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  #1  
Old 04.08.03, 5:59 PM
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Question Minimum Ohm-age???

I'm looking at used bass amp heads for my son and I saw a 300 watt amp that said the minimum Ohms were 4. Does that mean it will or won't work with an 8 ohm cabinet?

Thanks!
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Old 04.08.03, 6:33 PM
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I want to say you can go lower head OHM (4) into higher cab OHM (8) without any trouble....

Anyone concur/disagree ?

Nobody...what's the good word ?
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Old 04.08.03, 6:50 PM
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Yeah, you can run a higher load than the minimum load for the amplifier.

You just can't run a lower load than required by the amp.
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Old 04.08.03, 11:00 PM
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Another question...

Thanks, I wasn't sure if 8 ohms was more or less than 4 ohms (in electrical terms that is).

So let me ask another question:

The amp I looked at was 300 watts @ 4 ohms, so it'll be half that for the 8ohm cabinet? So why does the power output drop from 4 - 8 ohms?
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Old 04.09.03, 5:30 AM
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as far as i know it depends on the way the speakears are wired .
my cab is 16 ohms ,thats 4 ohms per speaker wired in a daisy chain like maner , however if you have the input wired to two speakers then to the other two it would be 8 ohms ,or if the input went directly to 4 it would be 4 ohms .
corect me if im wrong sombody ?
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Old 04.09.03, 7:28 AM
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It would be more likely that your speakers are 16 ohms and are wired in series parallel.

Series-parallel means two sets of speakers run in series are run in parallel.

Speakers run in series have a total load of the sum of the impedance rating of each speaker.

The impedance of speakers run in parallel can be calculated like so:

Invert the impedance rating of each speaker, and add them.
Then invert the result.

Example: 1/16 + 1/16 = 2/16
16/2 = 8.

To make life easier, you could just say that with equal impedance rating for each speaker, it is the rating of the speaker divided by the number of speakers there are.

I can't give you a good explanation of why the power output drops with increased load, it does the same thing with P.A. amps.
It might have something to do with the fact the amp is putting out a certain amount of power for a specified load, and an increased load will spread the power thinner, so to speak. This is just a stab in the dark though.

I'll try and find out, and get back to you, it's something I've been wanting to know myself.
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Old 04.09.03, 10:29 AM
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If you run an amp into a higher impedance than it can take, then it will work extra hard and will **** it up faster--this means an eventual meltdown in SS terms and a quicker new set of tubes in Tube terms. It is possible to run an amp into a lower impedance cab, but only if the power handling of the cab is sufficiently higher.

In terms of power: It takes more power to run a higher impedance cabinet. Lower impedance is nice because the amp runs more efficiently, though this may mean that your sound will be cleaner longer.

Most cabs are wired series parallel. In this configuration, the total impedance is equal to the impedance of one speaker. When something is wierd in series, you add up the impedance of all speakers. When
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Old 04.09.03, 11:03 AM
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Wantowail,

Let me take a stab at this for ya'.

Impedance is a measure of the forces that oppose the flow of electricity. The term resistance is for D/C circuits and impedance is for A/C circuits. Without getting too complicated, it measures how hard the elctrical signal has to push to move through the circuit.

Akin to riding a pedal bike.

Flat ground is a low impedance load, a hill is a high impedance load. You only have so much power to push (or pedal), and that would be your voltage, so if you go up a hill, your speed (wattage) will be less. Less wattage will equal less volume.

Strictly speaking Wattage is noted as Volt/Amperes. It is a measure of electrical work being done due to the interaction of voltage and the amount of amperes that pass through a circuit. The formula is P= I/E. P=power or watts, I= amperes, and E= electromotive force...or voltage.

So why does any of this matter?

If you change the load (Ohms), you change the amperes that flow. This can give you more volume....but.....what creates heat in a circuit is ....yeah, you guessed it, the flow of amperes. Too much, and you fry your amplifier. Too little and you lose volume.

Your amplifier said you needed at least a 4 ohm load. Above it and your output drops (an 8 ohm would be 150 watts). Below it and your wattage goes up the same way. (a 2 ohm load would give you 600 watts, untill it burned out.)

Solid state, PCB amps are the most easily burnt up by too many amperes. The far other end of the spectrum are PTP tube amplifiers.

A higher Ohm value in solid state amps will only cause a drop in volume. NEVER go lower in an SS amp. The best thing to do is to try and match the load. Your amp will sound & work best.

In a tube amp, a small ohm drop can be tolerated if you don't push the amp too hard. (Too loud) Tube amps work on large differences in ampere flow during normal operation, so if it's a decent amp, should be okay for a short time.

I read recently from Doug Roccaforte, that in tube amps, a higher load will harm the output transformers. His explanation as to why was beyond me, so I can only pass on what he said.

In the end of my long winded post, I hope this helps some and I'd recommend that you try to match the loads for best performance of the amp. Rick.
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  #9  
Old 04.09.03, 4:57 PM
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Thumbs up WOW

Thanks guys! That answered my question AND gave me an understanding of amps and current that I definitely didn't have before. Now I can look at amps and have a clue what I need, woohoo!

Ciao
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