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previous: dave In the thread "Gain vs. distortion ... -- 2/13/2000 9:22 AM

Re: presence

2/15/2000 1:45 AM
SteveR	Re: presence Dave, The short answer is, yes. You could take the pot/cap combination and insert it into a local feedback loop. But it wouldn't behave the same way. It would be an active tone control. Some say "to-mait-oh" others, "to-mah-toe". Call it what you want because they are the same thing. 'Cept difr'nt. The Marshall presence is a control for rolling off highs from a global feedback loop. The whole NFB loop incorporates the driver/phase inverter, power tubes, and transformer. Each of these has its own limitations which will in some way affect the sound. NFB is a attempt to compensate for this and make the output more linear by extending and flattening the frequency response. Rolling off the highs from the feedback signal will allow the highs to be amplified without the damping factor of the feedback. They will become more prominent in the output signal and will be subject to some of the non-linearities of the whole power amp (PI/output tubes/transformer combination). Putting a tone control in a local NFB loop is essentially the same, but here there's only one device. And I think a single 12AX7 stage is very likely already much more linear and flat than a whole power amp section. I don't know the math here, but if we just look at complexity we have on the one hand a single triode stage taking some voltage at its grid and amplifying it to a greater voltage at the plate. This signal is attenuted a bit by a resistor and fed back to the grid (plate and grid are out of phase). On the other hand we have a driver/phase inverter which amplifies voltage, but also splits the signal into an additional out of phase component. Unfortunately the two signals usually aren't quite mirror copies. These two signals are then amplified by the power tubes to generate current. Power tubes (pentodes) have a different response curve than preamp (triode) stages. This high voltage, low amperage current is then transfered into low voltage, high amperage current through the transformer which can have a whole host of limitations. Then part of this signal is squeezed through a resistor to lower the amperage and is fed back, inverted, into the driver at the cathode. I think the idea behind putting a tone control in a local NFB loop is to provide gain boost at specified frequencies rather than the more trandition Marshall/Fender passive roll-off controls. This is generally though of as an active tone control. Here are a couple of references to people who know way more about this than me: http://www.aikenamps.com - Tech Info / Advanced / Designing for global negative feedback Designing single-stage inverting feedback amplifiers What is "negative feedback" http://www.duncanamps.com/technical/tonestack.html also look at the link for Baxandall Tone Stack SteveR

2/15/2000 1:45 AM

SteveR

Re: presence

Dave,

The short answer is, yes. You could take the pot/cap combination and insert it into a local feedback loop. But it wouldn't behave the same way. It would be an active tone control.

Some say "to-mait-oh" others, "to-mah-toe". Call it what you want because they are the same thing. 'Cept difr'nt.

The Marshall presence is a control for rolling off highs from a global feedback loop. The whole NFB loop incorporates the driver/phase inverter, power tubes, and transformer. Each of these has its own limitations which will in some way affect the sound. NFB is a attempt to compensate for this and make the output more linear by extending and flattening the frequency response. Rolling off the highs from the feedback signal will allow the highs to be amplified without the damping factor of the feedback. They will become more prominent in the output signal and will be subject to some of the non-linearities of the whole power amp (PI/output tubes/transformer combination).

Putting a tone control in a local NFB loop is essentially the same, but here there's only one device. And I think a single 12AX7 stage is very likely already much more linear and flat than a whole power amp section. I don't know the math here, but if we just look at complexity we have on the one hand a single triode stage taking some voltage at its grid and amplifying it to a greater voltage at the plate. This signal is attenuted a bit by a resistor and fed back to the grid (plate and grid are out of phase).

On the other hand we have a driver/phase inverter which amplifies voltage, but also splits the signal into an additional out of phase component. Unfortunately the two signals usually aren't quite mirror copies. These two signals are then amplified by the power tubes to generate current. Power tubes (pentodes) have a different response curve than preamp (triode) stages. This high voltage, low amperage current is then transfered into low voltage, high amperage current through the transformer which can have a whole host of limitations. Then part of this signal is squeezed through a resistor to lower the amperage and is fed back, inverted, into the driver at the cathode.

I think the idea behind putting a tone control in a local NFB loop is to provide gain boost at specified frequencies rather than the more trandition Marshall/Fender passive roll-off controls. This is generally though of as an active tone control.

Here are a couple of references to people who know way more about this than me:

http://www.aikenamps.com - Tech Info / Advanced /
Designing for global negative feedback
Designing single-stage inverting feedback amplifiers
What is "negative feedback"
http://www.duncanamps.com/technical/tonestack.html
also look at the link for Baxandall Tone Stack

SteveR