No NFB equals no presence, right?

[ted01]

I have replaced a 4.7K feedback resistor with a 1M pot. When I run the pot all the way up (i.e. little to no feedback), my presence control doesn't seem to do a darn thing, which seems to make sense as it appears that the presence circuit is dependant on the feedback voltage. Right?

ted

[ampdoc1]

Correct.

[Alexo]

It might be more accurate to say that no negative feedback equals 100% presence, as the presence control typically dials out nfb over a given range of frequencies as you turn it up.

[Abstract]

More correct.

[Bob-I]

So if I'm standing on my head am I upside down, or downside up?

[drhulsey]

So if I'm standing on my head am I upside down, or downside up?

Yes

[Alexo]

Is it still negative feedback if you're standing with your back to the amp?

[tonelab2]

Constructive criticism is more important than negative feedback but whether your present or absent is another story. Roll with it

[butwhatif]

Negative feedback is ok as long as it's just a phase you're going thru

[mhuss]

Forgive me if I bring the thread back on topic.

As other posters have stated, a conventional LTP NFB-based presence control won't work without NFB. However, there are other ways to add a control which provides the same frequency control, basically a "high treble."

--mark

[benoit]

Forgive me if I bring the thread back on topic.

As other posters have stated, a conventional LTP NFB-based presence control won't work without NFB. However, there are other ways to add a control which provides the same frequency control, basically a "high treble."

--mark

So.. why is this almost never done? Does it have more to do with how an amp behaves in general with a little NFB? Or would the cap required for the presence (non-nfb style) be prohibitively expensive or hard to find?

[jaysg]

Mark's the expert here and hopefully he'll correct me if I'm misunderstanding things -- the basic Hiwatt design from the 60's has a Presence control that is not directly in hte NFB loop. The amps have NFB, but it looks to me like the presence would work without it.

[Wayne]

Forgive me if I bring the thread back on topic.

As other posters have stated, a conventional LTP NFB-based presence control won't work without NFB. However, there are other ways to add a control which provides the same frequency control, basically a "high treble."

--mark

One that I've seen is a partially bypassed cathode R with a rheostat-wired pot in series with the bypass capacitor - that's the presence control in the JCM 900.

P. S. I know I always post examples from that particular amp, as though I'm a big fan or something - I'm not, but I know it's guts well and it's paid for!

W

[C Moore]

Can somebody explain the relationship between NFB and the Presence control? Assuming you have a capacitor on the presence pot and a resistor in series of the NFB. Like in a T Wreck Express. In theory, what happens if you increase/decrease the value of the cap, and what happens if you increase/decrease the value of the resistor?
Thank You

[ted01]

Hired Hand,

Keep in mind I started this thread, so I'm certainly no expert, but I think the impact of changing the values of either the resistor or the capacitor will be a shift in the rolloff frequency. I think the presence circuit forms a low pass filter. As I have learned from this thread, no NFB means that there is the "maximum" (i.e. unfiltered) presence in the signal. The RC circuit (that is the presence circuit) sets up a low pass filter and the pot varies the degree of effect the filter has on the signal. If you change either the cap or the resistors value, that'll change the frequency of the low pass filter. I think the formula is f=1||2{pi}RC.

Hopefully some part of this is right.

Ted