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Negative Feedback (NFB) should or should not
NFB is the keyword that you hear much beside SE or Pushpull in amplifier buiding field.
First of all, what is NFB ?
NFB is one of the technical method that help your amplifier become better. It makes noise and distortion become lower, reponse frequency of amplifier becomes more beautiful.
So how it works and why it is so great ? Saying easily, NFB is method that you use a small part of output signal combine with the input signal. The output signal for feedback must have reserved phase with input signal, so as result combination, it will delete the noise, delete the distortion and make the sound signal look better.
People make ampifier for commercial purpose like to use NFB because it makes technical data more beautiful to show off. The legend Western Electric WE91A, Luxman 38 series are representative for using much of NFB in amp and receive many good reputation.
First of all, what is NFB ?
NFB is one of the technical method that help your amplifier become better. It makes noise and distortion become lower, reponse frequency of amplifier becomes more beautiful.
So how it works and why it is so great ? Saying easily, NFB is method that you use a small part of output signal combine with the input signal. The output signal for feedback must have reserved phase with input signal, so as result combination, it will delete the noise, delete the distortion and make the sound signal look better.
People make ampifier for commercial purpose like to use NFB because it makes technical data more beautiful to show off. The legend Western Electric WE91A, Luxman 38 series are representative for using much of NFB in amp and receive many good reputation.
Almost vintage tube amplifier used NFB in ampififier because 50 years before, people did not have technical enough to make a good output transformer and they must use NFB to make the sound better. In WE91A amp, Western used too much gain in driver stage for much of NFB. But 50 years before, people used very good core and material for transformers, although the spec of transformers is not good, the sound is still good to someone.
So why some people say that NFB is not good ? Look at how NFB signal flow works, you can see the output sound is combine from input sound and a small part of output, and more or less the sound was changed. Much of NFB will make much of sound to be changed. NFB also change the phase of output singal. That is why some mania dislike the sound of NFB, it makes sound lost the refresh and become slower.
Using NFB in amp is very difficult, it will be the best if you use NFB that make the sound better but does not affect much to the sound. You have to choose the proper NFB, a little NFB will not change the sound but not make anything better, use much NFB will make sound data become better but the sound become not good. You have to calculate and hear by your ear to find a proper value of NFB. Because NFB make the phase of output signal changed so if you use too much NFB, sometimes it will become positive feedback and make your device damaged.
That is why in almost KIT amp, there is no NFB, because it is too complicated to handle and sometimes it makes your speakers damage if you solder one wire in wrong place.
So, what is proper NFB ? you can see NFB level in db, for example (-)3db, (-)6db vvv..v.. The lower NFB value (far from 0db), the more NFB is put on your amp. Personally, NFB should not be lower -6db, lower than that rate, the sound changing can be found clearly by ear and not good. Luxman SQ-38FD or WE-91A that I mentioned above used NFB -15db or more.
Almostly in my power amp, I dont use NFB because the technical data is good enough and no need to use NFB to make it more beautiful. But in some case from customer requirement or to stabilize the low frequency, I use very slight NFB(-3~-6db), almost can not see difference between NFB or no NFB, just a feeling only.
The attachment is example that in some cases I have to use NFB although the data is beautiful and vice versa. One is 71A drive 71A PSE with output power 1.9W, hum level is 0.6mV, response frequency 16Hz~40KHz, another is 71A PP, output power 2.5W, hum level 0,1mV, response frequency 10Hz~60Khz. Look at technical data of 2 amps, the SE amp needs use NFB to reduce the hum, PP is very good and no need NFB, but in fact, the SE amp is No NFB and the PP amp has -3db NFB, because I found that the low frequency of 71A PP is not stabilize because of high gain from driver stage.
You dont have common fomular to all amp, which is good or bad, just hearing and trying until you are satisfy with your amp.
So why some people say that NFB is not good ? Look at how NFB signal flow works, you can see the output sound is combine from input sound and a small part of output, and more or less the sound was changed. Much of NFB will make much of sound to be changed. NFB also change the phase of output singal. That is why some mania dislike the sound of NFB, it makes sound lost the refresh and become slower.
Using NFB in amp is very difficult, it will be the best if you use NFB that make the sound better but does not affect much to the sound. You have to choose the proper NFB, a little NFB will not change the sound but not make anything better, use much NFB will make sound data become better but the sound become not good. You have to calculate and hear by your ear to find a proper value of NFB. Because NFB make the phase of output signal changed so if you use too much NFB, sometimes it will become positive feedback and make your device damaged.
That is why in almost KIT amp, there is no NFB, because it is too complicated to handle and sometimes it makes your speakers damage if you solder one wire in wrong place.
So, what is proper NFB ? you can see NFB level in db, for example (-)3db, (-)6db vvv..v.. The lower NFB value (far from 0db), the more NFB is put on your amp. Personally, NFB should not be lower -6db, lower than that rate, the sound changing can be found clearly by ear and not good. Luxman SQ-38FD or WE-91A that I mentioned above used NFB -15db or more.
Almostly in my power amp, I dont use NFB because the technical data is good enough and no need to use NFB to make it more beautiful. But in some case from customer requirement or to stabilize the low frequency, I use very slight NFB(-3~-6db), almost can not see difference between NFB or no NFB, just a feeling only.
The attachment is example that in some cases I have to use NFB although the data is beautiful and vice versa. One is 71A drive 71A PSE with output power 1.9W, hum level is 0.6mV, response frequency 16Hz~40KHz, another is 71A PP, output power 2.5W, hum level 0,1mV, response frequency 10Hz~60Khz. Look at technical data of 2 amps, the SE amp needs use NFB to reduce the hum, PP is very good and no need NFB, but in fact, the SE amp is No NFB and the PP amp has -3db NFB, because I found that the low frequency of 71A PP is not stabilize because of high gain from driver stage.
You dont have common fomular to all amp, which is good or bad, just hearing and trying until you are satisfy with your amp.
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