Topic: Negative Feedback: what is it?

[nizmo]

to all distinguished DIYers

My integrated tube amp has a NF mode from min-max. I consulted the manual to know about it and its usage but it says just little info about it. I made reseach in the net but my mind went wild to understand those highly tech terms.
I just simply want to know how to employ NF.

So please enlighten me and others too who has the same mode in their tube amps.

Questions:

1. if i'm on triode mode, should the NF be at min or max function?

2. if i'm on ultralinear mode, should the NF be at min or max function also?

please, let it be explained in lay man's language ; ).... since i failed numbers and science during my college years.


mabuhay po kayo!!!!!

[ATJr.]

can you be more specific? what is your amp? and can you post that part that says you can select min-max NF?

negative feedback is a very touchy subject, normally this is not supposed to be user controlled as the amp has incorporated this into the design...

tube amps are not supposed to be handled beyond the volume control as dangerous voltages exist, and the user is supposed to just plug it in and play it...

well, sometimes you also have to change tubes, but that is not very often and hopefully far in between, at least for well designed amps...

triode mode requires lesser feedback compared to ultra-linear, but ultra-linear puts out more power than triode mode.

hope this helps,

[nizmo]

thanks sir,


I'm using Audio Space 8imklll. at the front panel there is the NF adj button from min - max.

[nizmo]

sir tonyT

 the adj is just min and max. You mean to say even if i choose not to change the adjustments the benefits of both ultra-linear and triode modes of the amps will remain the same? 

Bakit po kaya may ganung adj pa? 

[JojoD818]

There is an rc network used in the nfb to couple the output of your amp to the it's input, often times referred to as global fb and the ultralinear/triode circuit as local fb. There are several ways to couple the output to the input like using a differential input section (more popularly known as long tail pair) but the most simple way is to couple it to the cathode of the input tube. Nfb is used to reduce distortion, increasing the resistance in the feedback rc network decreases feedback and vice versa, this is probably what that switch in your amp does.

As what Tony has pointed out, nfb is an incredibly debatable subject. The most popular of all discussions regarding nfb is that it is said to be utterly useless since what would be the purpose of using nfb if it is taken from the output? Hence the question, what is the point of correcting the signal if it has already gone out of the circuit and on to the next stage of the signal chain? (for an amp, the next stage is the speaker)...

Aside from that, effects of nfb on the overall signal has been a never ending battle between different forces in the audiophile community.

[ATJr.]

sir tonyT

 the adj is just min and max. You mean to say even if i choose not to change the adjustments the benefits of both ultra-linear and triode modes of the amps will remain the same? 

Bakit po kaya may ganung adj pa? 

this is your amp right? http://www.audiospace-hifi.com/detail.asp?subcatid=0&private=&catid=2270&PageNo=7&PageSize=1

way i see it, this is more likely a tone control of sorts....there are negative feedback type tone controls.....

it is never a good idea to tamper with the "global" negative feedback, as misapplication of feedback could lead to instabilities, if i were to design an amp, i would not leave NFB to the end user to tinker with it...

[nizmo]

jojod818

thanks.   well in that case, that selector/NF is just a cosmetics at the front panel.  And then another marketing tactics?
The more knobs/selector buttons the higher the price... hahaha i'm bad!!!

[nizmo]

sir tonyT

thanks again... maybe your right. It's like the "loudness" button that we can find on some amps or car stereos. everytime i put it on min. toned down ang volume and viceversa.  Correct me please.

[ATJr.]

sir tonyT

thanks again... maybe your right. It's like the "loudness" button that we can find on some amps or car stereos. everytime i put it on min. toned down ang volume and viceversa.  Correct me please.

yes, sort of......

[nizmo]

what would then be the best thing to do? keep the NF selector at the min. function "in aeternum"?
i dont want to mess with my amp ....and of course my economy....

i'll do what you would suggest coz you know well than me. thanks.

[JojoD818]

jojod818

thanks.   well in that case, that selector/NF is just a cosmetics at the front panel.  And then another marketing tactics?
The more knobs/selector buttons the higher the price... hahaha i'm bad!!!

don't worry, most audio space amps that I have seen has that switch. it's just there to switch between the highest and the lowest nfb allowable in your amp's design without sacrificing stability.

the reason why it acts like a loudness switch is that when you add more nfb it cancels out more of the signal and vice versa. less nfb means less correction, hence, stronger signal. i also incorporated this in my tube amp, works well with my ever varying choice of source material.

[alexg]

I used to have the same amp, that toggle switch for nfb just toggle the two values used for NFB of the amp. (I forgot the values of the feedback used). This switch just vary the feedback resistor of the feedback circuit (resistor || with a cap).

What it does is inject some signal from the output into the input tube, and to put simply, if we sum the feedback with the input signal, it reduces the distortion, at the expense of lower gain.

It is not cosmetics, if you listen carefully there is a difference in the sound, one is more relaxed (more nfb).

I usually leave mine on Minimum settings.

[nizmo]

thanks for the well rounded enlightenment about NF settings, all of you had shared the best for me to get away with my curiousity about the NF. Long had been thinking if i must match the NF with the modes, i.e., triode or UL, that i may be listening with.

Cheers for jojod, tonyT and alexg!!!

in case there are still things i should know re. NF,the best way to use it... i'll be glad to hear!!!

[old_age]

meron din ako nitong NFB switch sa Duntonic DU-2i amp ko since 2005 . .. yun pala yun! hehehe

[Tempter]

Akala ko negative feedback sa user.

[oznola]

masyado na kasi madaming nega dito! lol

[tony]

meron din ako nitong NFB switch sa Duntonic DU-2i amp ko since 2005 . .. yun pala yun! hehehe

pentodes without negative feedback have this "pentode nastiness" that is why global negative feedback is used...
another trick is to O'Schade, a local feedback technique that mimics bestrides....
but this is too technical to understand, just enjoy your music....

[old_age]

pentodes without negative feedback have this "pentode nastiness" that is why global negative feedback is used...
another trick is to O'Schade, a local feedback technique that mimics bestrides....
but this is too technical to understand, just enjoy your music....

yeah!  masyadong nagfofocus sa pasikot sikot sa audio, nakakalimutan ng ienjoy ang music.  cheers!