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General => Car audio => Topic started by: Colin on 01 July 2004, 13:41
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A not so daft question but i have a 2ohmn stable amp but as most subs are 4ohmn, can i apply another 4ohmn load in the form of a seperate resistor to trick my amp into thinking it's running a pair of subs at 2ohmn. i only really want to run one sub but would like to squeeze the extra power out of the amp.
Can this be done anyone?
or am i heading for an expensive meltdown?
cheers
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If your amp is stable at 2 ohms it will be stable at 4ohms.
Putting a 4 ohms resistor in parallel with your sub will show up as a 2 ohm load to the amplifier and will theroreticaly pull twice as many watts out of the amp. In practice it won't quite be double. The problem your solution has is that even if your amp was putting out twice the power, half of it would be going through the resistor and the other half through the speaker. So no gain there.
The only way to exploit the power from the amp is to use a 2 ohm speaker or two 4 ohms in parrallel. If you go for the 2 speaker option you could go for the isobaric set up which won't take up any more room because this setup halves the Vas which directly relates to the volume of the box, in other words halves the volume.
Hope this helps.
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that's confused even me! lol
I've never come across anyone doing what you want to do. If you run at 2 ohm you create more power yes, but the more power goes through 2 speakers. my Genesis amp creates 2 x 95w @ 4ohm. when run in 2 ohm mode it creates 2 x 145w. Not double power is it. I wanted to run 2 SVC subs off it in 2 ohm mode so they'd both get 145w each, but there was no way I could do that so I now run them in 4ohm mode and it's sounding awesome. even with a mere 95w going into them.
if you buy a dual voice coil (DVC) sub you can wire it up to run in 2 ohm mode.
The best thing to do would be to buy the biggest baddest amp you can afford. that way you can run it in 4ohm with the gain turned right down and draw less current from the battery and create less heat!
Also, forget PMPO figures. these are useless. The RMS output figures are where it counts.
I was previously running 2 12" Alpine '1000w' subs off a Kenwood '1600w' Amp in 1ohm (I think). The bass was constantly cutting out under high load even with a power cap in there and a 90amp alternator! I'm now trial running 2 12" JL 125w subs with a little 2 channel Genesis amp kicking out 95w a channel and it's almost as loud as before. Once I get the front speakers running and have the ability to set the headunit right it'll be better still!
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Also, forget PMPO figures. these are useless. The RMS output figures are where it counts.
FYI: RMS = peak power x 0.707
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shouldn't that be the other way round Ad?
RMS is always lower ???
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Yeah, so thats right.
If you multiply one number by another number (thats less than 1) the end figure will be lower.
ie: 10 * 0.5 = 5
So if peak was 100 multiply it by 0.707 RMS equals 70.7
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The other way (to convert RMS to Peak) is RMS x 1.414
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Yeah Diz multiplying peak power by 0.707 will result in a lower value.
In answer to Colins question you can't simply put a resistor in parallel with your sub to trick it into thinking it's running a 2ohm load. Although the voice coil on a speaker is a resistor because it is moving in a magnetic field the load is an inductive one, a pure resistive load would behave very differently and the amp wouldn't like it. Besides have you any idea how big the necessary resistor would be (100+ watts) and how much heat it would generate (if it were possible to do such a thing). Basically 2ohm stability is a nice feature to have if you need to run two subs otherwise just forget about it, by the way I am on the right forum here aren't I electronic geek weekly yeah??? ;)
Cheers
Toby
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sorry ads. it's too early for the educational answers from me :P
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^ You should try figuring electronics out at 3 in the morning after a 12 hour shift, when the brain is on auto pilot!!
11 years as an electronics engineer, before taking to the air, gives me a bit of an advantage though :)
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The other thing is, if you want to get more power to go louder, you should look at the sensitivity of the drive unit. This is listed as dBs for 1 watt of input power at 1 metre away. The higher the dB the louder it will be for the same input power. 3dB is a doubling of power so a driver with 93dB sensitivity will bejust as loud with 1 watt driving it as a 90dB one with 2 watts driving it.
Sensitivity isn't a measure of quality though! Enclosure design will also make a huge difference in SPL levels, for out and out SPL without quality or extension, huge peaks in the response are easily acheivable.
In my experience in building subwoofers there are 3 things to aim for;
quality
extension
SPL
Any 2 of the above 3 are easily acheivable. All 3 costs money.
I agree about the speaker being an reactive load, whereas a resistor is purely resistive. 4 ohms for a speaker impedence is an average at best. It will probably go down as far as 1.5 ohms and up to about 16 ohms at different frequencies. So 2 ohm stable isn't such a bad thing when driving a '4 ohm' speaker.
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thanks for all your help everyone it's been interesting reading.
I guess i'll be simply going for the single sub set up, but if Seyles comes back on or anyone else knows what he was talking about in message 2 and the Isobaric set up, i believe it's two speakers run face to face but a laymans term breakdown would be interesting.
In regards to RMS power, i still can't understand why manufacturers still quote other "mickey mouse terminology" (peak, mpo, pmpo, etc)apart from making a sh!te product sound more impressive in a brochure. If your dealer can't quote an RMS figure then go and buy something else, or you'll only be disappointed.
If anyone wants a fair product for fair money, check out the mutant range at halfords. or try www.mutant.uk.com.
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Isobaric is 2 drivers face to face.
Vas is one of Thiele Small parameters which are used to work out the ideal box volume and port lengths etc. It is the volume of air which has the same compliance (springyness) as the cone of the driver. This value is directly related to the ideal volume the drive unit needs for the flatest reponse. If you put two drive units together in a clambshell the result in effect is a single drive unit with half the Vas value and this means the ideal volume is halved.
To work out the ideal volume and port length you only need 3 Thiele small parameters for the drive unit
Vas-as above.
Fs the resonant frequency.
Qts the total damping.
The maths can seem a bit difficult but is pretty straight forward.
If you want, tell me what the parameters are, and I'll tell you what size enclosure to use and what length ports to use for the best response.
The best response in this case means the flatest.