#1
Why do bass amps depend on ohms for watts unlike guitar amps?
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#2
Every amp is different but the rough general rule is "half the resistance results in double the power output." It's a physics thing that often does apply to guitar amps, prosound amps, as well as bass amps.
"Your sound is in your hands as much as anything. It's the way you pick, and the way you hold the guitar, more than it is the amp or the guitar you use." -- Stevie Ray Vaughan

"Anybody can play. The note is only 20 percent. The attitude of the motherfucker who plays it is 80 percent." -- Miles Davis

Guthrie on tone: https://www.youtube.com/watch?v=zmohdG9lLqY
#3
it doesn't just often apply to guitar amps. It always applies to guitar amps. It always applies to all amplifiers.

TS, what is your basis for the statement that guitar amps don't 'depend' on ohms for wattage? The statement itself doesn't really make much sense.
#4
I think you may be thinking of the difference between valve and solid state amps. Solid state amps will double their power if you halve the impedance until the power supply gets overloaded. Valve or tube amps have a transformer which matches the speakers so the maximum output is available all the time.

https://www.ultimate-guitar.com/lessons/the_guide_to/matching_speakers_to_amps.html
#5
Quote by H377F1R3
Why do bass amps depend on ohms for watts unlike guitar amps?


Most bass amps these days are solid state -- they put out a different rated power when the impedance changes.

My Carvin BX1500 is actually a stereo amp (two separate amps) that can be bridged. At 8 ohms, bridged, mono, it puts out about 900W. *One* of my cabinets will handle that much power. If I put two of those 8 ohm cabinets out there (in parallel), the impedance is 4 ohms (bridged, mono) and the amp is putting out 1500W or so to the pair of them.
#6
What must be remembered is that there is a minimum ohmage that solid state amps will work into.
Take the ohms too low and you can damage the output stage of your amplifier.
Most solid state amps have a minimum working load of 4 ohms.
G&L L2500
Squier Affinity Jazz Bass 5
Ashdown RPM pre-amp
Ashdown Little Giant 1000
300 watt 15" powered cab
450 watt 15" powered sub bass cab
2x10 + horn
1x15x10 + horn
#7
Phil StarrI think you got it. I meant why do solid-state amps say 300 watts at 4 ohms but then has a different wattage if you put it into a 8 or 16 ohm cab
Together we climb any mountain, rent any video, dial any phone. And not just our phone, other people's phones. Decent phones, God-fearing phones, phones that everybody else gave up on, but we knew better because we were a team!
#8
It all has to do with Ohms law and physics (see we all shouldn't have nodded off in HS science class, myself included..)

http://blog.hughes-and-kettner.com/ohm-cooking-101-understanding-amps-speakers-and-impedance/

Check out this handy dandy link from Hughes and Kettner's amp blog!
#9
H377F1R3
Quote by H377F1R3
Phil StarrI think you got it. I meant why do solid-state amps say 300 watts at 4 ohms but then has a different wattage if you put it into a 8 or 16 ohm cab


Solid state amps are essentially limited by their power supplies, The amp can swing the voltage that drives the speaker up to the power supply voltage but no higher. (In fact this is where the RMS bit comes in but that gets a little complex, RMS is how you work out the 'average' of a varying voltage)

Anyway if your amps power supply allows a maximum of 40V RMS then the formula is that the power is the square of the voltage divided by the impedance of the speaker. For 8 ohms that is (40x40)/8 or 200W and for 16 ohms it will be (40x40)/16 or 100W. Theoretically it ought to give 400W into 4 ohms and 800W into 2ohms but in practice most power supplies run out of puff (amps or current) and so you might only get to 350W through 4 ohms and the current through 2 ohms may be enough to damage the amp so the inbuilt limiters will usually cut in to save you from yourself.

So basically halving the impedance will give you twice the power but only if you have a beefy enough power supply in your amp.
Last edited by Phil Starr at Jul 29, 2016,
#10
If you think of it this way jack car up put it into gear, 4 wheels 16ohms ok, 3 wheels 12ohms ok, 2 wheels 8ohms ok, 1 wheel 4ohms ok, 0 wheels no ohms engine races and burns out just as most amps will.
G&L L2500
Squier Affinity Jazz Bass 5
Ashdown RPM pre-amp
Ashdown Little Giant 1000
300 watt 15" powered cab
450 watt 15" powered sub bass cab
2x10 + horn
1x15x10 + horn
#11
Delerium called it!

It was a weird assumption without actual technical knowledge. I have worked with Guitar amps with Ohm Switches, and one with pretuned/designated Ohm load Output Jacks.

Thus, Phil Starr cannot be completely correct. The answer in the direction of Phil's thought is not Valve vs. Solid State, but Class vs. Class [A, B, AB, C. D, etc.]

Phil's second answer misapplied Voltage in place of Wattage at first, or did he mean wattage when he used the word "power"., but is over all a well reasoned presentation, within the limits of Class.

The answer is that Guitar Amps do care about the restistant Load (ohms) just as much as Bass Amps do.

Bassists on the otherhand to Guitarists have been more concerned about this issue when facing the acoustical phenomena of power requirements for thier frequency range to hit the same decibel levels as a Guitar's frequency range which requires about 1/4 the energy to produce verses the bass.

I had a guitarist 10 years ago who insisted on running 6 8 Ohm cabinets with his 100w crate amp.

He had all the tone of a Norelco Electric Razor.

I got him to get another Amp and get the 100w Amps running 3 8 Ohm cabinets to get him up to 3.2 Ohms of resistance.

He still sounded like an electric razor as I am sure he was melting his power transistors, and his 24 speakers at 200w, probably more like 350w., were dominated by my 425w out of 2 12" NeoX Cabinets.

I destroyed him, and he fired me for being too loud.

If he had gotten one more Amp, it would have been a fair fight, And his power transistors would have had a fighting chance.

By the way, I got thrown off of a Bass Forum for pointing this stuff out.
Ibanez BTB 1006 Fretless and 405 (no Barts)
456 & 455(w/Barts)
Genz Benz NeoX400 112T & NeoX 112T cab.
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Yamaha PB-1
Boss: SYB-5, PS-2, OD-20, EQ-20, PH-3,BF-3, CE-20, DD-20
Morely A/B
#12
^Then there's also the whole thing about different frequencies having different impedances in the loudspeakers (my hifi speakers are 8ohm nominal, but dip as low as 4.8ohm at 60Hz for example), but that's going far beyond what needs to be understood by most.

Class G would be an interesting development for bass amplification.