How big do the capacitors have to be?

[MikeW]

Could somebody explain to me why you need such big caps for the power supplies? What would be the difference between having 20,000 uf. and 40,000 uf.?

[jwb]

20,000µF



Thank you, thank you. I'll be here all week!

The difference is, the power supply with more filtering will have less ripple. The ripple is proportional to current drawn and inversely proportional to filter capacitance. 20,000µF is a very modest amount of filtering for a power amplifier, but you get to choose, since you are designing the thing. You could also use some other method of power supply filtering, using pi filters or some other method. You could even try a regulated supply.

[MikeW]

At what level does the ripple become acceptable?

[eLarson]

At what level does the ripple become acceptable?

That's up to you, of course.

For me if I can sit at my normal listening seat with the gear powered up but no signal playing and I can't hear the hum, that's good enough for me.

Other folks have stated that 1 - 2J per 10W of power is a good rule of thumb -- 0.5*C*V-squared, where V is the voltage across the capacitor and C is the capacitor.

Erik

[janneman]

Originally posted by MikeW
At what level does the ripple become acceptable?

One thing to remember is that the bigger the cap, the lower the ripple amplitude, but the more higher harmonics there will be in the ripple. Power amps normally have power supply rejection ratio that rapidly falls with frequency. You end up with less hum (which should not be audible in the first place, if you do your wiring correctly) but with more higher freq junk, which tends to muddle the sound field.
I would use the 20mF. If you hear hum with a well designed and executed amp, there definitely is something else wrong.
But, indeed, it's your call in the end.

Cheers, Jan Didden

[Steve Eddy]

Originally posted by MikeW
Could somebody explain to me why you need such big caps for the power supplies?

For the same reason you need such big power transformers. Because the AC line that's ultimately feeding them is operating at only 60 Hz (or 50 depending on which country you live in). With a 60 Hz line frequency, and full wave rectification, the caps can only be refreshed once every 1/120th (or 1/100th) of a second.

The longer the time between refresh cycles, the more current can be drawn from the capacitors which translates into a lower voltage across the caps before the next refresh cycle. That's what defines the "ripple voltage."

In switching power supplies, which typically operate in the tens of kilohertz, the time between refresh cycles is much shorter and the caps are drawn down less which translates into lower ripple voltage for a given capcitance and allows you to get by with considerably less capacitance.

se

[MikeW]

If you a drawing 2 amps from a 40 volt supply and have 20,000 uf. caps or 40,000 uf. caps, is there an audible difference? After reading this forum for the last couple of weeks there seems to be more problems from in-rush current than from ripple. Is it possible they are using to much capacitence?

[Steve Eddy]

Originally posted by MikeW
If you a drawing 2 amps from a 40 volt supply and have 20,000 uf. caps or 40,000 uf. caps, is there an audible difference? After reading this forum for the last couple of weeks there seems to be more problems from in-rush current than from ripple. Is it possible they are using to much capacitence?

Well inrush current isn't a problem as it's transient and only occurs during the initial power up.

However the companion to ripple voltage is ripple current. And as you increase the amount of capacitance, while you reduce ripple voltage, you increase ripple current.

And that can have consequences with regard to raidated magetic fields from the power transformer seeing as the magnitude of the magnetic field around a current carrying conductor (such as the windings in your power transformer) is proportional to the amount of current flowing through it.

So with regard to ripple voltage, its consequences depend on the power supply rejection of the amplifier topology, and with regard to ripple current, its consequences depend on how susceptible your circuit layout is to magnetic field interference.

se

[jwb]

Inrush is caused by two things: empty capacitors and torroidal transformers. You can have zero capacitance and a torroidal transformer will still draw 50amps when you apply mains power. But yes, more capacitance means more inrush.

[Steve Eddy]

Originally posted by jwb
Inrush is caused by two things: empty capacitors and torroidal transformers. You can have zero capacitance and a torroidal transformer will still draw 50amps when you apply mains power. But yes, more capacitance means more inrush.

Mmmmm. What have toroids to do with it?

se