Negative Feedback Reduction Switch?

[Guitar Adjuster]

The higher ohm rating of the neg fedback = more distortion. Think of if this way, a 67 100 watt marshall Plexi usually has a 47K ohm neg feedback resistor and a JCM800 has a 100K ohm neg feedback resistor. It also makes a difference which tap you place the NFB resistor too, the 4, 8 or 16 ohm taps. 4 ohm tap = more distortion, 16 ohm tap less distortion. Lots of info on NFB at

www.metroamp.com

Hope this helps.....

Allen

 

[Guitar Adjuster]

Thanks Allen that's great.

Would or could the NFB resistor be used for controlling the distortion level, perhaps in an attempt to eliminate distortion create more clean headroom? I get the impression from your response they are used to "add" distortion.

I'll check the website perhaps the answer is there.

I believe you can. David Brey has an adjustable NFB that lets the amp go from an older spec plexi--very loud and clean to a metal panel type with lots of distortion. I have heard of people installing an adjustable pot for the NFB for this purpose. Check out Brey site maybe some info there as well.

 

[Blueracer]

Here is a short explination of negative feedback (NF). Basically NF is where a portion of the output signal is phase reversed and added back into the input. The phase reversal tends to cancel part of the input. The question is why do this. In high fidelity amplifiers the aim was to flatten the gain of the amplifier over frequency and to make the amplifier more linear. If the amplifier has a gain that is higher at some frequency than some other then the portion of the signal sent back through the NF is also larger and will cancel more of the input. Hence the gain will be flattened out over frequency as well as reduced. If there is nonlinear amplification being introduced by the amplifier it is also compared to the input and compensated by the NF circuit.

The resistor in the NF determines how much of the signal from the output is fed back to the input. The larger the resistor the less NF, the smaller the resistor the more NF. It was also mentioned that the tap on the OT used affects the NF and this is true. The smaller ohms taps will have less voltage and therefore less feedback and conversely the higher ohms taps will have higher voltage and greater NF. So again from the first paragraph if there is less NF then the gain of the amplifier will increase as well as nonlinearities (distortion) and the variations of gain with frequency will increase.

One other thing that is worth mentioning is that the presence control is in the NF circuit. Basically what happens is that the high frequency part of the feedback is diverted to ground through the presence potentiometer and a capacitor. This makes the high frequencies not part of the NF which in turn makes the amplifier amplifiy these frequencies more. This is different than the treble on the tone stack as it is happening in the power stage.

This probably is more technical than the orignal post requested but is offered up for those that are interested.

Cheers,
Gary

 

[phaboo]

I was going to post a definition for nf, but the above post covers it.

Here's what you'll HEAR with nf:

Negative feedback tends to make the sound more "closed" where less feedback leaves it more "open" and "transparent." One reason tubes souind better than xistors is that xistors need a LOT more nf to produce linear results.

The downside to nf is that the amp has to first produce the distortion in order to feed it back to the input. If you have a very dynamic signal (like the pick attack) an amp with little nf (like a tube amp) will gave a very different attack than a xistor amp.

I have an old Crate xistor power head where you can switch the nf off COMPLETELY. It opens the sound up (it sounds like taking a blanket off the speakers) but then it needs some pretty extreme eq to clean it up.

Active eqs also use nf, especially when cutting bands. This is something to consider if adding an active eq somewhere--cutting a band may not give you quite the results you expect (the graphic eq on boogie preamps isn't active, though). For example, if you have a graphic eq and set all bands to -5db and compare it to 0db settings (with the volume normalized) you WILL hear a different tone. At -5 there nf going on in the eq, at 0db there is none. Set everything to +5 and you'll get another tone. This can be quite handy if you're trying to get a unique sound, though too much of this nonsense and it'll turn to mud.

 

[phaboo]

Not sure what an xistor is but perhaps somemore research will reveal that answer as well.

Sorry--xistor = transistor. It's a nerd thing where I work. Anything with a "trans" in it gets an "x" instead. Transfer = xfer, etc.

 

[phaboo]

So I take it that in general, NF resistors are typically used to Add/ Create/ Control distortion at a certain volume level depending on where it's placed and what resistor value is used?

The value of the resistor controls how much of the output signal is fed into the input. Other components in the amp will affect this as well, so the resistor is chosen with the rest of the amp in mind. So, a resistor of one value in one amp won't necessarily have the same amount of feedback if it was put into another.

 

[Blueracer]

So I take it that in general, NF resistors are typically used to Add/ Create/ Control distortion at a certain volume level depending on where it's placed and what resistor value is used?

Just to clarify, when distortion is discussed with regards to NF it is not the type of overdriven tube saturation distortion that is usually thought of in guitar amplifiers. This type of distortion is more of a linearity thing which the NF helps reduce. Think of it as how well does the amplifier reproduce the signal going into it. If it is a very accurate reproduction the amplifier is said to be linear. Note that this is for when the amplifier is running linear and not in saturation. I would characterize the difference caused by NF as whether the amplifier sounds more "open" which would correspond to less NF or "tighter" which would correspond to more NF. I hope this helps and trying to describe these things in words is at best difficult.

Cheers,
Gary

 

[Blueracer]

Clarifying the tube saturation distortion vs that which NF is addressing helped as well. I was thinking tube distortion!

Is it a "safe" perception to view NF as something that helps keep all frequencies tighter, meaning none will necessarily stand out when NF being used. They all sort of get "balanced".

Yes this is part of the function of NF. There is the gotcha of the presence control which will allow the highs to be amplified more. It should be noted that by manipulating the feedback network that certain frequncies can be modified and it is not just limited to the high frequencies. But for your more plain vanilla NF it will tend to even out the frequency response of the power stage. This would be helpful in the case of the "flubby" bass response in which more NF should tighten this up. So I would not go into modifying the NF if you are looking for more power tube type distortion. If you do want to have a "tighter' sound then it is appropriate. How much is part of the art of amplifer design. I would think that putting a potentiometer in the feedback loop can be instructive in hearing the differences in varying the NF.