Power supply in Arcam A19

[unsleepable]

I hope someone can shed some light over something that has been nagging me lately.

I have been looking at power amps as a the probably next step to upgrade my current kit—Arcam irDac, A19 and Kef R100—, as after that I may go for a DAC/preamp. I have only been checking specs so far, but when comparing things, I have found that the maximum power consumption of the A19 is rated at 350W, which seems to be quite high for a 50 Watts per channel amp. This should be directly proportional to the power supply it has, whose rating I haven't found anywhere.

To put things in perspective:

* Creek Evolution 2, rated at 75 Watts, has a maximum power consumption of 340W.

* Exposure 2010s2, 75 Watts, < 250VA.

* Naim XS 2, 70 Watts, 350VA.

All these figures, whether W or VA are power consumption—and I understand they are supposed to be equivalent.

The closest reference I have found is the Creek 50A, which has a maximum power consumption of 350 Watts, and publishes the rating of the power supply at 250 Watts.

So according to these figures, the A19 and the Creek 50A seem to have quite large power supplies for their rating, and on the opposite side the one in the Exposure seems to be quite small. Is that possible or am I not interpreting the specs well?

 

[Vladimir]

Output power depends on the output devices (transistors) and capability to maintain that voltage in the variable speaker impedances depends on the power supply's capability to deliver current.

Ohm's law, V=I * R.

I - current (amperes)

V - voltage (volts)

R - resistance or impedance (ohms)

A good practice is to have a transformer capable to deliver 2,5 times the power the amplifier is rated for. If the rating is 50W per channel RMS, then the transformer needs to be able to deliver at least 250W RMS (depending on the power factor PF, this can be 250VA or 350VA or other value, usually higher).

The 2.5 times is headroom needed to have enough power to run the preamp, the power amp, the digital sections (front plate display, logic for commands), safety circuitry, and still have free power headroom because if you go too close to the transformer's rating it will be stressed, saturate its magnetic field, hum, possibly distort or just poop out.

The other important partner in a PSU are the smoothing caps that also serve as reservoirs when large dynamic peaks in music demand more power and when impedances drop and more current is required to maintain voltage.

Some manufacturers go for huge transformers and not much caps, it has less phase shifts and caps charge and discharge faster. Some use small transformers and bigger capacity banks (cheaper way to go). And some like everything big.

What you see as sign of quality and very good design in amplifiers is when they have regulated PSU or even better, fully regulated PSU.

Anyways. I have no idea what VA rating is the FMJ A19 toroid.

 

[andyjm]

The other important partner in a PSU are the smoothing caps that also serve as reservoirs when large dynamic peaks in music demand more power and when impedances drop and more current is required to maintain voltage.

Some manufacturers go for huge transformers and not much caps, it has less phase shifts and caps charge and discharge faster. Some use small transformers and bigger capacity banks (cheaper way to go). And some like everything big.

Hmmn. Perhaps you can explain what phase shifts have to do with reservoir capacitors in a DC supply? and also how a large transformer / small cap combination supplies power to the output stage of an amplifier during the zero crossing of the input AC supply?

Anyway, that aside, amplifiers are pretty inefficient. The 'class' of the amplifier can give you an idea of how wasteful the amp is. Class A is the worst, class D (switching) amps are the best. There is no magic to the maths, Power in = power to the speakers + heat.

Googling would indicate that (as a general approximation):

Class A 20% efficient

Class B 50% efficient

Class D 90% efficient

 

[Vladimir]

@andy

Nope. That's all I know. Feel free to correct anything I said, I am not an EE and would like to learn more.

 

[andyjm]

@andy

Nope. That's all I know. Feel free to correct anything I said, I am not an EE and would like to learn more.

Fair enough.

Consider a linear DC supply like a bucket.

The output transitors of the amp are fed from the bucket, the more power the transistors require the faster the water leaves the bucket. Every 1/100 of a second, the mains transformer / rectifier combination sloshes water into the bucket to top it back up.

The point is that it is the bucket (reservoir capacitors) that supply the power to the amplifier, the transformer / rectifier combination only serve to keep the bucket topped up.

Clearly the average supply of water into the bucket needs to match the water flowing out, or the bucket will run dry. Thats why big amps need big transformers.

Big transformer / small cap in a large amp just doesn't work. The bucket will run dry before the mains replenishes the bucket. The consequence of this is a high ripple voltage on the output of the supply and hum in the amp's output.

Phase considerations do not come into play with reservoir caps in power supplies.

 

[Vladimir]

Question.

The Harman Kardon HK990 has 2 toroids, 600VA each, but 'only' around 16.400 uF caps per channel. This is a big integrated with high current design and wide bandwidth. Why not at least 40.000uF per channel?

 

[andyjm]

Question.

The Harman Kardon HK990 has 2 toroids, 600VA each, but 'only' around 16.400 uF caps per channel. This is a big integrated with high current design and wide bandwidth. Why not at least 40.000uF per channel?

The appropriate value of reservoir cap depends on a number of parameters, acceptable ripple voltage, power supply voltage, expected load and so on. If you are keen, there are a number of 'how to' guides on the web. I came across this, which seems quite informative and explains how a linear supply works in some detail:

http://sound.westhost.com/power-supplies.htm

Good luck.

 

[Vladimir]

Excellent material. The Capacitor Value section half way in has the answers I need. Thanks! :beer:

 

[unsleepable]

Well, thanks for the paper! It made an interesting read—even if I couldn't grasp a large part of it.

So far it seems that yes, the A19 indeed has quite a large power supply although I will see if I can the exact details somewhere. I wonder where this idea that it is not suited to drive difficult speakers comes from then. I already thought that since Kef had been using A19s to demo LS50 speakers in a number of fairs, that couldn't be whole story—as the Kefs are not easy to drive and at fairs you don't exactly play music low.

This also means that my idea of trying an Exposure 3010s2 power amp as a possible and to use it later with a DAC/preamp is probably a no-go. If the capacity to drive speakers is proportional the power supply in the amplifier, the upgrade would be small.