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RE: A curiosity about capacitance amount in the power supplies

Posted by Palustris (A) on December 8, 2013 at 09:19:32

In Reply to: A curiosity about capacitance amount in the power supplies posted by beppe61 on December 8, 2013 at 03:37:51:

You need to view the entire power supply of an amplifier as a cohesive unit. You can't just look at the capacitors and say it needs more or less. The entire power supply is engineered as a whole to fulfill a specific purpose e.g., to provide a specific voltage over a limited range of current draw.

The capacitors provide two separate functions in the power supply. They provide filtering from the AC mains and they serve as an energy storage device. There is a balance to designing power supplies between having a "quick" supply able to provide instantaneous current to the circuit from the power transformer and a "slow" supply able to provide current from the capacitor storage bank.

I can't speak to solid state amplifiers, but many tube designers build supplies for low impedance. These supplies provide adequate filtering from the mains AC ripple but don't store much energy in the capacitors. Current demands are drawn from the power supply transformer through low ESR capacitors and low impedance choke. In theory, current flow is instantaneous because of the low impedance of the supply as a whole. The alternative would be to store many joules of energy in a capacitor bank and when the audio circuit demands current, it is drawn off of the capacitors, which then compete for current with the audio circuit to recharge from the power transformer. This would require very low ESR capacitors, naturally, since the series resistance of the caps will inhibit their ability to provide current.

So, in the end power supply design is entirely up to the engineer, but one thing is clear: high quality low ESR capacitors are necessary for the best performance from a power supply. Many designers eschew electrolytic capacitors for that reason (I have a couple of amps with no electrolytics in the entire amplifier).


The link below is a tool you can use to design your own tube power supply and experiment with different capacitances to see their effect on the power supply performance.

• PSU Designer II   (Open in New Window)

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Atma-Sphere Music Systems, Inc.

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