8 Ohm resistor question...

[TazSob]

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.

[Firebottle]

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.
:)

[TazSob]

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/252861423404?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT?

[Firebottle]

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?

[TazSob]

Thank you again Alan,

I'm not sure what the REL approach is, tried to Google it with no luck :rolleyes:
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

[RothwellAudio]

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.

[TazSob]

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

[Radford Revival]

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.

[walpurgis]

Interesting.

[Firebottle]

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?

[TazSob]

Thanks Will, it was what I thought too.

The reason for my post is when I googled the speakers a came across a thread on another forum where the guy bought a pair only to find one sub not working, when he opened it up he saw the inductor had cooked.

After reading the Kef C200 installation instructions and it stated not to use the subs without bookshelf speakers attached.

I'm not sure of how to split the crossover and thought it'll be wiser to follow kef's instructions.

So now my question is which resistor should I use to take the load of the bookshelfs and where would be best to purchase it?

My apologies for my ignorance I've not come across anything like this before,

Regards
Taz

Cooked inductor (images from the web)

https://uploads.tapatalk-cdn.com/20170601/4bf37017ee0a442cd9c0f9941e6a22fe.jpghttps://uploads.tapatalk-cdn.com/20170601/56a4d8494f9177ec3c330de09a2ad95f.jpg

Screenshot from installation manual

https://uploads.tapatalk-cdn.com/20170601/80b7201af8bc5b5fdd68320439f213c1.jpg

Sent from my Hudl 2 using Tapatalk

[Radford Revival]

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?

I'm unfamiliar with the speakers but assuming there's a high pass section for the satellites, then yes the whole section of the filter could be disconnected (ie no filter in circuit not just an unloaded output), although if it is PCB based this could prove awkward.

[Radford Revival]

Thanks Will, it was what I thought too.

The reason for my post is when I googled the speakers a came across a thread on another forum where the guy bought a pair only to find one sub not working, when he opened it up he saw the inductor had cooked.

After reading the Kef C200 installation instructions and it stated not to use the subs without bookshelf speakers attached.

I'm not sure of how to split the crossover and thought it'll be wiser to follow kef's instructions.

So now my question is which resistor should I use to take the load of the bookshelfs and where would be best to purchase it?

My apologies for my ignorance I've not come across anything like this before,

Regards
Taz

Cooked inductor (images from the web)

https://uploads.tapatalk-cdn.com/20170601/4bf37017ee0a442cd9c0f9941e6a22fe.jpghttps://uploads.tapatalk-cdn.com/20170601/56a4d8494f9177ec3c330de09a2ad95f.jpg

Screenshot from installation manual

https://uploads.tapatalk-cdn.com/20170601/80b7201af8bc5b5fdd68320439f213c1.jpg

Sent from my Hudl 2 using Tapatalk


Two of these should do the trick (one per channel assuming these are stereo subwoofers):

http://www.ebay.co.uk/itm/100W-8-Ohm-Wirewound-Power-Resistor-Aluminum-Shell-Case-/112395335569

If you are able to solder, solder wires to either end of the resistors and connect them to the satellite outputs of the subwoofer as if you would connect the satellites (presumably there are binding posts for this?) Polarity doesn't matter, they are just being used to absorb power. Mount the resistors away from anything heat sensitive and even better bolt them to some kind of heatsink. Ensure it is difficult or impossible for the resistor terminals to make contact with the other resistor's terminals or anything else electrical or ground.

[Radford Revival]

On further inspection they appear to be a fair bit physically smaller than I thought they were. I would recommend heat-sinking them. Mounting them both on a sheet of aluminium would probably do the trick to be honest. In the real world they wont see much dissipation.

For a bit of overkill there's always these: http://www.ebay.co.uk/itm/8-ohm-100-Watts-Ceramic-Wirewound-Tube-Resistor-100W-/150665410056

These would be easier to mount free air. You'd really have to be going for it for them to warm up that's for sure.

[TazSob]

Thank you for the info and links Will, I was looking at the same wire-wound resistor too but as you mention could be a little on the small size and suffer from heating up.

I'll be grabbing a pair of the ceramic ones and add a heatsink to them.

Thanks again, very much appreciate the help

Regards,
Taz

[Radford Revival]

No problem :)