"HF Drive" on orange or amps

[Xander8280]

The presence control seems to be a RLC filter in front of the phase inverter that is either grounded or connected to the negative feedback.

I've seen videos of the HF control in use. It seemed like a tight presence control that gave sharp crunchiness to the high high mids.

Anybody know more about this? I'd love to use this in my marshall instead of the stock presence.

[roberto]

Well, it's a 5kHz LC filter, what do you whant to know exactly?

[Xander8280]

Is it a notch or low pass? And what happens when the wiper it on the negative feedback side?

On videos is sounded like a dirty presence, it way cool. Very harmonic rich and sparkly

[gingertube]

I'll have a go at explaining it.


The 2mH + 0.47uF form a circuit with a resonance at 5191 Hz.
F = 1/( 2 pi root LC) (As Roberto said above).

The 2mH, 0.47uF + 220K form a positive feedback circuit (which provides boost).

What POSITIVE feedback you say, it will oscillate - no it won't becauase the unbypassed cathode resistor is providing lots of NEGATIVE feedback and we are just subtracting from that to arrive at a smaller amount of NEGATIVE feedback - so long as the TOTAL feedback remains NEGATIVE then we are OK.

It will have minimum impedance equal to that 220K value at that resonant frequency (Capacitive and Inductive impedances cancel at resonance, they are equal magnitude but of opposite phase) with the impedance becoming higher either side of the 5191Hz.

The upper side, 5190Hz and up, is of no intertest to us except to note that the POSITIVE feedback is reducing again as frequencies go higher. This will help prevent the positive feedback causing stabilty problems.

On the lower side you will get a boost according to the impedance of that network, that is, maximum boost at 5190Hz with its level set by that potentiometer. AS frequency falls from the 5190 resonance (impedance increases) then the boost also falls.

The 220K sets the slope with frequency of the boost by setting the "Q" of the resonant circuit.

Hope this makes sense to you.

Cheers,
Ian

Edit: ***** See Corrections Below *****

[Xander8280]

Awesome, thanks!

[roberto]

Maybe I'm wrong, but I remember from some pictures that LC filter is just grounded. The 220k is grounded too to give gnd reference to the input of the gain stage.

So the LC is just a notch on the NFB, so acting like a boost.
The amount of boost is decided by the 1k lin pot.

[matt h]

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[gingertube]

The posters above are correct.
The 220K goes to 0V, that is, it is a normal grid leak resistor.
Similarly, the bottom of the LC circuit goes to 0V.
SO - half (at least) of what I said above is WRONG.

The LC circuit is providing a frequency shaped bypass of the cathode bias resistor combination (1K pot parallel 15K parallel 15K = 882 Ohms). The pot sets the amount of that bypass which is dialed in.

There is a second action which is a voltage division of the NFB voltage.

The frequency vs impedance slope is then set by the source impedance.

At very low frequencies (way below the 5191Hz minimum impedance at resonance) and at very high frequencies (way above the 5191 Hz resonant frequency) there will be little or no bypass action, and hence little or no boost regardless of the pot setting.

Max. boost is at the 5192Hz resonant frequency.

The gain for that stage with an unbypassed cathode is about 40 rising to 58 when fully bypassed. This moderate gain change suggests to me that the feedback voltage division is a significant part of the circuits action.

Sorry for any confusion caused.
Ian

[matt h]

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