ohms?

[bal]

im currently looking for an amp to power my sub but i cant get my head around these ohms.

i recently saw this in an amplifiers stats and i cant make sense of it

2 channel amplifier/switchable mono
Continuous power @ 4 ohm: 50W x 2
Continuous power @ 2 ohm: 75W x 2
Max Power Rating: 300 watts

i also have seen the word bridgeable does this mean speakers can be connected in parallel, to one output terminal on the amplifier?

can some one please explain the ohms concept and make sense of the above

thanks

 

[wildti]

ohms is the resitance of the speaker in car audio useally uses 4ohms as standard , obveously the less resitance the more power the amp can deliver.

if you wire 2 4ohm speakers in parallel the resiance become 2ohms and so on.

bridgeing is when you say for example take a 2 channel amp if the amp is in bridged mode there will be only one speaker output with twice the power great for subs etc.

to use a amp in bridged mode there will be writeing indicating which termanals to use pro bridged mode useally the - of channel a and the + of channel b or vise versa!

hope this has helped !

 

[Retepetsir]

if you wire 2 4ohm speakers in series the resiance become 2ohms and so on.

Wrong.

If you wire 2 x 4ohm speakers in series, you get 8ohms (add the values together). If you wire 2 x 4ohm speakers in parallel, you halve the values and get 2ohms.

But yes, bridging is where you combine 2 channels to make 1, with 'roughly' (but not always) the combined power of two. However, with this you then have to be careful with the ohms.

If an 2 channel amp is stable down to a 2ohm load on each channel (i.e. 2 x 50w at 2ohm), when you bridge these channels, the amp will now only be stable down to 4ohm (i.e. 1 x 100w at 4ohm bridged).

Ideally you want to be able to match the ohm rating of a speaker to the minimum ohm rating that the amp can take, this will give you maximum power transfer, which is what you ideally want!

If you can post up the specification of your sub (rated watts (rms, ignore max power), ohm rating) then we cna suggest an amp.

Hope that kinds of helps :suspect:

 

[Ed]

Another thing to point out is the rough quality of the amp and its powersupply can be seen from the power ratings given. For example:

2 channel amplifier/switchable mono
Continuous power @ 4 ohm: 50W x 2
Continuous power @ 2 ohm: 75W x 2
Max Power Rating: 300 watts

Max power into 4 ohms = 50w. To achieve this assuming the speaker was constant at 4 ohms (not possible) this would mean the current and voltage at the amp output terminals was 14.1v at around 3.5 amps. (14.1Vx3.5A=50w)

If you half the impedance you should get double the current.. so..
14.1x7 ~100w.

Holdon why is the amp rated at only 75w @ 2 ohms? What gives...

Well, the amp is rated at 75@2 ohms, so from this we can work out that to achieve this you would have 12.24V @ 6.12A into a 2 ohm load, which is some way off 14.1V @ 7A you would expect.

The reasons for this are due to resistance within the amp output stage. A high quality amp will have a very low resistance. From what we now know it would infact be possible to work out its internal resistance. so...

Total system resistance = Internal resistance of amp + resistance of speaker. This is now getting onto the Damping Factor of an amp, which is basically a measurement of the control over the speaker the amp has. (high = good). I have just done a few really rounded bits of maths so these wont be that accurate, but it seems that the rated power of that amp is at 16v. From this and the two output power ratings I get an internal resistance of around 0.5-0.6 ohms. (thats pretty poor) From this also as mentioned you can work out the damping factor. Which DF=(Zload/Zsource) Or for 4 ohm driver 4/0.55 =~7(approx)

These may be quite far out from what the manufactuer says. But then it just goes to show even more that this amp is pretty average/low quality at best.

Good amp = low internal resistance = high damping factor = double power into half impedance load.

I can tell you now that this amp probably does not even have a powersupply as such, and it pretty much would run straight off the cars battery. Infact I bet its fully intergrated as well, so if you were to buy this amp it would probably almost be empty inside, it will be rated at higher voltage than is possible to get in a car and in reality I would be surprised to see 35-40 watts from it let alone 50.

Ed

 

[bal]

really great replies thanks very much
they have helped alot

 

[Ed]

No problem!