Low damping factor but fast& high current SS amps?

Magfan - Deep feedback lowers output impedance (also lowers THD and IMD). Amplifier's configuration affects initial output impedance. Class D amps for instance connect speaker to power supply and GND (zero impedance points) all the time, only polarity changes. Without feedback output impedance would consist of Mosfets' resistance (in order of 0.1ohm) and zobel network's impedance (common mode choke + capacitor and resistor to filter out carrier). Inductance of this choke is the reason of class D higher output impedance at high frequencies. On the other hand this choke has only few turns of wire and very low resistance (high DF at low freq.). I would estimate that since 0.1ohm is reduced to 0.001ohm feedback is at least 40dB deep but it is not too bad.

There is nothing wrong with feedback, if you know how to use it. Amplifier has to be as linear and fast as posible to start with. Small amount of feedback should reduce THD only to about 0.2-0.5% while bandwidth at the input should be limited to one that amp had without feedback (to prevent TIM).

I'm trying to make sens of it and so far I've learned that things are extremely complicated. Very high DF means deep feedback but Soulution 700 amp has DF=10000 and bandwidth of 1MHz with no feedback. If I would only know how to do it I would build such amps and sell them (for $100k) myself.

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.