Amplifier-speaker matching impedance

[djrama]

Hello!

There is a question in wich I was thinking a while in the past weeks, imagine you have an amplifier wich specifications say that it can driver 2 ohm, 4 ohm, and 8 ohm speaker, and you have 3 speakers of 8 ohms each one.

If you connect this speakers in parallel, you have a teorically impedance of 2.666 ohms (8/3 ohms), I know the amplifier will work, but, it will work fine?

In think that the past amplifiers with a output transformer needs matching impedance but I have been told that with newer solid state designs this isn't necessary but for some reason I'm not convinced at all... why the manufacturer talks only about 2,4, and 8 ohms? It is needed to match some of this impedances?

Any help would be apreciate!

Thanks!

[raypalmer]

It's thermally stable to run down to 2 ohms. ie the thing won't melt itself to the rack driving a 2 ohm load.

Is the most simple explanation, more learned people here will surely chime in with more profound insights.

[stratus46]

Originally Posted by djrama

Hello!

There is a question in wich I was thinking a while in the past weeks, imagine you have an amplifier wich specifications say that it can driver 2 ohm, 4 ohm, and 8 ohm speaker, and you have 3 speakers of 8 ohms each one.

If you connect this speakers in parallel, you have a teorically impedance of 2.666 ohms (8/3 ohms), I know the amplifier will work, but, it will work fine?

In think that the past amplifiers with a output transformer needs matching impedance but I have been told that with newer solid state designs this isn't necessary but for some reason I'm not convinced at all... why the manufacturer talks only about 2,4, and 8 ohms? It is needed to match some of this impedances?

Any help would be apreciate!

Thanks!

Solid state power amplifier output impedance is way below the load impedance. The ratio of output to load is the 'damping factor' so to get 'optimum power transfer' meaning matched impedances, the solid state amplifier would fail. The output currents would be insane and the heat output would simply burn up. Too low an output load will cause more current and heat. Some amps are rated for low impedance loads but not all.

Tube amps try to match impedances - different constraints.

G²

[Jay]

Originally Posted by djrama

Hello!
There is a question in which I was thinking a while in the past weeks, imagine you have an amplifier which specifications say that it can driver 2 ohm, 4 ohm, and 8 ohm speaker, and you have 3 speakers of 8 ohms each one.

If you connect this speakers in parallel, you have a theoretically impedance of 2.666 ohms (8/3 ohms), I know the amplifier will work, but, it will work fine?

Amplifier needs only specified to be able to drive a minimum load impedance. In your case, if it can drive 2 ohm load, of course it can drive higher impedance loads.

Speaker impedance is not constant, it changes with frequency. There is a minimum impedance for every speaker. For example, in range 300Hz-700Hz an impedance may be lowest at 4 ohm. But speaker impedance is often specified as "nominal impedance", usually 4/8/16 ohm, not necessarily its minimum.

Three speakers specified with 8 ohm nominal impedance may have each 5 ohm, 6 ohm and 7 ohm of minimum impedance that may occur in different frequencies, so that formula of paralleling impedance and the nominal impedance spec will not be too useful in practice.

All amplifiers in general will have difficulties in driving low impedance load (too much current will be drawn from the amplifier), especially for tube amps. In multi-driver speakers, some drivers are wired in series and some are wired in parallel to get a reasonably high total impedance. In your case, if you parallel the 3 speakers you can "expect" the speaker system impedance will fall to 4 ohm at certain frequency. Almost all solid state amplifiers can drive this speaker system. You just need to watch out the heat dissipated by your amplifier before you decide to crank the volume up.

Paralleling speakers are not common because you cannot predict the result sound-wise, but if you're lucky you will get a better sound because you will have a "complete" signal (flatter response).

Originally Posted by djrama

I think that the past amplifiers with a output transformer needs matching impedance but I have been told that with newer solid state designs this isn't necessary but for some reason I'm not convinced at all... why the manufacturer talks only about 2,4, and 8 ohms? It is needed to match some of this impedances?

