Switchable impedance amplifiers?

[foetus66]

In the case of an amplifier with switchable impedance (ie the Orange Terror 500 which is switchable, 4 or 8 ohms), can I assume that the wattage is the same at both impedances, and, is that the whole point?

Also, what happens if your cab does not match the impedance you select? Let's say I have it set to 8 ohms and plug it into an 4 ohm cab? What's gonna happen? What if it were the other way around?

What if I want to plug it into a 16 ohm cab and the only options are 4 and 8?

Sorry so many questions, but thanks for any info. Note that I'm specifically asking about solid state amps -- I know an amp with a tube-powered output stage may have different issues running at a different impedance than solid state. The Terror 500, for example, is Class D.

 

[eukatheude]

Also, what happens if your cab does not match the impedance you select? Let's say I have it set to 8 ohms and plug it into an 4 ohm cab? What's gonna happen?

KABOOM!!!

 

[T-Bird]

Hi.

In the case of an amplifier with switchable impedance (ie the Orange Terror 500 which is switchable, 4 or 8 ohms), can I assume that the wattage is the same at both impedances, and, is that the whole point?

Also, what happens if your cab does not match the impedance you select? Let's say I have it set to 8 ohms and plug it into an 4 ohm cab? What's gonna happen? What if it were the other way around?

What if I want to plug it into a 16 ohm cab and the only options are 4 and 8?

When an output transformer is used in an amplifier circuit, doesn't matter if tube or SS, it's done to match the generator (amp) with the load (speaker).

Most -if not all- of the early power transistors were high impedance devices just like tubes, so all of the designs using them had OT's.​

The "gear ratio" of the OT ensures that the output device(s) - again tube or SS, doesn't matter- operate at their designed operating point/range, in respect with the load.

Simplified a lot, since the generator (output devices) usually wants to see impedances in the KOhm or 10KOhm range, and the speaker loads are Ohms, the "gear ratio" is pretty high.
The impedance ratio is not the turns ratio N:1 but N^2:1 BTW.​

Now, if the load is significantly different from the specified, the output devices do not "see" the correct load and therefore do not operate as designed.
Very serious damage can occur if the design doesn't allow that mismatch.


Regards
Sam

 

[foetus66]

OK, so for comparison, my current bass head just has one speaker output and has no impedance switch. I can plug in any impedance speaker as long as it is 4ohm or above, though the higher the impedance, the lower the power output.

In the case of a switchable impedance, can I still use a higher impedance speaker, as long as I don't go BELOW what I have it switched to?

Also if it doesn't matter to the output transformer whether the 'generator' is tube or SS, then why do I have to worry about not turning on my tube amp without a speaker load -- but on my SS amp it's fine?

 

[coreyfyfe]

Solid state amps - with very few exceptions - don't use output transformers. As to why you can't run a tube amp without a load but you can run a solid state amp that way that comes down to design. A tube amp at infinite impedance will cause voltage spikes that will damage your tubes/OT. A solid state amp at infinite impedance will have not produce any power at the output as they're usually directly coupled.

On the Terror Bass the impedance switch is part of the output limiter circuit, not an output transformer. There's a good write up on it in Bass Gear Mag.

 

[T-Bird]

Hi.

In the case of a switchable impedance, can I still use a higher impedance speaker, as long as I don't go BELOW what I have it switched to?

Depends on the design.

I would not.

Without an OK from the manufacturer anyway.

Also if it doesn't matter to the output transformer whether the 'generator' is tube or SS, then why do I have to worry about not turning on my tube amp without a speaker load -- but on my SS amp it's fine?

The topology is different.
The OT is there to match the impedances, and probably in the Orange's case (haven't looked up the schematics yet), providing "tube like" characteristics, so it's the output devices one has to be worried about.

Your transformer coupled SS amp won't probably like being run without a load either.

On quite a few transformer coupled tube amps the output jack is a shorting one, so the damage (if any) isn't immediate.

EDIT:

On the Terror Bass the impedance switch is part of the output limiter circuit, not an output transformer.

Thanks, didn't know that.

Regards
Sam