Testing Ohms

[azzfox]

Im about to install 2 subs and a 2ch amp for a mate, the subs are cheapies and the amp was second hand and i also think its a cheapie but its got no info on it about its impedance (ohms). by the way hes not really looking for a crash hot system.

I got a question, is it possible to test the ohms on a amp? and if so can you use a DMM? (digital multi-meter)
I know it can be done on speakers so now im wondering if sumthin can be done on amps... Thnx

[StoneX]

Nope, A DMM can't measure what impedence an amp can run at. Find out the amp model and you should be able to find info about it.

If in doubt, assume it will only do 4 ohm bridged. If they are cheap subs, they probably have a single 4 ohm coil each, so you'll have to run them from individual channels.

[azzfox]

hey again, iv managed to get a hold of the amp to have a good look at it. It doesnt have much info about it but its a JSE (Jet Sound Electronics) Model no. 2161. It has written on it "60w+60w" obviously meaning 120w bridged but no ohm info. Now is would that be 120w max or rms? Its fused at 30amp if thats any help. It says its bridgeable but its got a diagram on the output with "Normal" and "Bridged" but the "Bridged" is pointing to the + of the Right Channel and the - on the Left Channel so is that where I connect it to to bridge it? Thanx heapz :-)

[azzfox]

and another thing... i was wondering why you wrote that it will prob do 4ohm bridged? seeing its a car audio amp it will most likely be 4ohm/channel and all the amps iv looked at eg. a 2ch amp always has: "4ohm stereo and 2ohm bridged". iv also had a read about 12voltking's post about bridging but im still confused. because iv always thought that bridging halves the ohms. a little help would be greatfull. and by the way, because the amp is fused at 30amps, should the fuse in the line near the battery be fused at 30amp?

[Tasmaniak]

bridging does not halve the ohms.

What StoneX said about it doing 4ohm bridged is very correct. Without knowing the full details on the amp no-one can say that it will safely do 2ohm loads bridged. It most likely will....but it may NOT a good amp wil but a cheap amp won't. We don't know what it is.....

If a single channel can do 2ohms then it's a fair bet that bridged can do 2 ohms. But if it doesn't list the power rating for a single channel @ 2ohms. Only 4ohms then it most likely can't do 2 ohm bridged.

Lets break it down.
It's 2x60 @ 4ohms.
So, in theory we should have approx 1x120 @ 4ohms.
Now if we bridge it and then lower the impedance to 2 ohms. We effectively lookat 1x240 @ 2ohms.

Now, 240wrms is a BIG power claim for a small amp. Something I highly doubt it can do safely/effectively.

[azzfox]

thanx for that, im goin to assume that it will do 4ohm bridged. but if any chance its not what will happen? and once again from my last post... caus the amp is fused at 30amp, should the fuse in the line near the battery be 30amp too?

[StoneX]

I would say that 100% of car audio amps can run at 4 ohm bridged. That is pretty much the highest impedence and car audio speaker is, so they amps are made to suit.

You can put a 30A fuse at the battery if you want. The idea of the main fuse is to protect the car from the power wire shorting out, so as long as the fuse is a lower rating than what the wire can handle, you'll be fine.

[azzfox]

thanks heapz for all this but i got three final questions that two iv asked in previous posts that no1 has answered yet and one other.
The amp says its bridgeable but its got a diagram on the output with "Normal" and "Bridged" but the "Bridged" is pointing to the "+" of the Right Channel and the "-" on the Left Channel.

1. So is that where I connect it to to bridge it?
2. If the ohms arent right eg. 2ohm amp and 4ohm sub, what will happen? and is it possibe to tell if its wrong? eg. bad audio quality.
3. StoneX said that i would have to run the two subs from the two individual channels, but what if i bridged the amp than made two channels out of the bridge to be able to get L + R channels through both subs? would that work seeing the ohms would still remain the same?

[StoneX]

1. Yes.
2. An amp will run a high impedence no problems, it's only when you use a speaker that is lower than the amp can handle. So a 4 ohm speaker on an amp that can run at 2 ohm is fine, it just means the amp will be flowing less current, less power. If the speaker is 2 ohm and the amp can only run 4 ohm, then the amp will try to send too much current to the speaker which will make the amp hot and shut off or cook itself.
3. If you have 4 ohm subs, you can run them at 4 ohm from each channel. If you try to hook two 4 ohm subs up to the amp when bridged down to one channel, the subs can be wired in series (+ from amp to + on sub1, - from sub1 to + on sub2, - on sub2 to - on amp) or parallel (+ from amp to + on both subs, - from amp to - on both subs). Wiring in series will make 8 ohm overall which the amp could run fine, but won't make much power. Wiring in parallel will make 2 ohm overall which could damage the amp.

Go to the site below and have a read...
http://www.bcae1.com/

[Tasmaniak]

btw, I would suggest using something around the 40-60amp mark for the short protection fuse near the battery. If you match it to 30Amps then you may be robbing yourself of some power throung cable resistance.