4ohm Sub driver to 8ohm Sub amp? - diyAudio
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Old 23rd March 2007, 03:08 PM   #1
WBFAir is offline WBFAir  United States
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Default 4ohm Sub driver to 8ohm Sub amp?

Hello all and sorry if this is a dumb question for this board but.

I have a need to replace my 100W powered sub's 8ohm driver.

I see more choices in the 4 ohm range though so I was wondering if there was anything I could do (besides not turning it up to much) to use a 4 ohm driver in place of the 8 to not overpower the Amp? Can I add a resistor of some kind?

Thanks
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Old 23rd March 2007, 03:49 PM   #2
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...you could try to use a dual 4 ohm voice coil version, then series the coils. ....If your amp isn't so current or thermally rugged.
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Old 23rd March 2007, 08:28 PM   #3
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Adding a resistor isn't practical.

I wonder if the amp is capable of a 4 ohm load? Anyway to find out?
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Old 24th March 2007, 12:10 AM   #4
WBFAir is offline WBFAir  United States
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Beats me, I barely understood what I've already wrote so far, I'm afraid I'm kind of a learning as I go along kind of a audiophile.

I have seen that there are a few jumpers on the main board and I did get the drawings for the main board & controls so I'll try to attach them for others to see.


Well sorry it telling me its to big.
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Old 24th March 2007, 12:35 AM   #5
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If the woofer is direct-coupled to the amplifier, you could just connect a 4-ohm woofer and play at moderate volume for a while. Check the heatsink temperature by carefully touching the heatsink with your fingers. If the heatsink is very hot, the amp is not a good choice for a 4-ohm load (speaker). If the heatsink is mildy warm, turn up the volume a little bit. Check for heat rise in the heatsink. This is not an accurate method of determing if the amp is right for a 4-ohm load, but you can prevent a disaster.

...when I say "direct-coupled", I mean "are there any passive components (Coils, etc) in series with the voice-coil?". If there are other components, the voice coils new value (4-ohms) will have to be used to calculate a new component (coil) value.

The good news is, many amps can deal with 4-ohms pretty well.
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Old 24th March 2007, 03:12 AM   #6
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What brand of amp is it? Make and model, colour not crucial.
If you can't get the manufacturers specs then follow mrshow4u's very solid advice.
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Old 24th March 2007, 05:13 PM   #7
WBFAir is offline WBFAir  United States
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Hello all and btw thanks for all the help so far

The sub is a Athena P2 Sub with a 400 watts (100 watts RMS), MOSFET Amplifier Structure

The Sub has
Auto-On/Off
Internal Protection Circuitry
Anti-Clip Circuitry
Isolated Power Supply

It's frequency response range is 25Hz - 150Hz

Manf Link: http://www.athenaspeakers.com/modelP2specs.htm

I have manged to compress the schematic of what is called "The Main Control Board" which I believe is the Amp which I will try to upload although its getting pretty messed up by how small I'm being forced to make it. Also there is a remote "Control Board" which has the level adjustment parts on it that I don't think is part of the amp which I don't think I can upload.

The boards power is fed from a remote center tap transformer in from the SP1, 2 & 3 points to the right and upper right of the schematic, the speaker is fed via the Red & Black points on the right of the the schematic.

Again there are two jumpers on the board but I have no clue what there for, one has a jumper pin on it, the other has none.

As far as I can tell all the parts for this Sub are made by Athena.

Thanks
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File Type: jpg p2 sub.jpg (87.8 KB, 83 views)
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Old 24th March 2007, 05:35 PM   #8
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With just a single pair of TO-220 MOSFETs (IRF640/9640) I would struggle to call this 100 watts myself. If you use a 4 ohm driver you will have to be very careful with the volume control. Maybe it would be wise to change the transformer for one with a lower voltage also.
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Old 24th March 2007, 06:02 PM   #9
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Not being a big MOSFET man (my handle), I'm not sure. Your upload is very fuzzy, but I see only 2 output devices - the IRF648/649? I would think that there should be more, at least 4 to dissipate the heat, as you will double the current draw with 4ohms.
My guess is that Atena purpose designed this for 8 ohms.
To be safe I'd stick with 8 ohms.
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Old 25th March 2007, 08:00 AM   #10
WBFAir is offline WBFAir  United States
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Yeah richie00boy I agree with you about the 100W thing.

One of the big reasons I bought this sub was its size and shape, of course I read many praising reviews on it too and trust me its a great sounding sub, but the room its in doesn't have much space to spare and given this even though I knew it was small (the sub) I figured it would be enough for the room but as many on this board would probably agree theres never "enough" sub is there, so of course I pushed it hard right of the bat and thats why I think the speaker choked

BTW in case any might not be aware this sub was really designed to be a interchangeable component with matching bookshelves that we made at the same time, theres even two slide bars on the top of it that are actually live terminals that the books are designed to fit into. The idea was to mimic a tower look but have it be a real sub and book all in one.

Basically I've already ordered two Seas drivers MN:P21RF/P as replacements, I don't think there an ideal match, mostly as they are woofers and not subwoofers, but it was the highest watt, 8", 8ohm woofer Madisound had so I figured I give them a try. I did find a few nice 8"ers at PartsExpress but they were all 4 ohm which is what brought me to make this post so I guess I'll stick with the Seas as they are 8 as MJL21193 suggested.

Also to help...I've bought a second sub, (and actually a third if any one wants one). I found a place in CA that had the last two new's in a box that could be found anywhere so I figure I'll pop the two Seas, one into mine and one in to one of the new subs and with two subs now that better be enough.

Again if any one is interested in the third let me know as it really is a great sounding & looking sub for the right app.

Thanks again for all the help
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