Presence control

Jewellworks

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im wanting to add a presence control to one of my amp designs, but other than copping whats on the JCM schematic, ive never done a presence control before, and could use a little help with determining what values work best. im using an 8ohm OT @ 5k. i read a discussion on this forum at some point talking about the resistor value needed for best NFB, depending on the speaker load, but i can find it anywhere.

on the whole, im not a fan of NFB. i would want some way of dialing it down (preferred) or switching it off (less desirable). so rolling the presence control to "0" -more or less- turns off the NFB. -is that a thing?

in a perfect scenario, i want to put this on a push/pull DPDT pot, which would bypass the TMB tone stack when pulled. when pushed in (normal) the TMB is engaged. and if possible, have the Presence available on both TMB and bypassed modes. turning the knob engages NFB and Presence.
if thats not how it works, then ill come up with a plan B.

my first thoughts are to use a 22k resistor off the OT 2ndary "+", through a 10k pot, with a .1uf cap on the other side, then tied to the cathode of V1b (SE amp w 1, 12AX7 preamp stage to the Power tube). assuming that 32K of resistance is enough to keep the NFB out when dialed to "0".
but thats just a guess.
but then comes the question of how far will i need to turn it to hear any difference at all? if i dont hear anything till 8 or 9, then i need to find a happy medium. maybe change the 22k to something else? or bumping up the pot value?

hope this is making sense...
thanks
 

chas.wahl

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Here's the thread you refer to in your 3rd sentence:
It's upshot is that the narrow panel tweed Champ and Princeton designs both use a 22k NFB resistor (the resistor cutting down negative feedback from full-on to something useful), but they had (in most cases) different speaker impedances; 4 vs 8 respectively. Based on reasonable calculations, to have the same degree of NFB, the Princeton should have had an NFB resistor with a higher value, somewhere around 33k. NFB is, of course, a matter of taste, since it gives with one hand (clean) and takes away with the other (animal energy in fullness). And what works for an SE Fender may not work for a "JCM" or any other design with a different preamp layout, a different OT primary impedance, and so forth.

Uncle Doug has a video where he put the NFB on a pot control (with a fixed resistor to determine the lowest value of resistance in that part of the circuit, in series with a pot -- wired as a simple variable resistor -- of a reasonable value to add resistance; that is, reduce NFB from that fixed value's resistance). He used a 50k pot.
Uncle Doug's variable NFB circuit

In this thread
there's more discussion about what the value of the pot might be -- also Fender-centric. I suggested that, in order to turn off NFB entirely (disconnect the circuit there) it's possible to disassemble the pot, and put some nail polish over the resistance track at the "high" end of the wiper's sweep, to lift the wiper at the very end. This is not something I invented; people do this to make a "no load" guitar tone pot.
 
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Jewellworks

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@chas.wahl :
thanks for finding that post. when i search for "NFB resistor" i get results from nearly every post on this forum.
i like the idea of putting a bit of nail polish on the wiper. and now that you mention it, i remember Uncle Doug doing a variable NFB control on a few of his amps.
but how does that "relate" to Presence? does adding the cap to the wiper of the pot simply make it a Presence control?
from what i can tell, this makes it a variable NFB/Presence control. the lower the setting, the less NFB and less presence. the more i turn it up, the more NFB, the cleaner it will get, and with more presence, and brighter it will get. correct?
hhhmmmm... not sure thats what im really going for.
 

chas.wahl

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I know nothing about presence controls. You asked for a way of dialing NFB down or turning it off, and that's what I was responding to. A presence control may or may not interact with or be integrated into an NFB circuit; see Rob Robinette's section on "Presence Control is a Late Semi-Active HF Boost" within his extensive "Amp Voicing" article, which appears to be focused on higher-powered, higher-gain amp circuits than the sort that I know anything about. His examples of presence and resonance controls are, however, associated with the NFB circuit (resonance apparently before the NFB resistor, presence on the other side of the tap into the amplification circuit [?]).

Merlin Blencowe (Valve Wizard) also covers presence controls, resonance controls and combinations with feedback circuits in his "Designing Tube Preamps" book, pp. 166 ff. in the 2nd edition -- in the LTP PI chapter 9.

