formula for getting 8ohm 4ohm rating

[mike c]

the Rotel RB-03 has a rating of 70w x 2 (20-20 .03% 8ohms)
and then say it is bridgeable to 150w x 1 (20-20 1% 4ohms)

the Rotel RB 1070 has a rating of 120w x 2 (20-20 .03% 8ohms)
and then say it is bridgeable to 330w x 1 (20-20 1% 4ohms)

so what is the bridged output in 8ohms?

why put the 4 ohm rating when it says in the manual to only use 8 ohm and above for the speakers.

isn't that the same as saying this car can go 300kph if you remove the weight that is the engine?

 

[markw]

Bridging amps does make some denmands on the speaker.

Mainly, it "sees" the load impedance of the speaker as half of what it's rated at. So, it "sees" an 8 ohm speaker as 4 ohms. Below this, they can get flakey.

A 4 ohm speaker wiill be "seen" as 2 ohms and that most definitely can stress an amp. So, don't use 4 ohm speakers on a bridged amp unles you're really, really sure it can handle it.

The fact that the mfgr somewhat shies away from recommending 4 ohm speakers would cause me concern.

 

[highfihoney]

the Rotel RB-03 has a rating of 70w x 2 (20-20 .03% 8ohms)
and then say it is bridgeable to 150w x 1 (20-20 1% 4ohms)

the Rotel RB 1070 has a rating of 120w x 2 (20-20 .03% 8ohms)
and then say it is bridgeable to 330w x 1 (20-20 1% 4ohms)

so what is the bridged output in 8ohms?

why put the 4 ohm rating when it says in the manual to only use 8 ohm and above for the speakers.

isn't that the same as saying this car can go 300kph if you remove the weight that is the engine?

from the specs you posted it looks like both amps are not capable of being ran in bridged mono at 8 ohms.

usually when manufacturers dont spec an 8 ohm rating for mono the amp cant do it.

 

[Wid]

from the specs you posted it looks like both amps are not capable of being ran in bridged mono at 8 ohms.

usually when manufacturers dont spec an 8 ohm rating for mono the amp cant do it.

The Rotels can be bridged and handle an 8 ohm load. As previously stated they don't spec the amp in bridged mode at 8 ohm because when using it like this the amp sees the 8 ohm load as if it were 4.

 

[mike c]

hmmm, let me digest this first. so how do I know if that 4ohm rating is a balance for my left and right 8ohm rating?

 

[Wid]

hmmm, let me digest this first. so how do I know if that 4ohm rating is a balance for my left and right 8ohm rating?

That 4 ohm rating is what the amp will do for your 8 ohm speakers. Here is an article to read that will help explain what happens when you bridge an amp. Note this pertains to car audio but the basics are the same, scroll down to section 3.5.

 

[PENG]

The Rotels can be bridged and handle an 8 ohm load. As previously stated they don't spec the amp in bridged mode at 8 ohm because when using it like this the amp sees the 8 ohm load as if it were 4.

I agree with your first sentence but Rotel should not have stated their 4 ohm rating when bridged, because it is confusing at best. In the owner's manual, they clearly stated that when it is bridged, the speaker connected to it must have an impedance of 8 ohms or more.

 

[Wid]

I agree it is confusing at best, no argument there. What amp manufacture makes an amp that can push a 4 ohm load when bridged?

 

[PENG]

the Rotel RB-03 has a rating of 70w x 2 (20-20 .03% 8ohms)
and then say it is bridgeable to 150w x 1 (20-20 1% 4ohms)

the Rotel RB 1070 has a rating of 120w x 2 (20-20 .03% 8ohms)
and then say it is bridgeable to 330w x 1 (20-20 1% 4ohms)

so what is the bridged output in 8ohms?

why put the 4 ohm rating when it says in the manual to only use 8 ohm and above for the speakers.

isn't that the same as saying this car can go 300kph if you remove the weight that is the engine?

I totally agree with you. The way they spec the RB1070's output is confusing. Having said that the formula you asked about would be:

P=(V²/Z)*P.F. (or P=I²*Z*P.F., or P=V*I*P.F.)

P=Power, I=current, V=Voltage, Z=impedance, P.F.=cosine of the phase angle between the Voltage and current phasor, this angle is dependent on the characteristics of the impedance, that is how resistive, inductive, and capacitive the load is)

In this case it is easier to explain using P=(V²/Z)*P.F.

Normally (not bridged), 330W into 4 ohms would imply 165W into 8 ohms because given the same maximum voltage output, doubling the impedance will result in halving the output power.
When bridged, the amp's output voltage would double, since P=(V²/Z)*P.F., power would increase by 4 times and current would double. It is not realistic to expect the 26.25 lbs or even much larger amps, to be able to handle that kind of current, and that's why 4 ohm speakers should not be used for bridged mode. I strongly suspect the 330W into 4 ohms spec is an error, it most likely meant 330W into 8 ohms.

 

[PENG]

I agree it is confusing at best, no argument there. What amp manufacture makes an amp that can push a 4 ohm load when bridged?

I am not aware of any. If you do, please tell us.

 

[Wid]

I don't know of any either, I thought maybe you did.

 

[PENG]

I don't know of any either, I thought maybe you did.

Okay, I think I got the message. I edited my posts accordingly. Thanks!