I'm in WAY over my head...

[Chuck H]

for most g amps perhaps... I have a different approach for introducing "presence".

-g

For the purposes of discussion the term "presence control" was referenced due to it's typical incarnation. Which is a NFB filter. Whether a different circuit control can be labled "presence" or not doesn't really make a point. I could label every control on my amp "presence" and it wouldn't change this subject matter. Just sayin'.

[pdf64]

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.

[J M Fahey]

Pete, congratulations !!!
First original tube circuit I've seen in quite a few years.
COOL !!!!

[pdf64]

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.

[R.G.]

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.

[mooreamps]

Mmhhhh , interesting.
Can you elaborate?
Thanks.

I'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

[markr14850]

I'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....

[J M Fahey]

I already tried that,

Where did you get the schematic ?

[markr14850]

Where did you get the schematic ?

Well... I figured that RG would write up a rational and kind explanation encouraging Gary to share with the community, rather than just taking driveby shots. Then Gary would reply with some paranoia about someone stealing his work. More rational replies would be written, and ignored. We'd all be forced to wonder whether said circuit really exists. So, I just skipped all that, and resolved it by claiming "golden ears". Now we can get back to real life and not illusion.

[J M Fahey]

You're a funny guy, ha ha.

[mooreamps]

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

[pdf64]

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.

[Steve Conner]

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.

[Rhodesplyr]

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.

[J M Fahey]

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.