Watts; the worst measurement unit ever for amp output?

[agedhorse]

Please don't try to cloud the issue with facts.

This is an engineer's job... much to the disappointment of the folks in marketing

 

[B-string]

This is an engineer's job... much to the disappointment of the folks in marketing

You are being so kind today. Not that you are not usually, maybe generous would be a better word.

 

[acmebass]

Wow. Get away from the computer for a few hours, and a thread can really come alive. Just a few comments, and since agedhorse showed up, a question, as well.

It would be great if a simple spec could be used, but too much information is needed in order to make an informed decision.

I’ve never had a problem with the watt as a unit that can be used to compare amplifiers. I think the OP’s concern has more to do with his lack of understanding than with the unit itself. I’m not sure this thread, which is all over the place, is likely to be very helpful to him.

Since a moving coil loudspeaker is a varying complex load, any impedance rating is necessarily an approximation, and since amplifiers drive these complex loads, actual dissipation in the system will be devilishly hard to calculate, and the actual power will be an approximation as well. So in this context, a watt is a flawed tool, but in the real world is still very useful when comparing amplifiers.

The amount of power used by an amp has very little to do with its output power. Yes, it is true that the amp cannot put out more power than it takes in . . . continuously . . . but it can, in fact, provide bursts of power (peaks) that far exceed its continuous input.

Interestingly, I have a Sunfire subwoofer rated at 1500 watts output, which only has a 6 amp fuse. Seems impossible, but the tiny transformer inside actually charges two monster capacitors on a continuous basis, and the capacitors drive the amp. So your use of the word continuously is illustrated to be appropriate here.

There's a difference between impedance & resistance, even though both are measured in ohms. When you make a static measurement across the terminals of a speaker, you're measure the DC resistance of the voice coil. When you apply an AC signal to that same speaker, it moves the coil back & forth through a magnetic field to produce sound. That same motion, of a coil through a magnetic field, also generates AC power in the reverse direction, sort of an electronic back-pressure.

This would be the oft-misunderstood Back EMF, which to be precise, it not “power,” but voltage. In the context of a moving coil loudspeaker, Back EMF expresses itself as complex impedance, or “ohms,” if you will.

Excellent comments.

I'm sitting here laughing thinking of how one explains that complex relationship where designers work with real and imaginary numbers. Indirectly, when an amp is connected to a speaker cab, real watts and imaginary watts. How confusing is that.

Made me laugh, thanks for that, but to be precise, impedances and currents can be complex, but watts are just watts. Power is a scalar quantity.

Tube amps are (generally) more foregiving, a byproduct of higher impedances and more lossy coupling. This is also part of what gives a tube amp it's feel.
The real issue with tube amps is (much) higher impedance mismatches, where the output circuit can become underdamped and can not absorb the back-emf of any ringing that may occur. The voltage may rise to a point that compromises the insulating system inside the transformer of output tube circuitry causing arcing and then catastrophic (and costly) failure.

A question: Why in the world would the designer of a solid state amp use an output transformer? It seems too heavy, too expensive, and too superfluous to me. I’m not aware of many, only some vintage SS McIntosh amps, and some recent Warwick models which seem to have fallen off the radar. As an amp designer, can you tell me what motivates anybody to do something which seems so dumb?

And a comment: As I said, Back EMF, although it’s a voltage, appears in DC motors as a resistance (R = E/I), and in moving coil loudspeakers as a complex impedance. I’m not sure how the word “absorbs” applies any more than it would to any other impedance.

As Andy already said, the frequency spectrum of a bass guitar is very time sensitive, starting at the first moment of attack during decay up to the sustained note it is impossible to state a steady spectrum.

I’m not sure what you’re referring to. Maybe it wasn’t me, and maybe it was that other thread with the screwball pdf file relating to transient response.

BTW, agedhorse, in case I haven’t told you, my customers love those little amps you’ve been making.

Andy

 

[Major Softie]

Interestingly, I have a Sunfire subwoofer rated at 1500 watts output, which only has a 6 amp fuse. Seems impossible, but the tiny transformer inside actually charges two monster capacitors on a continuous basis, and the capacitors drive the amp. So your use of the word continuously is illustrated to be appropriate here.
Andy

I've opened up a Crown PA amp to find that most of the space inside is taken up by two beer can sized caps. Dem's some caps!

 

[megafiddle]

A question: Why in the world would the designer of a solid state amp use an output transformer? It seems too heavy, too expensive, and too superfluous to me. I’m not aware of many, only some vintage SS McIntosh amps, and some recent Warwick models which seem to have fallen off the radar. As an amp designer, can you tell me what motivates anybody to do something which seems so dumb?

Some early solid state car radios also had output transformers. I believe they were required
because of the low current gain and low collector current of the early power devices. Much
easier to design a circuit to drive the higher impedance transformer primary.

But here is one article about McIntosh's use of the output transformer:

http://www.pstracks.com/pauls-posts/mcintosh-transformers/9843/

Possibly it also solved some design problems, like DC offset on the output.

.

 

[acmebass]

Some early solid state car radios also had output transformers. I believe they were required
because of the low current gain and low collector current of the early power devices. Much
easier to design a circuit to drive the higher impedance transformer primary.

But here is one article about McIntosh's use of the output transformer:

http://www.pstracks.com/pauls-posts/mcintosh-transformers/9843/

Possibly it also solved some design problems, like DC offset on the output.

.

Cool! Thanks for the link.

