# Different impedances on the same amp?



## TheMayer (Dec 19, 2012)

Ive always wondered if this will work or if the amp will commit suicide.

Specifically, taking a Alpine MRP-F300 and have channels 1/2 running on a 2 ohm load, while channels 3/4 are running on a 4 ohm load.

Anyone know anything on this subject?


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## l3ackfire1144 (Dec 7, 2013)

id say go for it if you want the amp to commit suicide


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## cajunner (Apr 13, 2007)

TheMayer said:


> Ive always wondered if this will work or if the amp will commit suicide.
> 
> Specifically, taking a Alpine MRP-F300 and have channels 1/2 running on a 2 ohm load, while channels 3/4 are running on a 4 ohm load.
> 
> Anyone know anything on this subject?


some amps can do it, I don't know especially about this Alpine, but I assume it's a 2 ohm stable 4 channel variety.

in that case, lowest recommended impedances should be 4 ohms on either 1/2 or 3/4 bridged.


however, if you're running limited bandwidth, and you're attenuated, like rear channel duty you might not tax the power supply by running a 2 ohm mono load on one pair of channels in a multi-channel setup.


But if you're wanting to run one subwoofer at 2 ohms, and another at 4 ohms, you might get away with it for a while but you'll likely heat the amp a lot more due to the extra current demands, and eventually shorten the amp's life.


so, in specific limited applications it's probably going to be okay but those situations are much fewer than most find themselves trying.


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## l3ackfire1144 (Dec 7, 2013)

said that better than i could have, for most cases you want to run the same impedance across all channels, usually only higher end amps the power supplies can handle it, for instance the focal symmetric line of amps use dual power supplies so in that case do it without issue, but i had a lower end focal solid 4 running front compenents and a sub bridged at the same load and the amp overheated many times, and everything speaker was below the power requirement that the amp was putting out


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## TheMayer (Dec 19, 2012)

Thanks, I figured if it was possible it would only be certain amps.

And sorry should have specified beforehand, Im running active so two channels would be playing 60-1500hz (4 ohms), the other two would play everything up from there (2 ohms). I just figured since its stable at 2 and 4 ohms it could possibly do both at the same time and feed my highs the extra 25W.


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## cajunner (Apr 13, 2007)

l3ackfire1144 said:


> said that better than i could have, for most cases you want to run the same impedance across all channels,


not necessarily.

an amp is like a reservoir of power, and the channels are the pipes that lead the power out.

some channels can run at high pressure and low volume, while other channels run at high volume and low pressure, and the amp is okay.

it's like when a note happens in your midrange's impedance dip, and the amp is drawing 2 ohms to that speaker, while your sub is in it's resonant peak and the amp is drawing 15 ohms to make low notes at resonance.

if too many channels are running in low impedance conditions then the amp is going to run out of reservoir, and what happens is the transformer gets saturated, and you throw too much and the wrong kinds of power into your transistors.

being very simple with my explanation, of course a search would find better answers, but it's not necessary to run an amp's channels at the same impedances on probably 98% of amp designs out there, as a general rule.


You can run 4 ohm on one channel, 8 ohm on another channel, bridge the other 2 channels on a 2 ohm mono load and the amp will push watts until it reaches it's limits, where the entire amp's outputs are vulnerable because the power supply is overloaded.


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## l3ackfire1144 (Dec 7, 2013)

to be honest highs require much less power as is to blend with the lower frequencies, i honestly wouldnt worry about it. some high end tweets require much more power to sound how they are suppose to, high end comp sets running active often times are running 200rms to each tweet...but for your setup like i siad... no big issue

for instance my midbass drivers are rated "rated" 150rms, with my tweets only want 15 rms,,, huge difference in power there, 1/10th of my mids


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## cajunner (Apr 13, 2007)

TheMayer said:


> Thanks, I figured if it was possible it would only be certain amps.
> 
> And sorry should have specified beforehand, Im running active so two channels would be playing 60-1500hz (4 ohms), the other two would play everything up from there (2 ohms). I just figured since its stable at 2 and 4 ohms it could possibly do both at the same time and feed my highs the extra 25W.


more explanation is necessary here.

you're saying you want 2 channels to run at 4 ohms a piece, in mid bass frequencies.

the other two channels, are bridged?

you can do both at the same time, but not on the same speakers.

it sounds like you are confused on what bridged operation is about.

mixed-mono capable amplifiers, which most car amps are, won't be able to parallel different speakers such that the impedance the amp sees, is too low.

every time you place a speaker into a circuit in parallel, you decrease the impedance and unless you divide those frequencies correctly, you'll put the amp into a stress condition.

If you're running active and you want to drive tweeters at 2 ohm mono, say, on 2 of the 4 channels, that's bridged operation and you can't run more speakers on those channels in their unbridged modes.

