Originally posted by mooreamps
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I'm in WAY over my head...
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"Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
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Presence / Raw Control
I came up with a simple way to adapt a regular presence control to pan between presence and open loop.
I find it useful when setting the amp up for LP (presence full) and Strat (about halfway).
This was on a JTM45 based amp, which has about the highest NFB ratio of regular guitar amps.
Full open loop does add a chunk of gain, but it seems manageable on a JTM45, so shouldn't be a problem for amps with a lower NFB ratio.
Regular presence controls seem pretty flat now that I've got use to this.
Pete.Last edited by pdf64; 02-17-2012, 07:19 PM.My band:- http://www.youtube.com/user/RedwingBand
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Thanks JM.
In use, it seems to be most intuitive to be wired up the opposite way around to normal, ie the ground is at the CW / hot track end.
It's then kind of like a volume control with a big 'bright' cap.
Pete.My band:- http://www.youtube.com/user/RedwingBand
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As an illustration of gain compensation, I jotted this down:
It's mildly complicated to do, as you have to find or (more likely) make a dual non-matching-sections pot, but it illustrates the concept. As the amount of feedback is changed, the input pot also changes the input signal and so the overall gain.
I did it on the presence control schemo, so it could be jiggered to change the apparent volume there, but it also works for the case where the feedback in toto is dialed in or out. The amount of compensation depends on the relative values of the input pot and the fixed resistor left at the input. It's shown for 10:1.
Like all the stuff I jot down off the top of my head, I probably missed something, but that's the idea.Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!
Oh, wait! That sounds familiar, somehow.
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Originally posted by J M Fahey View PostMmhhhh , interesting.
Can you elaborate?
Thanks.
If they like it, I'll have to think this over.....
-g
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Originally posted by mooreamps View PostI'll just incorporate the circuit into my next amp, have the guys play through it and see what they think of it...
If they like it, I'll have to think this over.....
-g
I already tried that, and you must have tin ears if you think it sounds good....
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Originally posted by J M Fahey View PostWhere did you get the schematic ?
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It exists on paper only... Thus I can only say the current incarnations of presence are passive circuits.. The only thing I can say at this point is this will be an active presence control...
-g
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An aspect of a presence control (in the common 5F6A onwards implementation), that isn't obvious to an initial analysis, is the way it interacts with speaker impedance and the amp's output impedance (which will rise with frequency as the NFB ratio decreases, due to the presence control).
My JTM45 based amp has an open loop output impedance of about 50 ohms, closed loop of about 3 ohms (on 8 ohm OT output).
With an 8 ohm resistive load, max presence gives a voltage gain (at the amp output) of about 5 at 10kHz (compared to min presence on a stock circuit), 3.76 (3dB down) at about 2kHz.
However with an 8 ohm speaker load, max presence gives a power amp voltage gain of about 16 at 10kHz, 12 (3dB down) at about 7k4Hz.
So, depending on the speaker characteristics, somewhere in the range 2kHz - 8kHz and above, the power amp will be operating pretty much open loop (which will impact on speaker damping).
So, from the above, despite appearing to be a simple little thing, the way that presence control operates in practice is rather complex; I can't see how that operation could be mimicked in any other way. Boosting treble somewhere earlier in the amp would lose that speaker / amp interaction.
Pete.Last edited by pdf64; 02-19-2012, 10:42 PM.My band:- http://www.youtube.com/user/RedwingBand
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Pete is right, the presence control really does interact with the speaker. I've measured similar results on the bench, and I also feel that boosting treble earlier in the circuit, without increasing the power amp output impedance, doesn't sound as good.
In the light of this, I think calling the presence control "passive" is a little unfair. Surely it's an active tone control, using the power amp as its gain block."Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"
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I'm guessing you're familiar with the story of Dean Markley and the JMF Spectra amps. I was told that compromises in build quality resulted in many of those DM amps breaking down before they left the showroom floor. Of course, that was 30 years ago, and they may have learned from the experience.
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Agree that classic Presence controls era active.
They modify the response of a gain stage by modifying its NFB net.
Can't get more active than that. !!
There are also some passive "so called" Presence circuits .. which they are not, just old passive treble cut controls.
Check "Presence" in classic Randalls and in MB DR.Juan Manuel Fahey
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