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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
Sag vs Loudness

If you have a simple amp that you are driving hard, the signal from the preamp into the power amp may be already compressed or even be distorted into a square wave. EQ will change the square wave into something much spikier that will have more "edge". The collapsing power supply will cut off those spikes first, and ultimately may clip the square wave into a better "square".

At the first tonal transition we have "limiting", and at the second tone change we have hard clipping.

Players who are trying to have an over-driven but not too saturated distortion sound want to play at the "sweet spot" of when the amp is limiting. depending on the power of the amp and the speakers used, this may occur at a loudness level that the player or club owner may not desire, so some players end up with a few amps of different power for use in different venues. The alternative is to add appropriate circuitry in the amp to allow control over the loudness and over the limit or sag point.

During the 1990s, Mavin Peal amps had their Sag Control and Wattage system built into their own products, designed by David Zimmerman (RIP). David concentrated his efforts on the sag aspect and came up with a fairly intuitive but complicated circuit to achieve his goal. Unfortunately, he chose to limit how quiet the amps could play by setting a too-high low-power output. This was likely related to a common mistake he made regarding how much of the amp's circuitry to control: control too much and the basic tone will change. To minimise the tone change he had to minimise the power reduction, and therefore the benefit of his system was crippled.

To me, the control over loudness through Power Scaling was the main goal and the limits were set to be "unusably loud down to unusably quiet". The sag aspect was secondary, but easily set up to follow the loudness set by the player. Our SUS (sustain) kits and circuits alter only the attack of the note, which is the limiting effect of a sagging supply. The player can either use Power Scaling alone to get the loudness under control and hit the output stage hard or soft, or add the SUS effect to be able to soften the attack automatically.

Overall, the natural limiting effect of the sagging power supply can be subtle, as our SUS kits are, but it can make your playing seem more fluid. As we show in the TUT-series, one can begin with a stiff supply and add sag elements that are switchable or continuously variable, or one can use a saggy power supply. There is no wrong way to approach it except for whatever limitations the decision imposes.