It's not that intuitive and you would do well coming back to the basics in BCAE's tutorials, several times until it starts to click.


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## TheMayer (Dec 19, 2012)

l3ackfire1144 said:


> to be honest highs require much less power as is to blend with the lower frequencies, i honestly wouldnt worry about it. some high end tweets require much more power to sound how they are suppose to, high end comp sets running active often times are running 200rms to each tweet...but for your setup like i siad... no big issue
> 
> for instance my midbass drivers are rated "rated" 150rms, with my tweets only want 15 rms,,, huge difference in power there, 1/10th of my mids


damn! that is a big difference

The reason I am wanting to squeeze a few more watts out to the highs is I'm about to play around with a 3-way setup by reinstalling my old tweeters on the same two channels as my current "tweeters" (CDT ES-02 mid/tweeter) with an inline ~4kHz hpf to see if it helps a slight blending issue im having.

I realized with the two 4 ohm drivers i could now run a 2 ohm load and draw the 75w so I'm not giving the CDT's that much less power.


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## cajunner (Apr 13, 2007)

also, when you're dealing with a lower powered amp like a 40W X 4 model, or something similar, it's much easier to pop tweeters when you're paralleling pairs of speakers, because you'll be in clipping conditions for longer periods, where damaging power can get to the tweeters. 

Everyone who is just starting out, needs to understand that just because the speaker is rated for a specific amount of watts, you can't just hook up an amp with that power rating and expect to have everything last.

that's the learning curve, when you find out you've blown your speakers and you're puzzled why that happened, it's usually because you didn't know your amp was in an overloaded condition for too long a duration.

I can blow a 100W rated tweeter with a 25W/ch amp, even if it's crossed over right just by being careless with the volume knob.

Car audio is not capable of a perfect record of idiot proofing, even if there's 5 different protection circuits in the amp and the salesman said you're fine.


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## cajunner (Apr 13, 2007)

TheMayer said:


> damn! that is a big difference
> 
> The reason I am wanting to squeeze a few more watts out to the highs is I'm about to play around with a 3-way setup by reinstalling my old tweeters on the same two channels as my current "tweeters" (CDT ES-02 mid/tweeter) with an inline ~4kHz hpf to see if it helps a slight blending issue im having.
> 
> I realized with the two 4 ohm drivers i could now run a 2 ohm load and draw the 75w so I'm not giving the CDT's that much less power.



this sounds safe enough. 

you're taking everything above 4 Khz into 2 ohms at the amp, when you parallel the CDT with the old tweeters. However, if you're using passive crossovers that came with a component set, on those old tweeters for your 4 Khz and up, you could put the amp into a near short condition because of how some passive crossover networks are configured.


If you're just adding a capacitor to the old tweeters for your 4 Khz and up crossover, then it's okay.

You see, it's not always obvious if a change in your set-up is harmful or not.


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## TheMayer (Dec 19, 2012)

cajunner said:


> this sounds safe enough.
> 
> you're taking everything above 4 Khz into 2 ohms at the amp, when you parallel the CDT with the old tweeters. However, if you're using passive crossovers that came with a component set, on those old tweeters for your 4 Khz and up, you could put the amp into a near short condition because of how some passive crossover networks are configured.
> 
> ...


Hmm, ill be honest I didn't know there was another way besides a passive crossover. :stunned: Could you point me in the direction of a write up on the capacitor method? And possibly explain why a passive crossover could short it out?


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## cajunner (Apr 13, 2007)

TheMayer said:


> Hmm, ill be honest I didn't know there was another way besides a passive crossover. :stunned: Could you point me in the direction of a write up on the capacitor method? And possibly explain why a passive crossover could short it out?


a 12 db/oct passive, is a capacitor and inductor together in circuit.

a capacitor is still a passive crossover, it's just a 6 db/oct slope.

when some passive crossovers are connected only to the woofer, or only to the tweeter, depending on the amount of circuits involved you could have a portion of the frequency spectrum running an impedance near 0 ohms. This could cause an amp to go into protect or just overheat without any warning signs and self destruct.

there's no real method, it's just practicing safe crossover sex. Make sure your passives if they have outlets for woofer and tweeter, have both outputs connected to speakers.

You might never run into this problem but I've damaged amps myself because I didn't know it was possible. 

You will see warnings and things in component speaker manuals but it's easily missed in the heat of installing, and easily forgotten if you don't go back and read the manual before re-installing later.


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## TheMayer (Dec 19, 2012)

cajunner said:


> a 12 db/oct passive, is a capacitor and inductor together in circuit.
> 
> a capacitor is still a passive crossover, it's just a 6 db/oct slope.
> 
> ...


Im glad you brought that up, I would never have thought that was possible. I was going to get one of those inline one way high pass filters, for the tweeter wire only, from parts-express to test the idea before I bought a nice one. Bad idea?


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