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Discussion Starter #1
Good morning everyone! I stumbled upon this forum looking into possible options for my DIY subwoofer retrofit I was working on, and found it very useful, so I joined! As my first post I pose a question to those audio / electronics gurus out there.

I have picked up an older Acoustic Research AR 108PS 8" Subwoofer and replaced the amplifier board with a Velleman Discrete 200W mono amp as the existing one was toast. After switching out the Amp, the sub played only as a loudspeaker, so I added a PASSIVE low pass filter rated for 50Hz. Since I had installed said items, I get alot of woofer extension / movement, but very little sound. The amp board gets really hot after about an hour of use... Does anyone have some ideas to how to get my Sub thumping properly? I can provide pictures and whatnot soon. Thanks for the help in advanced!

-Nick
 

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If you are using a low pass at 50hz with an 8" woofer, the range your sub is actually playing is going to be extremely small. I'd wager it's rolling off somewhere in the mid 30's, and then it's going to already be down by 50hz as well, so unless you are listening to something centered around 40hz, you won't get much.

As for the cone movement, you need to make sure your plate amp has a high pass filter to protect the small driver from overexcursion below 30hz. If your plate amp has one built in, it's probably centered around 20hz, a bit too low.

Ultimately, the manufacturer had taken the limits of the sub into account and built the proper filters into their plate amp. You went with a different plate amp and now you'll have to essentially guess where to center your filters at. If it were me, I'd set the low pass to 150hz and the high pass to 30hz.
 

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Discussion Starter #3
Interesting, well as far as I know, there is no high pass built into the amp. It is not a plate amp btw, it was an amp board with heatsink only. I reused the existing power supply & speaker / enclosure.

Its a real frankenstein subwoofer right now, but I was using it as a project to try seeing as it didn't work before anyhow. If I add the high pass, how do I adjust it? Sorry for the noob question(s) I still new to this and learning as I go.

Also, the low pass filter has a 188uF filter cap with a coil inducer. I do not know the h rating of the coil but the filter wasn't rolling off at thte freq. I wanted, so I changed the 188uF to 3300uF and thats when I got the huge speaker extension but almost no sound.
 

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I'm assuming you have the capacitor in line with speaker and the inductor across it.
So your inductor works out at 40 mH for a 50Hz cutoff.
With a 3300 uF capacitor it's cutoff is now 14Hz - way too low for an 8" speaker.

You really have to know what you are doing when you change crossover components as the load seen by the amplifier can get really scary and changing the ~200uF to ~3000uF could explain the reason the amp is getting hot.
We really need to know the inductance of the coil to work out what the true story is.
 

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I understand using such a high filter cap without knowing the results can be bad, I can change it back without issue, but the sub still plays like a loudspeaker only slightly muffled with the 188uF cap in place. Will this high pass filter aid in making this speaker work the way it is supposed to? I will post a few pics shortly to show what it is like now, to help paint the picture.
 

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I do not have any specs for the existing speaker other than it is 4ohm rated for 220Wrms. The enclosure is small, ported down firing.
 

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New Input

First off, thank you everyone for all your help and ideas thus far, they have been helpful.

So far, I got the subwoofer frankenstien working! :)yay2:) I swaped the 3300uF filter cap with the original 188uF NP cap, and hooked the sub back up matching the polarity of the wires... low and behold, the sub played sound!

The only problem is, the amp get really hot still after about 30 minutes of play, and the sub isn't nearly as loud as I was hoping it to be... The speaker is rated at 4ohm, which would make the amp output rated for 100W. I do have a 47k variable potentiometer on the board for volume adjusting, could that be limiting the input to the speaker? Or do I need something else in there?!

Please any input is greatly appreciated and welcome!

-Nick
 

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Good morning everyone! I stumbled upon this forum looking into possible options for my DIY subwoofer retrofit I was working on, and found it very useful, so I joined! As my first post I pose a question to those audio / electronics gurus out there.

I have picked up an older Acoustic Research AR 108PS 8" Subwoofer and replaced the amplifier board with a Velleman Discrete 200W mono amp as the existing one was toast. After switching out the Amp, the sub played only as a loudspeaker, so I added a PASSIVE low pass filter rated for 50Hz. Since I had installed said items, I get alot of woofer extension / movement, but very little sound. The amp board gets really hot after about an hour of use... Does anyone have some ideas to how to get my Sub thumping properly? I can provide pictures and whatnot soon. Thanks for the help in advanced!

-Nick
If you want thumping you need a much higher crossover frequency.

I would suggest starting with no crossover and measure the frequency response of the amp/woofer combination and then decide where to go with the crossover.
 
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