Andy

 

[agedhorse]

A question: Why in the world would the designer of a solid state amp use an output transformer? It seems too heavy, too expensive, and too superfluous to me. I’m not aware of many, only some vintage SS McIntosh amps, and some recent Warwick models which seem to have fallen off the radar. As an amp designer, can you tell me what motivates anybody to do something which seems so dumb?

Andy

Hi Andy... yes this does appear to be a very odd decision.

My understanding is that they chose to use an output matching transformer to allow for driving very low impedance loads, down to 1 ohm or less. If this is indeed the driving factor behind the decision, then a matching transformer can increase the amplifier's efficiency and reduce the thermal load on the output stage at a time when the cost of quality silicon in that quantity (I'm pretty sure this was well past the germanium era) was very, very expensive. This is even more significant if bridging was an option. This also allowed the amp to deliver "rated power" into a much wider range of loads.

The historical perspective is easily forgotten once we remember how things were in "the good 'ol days". In fact, they weren't all that good when it comes to the availability of electronic parts even if cost was not factored in.

 

[JimmyM]

In the BP article that came out when Warwick introduced the Hellborg rig, Jonas Hellborg said he used an OT in his power amps because he felt that it was what gave tube amps their signature sound and not the power tubes. His claim is that tubes should have no sound of their own, and that the OT is responsible for that sound. Have never played a Hellborg rig to compare, but I must say that the REDDI only has one preamp tube and a massive OT and it sounds like a tube amp being speaker DI'd to the board. BUT...there's still a tube involved.

Only other person I ever spoke to who said he's used a SS power amp with an OT was Psycho Bass Guy (an old McIntosh), and he didn't think it sounded like a tube amp.

 

[ThisBass]

A DI schematic design and OT is running into resistive load whereas a tube amp design and his OT is running into reactive load, that is the difference.
The so called "vierquadrantenbetrieb" of an power amplifier.
Try to translate for yourself. I failed to find out a proper translation on the web.

 

[1958Bassman]

This is an engineer's job... much to the disappointment of the folks in marketing

But engineers aren't the ones who come up with all of the catchy slogans, buzzwords, hyperbole and BS that grabs people by the schnoz and makes them pay more than they would ever conceive of, for something that usually isn't right for them, for reasons that are lost on the buyer.

Who first used the word 'organic' to describe the sound of an amp?

 

[Space Pickle]

DOWN WITH WATTS

 

[1958Bassman]

A question: Why in the world would the designer of a solid state amp use an output transformer? It seems too heavy, too expensive, and too superfluous to me. I’m not aware of many, only some vintage SS McIntosh amps, and some recent Warwick models which seem to have fallen off the radar. As an amp designer, can you tell me what motivates anybody to do something which seems so dumb?
Andy

Mac amps have three separate taps for speaker load- 4, 8 and 16 Ohms.

 

[Garey]

Mac amps have three separate taps for speaker load- 4, 8 and 16 Ohms.

As do the Hellborg rigs - Jonas believes it adds the feeling of a tube amp, and also utilizes full power at different loads.

 

[Major Softie]

DOWN WITH WATTS

Was it the riots that turned you against them?

 

[1958Bassman]

For those who don't like the conversions from Metric to English, or the other way around,

"The metric system is the tool of the devil! My car gets 40 rods to the hogshead and that's the way I likes it."

-Abe Simpson

 

[Major Softie]

For those who don't like the conversions from Metric to English, or the other way around,

"The metric system is the tool of the devil! My car gets 40 rods to the hogshead and that's the way I likes it."

-Abe Simpson

That is some pretty horrible mileage. I come up with 0.00231 miles per gallon.

 

[1958Bassman]

Was it the riots that turned you against them?

Nope, but this wouldn't strike me as a good reason, either-
Link Removed

 

[ThisBass]

sarcasm is the truth.

 

[acmebass]

Some early solid state car radios also had output transformers. I believe they were required
because of the low current gain and low collector current of the early power devices. Much
easier to design a circuit to drive the higher impedance transformer primary.

But here is one article about McIntosh's use of the output transformer:

http://www.pstracks.com/pauls-posts/mcintosh-transformers/9843/

Possibly it also solved some design problems, like DC offset on the output.

.

As I said, thanks for the link, but I read that piece, and while I don't want to slag another Boulder boy, it's almost completely nonsense. He lost me at:

"As the Ohm number goes down, the need for power goes up proportionally. So the power needed to make an 8 Ohm speaker produce sound in the room needs to double when the Ohms cut in half to 4 – and double again for 2 and so on. Lower Ohms, higher power to achieve the same sound pressure."

Bull.

I don't know the guy, and have no idea if his amps are any good, but suffice to say I probably wouldn't drive very far to audition one.

There was a comment below the piece from a "Soundminded" which echoed some of my concerns about what the author asserted.

I think agedhorse's explanation makes more sense, and like me, he seems to be not convinced. Of course, I don't design amps anyway, I was just curious.

The other comments on the subject are interesting, and appreciated!

Andy

 

[megafiddle]

The output transformer on at least some of the car radios was pretty unusual.
There were no separate primary and secondary windings, but a single tapped
primary. As the primary had some DC current standing in it (class A output),
and some resistance, there was also a DC voltage on the output tap.
When powered up, the speaker cone would jump outward! I always wondered
if the cone suspension had to be pre-biased off center, so that the DC offset
moved it into it's center position (where normal cones usually sit).

Partial schematic for one here:

Invalid Link Removed

Another interesting thing about the old stuff, is that the tube car radios used one of
the first switching supplies. An electro mechanical interrupter switched the 12 VDC
into a step up transformer to provide B+ voltage for the tubes.

And they did it all without the interwebs.

.