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Old January 29th, 2013, 07:46 PM   #1 (permalink)
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5F2-A NFB resistor value question

Beginner question:

If I understood correctly, the NFB resistor in 5F2-A is 22K. The amp has a NFB switch. When bypassed, the amp is a little too bright and 'wild'. With NFB - little bit too tamed. I figured out that the optimum is somewhere between. Brightness and volume difference is quite significant between NFB on/off.

Could a resistor of 10-15K be the answer? If yes, do I need to tweak something else besides the resistor?

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Old January 29th, 2013, 08:32 PM   #2 (permalink)
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You must increase the resistance to decrease the gain lost to NFB. Try a 50K ohm resistor.
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Old January 29th, 2013, 08:50 PM   #3 (permalink)
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be careful here,

Put say a 10K resistor before your pot.

Reason, it would be possible to dial the pot to 0 resistance and the PI would see the full output of the output transformer.

Also use a linear taper pot. This is one of the few areas you don't use audio taper.

It is common for boutique amp builders to put a pot on alligator clips and dial in the negative feedback. They then remove and measure the resistance of the pot and replace it with a resistor of similar value.

Ray
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Old January 30th, 2013, 12:23 PM   #4 (permalink)
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Quote:
Originally Posted by hackworth1 View Post
You must increase the resistance to decrease the gain lost to NFB. Try a 50K ohm resistor.
I don't follow. Does 22K --> 50K give more or less NFB? I want to have less.
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Old January 30th, 2013, 12:30 PM   #5 (permalink)
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Quote:
Originally Posted by stingray_65 View Post
be careful here,

Put say a 10K resistor before your pot.

Reason, it would be possible to dial the pot to 0 resistance and the PI would see the full output of the output transformer.

Also use a linear taper pot. This is one of the few areas you don't use audio taper.

It is common for boutique amp builders to put a pot on alligator clips and dial in the negative feedback. They then remove and measure the resistance of the pot and replace it with a resistor of similar value.

Ray
Sorry, I am I am little bit confused about this also. I would like to just chance the 22K resistor to a slightly different value, that would "open" the sound a little bit. I don't have extra pots at the moment.

If this can be done just by chancing the 22K resistor (bigger or smaller?), then I will remove the NFB bypass switch (=the amp would have constant, less than stock 5F2-A, NFB).

If I just replace the 22K to 10K or 50K, what would likely happen?
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Old January 30th, 2013, 12:50 PM   #6 (permalink)
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If you want less NFB, use a value higher than 22K. I can't tell you what value to use, you might have to experiment and find a value that sounds good to you. Look at it this way, when you use the bypass switch to remove the NFB, it is like switching in a resistor with an infinite value. There's a whole range of values to choose from between 22K and infinite resistance. Or you could wire in a pot and have it adjustable, which is what I did in my amp.

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Old January 30th, 2013, 04:45 PM   #7 (permalink)
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The NFB circuit is not intuitive at all.

But I have to say it is a useful and fun circuit to play around with.

it works by introducing SOME of the output back to the cathode of the gain stage. that signal is inverted from the signal going in and wiggles the bias nearly exactly opposite of the input.

so what happens is sort of a compressor effect, the louder the output, the less the gain stage amplifies, giving a lot more "clean" headroom.

This was great when Leo and crew designed this amp, cleans were mean!

SO, by increasing resistance to the NFB resistor you lessen the signal to the cathode, earlier overdrive, a switch would be a resistor of very high magnitude.

Lowering the resistance of the NFB resistor increases the signal to the cathode increasing the compressor effect.

It wasn't a very useful mod, but it was really fun, I placed a tone stack in place of the NFB.

What this did was introduce only part of the signal back to the cathode. When the knob was turned to treble cut, that let more bass to the NFB and the over all voicing got brighter. When the knob was turned to bass cut, the NFB signal was brighter and the over all voicing darkened.

Fun stuff, but useless. like twice pipes on a Pinto wagon.

Ray
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Old January 31st, 2013, 07:27 AM   #8 (permalink)
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Thanks, now I got it!

I have 56K - 200K / 0,5W resistors available. Is 0,5W enough?
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Old January 31st, 2013, 08:02 AM   #9 (permalink)
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0,5W is de rigueur.
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Old July 29th, 2013, 11:56 AM   #10 (permalink)
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I have been testing NFB resistor values and have found out that this is the key tweak when changing speaker in the amp. With my Weber 8", the stock 22K is perfect, but with Eminence Legend 1058 the most pleasing sound comes with 180K resistor.

I was wondering: I have a NFB on/off toggle switch which is cutting the NFB signal. The switch is actually ON/ON. Instead of cutting off the NFB, I am thinking of keeping the stock 22K resistor on other side of switch, but adding a 150K resistor in line with the 22K "nfb off" = a NFB switch with 22K or 172K resistor values?

Will this work?
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Old July 29th, 2013, 03:31 PM   #11 (permalink)
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Quote:
like twice pipes on a Pinto wagon
\

that's a new one on me!

we always used "twice pipes" to refer to a certain sexual act... but hey, this is why I like the TDPRI, I always learn something new.
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