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12" subs; 1x100W or 2x50w?

Discussion in 'ICE, Sat Navs & Dash Cams Forum' started by Slate8, Aug 1, 2005.

  1. Slate8

    Slate8
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    Hi all,

    I have an oldskool denon DCA-800 amplifier that I would like to buy a 12" sub for. It's a 300w 6 channel amp with 100W used for the sub.

    My question is which would sound better; running one 12" with 100W (in bridged mode) or getting two 12"s running at 50W?

    Any suggestions?
     
  2. MGDisco

    MGDisco
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    Good choice of amp, Slate8, have had one of these myself for the last 11 years, still going strong after being in 6 cars. Used to have a Pyle 8" Bass tube and now have a 12" Kenwood in a ported box.

    Has always been run as bridged for the sub, 1 is plenty IMO, current vehicle is a Discovery TD5. It's a noisy brute, being an auto it can rev quite high at times depending on engine load.... Denon manges to totally dispel the noise of the car, and after about 6-7 minutes at that volume, ears are going numb!

    So, I would say get a single sub with decent sensitivity, rated about 250W RMS (the DCA800 is renouned for producing far in excess of the quoted ratings, in bridged mode it is around 150W, maybe a bit more), with a decent box for it (follow the manufacturers reccomendations, they know their own kit) and you should benefit immensely from the installation. Remember to set it for 2ohms if your sub is rated at 4ohms (bridging needs this as half the impedance is seen by each channel). If the sub is 8ohms, you best leave it at 4ohms on the amp, or you run the risk of damaging both sub and amp!

    Are you using an external crossover or will you be relying on the internal one built into the amp? I have used an external in the past, and it gave much more flexibility but the internal one is fine.

    Cheers

    MGDisco
     
  3. Slate8

    Slate8
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    11 years! Excellent! :thumbsup:

    I've run this in a couple of vehicles before with one Kenwood 12" 250W in bridged mode and it's been excellent. I've recently purchased a Toyota Hilux Surf and had to sell the old sub with my previous car. I thought as I now a huge boot that 2x12" might be better, but really have no idea.

    Interesting what you say about the power rating being generally higher (yay :clap: ). Think I will probably take the 1x12" route. Also, are you sure about the ohm settings? I think on the switch it says "2ohm - 4ohm (bridged)" so I just assumed 4ohm should be used with one sub. Again, not really sure.

    I'm using the built in crossovers, they seem to do the job so I'll probably just stick with them.

    Thanks for the excellent advice; I really had no idea what rating of sub to get. 250 it is!
     
  4. MGDisco

    MGDisco
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    I'll check that manual tonight and see what it says about the 2 - 4 ohms setting for the bridging, I've always had the switch in the 2ohms position (from what I understood, this makes the drive circuits work properly when bridged with respect to the impedance of the attached speaker).

    With the switch in the 4 ohm position, the impedance match of each channel would need to be 4 ohms, and with a 4 ohm speaker, this will give 50W as stated. If you connect a 2 ohm speaker, the switch needs to be moved to the 2ohm position to match this new impedance. The formula for power is P = Vsquared / R. As the voltage is constant, and the resistance has halfed, the power will now be 100W per channel.

    In bridged mode, ie Left + to right -, the channel power is added together and so is the impedance it needs to see. Therefore, if the new speaker is 8 ohms, the switch stays at 4ohms and the power will be 50 + 50 = 100W. If the speaker is 4 ohms, each channel has to see 2 ohms (2+2....), so the switch should be in the 2 ohms position and the power will increase to 100 + 100 = 200W!

    Most subs are 4 ohms, when bridged the amp should be set to 2 ohms, thus the power will be 200W. This is extremely useful when you then take into account the sensitivity of the sub. If it is rated at 90dB/W/m, then doubling the power will obviously increase the volume. But if you had 2 singles at the same sensitivity and connected to each channel on its own, then the maximum power is 100W. In bridged mode, you have 200W, and the rule is that twice the power = twice the volume. So 2 singles, at 4 ohms and 50W each will sound half as loud as 1 4ohm sub bridged with 200W. (power and volume are logarithmic, which just means twice power means 3 dB increase in volume + twice volume).

    Hope this isn't teaching your granny to suck eggs or too techy but will hopefully get you setting the Denon up correctly.

    Cheers

    MGDisco
     
  5. Slate8

    Slate8
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    Wow, some really useful info there! Thanks!

    I understand what you're saying (though did have to ready every sentence twice :D ) and it sounds as if one 4 ohm sub bridged with 200W would be the way to go.

    Did you get a chance to have a look at the manual?
    I have some 10cm speakers in the front powered from the amp. These are 4ohm, if I'm planning to run the amp in 2ohm mode does that mean I should replace the front speakers with 2 ohm ones as they are not bridged like the sub?

    Many thanks again for all your useful comments. Would be doing totally the wrong thing without them! :smashin:
     
  6. roversd1

    roversd1
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    Just as an aside, to get the best from your amp, run thickest power lead possible, weedy leads will starve the amp of power.
     
  7. Slate8

    Slate8
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    Thanks for the info. I have already fitted the amp and used the thickest power cable i could afford (some were silly money).

    Tucking it under the trim was interesting ;)
     
  8. MGDisco

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    Slate8,

    Even better than me regugating the spiel from the manual, I have managed to scan it to a PDF file (split in 2) so you can see exactly what it says re: bridging.

    Happy reading (not much to read but it's all there!).

    Note that the crossover only works for the subs if you use the internal select switch - if using three sets of phono leads, with the switch to ext, the crossover only applys to the front/rear channels and not the sub.

    Page 6 is what you need to look at for the concerns of bridging......


    Cheers

    MGDisco
     

    Attached Files:

  9. carbentony

    carbentony
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    Hi

    Dont know whether you are still around, but thanks so much for the DCA-800 manual, in desperation I was about to buy a service manual.

    Cheers

    Tony
     
  10. Andy8030

    Andy8030
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    As with the previous reply you saved my bacon, this is a great amp does exactly what I need - and now I have the manual - bonus! Thanks:thumbsup:
     

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