the 4.7k in the nfb...explaination?

[daz]

while eyeballing a schematic of a jcm channel switcher i noticed something different... instead of the usualy 4.7k resistor to ground coming off from the tail 10k, there was a 10k instead. So a 10k tail and the 10k to ground in place of the 4.7k you usually see. I had been messing with the NFBL lately and i realized i never played with that value. So i trie a 10 k, then finally i just put a 25k pot there. the more resistance the softer the sound much like if you install a pot in place of the NFB resistor and turn it to smaller resistance, tho in this case that tone happens as you go higher in resistance. Can someone try and explain to me in laymans terms as much as possible whats going one there?

[raiken]

The amount of negative feedback is controlled by a voltage divider consisting of the series resistor, commonly called the "feedback" resistor", and the shunt resistor, either a fixed resistor or the resistance of the presence control pot.

The ratio of the resistors in this voltage divider determine the amount of voltage "fed back", and thus, the amount of gain reduction, bandwidth increase, noise reduction, and output impedance reduction.

There's a surprise for you - I'll bet you thought the 16 ohm tap had an output impedance of 16 ohms, right? It actually changes, depending on the amount of global negative feedback you apply. This doesn't mean you connect a different load impedance when it changes, it simply means the effective output impedance changes depending on how much feedback you apply.

The ratio of the internal "virtual" output impedance to the speaker load impedance determines the damping ratio, which in turn determines how much the amp reacts to the varying impedance characteristics of the speaker. The more feedback voltage you apply, the higher the damping ratio, and the "tighter" the sound.

Therefore, a feedback resistor of 100k and a shunt resistor of 4.7k is roughly the same amount of feedback you would get if you used a 200k and 10k. If, however, you left the 100K alone and changed the 4.7k to a 10k, you would then have more feedback voltage. You can vary either the feedback resistor, the shunt resistor, or both. Normally, you leave the shunt resistor alone, because it also controls the headroom and balance of the PI, along with the 10K "tail" resistor.

Randall Aiken

[daz]

Thanks Randall, that does help. But theres one thing i'm not sure of. When you say a 10k there will cause less FB voltage, wouldn't that make the sound closer towards how it sounds with no NFB, IE:more high end and looser? Because when i make that resistor larger, the larger it is the more NFB i seem to have because as i always thought, more NFB=softer less cutting high end, and less means closer to no NFB high=more high end and looser, more cutting. So i'm still misunderstanding this even tho your explanation did help me understand a bit more about NFB.

[raiken]

Thanks Randall, that does help. But theres one thing i'm not sure of. When you say a 10k there will cause less FB voltage, wouldn't that make the sound closer towards how it sounds with no NFB, IE:more high end and looser? Because when i make that resistor larger, the larger it is the more NFB i seem to have because as i always thought, more NFB=softer less cutting high end, and less means closer to no NFB high=more high end and looser, more cutting. So i'm still misunderstanding this even tho your explanation did help me understand a bit more about NFB.

Sorry, that was a mistake on my part, I should have said changing the 4.7k to a 10k would increase the feedback voltage, as you surmised.

More reading:

http://www.aikenamps.com/NegativeFeedback.htm

http://www.aikenamps.com/GlobalNegativeFeedback.htm

http://www.aikenamps.com/FeedbackAmp.htm



RA

[daz]

Thank god....if it wasn't a mistake i'd be more confused than usual, and thats some pretty serious confusion. So basically, increasing it is going to yield the same sounds as decreasing the series R aside from any affect it has on the PI, correct?

[raiken]

Thank god....if it wasn't a mistake i'd be more confused than usual, and thats some pretty serious confusion. So basically, increasing it is going to yield the same sounds as decreasing the series R aside from any affect it has on the PI, correct?

Correct.

[Enzo]

And as always, remember, it is the whole circuit, not the parts.