Radio Shack MPA-250 to run a subwoofer - AVS Forum
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Old 03-17-2014, 09:29 PM - Thread Starter
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Hi,

Has anyone have experience in running a Radio Shack MPA-250 stereo amplifier. I can buy one for $50 to run my subwoofer with died amplifier . My dead sub is a PSB subseries 6i, which was reviewed by Tom Nousaine to the best sounding subwoofer for less than $700 price range. The driver is still in great condition, a new plate amp from PSB is well over $300 plus shipping from Canada.

The driver on PSB 6i is 4 ohm. .Radio Shack MPA-250 can run either 8 or 4 ohm in stereo. Can anyone tell me if I can bridge the MPA-250 to run PSB 6i.

Thanks
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Old 03-18-2014, 12:46 AM
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Here's the manual for the amp. It looks like you can run in bridged mode: http://support.radioshack.com/manuals/ome32-2055.pdf
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Old 03-18-2014, 09:41 AM
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A number of individuals (including me) have used standalone amps/receivers to drive a subwoofer. In my case, I used an HT receiver to control the output frequencies sent to the subwoofer (LFE) which I plugged directly (via rca cables) into the extra receiver. Because my subwoofer (at the time) was passive, it was simple to connect one speaker channel from the receiver to the subwoofer. In you situation, you will likely need to connect your speaker cables directly to the speaker in the subwoofer.

Parts Express has plate amplifiers that may be drop-in (i.e., correct size) replacements for your plate amp but unfortunately may be out of your price range. It is a solution you can consider for the future because the plate amps have a high cutoff that is used to control the frequencies sent to the speaker driver.
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Old 03-18-2014, 09:51 AM
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Quote:
Originally Posted by maxcooper View Post

Here's the manual for the amp. It looks like you can run in bridged mode: http://support.radioshack.com/manuals/ome32-2055.pdf

It can be bridged but is only good for 4 ohm in stereo or 8 ohm bridged so it will not be a good match for your sub unless you want to run with only 125 watts from one stereo channel.
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Old 03-18-2014, 10:28 AM
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It can be bridged but is only good for 4 ohm in stereo or 8 ohm bridged so it will not be a good match for your sub unless you want to run with only 125 watts from one stereo channel.

I had the same concern, but I couldn't actually find the minimum impedance spec for bridged mode in the manual. Did you see the spec in the manual? Oddly, the manual shows running two speakers (in mono, in parallel) in bridged mode.

-Max
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Old 03-18-2014, 11:49 AM
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I saw in the manual that they only show ratings on it down to 4 ohm. That's a good indication that it's only good to 4 ohm.
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Old 03-18-2014, 01:41 PM - Thread Starter
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Thanks for all your replies.

The driver on my subwoofer was measured by my local, small one person electronics repair store to be 3.94 ohm. MPA250a manual does not state it is stable at 4 ohm when bridged, nor it gives out an specific output when bridged. Is may be ok at low volume, at high volume, should I concern the amp will damage my sub driver?
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Old 03-18-2014, 01:46 PM
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You need a highpass filter to protect the subwoofer from over excursion below its tuning point.

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Old 03-18-2014, 02:25 PM - Thread Starter
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Here is the info for my sub, can anyone figure out the tuning port, thank. It was tested to put out 96db at 20hz with 10% THD.

http://www.audioproducts.com.au/downloadcenter/DOCs/Manuals/PSB/ss6-5-0i-2003.pdf

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You need a highpass filter to protect the subwoofer from over excursion below its tuning point.
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Old 03-18-2014, 02:30 PM
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Specs say the -3db point is 29hz so you want to set a highpass around that frequency.

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Old 03-18-2014, 03:21 PM
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Quote:
Originally Posted by tsloms View Post

I saw in the manual that they only show ratings on it down to 4 ohm. That's a good indication that it's only good to 4 ohm.

It seems weird that they specify 4 ohms minimum impedance stereo, but say nothing about minimum impedance in bridged mode.

The driver is 4ohms, so by the letter of the manual, that would be okay.

However, I do recognize that the minimum impedance for bridged mode is probably 8 ohms, considering that the minimum impedance in non-bridged mode is 4 ohms. But this is an assumption that seems unspecified in the manual.

-Max
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Old 03-19-2014, 12:37 PM - Thread Starter
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at 27 lbs, this amp is heavy, it is heavier than my Denon receiver.


The power consumption is 650 watts. at 50% efficient, it should be able to put out 325 watts.
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Old 03-20-2014, 03:35 PM - Thread Starter
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Quote:
Originally Posted by swllz View Post

Quote:
Originally Posted by Mrkazador View Post

Specs say the -3db point is 29hz so you want to set a highpass around that frequency.

What if I plugs two ports and make it sealed passive sub?
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