Powering Subwoofer

[Buzzy]

I have 4 ohm single voice coil subwoofer. I have a spare amp at home but it can only drive 8ohm speaker. Can i connect a resistor in series with the subwoofer so that it becomes 8ohm load for the amplifier ?

[kelticwizard]

What kind of amp is it? Is it a stereo amp or receiver, and what kind of power does it deliver?

[Buzzy]

its a kenwood stereo amp with about 40W power 8-16ohms. i am looking for a temporary solution to power my subwoofer.

[nunayafb]

a 4 ohm resistor is not going to hurt anything as long as it has a high enough power rating. The power rating should be about half of what the amp can deliver. Madisound's 25W Wire-wound resistors are cheap enough ($0.65) that I would get two 8 ohm resistors wire them in parallel to eliminate any risk overheating even with extensive high volume usage. This would allow the resistors to easily withstand the ~80W peak power from the amplifier when wired in series with a 4 ohm driver.

[ocool_15]

Keep in mind adding a series R will change the frequency response and you will be wasting ~half what the amp is capable of.
If you are using a passive crossover it will also be affected.

[kelticwizard]

I was going to recommend "bridging" your amp so you can drive the sub from both channels, but that won't work here. The sub would either have to be 16 ohms, or your amp channels would have to be able to drive 2 ohm loads. That is not the case here.

Here's the link to the discussion, not that it is going to help you in this application:

How To "Bridge" A Stereo Amp to Just One Channel

As long as this is just temporary, then a 4 ohm resistor is the way to go.

[kelticwizard]

One more thing. The amp that is going to power your main speakers, not the sub-is that a stero amp or an AV amp?

The reason I am asking is that many AV receivers have a jack in back which carries a bass-only signal to a powered subwoofer. If that is the case, then of course you don't have to worry about a passive crossover for your sub.

[nunayafb]

Originally posted by ocool_15
Keep in mind adding a series R will change the frequency response and you will be wasting ~half what the amp is capable of.
If you are using a passive crossover it will also be affected.

How will it change the frequency response?

how can you waste amplifier power when it is rated at 8 ohms and it will be seeing 8 ohms, with a 4 ohm driver he'll be most likely fry the amp, that would be wasteful.

[sreten]

Hmmm......

Sticking a 4R resistor in series is a very bad idea, driver Qts will
approximately double, not good at all, boombox will be the result.

A stereo amplifier can be wired in parallel (not bridged). Parallel
the inputs and add to each output 0.22/0.33R series resistors
before they are paralleled together. The resistors will ensure
current sharing and take up any gain / DC offset issues.

Each channel will see an 8ohm load (with a 4ohm load).

/sreten.

[kelticwizard]

Just out of curiosity, is your subwoofer a closed box or a ported box?