As for me, I will wait until people with more experience in this direction show up, and offer suggestions.
 
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Jewellworks

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Rob has a diagram/drawing on how to add a presence control on his Amp Voicing page. its a simple mod, and i copied that verbatim. Presence IS tied to NFB, and the way i had it drawn out wouldnt have worked.
 

JohnnyCrash

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Adjustable Negative Feedback isn’t the same as a Presence control. It sounds like you want NFB, not Presence.

On my 2x10” JCM800 build I used a 3-way switch for no NFB, light, or heavy.

I like them for their different feel.

Instead of a fixed resistor (if I recall, the JCM800 uses a big resistor for light NFB: a 100k resistor), you could use a potentiometer as an adjustable resistor. This could give you a wider range from heavy NFB (a cleaner, smoother power section) to light (for a very raw rockin feel).
 

dan40

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The presence control is simply a filter on the nfb circuit that will make your amp slightly brighter in the really high frequencies or slightly duller. You can have nfb without a presence pot but you can't have presence without an nfb circuit. A resonance control is very similar but it works with the lower frequencies.
 

Lowerleftcoast

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im wanting to add a presence control to one of my amp designs, but other than copping whats on the JCM schematic, ive never done a presence control before, and could use a little help with determining what values work best.
I assume you are wanting a presence control on an SE amp. I haven't tried a presence control on a SE amp. I have read it wouldn't be very useful. YMMV.

Idk which JCM schematic you are referring to... what is important: there should not be VDC on the presence pot. The cap associated with the presence pot should be in series with the pot. (At least one Marshall schematic shows the presence pot in series with the pot. The pot is wired as a variable resistor.)
I would think the value of the tail resistor will have to be more than the small 47R - 100R we typically see when no Presence control is used. If the tail resistor is larger the NFB resistor will have to be larger to create a workable NFB voltage divider. Since you want a variable NFB resistor, the pot for NFB would probably be 50k - 100k and it would be in series with a resistor.

Below is a diagram of a Presence pot in series with the cap. The resistor in this case is 4.7k. The pot is essentially in parallel with this resistor so you can pick a value. The standard 25k will do. (The Marshall schematic shows a 22k pot.)

pignoseg60vr2.jpg
 

Jewellworks

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ive pretty much abandoned the idea of a variable NFB WITH Presence, since after thinking about it, thats not really what i want. now im thinking about an on/off switch at a set value, and a Presence control.

the following was lifted from RobRobs Amp Voicing page.
now if im not going to get much effect because its a SE amp, then thats completely different, and ill have to re-think what (if anything) to do next.

Image1.jpg

the "Marshall" reference was just a glance at the Marshall schematic, but i didnt copy it or anything. theyre the only amps i know that has a Presence. ive been looking elsewhere since...
 

dan40

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Many amps use nfb without the added presence control while many of the larger amps with more features have a presence control. Are you simply looking for a way to increase the higher frequency content or a way to remove some of the higher frequencies? A "cut control" mod is a simple way of removing a bit of the higher frequencies if that is your goal.
 

Jewellworks

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Many amps use nfb without the added presence control while many of the larger amps with more features have a presence control. Are you simply looking for a way to increase the higher frequency content or a way to remove some of the higher frequencies? A "cut control" mod is a simple way of removing a bit of the higher frequencies if that is your goal.
just looking for a new "feature" to try out. someone suggested adding a Presence control.

in the 1 or 2 amps ive built (all SE) i put the NFB on a switch, and 99% of the time, i leave it off. it dulls the sound. if there was a simple way to have it on and keep it bright, id add it. ive seen caps that go around the NFB resistor... but id guess at 500pf.
-the important word being "guess"...
 

dan40

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Adding a cap across the feedback resistor will only dull the highs. It allows the higher nfb frequencies to go around the resistor and act on the power amp by dulling the high end.

Introducing nfb is suppose to extend the frequency response but I do agree that in some circuits, I have also noticed a dulling affect when introducing it.
 




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