What components determine amp wattage?

[Shimmer]

I'm not an amp builder (too scared of electric shocks!), but just curious. What are the components in, for example, a solid state amp, that determine the wattage? I mean, what are the major differences between what make an amp 10W or 100W.?

[Molly]

Interesting question. I've no bloody idea but I'm sure Nik will be along in a moment to explain. I do know that a solid state amp's 100w is quite different to a valve amp's 100w. But I've no idea why.

[Dharmajester]

Transformers, y'know Megatron and that lot

[Bg]

Output transformer and power transistors

[LucyTheSpud]

Power tubes affect it also right? Different types and 2 vs 4?

[sizzlingbadger]

Simplistically...

Power valve type & number of valves + power & output transformers in valve amps.

In solid state amps its the power supply and output transistors.


Wattage ratings are generally measured as being the point at which a certain amount of THD (say, 1%) is happening. So if you have two amps, one solid state and one valve that both are rated at 50 watts 1%THD, they'll both put out 50 watts cleanly. Beyond that, if you turn the solid state one up it'll very quickly start to sound nasty. If you turn the valve one up, you might find it gets to 70 watts at 10%THD and still sounds reasonably clean and musical, and if you really push it maybe it'll be really distorted, sound awesome, and be putting out 90 watts - whereas the solid state amp would have exploded and released smoke long before it got to that volume.

[druz15]

in a tube amp, a combination of the power tubes used, how they are used, and the transformers used to both power them AND pair them with speakers.

A properly designed dual pentode amp can put out close to 100W (not clean of course), and it goes the other way too, but typically
2x EL84 = ~15W
4 x EL84= ~30W
2x EL34= ~50W
2x 6l6= ~60W
2x KT88= 80-100W
and so on and so forth.

But that's a little simplistic. The power supply design and it's efficiency plays a big part as well.

For solid state it's a lot more complicated from memory, as the solid state opamps that are doing a similar job to the tubes can only dissipate so much power as heat before they overheat and stop working effectively, so it's about more chips (more $$$) or better and bigger heatsinking & fans (more weight and size). Class D designs are becoming very popular now as they are super efficient and the weight/wattage ratio is pretty incredible when compared to standard solid state and of course tube stuff too

[Shimmer]

Thanks for the replies. Makes sense - well as much sense as I can process. I had always wondered what made the price as wattage increased. Physics eh!

[GrantB]

2x KT88= 80-100W
and so on and so forth.

You're right, it's a complex thing...my BTR23 is 2XKT88's in class AB1/A and the amp produces 23 glorious watts.

[Terexgeek]

Simplistically...

Power valve type & number of valves + power & output transformers in valve amps.

In solid state amps its the power supply and output transistors.


Wattage ratings are generally measured as being the point at which a certain amount of THD (say, 1%) is happening. So if you have two amps, one solid state and one valve that both are rated at 50 watts 1%THD, they'll both put out 50 watts cleanly. Beyond that, if you turn the solid state one up it'll very quickly start to sound nasty. If you turn the valve one up, you might find it gets to 70 watts at 10%THD and still sounds reasonably clean and musical, and if you really push it maybe it'll be really distorted, sound awesome, and be putting out 90 watts - whereas the solid state amp would have exploded and released smoke long before it got to that volume.

THD = Total Harmonic Distortion

[Single coil]

THD = Tetra Hydra Distortionol

[Ears]

Ideally, the design determines the power output. Components are then chosen to meet the design requirements.

The performance of some components limit the output of the amplifier, meaning if you need greater performance you will need to use components that deliver required performance. This allows an alternative design strategy; one can start with components known to handle/deliver certain outcomes and base your design around them.

All components of the system must be rated to meet design requirements, including humble resistors.

But simplistically, in audio application power is about delivering required electrical current into a (nominally) fixed load (the speaker).

Power = Resistance of load (e.g 8 ohm speaker) x current squared.
To get 50 watts from an 8 ohm speaker you need drive the speaker with about 2.5 amps.

Power transistors are built to control and deliver large currents.
The combination of the output transformer and power tubes does the same thing in tube ccts.

Both systems need a power supply large enough to draw from (hence BG's answer).

Ultimately it's the design that "determines" output. Even if you substitute components, say put massive KT88 valves into a cct designed for medium rated tubes like the 6V6, you will get no increase in power output, you'll still only get the lower output as expected from the original 6V6 cct. Changing the components won't magically give you higher output. And note, you can't always substitute lower rated tubes/transistors into ccts designed for components with higher ratings, they'll probably cook.

PS It is because of the large currents under discussion that you must use properly rated speaker cable to connect speakers, audio small signal cable such as guitar leads will just melt.