VA vs Wattage

[Richardjhy]

I know that VA ratting is bigger than wattage, but why huge difference in this amp:

 

[DVDdoug]

That seems unusual, but (as you may know) it means there's a phase difference between the voltage & current. The voltage peak and the current peak don't happen at the same time... It's related to inductance of the transformer.

...You get billed for the watts. The current creates a voltage drop through the wiring (hopefully insignificant) and it's current that blows fuses & circuit breakers.

Power Factor

 

[AnalogSteph]

These also don't have just any old transformer power supply, it's a semi-regulated affair with sort of a dimmer on the primary. It's the exact type of amp for which I've described a power filter mod more than once lately.

 

[mhardy6647]

Power factor.
I guess the A-760's a pretty reactive load.
Now... I wanna know if those three small holes after the CAUTION are factory stock or aftermarket?
I don't know and I can't check -- because, amazingly, I don't have an A-760 in the Yamaha collection here?!

 

[Ken Tajalli]

OR
It could be that the amp can use 350W normally, but has an 850VA transformer inside.
They used to use the VA of the transformers as a selling feature.

 

[Richardjhy]

Yamaha had a warning on A-760 for 220-240v regions - A capacitor may cause fire.

 

[Richardjhy]

Power factor.
I guess the A-760's a pretty reactive load.
Now... I wanna know if those three small holes after the CAUTION are factory stock or aftermarket?
I don't know and I can't check -- because, amazingly, I don't have an A-760 in the Yamaha collection here?!

View attachment 393162

Holes are original. Actually there're 6 holes :

 

[audio_tony]

These also don't have just any old transformer power supply, it's a semi-regulated affair with sort of a dimmer on the primary. It's the exact type of amp for which I've described a power filter mod more than once lately.

There's a lot of additional complexity there which must generate a fair amount of noise, both into the amp itself and on to the mains supply.
No wonder you thought a filter was worthwhile!

I guess the PSU wasn't that successful as they obviously dropped it in the late models (and probably wouldn't meet with current EMC regulations).

For anybody else interested in the Yamaha power supply, there's a good thread about it on eevblog https://www.eevblog.com/forum/beginners/silicon-bilateral-switch-ig04080/

One of the posts (#18) has a link to a manual for the Yamaha B6 amp which uses a similar PSU, and this manual has a good description of how it works.

 

[Lambda]

Amplifiers of with a basic Rectifier to charge a capacitor have the problem that the current in the transformer is not at all smooth and sinusoidal but pulsed.
And the Transformer is somewhat Inductive.

So its normal

 

[voodooless]

I wanna know if those three small holes after the CAUTION are factory stock or aftermarket?

They seem to be stock, see here the service manual with the same holes:

The whole thing is here: https://www.hifi-archiv.info/Yamaha/A-760s/

Maybe it sheds some light on the Watt vs VA curiosity?

 

[audio_tony]

Maybe it sheds some light on the Watt vs VA curiosity?

It's the power supply design that's reactive. As @AnalogSteph has said, this is a regulated AC power supply - it attempts to compensate for high and low mains voltage, and maintain a constant, even supply to the transformer.

Some older CRT television sets used similar principles to regulate the incoming power. They were notoriously unpopular due to the amount of noise they created on the mains supply (due to thyristor switching).

 

[FrantzM]

Hi

It's the power supply design that's reactive. As @AnalogSteph has said, this is a regulated AC power supply - it attempts to compensate for high and low mains voltage, and maintain a constant, even supply to the transformer.

Some older CRT television sets used similar principles to regulate the incoming power. They were notoriously unpopular due to the amount of noise they created on the mains supply (due to thyristor switching).

View attachment 393304

+1
....

Nowadays PFC (Power Factor Compensation) is included in (many?, most?) power supplies. This amp is likely >20 years old...


Peace.

 

[mhardy6647]

Nowadays PFC (Power Factor Compensation) is included in (many?, most?) power supplies. This amp is likely >20 years old...

>> 20 years old.

(early 1980s)