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What is sag? - Printable Version

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What is sag? - K O'Connor - 08-22-2018

Hi Guys

"Sag" refers to the change in power supply voltage under load.

An ideal power supply exhibits zero change in its output voltage going from no load to full load. This is also the characteristic of a "voltage source".

Real power supplies have internal resistance, including that of the wiring, the solder connections, the rectifiers, the transformer windings, and deliberately added resistance. Also, economics drive the power supply size to be just what is needed and no more, rather than to be built to excess.

The voltage variation due to loading is called "regulation" and every power supply has an inherent regulation capability. This is measured as a percentage, where the change is divided by the unloaded value. Most PSUs have 10-20% regulation, meaning the supply can sag by 10-20% under load. For example, a 400V supply will be down to 360V if it has 10% regulation, and will be down to 320V if the regulation is 20%. You can see here that the higher the regulation number, the worse the performance of the supply!

This sag is important in a guitar amp because guitar amps are inherently economically designed AND guitar amps are often driven to their power limits. The sagging supply is a dynamic characteristic, responding to how hard you hit the strings and how the controls on the amp are set. So, the supply voltage "comes down to meet the signal" which itself is rising. There is a limiting and compressing effect that occurs if you can play right at the cusp of where the sag becomes simply a dropped loaded voltage.

Of course, the dynamics of sag and how it interacts with the guitar signal depends a lot on the shape of the guitar signal, as TUTs detail. More about this as Qs arise and discussions evolve.

Have fun