Syntrillium Backup Forums :: View topic

zeitgeist · Location: Slovenia

Hi~
I am looking at some monitor type speakers and the stats say they are 4 ohm speakers. My amp is designed for 8 ohms, I think. Will the 4 ohm ones work right with it?

AndyH · Posted - Thu Nov 21, 2002 3:43 pm

I think it would be a very unusual amplifier that would not work. Is this a old tube amp that doesn't have multiple output taps? Surely no solid state amplifier would even think twice about it. You could always make your first try at very low signal level, to avoid putting much strain on anything.

zeitgeist · Location: Slovenia

its a 1979 hitachi solid state with mosfet circuits.

SteveG · Location: United Kingdom

That 4 ohm figure will only be the lowest impedance - this will vary quite a bit across the audio range, as with all speakers. They will work fine, and if you have a MOSFET amp, which is inherently self-protected, you won't have a problem anyway. The worst thing that can happen is that you might draw a little more power out of your amp, but since most of the time it's running you will be nowhere near the dissipation limit, this hardly matters either - certainly not on a modern amp, anyway.

zeitgeist · Location: Slovenia

So if I order the 4 ohm monitor speakers I can be sure they'll work correctly for me? I thought it was ok but just want to make sure. I just learned today that the speaker impedence is simply the resistance of the length of coil wire used. The near-field monitors are smaller with less coil wire length, so the resistance is lower, like with car speakers, right? But the amp will drive them ok, but will need less volume power, right? I don't have a manual on the amp.

teflon20 · Location: Australia

jonrose · Location: USA

jonrose · Location: USA

teflon20 · Location: Australia

Butch · Location: USA

the only thing that i see a problem with. is you amp's age and the fact that it will run a bit hotter with the 4 ohms..

SteveG · Location: United Kingdom

Teflon's optimum power transfer theory is correct, but unfortunately it only applies in cases where there is a finite source resistance/impedance. With old valve(tube) amps that had output transformers, it made a difference, because the transformer winding actually had a finite value, like 15 ohms, and if you didn't match this with a 15 ohm speaker, you transferred less power. Fine. But MOSFET (and in fact pretty much every modern) amps are arranged differently. What happens is that the output devices (the MOSFETS) are acting as controlling devices that regulate the amount of current supplied to the speaker from the amp's power supply. And they do this whilst introducing an absolutely negligable series resisitance.

So ultimately, what gets fed to the voice coil is ultimately limited by what the power supply can actually supply, which should be arranged (in a good amp) to have a very low output resistance. But also, there is a limit to how much current it can supply to the loudspeaker before the components supplying it break down, or the components controlling this supply have a similar problem! The other limiting factor is the maximum voltage that the supply can run at, because this will ultimately limit the power that can be delivered into a given impedance.

But anyway, this is why speaker manufacturers provide the lowest impedance value when specifying the performance of a speaker. Because now, by a simple application of Ohm's Law, we can see that the power transfer to a loudspeaker is equal to, on the one hand, Power=V^2/R, which says that at any given impedance, the amount of power transferred goes up rather less quickly than the applied voltage (the amp starts to clip very early on a high impedance speaker, or, more importantly, headphones) and...

I=V/R which means that for a given impedance R, and a fixed voltage V, that I will go up as R goes down. So ultimately, if you put too much of a resistance load on your amp (too many speakers in parallel), it will blow up very quickly, because it will try to supply current to all of them. With a transformer-coupled system, this wouldn't happen, because the source and reciever wouldn't be matched any more. The losses would just dissipate as heat in the transformer, and probably wouldn't do any significant damage unless the speaker resistance was so low as to present a virtual short-circuit to the transformer secondary.

Incidentally, the headphone bit is quite important. If you plug 600 ohm headphones directly into the output of a power amp, the impedance will mean that the amount of power that they can absorb and dissipate is strictly limited by the high R value, so you won't blow your ears off. OTOH, if you plug in 8 ohm headphones, they will absorb, and attempt to disspiate the entire output power of the amp straight into your ears. That's why it's much safer to have 600 ohm phones. You can plug them into whatever you like, within reason, and not blow it, them, or you, up.

So basically, the lower the load impedance/resistance, the more power you can transfer at any given voltage, providing that the power source is up to it. Yes, you could say that the amp 'works harder', but it's actually a bit more complicated in the real world because of differences in speaker efficiency, etc. What you can say, with alsolute certainty, is that the lower the speaker impedance, the better chace you stand of either blowing the fuses, or output stage, of your amp (unless it has a transformer output, which will probably absorb the losses).

zeitgeist · Location: Slovenia

post78 · Location: USA

Oh, no you don't, zeitgeist! You asked a simple question about electronics, so you can bet that this one will go on for a few more pages. Black Eye

SteveG · Location: United Kingdom

There was a simple answer - and nobody has disagreed about it, I believe.

Q. My amp is designed for 8 ohms, I think. Will the 4 ohm ones work right with it?
A. Yes, unless they don't... Tongue

:D

But unfortunately, as usual with electronics, a simple statement often hides multitudinous levels of complexity. Speaker matching and impedance are no exceptions.

We have actually covered some of the related issues before (when I was recovering from being ill) 'Is CLIPPING bad for speakers??'.

post78 · Location: USA

motorhead6 · Posted - Sat Nov 23, 2002 12:38 am

Thats right it will work unless it burns up! Odds are it will just run a little bit warmer but it could burn up.

jonrose · Location: USA

zeitgeist · Location: Slovenia

errr---it says on the back of the amp in question to use 4-8 ohm speakers doh! LMFaO Thanks for the input, anyhow.

SteveG · Location: United Kingdom