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Alpine MRD-M300 strange distortion (oscilloscope pictures inside)


lunarfox

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Greetings SMD forum! This is my first post here so forgive me if it's in the wrong section. I wanted to share some interesting results from a benchmark I did on an Alpine MRD-M300 amp someone gave me a couple years ago. I was excited to try it out 'cause the DSP options are very nice and super easy to set up without additional test equipment. Not to mention anything with an ammeter built in gets lots of 'cool points' from me. Unfortunately, the power output is rather sad. I don't know if I have a defective amp or what, but needless to say I stuck with the cheap 400 watt 4 channel VR3 amp I found at the junkyard (which pushes out ~240 watts @ 4 ohms with two channels bridged; I use the other two channels for the 4 cabin speakers and I've been quite pleased with it, especially for $15).

So first how about some benchmarks? These are only rough figures I've gotten using an oscilloscope (BK 2120) and a multimeter (Fluke 27, older yellow face). I increase the volume until I see visible distortion, then take a reading from the voltmeter and use ohm's law to calculate the wattage. I only tested it into 4 ohms because that's what's relevant to my setup (only have one 12"; it's more than enough for a little Honda).

Visible distortion: 56 watts (see pictures)

Clipping: 144

Well the clipping figure isn't far off from the 150 watt rating, and I'm only testing at 13.8 volts. The distortion figure is just sad though. Anyways, picture time. Here's a 40hz sine wave on my scope from the MRD-M300 just before visible distortion (sorry my camera is so crappy):

scope1.jpg

And with just a bit more power:

scope2.jpg

See what's happening at the sine wave peaks? This happens on both a 4 and 2 ohm load at about ~15 volts output (scope is not calibrated in the pictures to better show the sine wave), and gets worse up to clipping. Anyone have any ideas what's up? My only guess is maybe the power supply is going bad and injecting ripple into the amplifier. It doesn't look like much, but it is noticeable; it makes the bass sound kind of fuzzy and weak (that's the best I can describe it). If anyone has any ideas on something I can do to fix this problem I'll give it a shot and post the results here!

Oh, and as a bonus, amp guts!

guts1.jpg

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