"PRESENCE" control on Marshall amps!!! Circuit sim

[Kleber AG]

Could a solid state circuit simulate that "PRESENCE" control on Marshall amps?
How should it be done?
I really would like to build a circuit for that.

I know Doug H. had simulated the wreck with his Meteor circuit, then I tought someone could have an idea...

Thanks!
Best regards
Kleber AG

[Aharon]

The Presence control in Marshall amps changes the freqs going through the negative feedback loop of the amp,I think.
Aharon

[Kleber AG]

Thanks Aharon,
Yep, that's right. If it was "only" on the preamp stage it would be easier to simulate from the RC filters frequencies and all that stuff we're used to work with...  :?

But it's a feedback loop from the output stage, back to the phase spliter section... :?:  :?:  :?:


thoughts???
Please
Kleber AG

[pippas]

Most 'presence' controls introduce some form of bandpass filter which increases the gain at (typically) between 2 and  3 KHz -

Of course, by how much, and at exactly what frequency gives each different variation it's own character! - there's been much debate over the years as to the 'right' way to get the best presence - it's usually a case of 'suck it and see'!!!

[The Tone God]

The principal that the Marshall presence control works on is the same as almost any amp with a presence control. A connection is made from the output transformer which is fed into an earlier stage such that the signal is 180 degrees out of phase thus causing some signal cancellation. This is called negative feedback. The presence control adjusts how much negative feedback is added. Nothing really special.

To simulate it is easy enough. Make a effect like a buffer, take the output, invert it, then use a mixer stage to control how much signal gets added to an earlier stage. To be honest I don't think your going to get much out of it. I think you would be better off with a good EQ or cab simulator.

Andrew

[Kleber AG]

To be honest I don't think your going to get much out of it. I think you would be better off with a good EQ or cab simulator.

:roll: But then I should know how this "negative feedback loop" affects the signal, how is the roll-of curve characteristics and point that the roll-of starts  :?:

And IMO this PRESENCE control is perfect for put you at the exactly level of a "live" band mix situation, adjusting how much your guitar will cut-through the whole band mix without having to raise your amp volume to ear-crushing levels :wink:

Thanks guys! Keep the ideas coming, Please!
"How can I build a similar PRESENCE circuit to have this control over the mix???"
:?:
Kleber AG

[The Tone God]

But then I should know how this "negative feedback loop" affects the signal, how is the roll-of curve characteristics and point that the roll-of starts  :?:

And IMO this PRESENCE control is perfect for put you at the exactly level of a "live" band mix situation, adjusting how much your guitar will cut-through the whole band mix without having to raise your amp volume to ear-crushing levels :wink:

Thanks guys! Keep the ideas coming, Please!
"How can I build a similar PRESENCE circuit to have this control over the mix???"

Are you trying to mod an amp or just simulate this in an effect box. If your modding an amp then chances are you already have a neg. FB loop. You would just add the necessary control. Its not a tough thing to do.

As for how it affects your signal the neg. FB loop was orignally put in to keep distortion under control. The lounder freqs would make more neg. FB thus attenuation that freq down while the other freqs would not have as much neg. FB thus allowing more to signal to flow. In a way it was like having compression.

If you just want to adjust your level you can use boasters, attenuators, DI boxes, etc. to achive this. I don't want to disscourage you from experimenting but just make sure your heading in the right direction for what your looking for. Besides ear-crushing is what guitar were suppost to do, or atleast I thought. hehe

Andrew

[pippas]

The principal that the Marshall presence control works on is the same as almost any amp with a presence control. A connection is made from the output transformer which is fed into an earlier stage such that the signal is 180 degrees out of phase thus causing some signal cancellation. This is called negative feedback. The presence control adjusts how much negative feedback is added. Nothing really special.

Do you mean the whole signal - or just a band passed section? - surely if you feed everything back, you would in theory cancel the lot! - or do the dynamics of the transformer make the feedback frequency selective?

[The Tone God]

Do you mean the whole signal - or just a band passed section? - surely if you feed everything back, you would in theory cancel the lot! - or do the dynamics of the transformer make the feedback frequency selective?

