It just occured to me - can a 4-channel Amp work with 4 Ohms (front comps set) on 2 channels and 2 Ohms (rear coax set) on the other 2 channels?

Is this something amps just automatically do?

sorry if this is a dumb question or has been asked before

 

It's not a dumb question, and yes you can do it. So long as your amplifier is rated at two ohms, you don't have anything to worry about.

Amplifier specifications can be tricky to understand if you aren't an audio geek like some of us.

A speaker 'load' is just that... a load on the amplifier. While it may seem odd that a "smaller number = bigger load" it's actually pretty easy when you look at the math.

Let's not go there right now, okay?

So lets say you had a truck that could haul a ton. Can you haul 1/2 a ton? Sure. No adjustments are necessary. What if you had four of the same trucks? Could each haul a different load without causing a problem for the other trucks? Yes.

Same thing with your amplifier. Each output channel is independent (unless bridged together) so it can haul up to it's rated load without doing anything special. (Just don't overload them. )

Here comes the math:

Power is defined as: Watts = Voltage * Amperage

Voltage is defined as: Volts = Amperage * Resistance (Or Impedance)

Substituting Voltage into the equation:

Watts = (Amperage * Resistance) * Amperage

OR

Watts = Amperage^2 * Resistance

Decreasing the resistance allows more amperage to flow for a given voltage... which increases the overall power output. When you adjust the volume control, you adjust the voltage going to the amp, when then adjusts it's output voltage accordingly.

Hope this helps.

Mark

 

Thanks Mark!

I understand the basics, kinda . It's just that I see for example most 4-channel amps rated 4 channels at x watts @2 Ohms or 4 channels at y watts @4 Ohms . (4 Ohms having half the x value, which makes sense)
I didn't know if they could run 2 channels @ 2 Ohms and 2 channels @ 4 Ohms at the same time or if I'd need 2 separate amps - which would screw my plan

The amp I'm looking at is rated at 2 and 4 yes.

 

Just bear in mind that if the amp is rated for X watts @ 2 ohms, the power it can deliver into 4 ohms will be less than the rated power


(that pesky Ohms Law thing again)

 

Here is an example of that KenHorse is talking about:

I have an Orion 250 HCCA amplifier, and here is how it is rated:

Stereo:
50 watts X 2 @ 4 ohms
100 watts X 2 @ 2 ohms
200 watts X 2 @ 1 ohm

Mono:
200 watts X 1 @ 4 ohms
400 watts x 1 @ 2 ohms

Those are the 'official' amplifier ratings. I can run a 4 ohm speaker on the left channel, and a 2 ohm speaker on the right channel. The left would get 50 watts at full output, while the right would get 100 watts at full output. The same thing applies to 4 channel amps.

Here's the cool part: I can run 8 ohm speakers on that amplifier without any problems, because an 8 ohm load is smaller than a 4 ohm load. Guess how much power I could get at full output?

Given our amplifier specifications:

Stereo:
25 watts X 2 @ 8 ohms

Mono:
100 watts x 1 @ 8 ohms

The reason: The amplifier has a maximum output of 14.14 volts.

At 4 ohms:

14.14 volts = Y amperes * 4 ohms
14.14 volts / 4 ohms = Y amperes
Y = 3.535 amperes

Watts = Volts * Amperes
Watts = 14.14 volts * 3.535 amperes
Watts = 49.99

At 8 ohms:

14.14 volts = Y amperes * 8 ohms
14.14 volts / 8 ohms = Y amperes
Y = 1.7675 amperes

Watts = Volts * Amperes
Watts = 14.14 volts * 1.7675 amperes
Watts = 24.99

The math is the same for all loads. The overall maximum output voltage of my amp is 14.14 volts, and we can pull more power out of it as the impedance drops. We need to be careful though... because those integrated circuits can only deliver so much current. Go past that, and the amplifier is toast.

What amplifier are you running?

Mark

 

I'm just choosing my gear now. I *want* an ED Nine.4 however reality says I can only afford a Kenwood KAC-8403 or the next model up, maybe.
The idea is to get a cheaper amp now and then upgrade later when I get the rear subs. This cheap amp then goes in my truck with the current Infinity 6x9s that I foolishly bought from Best Buy because I couldn't wait any longer :o

So looks like I will have Polk Momo comps (4Ohms) in the front, Infinity reference 3-way 6x9s in the rear (2Ohms) with the cheaper 60/90w amp.

 

any specific reason you are set on infinitys they are just so harsh and bright imo

id do polk db over infnity in a heartbeat and it shoudl be close to same price point

 

The infinities I have now and fitted - was a rush job when I got all excited last weekend and some cash burning a hole in my pocket They were the best sounding (to me) 6x9 set the store had with the most bass.

The cheaper amp and front comps I'm about to order ( have to keep budget around $300 for these)

Infinities will stay for the lows until I can afford a decent amp plus 8" subs. Then I'll move the cheapo 6x9s into my truck. Yeah I know now I did a bad thing

That's the plan so far anyway, it's been changing nearly every day