Sag? Please educate me?

Catthan

So yeah, what is sag?

Is it how a pushed power stage compresses? is it flabbiness in the bottom end? is it "mushy" distortion?

Not being a native English speaker doesn't help much..

Please amp gurus, enlighten me..

Thanks,

ESchap

This helped me understand "Sag"    B-)

pluckbuddy

In my opinion it is how a power stage compresses when pushed hard. I don't understand all the technical reasons for it but I have a rough idea that it is how the power stage (incl the power valves, transformers and rectifier) responds when it is stretched to the limits of what it can achieve. It will usually compress the sound - more so if it has a valve rectifier which has more internal resistance than solid state. 

Some amps can get flabby in the bass when pushed hard because the lower notes require the more energy to be reproduce so that's where it flags the most at high volume - but it depends on the amp and just how hard it is being pushed. Likewise some amps will deliver a mushy distortion sound at high volume - there power amp sag and distortion would be factors but other parts of the amp, such as the preamp and phase inverter may well also be distorting and contributing to an overall tone that has less definition. Just my observations - happy to be corrected. 

Catthan

Thanks fellas, so, it's some sort of compression caused by voltage drop. Is it more of a feel thing then ?Or is sag an inherent constituent of power stage saturation and thus can t be distinguished by the ears. Any amps which are more "sagy" than others? Valve rectified ones maybe?

ICBM

Yes, valve-rectified amps have more sag. It varies depending on the type of valve, so sometimes (as long as you check the spec to match what the amp calls for) you can change it by changing the rectifier valve type. You can usually substitute it with a solid-state one as well, to reduce sag - although not eliminate it because some comes from the power transformer.

Sag is probably easier to feel than hear, but you can still hear it in the sound if someone else is playing.

Catthan

thanks @ICBM. I was thinking it is a feel thing myself but now curious whether I could listen at any demo and see if there's anything I can recognise. A JTM vs a peavey or smth..