Presence Pot 5K vs 25K

[DaveWell]

Presence Pot 5K vs 25K

I need infos on this.
25K pot with a parallel 4.7k resistor to ground ?
or a 5K pot?

thanks !

[Colossal]

5k pot (in my opinion). Pro: wide range of usable control. Con: you get DC on the pot which makes for a little scratching noise at volume, but in practice, one does not sit there cranking the Presence knob back and forth. You set it and forget it.

25k pot with 4k7 has a more limited range to the control but no DC on the pot.

[DaveWell]

Thanks colossal for the info !

Is a 5K Pot with a 820ohm parallel will do the trick ?

[RockinRocket]

This thread is kind of a extension of the thread I stared in the Marshall section. This question I'm asking is probably more fitting for this thread.

Say both circuits use the typical .1uf.
5k DC on the pot/ .1uf
vs
25k/4k7 circuit/ .1uf

Does the 25k basically let you" keep going" after the 5k pot is maxed on 10?
SO basically you can get even more top end out of the presence at 6-25k on the pot??

[pompeiisneaks]

I think that sounds right, the more resistance you give it, the more it decreases the impact of the high frequency negative feedback, (and some mid) so you get more attenuation of the negative feedback in the higher range, up to the point where you'd have no NFB at some point, but only in the selective frequency range defined by the cap and resistance being shunted away from the NFB.

That is, if I've fully understood how it works myself

~Phil

[Colossal]

Thanks colossal for the info !

Is a 5K Pot with a 820ohm parallel will do the trick ?

Hi Dave,

I'm not sure of the context you are asking this question, meaning, in regards to what amp you would want to put it on. What are you referring to with the 820R? Do you want to change the value of the pot? If for a Trainwreck or Marshall, a 5k pot is added on to the tail resistor of the phase inverter and an 0.1uF cap is placed on the pot's wiper to ground. Negative feedback is applied at the junction of the pot and the tail resistor.

[DaveWell]

Hey
I mean, A 5K Presence Pot witn a Parallel 820Ohm resistor , to drain out the DC

[Colossal]

Hey
I mean, A 5K Presence Pot witn a Parallel 820Ohm resistor , to drain out the DC

Unfortunately not. In the traditional Marshall/Trainwreck setup, the presence circuit is attached to the tail resistor of the PI. Negative feedback is applied at this junction so there is both AC and DC at this point. The presence 5k pot is in between the tail and ground. So unless you use the later circuit with the 25k pot in series with the 0.1uF cap (blocking DC), you have DC on the pot. The alternative is to not locate the presence circuit on the PI tail in the traditional manner. This approach was borrowed from the Fender approach where NFB is injected at the tail, and others copied it.

[Mark]

I would have thought the value of the pot would have to be under 4.7K before the presence pot actually does something. I think a reverse taper pot would be more effective. I suspect Fender may have used these pots in newer amps as the presence control works so much better.


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[roberto]

As I wrote in the Marshall thread, I like the scratchy pot, otherwise I go for 10k+100n // 4k7.

[RockinRocket]

As I wrote in the Marshall thread, I like the scratchy pot, otherwise I go for 10k+100n // 4k7.

So is this 10k pot actually making or extending the amount of top end (compared to 5k pot) we can dial in the feedback circuit?

[Mark]

So is this 10k pot actually making or extending the amount of top end (compared to 5k pot) we can dial in the feedback circuit?

The short answer is no. Whether you have the 5K pot or the 25K pot the results will be the same. I have used a 25K pot on the presence control and the boost is bunched up from 3 o'clock on the pot to fully clockwise. The 5K pot has boost throughout its entire rotation.



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[pompeiisneaks]

So is this 10k pot actually making or extending the amount of top end (compared to 5k pot) we can dial in the feedback circuit?

The short answer is no. Whether you have the 5K pot or the 25K pot the results will be the same. I have used a 25K pot on the presence control and the boost is bunched up from 3 o'clock on the pot to fully clockwise. The 5K pot has boost throughout its entire rotation.



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I'm definitely trying to understand the presence a bit more, but as I understand it, (could be way wrong) the pot gives you variability on the effect of the tone shaping, and the actual NFB level is independent of it, due to it being a fixed resistor. Thus the size of the pot used for the connection to ground with the cap is what changes the tonality of the presense, (This is basically a tone pot used in the NFB in some way) therefore to me the amount of resistance coupled with that capacitor means it acts on the amount of treble and some mid that is NOT impacted by the NFB itself right? If what you're saying is right, though, that seems to imply that the amount of resistance doesn't change the frequency response? Isn't that against the rules of a cap/resistor filter?

[Mark]

So is this 10k pot actually making or extending the amount of top end (compared to 5k pot) we can dial in the feedback circuit?

The short answer is no. Whether you have the 5K pot or the 25K pot the results will be the same. I have used a 25K pot on the presence control and the boost is bunched up from 3 o'clock on the pot to fully clockwise. The 5K pot has boost throughout its entire rotation.



Sent from my iPhone using Tapatalk

I'm definitely trying to understand the presence a bit more, but as I understand it, (could be way wrong) the pot gives you variability on the effect of the tone shaping, and the actual NFB level is independent of it, due to it being a fixed resistor. Thus the size of the pot used for the connection to ground with the cap is what changes the tonality of the presense, (This is basically a tone pot used in the NFB in some way) therefore to me the amount of resistance coupled with that capacitor means it acts on the amount of treble and some mid that is NOT impacted by the NFB itself right? If what you're saying is right, though, that seems to imply that the amount of resistance doesn't change the frequency response? Isn't that against the rules of a cap/resistor filter?

Basically what is happening is, negative feedback from the secondary of the output transformer is applied to the P.I. this reduces the gain of the P.I. stage, by earthing out a particular frequency this frequency is boosted. The cap chooses the frequencies with the resistor provides a path to earth of lower resistance than the 4.7K resistor. For this reason the pot has to be turned up quite a bit before it is lower than 4.7K.



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[RockinRocket]

Basically what is happening is, negative feedback from the secondary of the output transformer is applied to the P.I. this reduces the gain of the P.I. stage, by earthing out a particular frequency this frequency is boosted. The cap chooses the frequencies with the resistor provides a path to earth of lower resistance than the 4.7K resistor. For this reason the pot has to be turned up quite a bit before it is lower than 4.7K.



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I thought the presence control with 4k7 to ground would have only a AC coupled cap/pot presence network. As Pompeii mentioned its a RC filter? So I'm trying to understand why the range of pot doesn't start being noticeable until it reaches 4k7??

When the presence control is turned to 10 or max what is the resistance the pot has 5k or 0ohms??

I really need to understand how these circuits work because I don't what to go in messing up my build to try a 25k no dc set up!