8ohms-->4ohms... how to?

[Little Grabbi]

Okay, computer aidio is the closest thing I can find that would fit my question:

i have a laptop I use to dj, this is hooked up to a preamp and a 1200 watt RMS main amp. Now, I got a hell of an offer for a 4 kilowatt @ 4 ohms amp. Problem is: I have used 8 ohm speakers until now, and I'd like to reuse them with the new amp.

Now, a buddy of mine told me that I could hook up the 8 ohm speakers in parralel, which would give me 4 ohms of resistance. Is this true? i thought parrel resistors give a total resistance of the larger resistor divided by the smaller one (in this case 2 ohms, which would blow the amp).

 

[GodsMadClown]

Just hook 'em up. An amp that is capable of driving 4 Ohm spekaers will have no trouble driving 8 ohm speakers.

 

[m1abram]

Just hook 'em up. An amp that is capable of driving 4 Ohm spekaers will have no trouble driving 8 ohm speakers.

As he said.

Also the the resistance of a parallel circuit is 1/R1 + 1/R2 = 1/Rt

where R1 and R2 = 8 will lead to Rt = 4

 

[jpmkm]

Also, the important factor when dealing with speakers is impedence, not resistance.

 

[GodsMadClown]

A speaker acts like a resistive load on the amp. That's why they are rated in Ohms. For the purposes of the simple circuit modeling math that M1abram cited, they can be treated as resistors.

Anyhow, the only math that you need to now is that 4 (rated impedence handling of amplifier) < 8 (rated impedence of speakers). Perhaps it would also help to know that speakers with smaller impedence ratings represent a more difficult load for the amplifier. A car can carry less weight than it is rated for, but not more.

 

[m1abram]

A speaker acts like a resistive load on the amp. That's why they are rated in Ohms.

Actually no, it is very much an impedance not a resistance. a Speaker is a voice coil which is an inductor, which an inductors resistance vaires with frequency. An amp puts out a varing frequency such as music.

 

[jpmkm]

A speaker acts like a resistive load on the amp. That's why they are rated in Ohms. For the purposes of the simple circuit modeling math that M1abram cited, they can be treated as resistors.

Anyhow, the only math that you need to now is that 4 (rated impedence handling of amplifier) < 8 (rated impedence of speakers). Perhaps it would also help to know that speakers with smaller impedence ratings represent a more difficult load for the amplifier. A car can carry less weight than it is rated for, but not more.

Impedance is essentially resistance in an alternating current circuit. Sound is obviously alternating, so impedance is the main measurement needed when dealing with audio circuits, not resistance. Impedance and resistance are both measured in Ohms, though.

 

[Corvette]

Actually impedance is measured in Henry. Abstract (<-- right word?) math can be used to calculate the impedance similar to resistance though. Ex. j4 Ohms. It has been weeks since I took circuits though and its all slipping away.

Vette

 

[jpmkm]

I'm pretty sure the Henry is a measure of inductance. Impedance is a result of resistance and reactivity, and is measured in Ohms.

 

[m1abram]

I'm pretty sure the Henry is a measure of inductance. Impedance is a result of resistance and reactivity, and is measured in Ohms.

Ding Ding Ding to the winner

 

[MetalDwarf]

fear not citizen
Pro audio monkey to the rescue

ok if i knew more about your DJ rig i could be more specific
so if you give me all the boxes and impedences etc i could help more

ok if you have 2 speakers that are both 8ohms you can run them in parrallel as mentioned previously

so im going to guess that they speakers are using a 1/4" (guitar style) jack
there are likely 2 jacks on the back of each speaker.

if you were to run 1 cable from the amp to the speaker it will be powering 8ohms
this is fine and dandy but the amp wont be able to push as hard usually about half as much as at 4ohms. (not exact but a good general rule) so driving 8ohms it will put out 2KW.

to drop to 4 ohms
run a cable to a speaker (speaker 1) then from the second jack on speaker 1 run another cable to speaker 2
you have now chained the speakers together and dropped to 4ohms

this will provide more power as the amp doesnt have to push as hard

however it wont be much louder

2 speakers powered by the same wattage will be 3db louder.
thats assuming same wattage to both
so assuming 2kw to each speker you get 3db louder so a little louder but at the cost of some soud quality.

also something look at
what are the quoted specs of the amp?
is it 4kw per channel?
bridged mono?
at 1khz?
20hz to 20,000hz?

that all makes a HUGE difference in what its really able to do
i would guess that the 4kW spec is 4ohms bridged mono (this means that both left and right of the amp are pushing one speaker, turing your stereo 2 channel amp into a mono amp)
if this is true this means that either side of the amp is essentially seeing 2ohms load
which is mightly low. you can run into danger here and blow up your amp if the resistance drops too low.

lastly how do you plan on powering the amp?
last time i checked most wall sockets provide 15amps x 120V
or approx. 1800W max
if you try to pull 4KW from that outlet the breaker will blow.
so unless you are plugging into something cooler then a regular outlet you wont get your full 4KW

some food for though

PM me if you want more info

 

[BO(V)BZ]

I'd be rather surprised if ANY normal pair of speakers can take 2kW apiece

I was just looking at some big, dual 15", horn loaded PA speakers, and they were rated for 600Peak, and around 250RMS. It would take a crazy speaker setup to handle even a thousand watts continuous to each speaker.

 

[jerb]

but if you hooked up two speakers in parrallel woundnt you lose stereo?
for example if you had the 2x 8 ohm speakers and you put them in parrallel and plugged them into the left channel on your receiver then wouldnt you be playing the music only on the left side rather than both left and right? sounds like you need four speakers if you do it that way... right?

 

[m1abram]

but if you hooked up two speakers in parrallel woundnt you lose stereo?
for example if you had the 2x 8 ohm speakers and you put them in parrallel and plugged them into the left channel on your receiver then wouldnt you be playing the music only on the left side rather than both left and right? sounds like you need four speakers if you do it that way... right?

Yes, but then you take 2 more 8ohm speakers and plug them into the right side and you have stereo sound again.

 

[jerb]

thats what i thought but i wasnt sure how many of these 8 ohm speakers he had

 

[MetalDwarf]

I'd be rather surprised if ANY normal pair of speakers can take 2kW apiece

your looking at woofers
most of those cant
throw it into some big freaking compression drivers and they will do it all day every day and blast you in the face and make you loose your hearing

Yes, but then you take 2 more 8ohm speakers and plug them into the right side and you have stereo sound again.

yup


like i said in my post if i knew his set up i could figure it all out perfectly

 

[figgie]

4kw amp?

Crown I (eye) series or K series amp?

Anyway.

you could also hook up the 8 ohm speaker as an 8 ohm speaker and stay with 2000 watts if the figure holds true.

K.I.S.S

 

[BO(V)BZ]

your looking at woofers
most of those cant
throw it into some big freaking compression drivers and they will do it all day every day and blast you in the face and make you loose your hearing

Ok, perhaps there are things out there that can handle that wattage... but why? I've seen PA's with sensitivities of 107dB...I take it this guy doesn't live in an auditorium =]

 

[MetalDwarf]

.I take it this guy doesn't live in an auditorium

actually i do work in an auditorium
big concert hall
just cuz something can be done doesnt mean it should....