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Topic: 4 ohm amp with 8 ohm speaker |
Dave Zirbel
From:
Sebastopol, CA USA
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Posted 11 Aug 2018 5:25 pm |
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Just wondering if there is a formula for how that affects the power of the amp. For example: my amp is 56 watts and wanting 4 ohms but I use an 8 ohm speaker. Does it cut the output by about one third? Like maybe it works more like a 40 watt amp instead on 56 watts?
Thanks
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Dave Zirbel-
Sierra S-10 (Built by Ross Shafer),ZB, Fender 400 guitars, various tube and SS amps |
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Tony Prior
From:
Charlotte NC
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Posted 11 Aug 2018 11:28 pm
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Dave the scientific theory of it all says it changes the total output WATTAGE.
The practical theory of it all says you probably will not even know it !
I yanked the 4 ohm speaker from the N112 and replaced it with an 8 ohm. Two things never happened, it didn't blow up the amp and I never recognized any difference in output volume.
Typically when we go from 8 to 4, on the same tap, it increases output by "something" , going from 4 to 8, theoretically reduces output.
There is the actual MATH and then there is our ears.
There are some amps that run 4 ohms where the manufacturer does not recommend 2 ohms. In those cases the amp can run in a stressed situation and possibly reduce the life of components.
Maybe Ken Fox will chime in here.
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Ian Rae
From:
Redditch, England
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Posted 12 Aug 2018 12:40 am
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I used to know all this when I was at college but forgive me if I'm rusty. Halving or doubling the load is going to raise or lower the power by 3dB which is the least difference the ear can detect. I think.
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Georg Sørtun
From:
Mandal, Agder, Norway
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Posted 12 Aug 2018 2:44 am
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For an "ideal" SS amp – an "on-paper model" without limitations; changing the load (speaker) from 4 ohm to 8 ohm will reduce the max Wattage that can be drawn by the speaker by 50% – an amp that can deliver max 56 Watt into a 4 ohm speaker can then deliver max 28 Watt into an 8 ohm speaker.
As there are no "ideal" SS amps on the market, the reduction will in most likelyhood be less than 50%. As I have no idea how near the OP's amp is to an "ideal" SS amp – or if it really is an SS amp (and not a tube construction), I'll simply estimate that for an average SS amp the max Wattage that can be drawn from it will drop by about 35% to 45% when changing from a 4 ohm to an 8 ohm speaker.
As for audible reduction; the speaker's efficiency and other characteristics must be calculated in. No such data are made available, so one can only guess.
- If the 4 ohm and the 8 ohm speaker have identical characteristics, the loss may be somewhere in the 2db to 3 db range. For our ears that is a barely audible drop.
- If the 8 ohm speaker is more efficient than the 4 ohm speaker, audible power may actually increase after change to 8 ohm.
I usually take all talk, and specs, about amp-Watt, speaker-ohm, and speaker-efficiency with a pinch of salt – they may be off by quite a large percentage if measured.
The signal-swing on a power amp's output terminals is in Volt, not Watt, and the speaker draws whatever Wattage it needs, and can, as it attempts to follow that signal-swing. In the process the speaker generates a Voltage-swing in the other direction – back to the amp, which acts as an impedance-altering signal and can be, and actually is, used as part of the feedback in better SS amps. Not much of these factors are utilized in instrument amp/speaker designs usually dealt with here on the Forum, and how many Watt an amp is said to deliver is usually of little value except when it comes to preventing amps and/or speakers from being blown up. |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 12 Aug 2018 8:09 am
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Tube amps need impedance matching. Solid-state amps often have limits ('4-ohm capable' etc). Tube amps with single output taps want one impedance for optimum... SS amps will increase their output with lower impedance up to the point where they overheat. Tube amps run at too high an impedance will arc over (the voltage climbs)... SS amps run at too low an impedance will melt or shut down.
Speakers have different sensitivity to signal. JBL's can be 102db/watt... big ceramics like TToy or PF-350 can be down 3db from this, or more. What this means is that changing speakers is not just about impedance, the sensitivity of the speaker has to be taken into consideration as well if ultimate audio output is the goal. More sensitivity with more impedance with solid-state might equal the same output.
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ajm
From:
Los Angeles
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Posted 12 Aug 2018 11:30 am
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Power output wattage versus impedance load: It might be a linear relationship, it might not be.
