Low damping factor but fast& high current SS amps?

Kij, buy one and take it apart?

As far as I can tell, feedback and DF are the 'Third Rail' of stereo discussions. Look at the 'beating' I took from a stickler for physics.

The take I've heard on feedback makes some sense. Global...from output to input, is a no-no. Minimally applied feedback....by stage...is OK. I've read a paper, complete with scope photos, claiming increases in certain distortion products as a direct result of feedback.

Now, I've worked in the Semiconductor manufacturing industry. Maybe you can explain something to me.....
We have a measured parameter called RDoN....Resistance of the Device in the ON state. Low enough that if the device were ON, it would yield a fairly high DF, especially if several devices were in parallel or P / N devices were in a push / pull configuration. How does feedback lower DF? Get as technical as you like.

Zuio: If you want a lower damping factor less than 100 on an amp that has more than 100, all you have to do is put a series 10watt resistor on the + terminal and the speaker wire on the other side of that resistor.
To get a damping factor of 50 on a 8ohm speaker you would use a .16ohm resistor.
To get a damping factor of 80 on a 8ohm speaker you would use a .1ohm resistor. and so on.
The only reason I can see for anyone lowering the damping factor of an amp is, if the speaker itself is an overdamped design, (eg: too tight in the bass)

Cheers George

"How does feedback lower DF"

Magfan – It increases DF (lowers output impedance).
Let’s take amplifier that has gain of 30 (31.6dB). When input voltage is 1V output voltage is 30V. Output voltage drops (for whatever reason) 1V under 1A load to 29V. That's 1ohm output impedance (DF=8).

Now, let's build this amp with gain of 300 but feed 3% of the output voltage back to the input in opposite phase. As a result amplifier’s output is the same 30V as before but input is the difference between 1V and 3% of 30V = 0.1V Let’s verify (1V-0.03*30V)*300=30V

Let’s load this amplifier with 1A. Our voltage drop inside is still 1V under 1A load, but output voltage will be higher than 29V because we subtract less from the input. Output voltage will be 29.9V and output impedance will be 0.1V/1A=0.1ohm (DF=80). Let’s verify. (1V-0.03*29.9V)*300-1Vdrop=29.9V.

Output impedance dropped 10 times. Expression 1+B*Aol is known as “Improvement Factor”. In our case B (“Feedback Factor”) = 0.03 (3%), Aol (“Open Loop Gain”) = 300 thus Improvement Factor = 1+0.03*300=10.

It is a little clumsy, but I didn’t want to bring whole feedback theory equations into simple example.

09-15-11: Kijanki
"So, I believe that high DF indirectly let's us know that the amp is trending towards high bandwidth (fast) & will be capable of high current delivery (if it's power supply is able & the music signal demands it)."

Damping factor and Slew Rate (speed) are not related.

Liquy - I never said they are related. That wasn't my quote.