NFB vs Presence?

[doveman]

I'm somewhat familiar with NFB ... my D'Lite 22/33 has it ... a Vibrolux reissue did not. I am familiar with what it does. My question is:

How does the presence control relate to no NFB? Is the 0 setting on the presence = no NFB?

[mlp-mx6]

Without negative feedback there is no presence control possible.

NFB changes the character, tone, and probably "feel" of the output section.

'0' on the presence control is NOT the same as no negative feedback. It is actually full negative feedback. As you turn up the presence control you shunt some of the feedback signal to ground above a certain frequency, thus removing that part of the signal from the feedback. If you remove the higher frequencies from the feedback they attenuate the final signal less. Make sense?

[doveman]

So is presence on full the same as no negative feedback?

[mlp-mx6]

No - it is frequency-selective. The lower frequencies are still fed back. It is only the highs that get shunted to ground, thus not fed back. (as much - there is still *some* HF feedback too)

[doveman]

Thanks that helps. One more question for anyone ...

My NFB is tied to the 4-ohm tap on the output transformer. What difference would it make (if any) to be on one of the other two taps? Mine has 4-8-16 tap choices but the Brown Note D'Lite 22/33 instructions pointed to the 4-ohm tap. Just wondering why? I asked over at BN too ... but I'm sure you guys know.

If it makes a difference, what might you expect from the other taps?

[Structo]

Moving the NFB wire to the 8 ohm tap will provide more negative feedback. 16 ohm tap even more

Some guys like it there.

You might want to try it to see how it sounds.

[doveman]

Is it possible to have a switch to disable the NFB? Anybody done it? If possible, what sort of switch could you use? I don't know that I would do it ... just interested.

[Sonny ReVerb]

My NFB is tied to the 4-ohm tap on the output transformer. What difference would it make (if any) to be on one of the other two taps? ... If it makes a difference, what might you expect from the other taps?

The relationship is not linear. This is from Randall Aiken's website:

Note that the tap voltage is not linear with respect to the impedance, it varies linearly with the square root of the impedance. That is, the voltage on the 8 ohm tap is not half the voltage on the 16 ohm tap, rather, the voltage on the 4 ohm tap is half the voltage on the 16 ohm tap.

[M Fowler]

I think it was Richie that said to convert the presence to a PIG control which works great.

[doveman]

I think it was Richie that said to convert the presence to a PIG control which works great.

I'm not sure I follow? What is a PIG control?

[iknowjohnny]

i imagine pig refers to using a bigger cap which boosts lower frequencies as the value of the cap is raised . Don't particularly like it myself, tho it's amusing at first.But i find it makes the tone harder to fit in a mix nicely.

[Cliff Schecht]

Negative feedback, when applied to just about any linear amplifier, is used to reduce the overall gain while increasing the amplifier bandwidth. So in the case of us guitarists, this means slightly less distortion from the PI/PA section but a more brilliant sound because, as others have said, the feedback we typically use is frequency selective (i.e. not broadband feedback).

[M Fowler]

Here it is

[rfgordon]

In the case of many Fender designs, there is simply a fixed resistor setting the NFB leve--typically 1k5 Ohms, which lets quite a bit of NFB in to the PI/PA. While this does increase headroom, it can result in a rather unpleasingly abrupt transition into PI/PA overdrive in some amps.

Using a larger resistor--even moving from 1k5 to 2k2 or 3k3--can dramatically alter the feel and touch of a BF/SF Fender amp. If one has no compunction about drilling holes in the chassis back, installing a switch or a pot in series or parallel with the NFB resistor are nice options for adding tweakability to those amps. Reducing NFB in a Fender Pro or Super Reverb can really help it "give up the goods" at lower output levels (read: keep the sound guy and venue owner and the cops on the street with the dB meter happier!).

[Jana]

One thing to keep in mind with fender NFB circuits is that even though the feedback resistor is a lot smaller than in a Marshall, for example, the other resistor that completes the voltage divider network is also a lot smaller.

A twin reverb AB763 has an 820 ohm feedback resistor. The other resistor is 100 ohms. A ratio of 8.2.

A marshall with a 47K resistor is going to a 4k7 resistor--a ratio of 10. Very close.