If you did an experiment with resistive loads and an oscilloscope you may be able to determine this, but then again, amps are intended to be used with speakers and not resistors. Therefore I don't know what parctical useful data you'd get from doing this.
Swapping one speaker for another: They had better be real close in specs, otherwise all bets are off as to your hearing a difference. As was mentioned, speaker efficiency is probably the main factor here.
The other possible factor is speaker size or area. Using an extreme example, swapping a single 12 inch 4 ohm speaker with a 4x12 cabinet that is 8 ohms will probably be a noticeable difference in volume and coverage.
All things being equal, you probably won't notice a significant difference.
Just for the record:
What model of amp is it?
Tube or solid state?
What is the root question that you're trying to answer? |
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Dave Zirbel
From:
Sebastopol, CA USA
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Posted 12 Aug 2018 4:58 pm
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I actually have two Fender tube amps wanting 4 ohms but getting 8...and they are both plenty loud. I have a modded Deluxe Reverb with a Bandmaster transformer and 6L6 tubes. I used a k120 for years . Sound men still could not tolerate the volume. I also have a modded Super, the ultra linear model. Swapped out the transformer so now it’s around 56 Watts and stupid loud with an 8 ohm speaker...🙄
Thanks for the replies!
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Dave Zirbel-
Sierra S-10 (Built by Ross Shafer),ZB, Fender 400 guitars, various tube and SS amps |
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Cartwright Thompson
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Posted 13 Aug 2018 1:06 pm
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I had a D-120 in a ‘66 Vibrolux for about ten years, used it on tons of gigs. It sounded like a million bucks, plenty of volume, never had a problem. Then, like a complete idiot, I sold it!
One step up or down on a Fender tube amp ( although I think I’d get a little queasy running a 4ohm amp at 2 ohms), from my experience, is ok. |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 13 Aug 2018 7:56 pm
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You want a formula? OK - the formula for power transfer from an electronic source to its load is given in this old thread:
https://bb.steelguitarforum.com/viewtopic.php?p=1561893
Quoting from that thread:
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The output impedances of both a tube amp and speaker are complex, having both resistive and reactive components. The rule about electrical power transfer is that maximum power transfer is achieved when the load impedance is equal to the complex conjugate of the output impedance of the amp - http://en.wikipedia.org/wiki/Impedance_matching. For purely resistive loads, this means that the load resistances are the same, but this is more complex for a reactive load. The power transferred is given by this expression
P = {(1/2)*|V|2*RL}/{{RS + RL)2 + (XS + XL)2}
where V is the applied voltage, X is reactance (S=source, L=load) and R is resistance (S=source, L=load). So maximum power is transferred when
(1) XS = -XL
and
(2) RS = RL
Remember that purely capacitive reactance and purely inductive reactance have different signs, so conjugate reactance matching is possible in principle, but not generally achieved in practice. But regardless of whether or not reactance matching is achieved, for a given source and load reactance, [edit - max] power transfer is generally achieved when source and load resistance are matched. There is a possible second-order effect that changing the load resistance may change its reactance in a favorable or unfavorable way, but it is generally a good idea to match source and load resistances. |
I generally prefer to run the correct load in my old tube amps. The issue for me is not power transfer - there are cases where it is an advantage to reduce power transfer efficiency, even at the expense of increased wear on the tubes. For me, the main issue is flyback voltage, which is explained in this thread referencing an explanation from a webervst link that no longer works - https://bb.steelguitarforum.com/viewtopic.php?t=170054.
So if one insists on mismatching the load deliberately, it is generally better in a tube amp with an output transformer to use a lower impedance (larger load) speaker than a higher impedance (smaller load) speaker. Again, this is because the higher impedance is what increases flyback voltage. See this thread - https://bb.steelguitarforum.com/viewtopic.php?t=228139.
This may seem counter-intuitive, and is opposite to the case for most direct-coupled solid-state amps, where the mortal sin is to connect too small an impedance speaker (higher load), which, if taken to extremes, will tend to cause too much current to flow through the transistors and likely overheat or even melt the substrate. It's OK to even run a direct-coupled solid-state amp with no load at all - no current flows, the amp is happy, but you don't hear anything. But run a tube amp with output transformer without a load, and you will likely damage the output transformer fairly quickly. |
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