem on my multichannel board, and I'm pretty sure it's not that it can't handle a 4 ohm load. What I have NO idea about is whether or not having two different sets of resistance (4 + 8 ohm) could be contributing.