Be aware that "matching" has two different meanings that may confuse you:

1. Matching between a speaker with specified nominal impedance and an amplifier with specified minimum impedance that it can drive safely.

2. Matching between "output impedance" of an amplifier and the "input impedance" of it's load (or the "load impedance")

Basic amplifier circuits with tube as amplifying devices have very high "output impedance". You want to lower this, or to match this with relatively lower "load impedance" (16 ohm of the past speakers is already too low), so you use transformer in the output to "match" these two impedance.

Modern solid state amplifiers do not need this output transformer because it can be designed to have a very low output impedance. Modern solid state amplifiers have the ability to supply high current to the output (big power supply and paralleled output transistors) that will give capability to drive impedance as low as 2 ohm.

[Jay]

Ups... that paralleled speaker will most probably goes below 4 ohm. Higher power amps (especially class B) usually have capability to drive lower load. The minimum impedance only happens with certain music or frequency, and not all amplifiers are specified with this low impedance driving capability. You should know some aspects first before trying to feed some low impedance load to your amp!

[raypalmer]

Worst comes to worst have some marshmallows and weenies handy for the fire

[djrama]

Originally Posted by Jay

Amplifier needs only specified to be able to drive a minimum load impedance. In your case, if it can drive 2 ohm load, of course it can drive higher impedance loads.

But speaker impedance is often specified as "nominal impedance", usually 4/8/16 ohm, not necessarily its minimum.

Three speakers specified with 8 ohm nominal impedance may have each 5 ohm, 6 ohm and 7 ohm of minimum impedance that may occur in different frequencies, so that formula of paralleling impedance and the nominal impedance spec will not be too useful in practice.

I see I was wrong with this, I used to think that the specified impedance was minimum. From this came all my error.

Originally Posted by Jay

some drivers are wired in series and some are wired in parallel to get a reasonably high total impedance. In your case, if you parallel the 3 speakers you can "expect" the speaker system impedance will fall to 4 ohm at certain frequency. Almost all solid state amplifiers can drive this speaker system.

Maybe there is not much problem connecting the speakers in parallel if the amp could drive 2 ohm loads. Althought I don't like connecting them in parallel, but the system is actually wired in this way by a partner.

Originally Posted by Jay

Modern solid state amplifiers have the ability to supply high current to the output (big power supply and paralleled output transistors) that will give capability to drive impedance as low as 2 ohm.

Yes, and this model in concrete (a cheap Yamaha one) is specified to a minimum of 2 ohms, but for some reason I used to think that the best is only connect 2/4/8 ohms loads, and not to connect another intermediate numbers (2.4,2.8,3.2...) but now I see that this is nosense.

Thanks!

[djrama]

Originally Posted by raypalmer

Worst comes to worst have some marshmallows and weenies handy for the fire

I think I could be happy that the amp isn't in my home, and, well...

[djrama]

Originally Posted by stratus46

Solid state power amplifier output impedance is way below the load impedance. The ratio of output to load is the 'damping factor' so to get 'optimum power transfer' meaning matched impedances, the solid state amplifier would fail. The output currents would be insane and the heat output would simply burn up. Too low an output load will cause more current and heat. Some amps are rated for low impedance loads but not all.

Tube amps try to match impedances - different constraints.

G²

Ah ok, I mean matching with the specifications impedance with a discrete way, for example, only connnect 2 ohms drives, or 4 ohms driver or 8 ohms drives, and never 2.6 ohms, or 4.2 ohms, for example.

Thanks for the reply!

[AndrewT]

Hi,
there is a big difference in amplifier performance whether it is driving a resistor as a load or a reactive speaker as a load.

A Solid State (transistor) amplifier that is capable of driving an 8ohm reactive speaker will probably also be capable of driving a 4r0 or 6r0 or 8r0 or 64r0 resistor. It might be capable of driving a 3r0 load and maybe even a 2r0 load, but this would be quite unusual for an 8ohm amplifier.

That ability to drive a 4r0 resistor well is absolutely no indication of how capable it could, or could not, be at driving a 4ohm reactive speaker.