Amplifier Voltage Sag...Myths & Clarifications

[Philippe]

From what I've heard...is it true/correct that (1) class A/single ended amps do not sag because they are always 'on' (as compared to an a/b push-pull amplifier. (2) SS amps do not sag. (3) an Orange Squeezer compressor simulates sag. (4) tube rectifiers create sag while SS rectifiers (diodes) do not. Or are we simply dealing with perceived degrees of of sag...that perceived 'spongy', compressed characteristic?

[merlinb]

From what I've heard...is it true/correct that (1) class A/single ended amps do not sag because they are always 'on' (as compared to an a/b push-pull amplifier. (2) SS amps do not sag. (3) an Orange Squeezer compressor simulates sag. (4) tube rectifiers create sag while SS rectifiers (diodes) do not. Or are we simply dealing with perceived degrees of of sag...that perceived 'spongy', compressed characteristic?

1) Yes, class A circuits draw almost constant average current, so there will be very little sag. (There may still be some though, due to nonlinearity of amplification).

2) Depends. If they have unregulated supplies (most do) then they will indeed sag under heavy output loads, but you won't typically hear the effect in the same way as for a tube circuit because SS circuits don't usually compress, they just clip.They also tend to have lots of feedback and other mechanisms that hold the operating points more constant.

3) Yes, sag basically results in compression.

4) Tube rectifier will sag more, all else being equal, because tubes have more internal resistance. SS rectifies will still sag, just less.

[R.G.]

I'd add a mildly different viewpoint in some ways.

From what I've heard...is it true/correct that (1) class A/single ended amps do not sag because they are always 'on' (as compared to an a/b push-pull amplifier.

Class A amplifiers do indeed have a much more constant current drain than Class AB. In fact, the reason AB/push-pull was invented was to try to get some of that constant heat generation done away with, and get more usable power out of the same tubes.

(2) SS amps do not sag.

What Merlin said. It's there, but it's small, and sounds different from tube amp sag.

(3) an Orange Squeezer compressor simulates sag.

An Orange Squeezer simulates some characteristics of sag. Matching tube amp sag closely will take some dinking and tweaking to get close to what you got from the tube amp. Even then, there's probably mild distortion from the lowered voltages and running at full-bore to make the power supply sag that the compressor does not do.

(4) tube rectifiers create sag while SS rectifiers (diodes) do not.

What Merlin said; to build on that, one can make SS rectifiers sag by inserting a power resistor similar to the internal resistance of a tube rectifier. This is the method used in Weber's tube-rectifier emulators, and many amps with SS rectifiers. It's a nice way to get the sound without paying for tube rectifiers. Many picky musicians like this just fine.

To extend that a bit, it makes huge sense to put SS "backup diodes" in series with the leads of the PT going into a tube rectifier. If the tube rectifier dies shorted, the SS diode prevent this from becoming a replace-the-caps-and-PT event.

Or are we simply dealing with perceived degrees of of sag...that perceived 'spongy', compressed characteristic?

I don't know about the word "simply". We're always dealing with micro-fine degrees of perception in music. Just ask any seller of highly-hyped boutiquery.