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Thread: 8 Ohm resistor question...

  1. #1
    Join Date: May 2016

    Location: Birmingham, U.K

    Posts: 211
    I'm Taz.

    Default 8 Ohm resistor question...

    Hello AOS family,

    Yesterday I picked up some Kef C200 subwoofers. These are stereo and are made to be used with the Kef C15 and C25's. After checking them over I had a read through the installation instructions and according to them the subs need to be connected to the bookshelf speakers and should not be used without them unless you have a " 8 Ohm resistor of at least 50 watts in place of the bookshelfs"...

    I'm thinking of using a 100watt resistor but have no idea of how it'll be used
    So my question is how would I do this..., Do I just connect the resistor in place of the bookshelfs using the pos (+) and neg (-) connections?

    Also where could I buy half decent resistors? Don't want to buy off the bay only to have them blow and damage the x-over.

  2. #2
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 3,421
    I'm Alan.

    Default

    As far as I can make out the suggestion of fitting an 8 ohm resistor in place of the satellites is only to keep an even resistance/impedance presented to the amplifier over the full frequency spectrum. The crossover shouldn't be bothered or damaged if no upper frequency load is attached.
    What speakers are you going to be using them with?
    Did or are you considering to connect sub and other speakers in parallel to the amplifier, in which case you need to be careful about low impedances, as the sub impedance is already down at 4 ohms.

  3. #3
    Join Date: May 2016

    Location: Birmingham, U.K

    Posts: 211
    I'm Taz.

    Default

    Thank you Alan.

    Originally I wanted to use these as subs only and add an amp and try make them active running off my AV receiver, then when I read that they can't be used on their own I got little worried that I've gone and bought another set of speakers Mrs T will moan about.

    If I can use them without a resistor could I in theory turn them active by adding a small 4 Ohm car amp, something like this http://www.ebay.co.uk/itm/2528614234...3AMEBIDX%3AIT?

  4. #4
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 3,421
    I'm Alan.

    Default

    I don't see any reason they can't be used on the own, fed from a separate amplifier as you are suggesting.

    The difficulty is integrating the response with your main speakers if you don't have variable or flexible enough filtering.

    I would always go with the REL approach of leaving your main speakers as full range and then add the lacking bass with the sub through a variable filter.
    I don't have any experience of the AV type amp/receiver, do they offer variable filter settings for the sub out or is it usually a fixed frequency?

  5. #5
    Join Date: May 2016

    Location: Birmingham, U.K

    Posts: 211
    I'm Taz.

    Default

    Thank you again Alan,

    I'm not sure what the REL approach is, tried to Google it with no luck
    I've a Yamaha rx-v675, a 7.2 channel, it has frequency settings for the sub and by the looks of it has settings for all speakers. I'm only using it as a stereo amplifier at the minute ( a pair of Kef K160s as fronts ).
    The frequency settings for the sub are set at 80hz and I can change this

  6. #6
    Join Date: Oct 2016

    Location: Bolton, England

    Posts: 504
    I'm Andrew.

    Default

    Quote Originally Posted by TazSob View Post
    I'm not sure what the REL approach is...
    One possibility with a sub is to roll off the bass from the main speakers and roll on the sub at the same frequency. However, the REL approach (if I'm not mistaken) is to leave the main speakers untouched and merely augment them with extra bass from the sub.

  7. #7
    Join Date: May 2016

    Location: Birmingham, U.K

    Posts: 211
    I'm Taz.

    Default

    Thanks Andrew,

    I'm going to try adding that little car amp and using these as active subs... Will have a play around and see if I can get it to gel together. If not I'll be on the lookout for some decent bookshelf speakers to run these with and get a proper sub when I can

  8. #8
    Join Date: Apr 2012

    Location: South West of England

    Posts: 193
    I'm Will.

    Default

    Quote Originally Posted by Firebottle View Post
    The crossover shouldn't be bothered or damaged if no upper frequency load is attached.
    Not necessarily true! Unloaded resonant filters, if excited at or near resonance, can potentially generate flyback voltages high enough to damage themselves and/or surrounding equipment.

    In general it's never a good idea to drive the input to a crossover if one or more outputs has no driver/load connected.
    Radford Revival
    www.radfordrevival.co.uk

  9. #9
    Join Date: Apr 2012

    Location: Southall, West London

    Posts: 25,404
    I'm Geoff.

    Default

    Interesting.
    Mr. Tact!

    Main system: MMs/ADCs/Low output MC's/One rare Japanese SUT/One scarce British phono stage/various tonearms/hefty Japanese DD TT and hefty Japanese BD TT and small British BD TT. 4 CD players/2 jitter buster/2 DACs/Valve buffer. TVC stepped attenuator or valve pre-amp or solid state pre-amp. Current dumping power-amp or either of two Class A SS power-amp or Class A EL34 valve monos or big Japanese (part Class A) integrated. Big dual concentric speakers/Smaller dual concentric speakers/Two way British compacts and full range speakers, amongst others. And too much more to list!

  10. #10
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 3,421
    I'm Alan.

    Default

    Thanks for that Will, we keep learning all the time.

    In the OP's case then, best to disconnect the satellites output side of the crossover?

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