[wilsynet]

This one has been nagging me for awhile, so I figured I'd just ask.

How come in some amplifiers, as the impedance of the speaker moves up, the amplifier power output also moves up and in some other amplifiers as the impedance load of the speaker moves up the amplifier power output moves down?

For example:

Class D Audio SDS-224
120 watts into 4 ohms
60 watts into 8 ohms

Transcendent Sound T-16
8 watts into 4 ohms
15 watts into 8 ohms

Wilson

[newzooreview]

Because Transcendent reversed their numbers by mistake?

Seriously, I've never seen a lower output given at 4 Ohms than at 8 Ohms.

[PSB Guy]

Has to be a typo. But, they did it TWICE on the same web page, once for the stereo unit, and then again for the monoblocks.
http://www.transcendentsound.com/Transcendent/Transcendent_Sound_T16_OTL_Tube_Amp.html
Weird.

Cornelis

[mfsoa]

And often, the mfgr will down-rate the pwr into 8 ohms to make it look like it doubles down into 4-


Better to have 120/240 watts than 180/240   

[richidoo]

It is common for OTL tube amps to do that. Futterman OTL does that and Transcendent amps are OTL. Some transformer tube amps do it too, a friend told me that his Music Reference RM9 did that, but I don't understand exactly why it happens. I assume it has to do with the source:load impedance ratio.

SS amps usually decrease power with rising load impedance, as ohms law predicts.

Ohms law cannot be defied, so the tube amps whose power increase with impedance are just unable to provide the requested power at low impedance loads, and finally catch up to ohms law when the impedance rises up into their comfort zone.

I hope somebody can explain in more detail cuz I would like to understand it better too. 

[Niteshade]

It all depends on the native output impedance of an amplifier. The closer you get to it, the better the match and more power will be transferred.

[*Scotty*]

The SDS-224 is a classic example of a constant voltage source amplifier, which when the load impedance decreases attempts to maintain a constant voltage output. As you can see when the loads resistance is cut in half the constant voltage value supplied results in twice the current into the load and thus twice the power.
 The other amp can only supply the same amount of current regardless of the resistance of the load it sees. When it sees a 4 ohm load which is half the value of the 8 ohm the voltage falls to half the output it can supply to the 8 ohm load which results in half the power delivered to the 4 ohm load.
Scotty

[wilsynet]

Has to be a typo.

Not a typo, the Atma-sphere S30 delivers 30 watts to 8 ohms and 45 watts to 16 ohms.

[wilsynet]

So would we say this is primarily a OTL versus a non-OTL thing?  Or are there other examples of non constant voltage source amplifiers aside from the RM-9?

[richidoo]

So would we say this is primarily a OTL versus a non-OTL thing?  Or are there other examples of non constant voltage source amplifiers aside from the RM-9?

Well, current source amplifiers have infinite output impedance and their power goes up with load impedance also, and they can be tube or SS, but they are very rare in the marketplace.

[wilsynet]

Thanks.  Very much appreciated.  Anything to say about benefits and drawbacks?

[JohnR]

I don't think it was very clearly stated in the thread, but it depends on whether the amp is current-limited or voltage-limited (in the range of operation). OTLs are generally current-limited - the output tubes will only deliver so much current, and as a result they "like" higher-impedance loads better (up to the point at which they become voltage-limited). Solid-state (Class A/B transistor) amps are voltage limited - they clip at the power supply rails, and so the maximum power increases as the speaker impedance drops.

I gather that class D amps are current limited below the rated minimum impedance. I can't find it quickly right now but I recall seeing some amps designed for 8 ohm operation that deliver less power into 4 ohms.

[JohnR]

BTW, it should be made clear that this is not all anything to do with current-source amplifiers. Both the amps given in the OP are voltage-source amplifiers. The question is what governs their maximum power output into a given load.

[JohnR]

And (sorry) I think it should also be made clear that it is fundamentally nothing to do with the output impedance of the amplifier. It makes no difference how high the output impedance is, if an amp is voltage limited then the maximum output power into 4 ohms is still going to be higher than the output power into 8 ohms.