Everything that comes out of the output transformer is sent back so take into account what tonal vartions the OT and power tubes attribute. The neg. FB usually gets added back in the phase inverter section after the tone controls. It would, and possible could, cancel all the signal but the amount of neg. FB added is fairly low. If you were to add more neg. FB then more of the signal would be attenuated. There maybe fancy master volume amp varitions on this but the vast majority of amps use this method of distortion limiting.

Just about every amp I get has this in there. I get requests regularly to "tweak" feedback loops in amps.

Andrew

[Kleber AG]

Thanks Andrew,
but I'm really going to try to reproduce "that" effect on a stompbox circuit.

I'm used to use that PRESENCE control, but because I gig too much and almost always not with my own amp, sometimes I don't have that PRESENCE control, or sometimes I even play through a PA Stereo Power Amplifier through guitar speakers cabs... and then with this PA Amps what I'd really need is the presence control that they don't have. Keep in mind I don't need a whole spk sim, as I'm actually really going through guitar spks cabs...

I need a circuit to simulate "Negative Feedback Loop From Tube Amps" :wink:

Kleber AG

[The Tone God]

Well then I'll work on the principal that you don't have an amp and that your hooking up straight into the board/PA. My earlier suggestion of a buffer with an internal neg. FB loop may still work. Tossing in a few controls to adjust freqency response to approximate possible tonal changes in an amp might help. Maybe a phase selection switch would be handy too.

I don't keep up with the cab simulator world but they may already have this one licked. That may be what your looking for.

Andrew

[Johan]

look at the FuzzFace. the fuzzknob adjust how much feedback you have and the result is more or less fuzz. ....the main difference is that in the fuzzface you controll the whole frequency spectrum, from about 16HZ and up. in a Marshall you go from around 500Hz and up
try changing the cap on the fuzzpot to 0,2-0,3uF instead of 20uF and it will do something simular, but I dont think it will sound lika a Marshall. as you know, its really hard to build a stompbox that makes a tubeamp behave like a tubeamp doing something else  :? ...and also this FF-like circuit would probably have to be Si, so that the result isnt rounded of and disapears in the Ge-smothness...

Johan

[J.Nadge]

Hi Kleber,

Here's a simple idea for you.  You need a cap and a new pot.  

Take your standard voltage dividing volume control that you see in most effects.  Now, connect your new cap to the Hot end of the volume pot (lug 3).  Now, connect the other lead to lug 2 of your new presence pot.  Connect one end of the presence pot to ground and the other end to the outgoing signal (lug 2 of the volume pot).  The values of your new cap and pot will depend on your volume control value.  If it's 100k, start with 1nF and a 100k presence pot, and mod from there.  

One end of the presence pot shunts high frequencies to ground, while the other end bypasses them on the volume pot.  That's it!

There are probably many ways to implement neg. feedback with opamps and discretes.  The most basic, which can be shared by both is to first boost your treble and then use a .01 cap and a 100k variable resistor in series from C/D/OUT to B/G/IN.

Good luck!

-J

[Kleber AG]

WOW  :idea:    

I liked the ideas! Thanks SOOO much!

Liked the fuzzface one and J.Nadge ideas as well,
Thanks Andrew, Johan, J.Nadge...

I'm gonna try that simple idea with the volume pot trick, but something came to my mind, from the fuzzface NFLoop idea:


Please, I just drawn that from the ideas you told me, and I think I'll be able to try it only on thursday.

Any modfications, thoughts, improvements, oh well, is it gonna work as a Presence Control???  

Thanks everyone!
Kleber AG

[The Tone God]

My thought on the passive pot suggestion is you have to become aware of what phase your signal is. You also have to aware if any effect you have that changes your signal phase otherwise your going get a not so nice surprise.

Loading your output signal is another issue. That is why I think an active version would be better. You could also then add some controls to EQ up things to simulate the various freqency changes that occur in the process.

That being said give the pot a try if you want. It may just do the job.

Andrew