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Vintage threads from the first ten years

presence

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2/9/2000 9:05 AM
dave	presence Hi everyone! Anyone have a particular favourite presence design?

2/9/2000 8:17 PM

The one from the reissue Bassman is pretty good. There is no DC voltage across the presence pot, so it doesn't get scratchy over time. The Marshall presence control has a few volts across it, so the pots get noisy and need to be replaced.

Another consideration is how much negative feedback is applied in the circuit. Negative feedback is required to make the presence control work, with more feedback making the control work "better". However, too much negative feedback hurts the basic sound of the amp. I've found that about -4 to -6 dB of negative feedback is enough to have a useable presence control without compromising the sound of the amp (my own opinion, of course).

2/10/2000 7:31 PM
dave	Does that mean no NFB, no presence with any design?

2/10/2000 10:42 PM
Don Symes	You gotta have NFB _somewhere_ in the amp to get a presence control... who says it HAS to be in the power amp? I think.

2/11/2000 12:57 PM
GFR	Some MESA boogie amps have a simple cap + pot to ground (like in a guitar tone control) labelled as a "presence" control.

2/11/2000 6:04 PM

Over what frequency range is the Presence control effective in the typical Marshall or bassman circuit?

Back in the early days of hifi, a Presence control was a midband boost, usually centered near 1khz, which was where the female voice is prominent. It was used to effectively bring the singer's voice forward, closer to the listener than the rest of the band or orchestra.

I have a circuit in an old audio projects book for a Presence Control, which is basically a passive, attenuative, bandpass circuit, with a maximum 6db boost. (The total signal gets attenuated 6db, in order to be able to raise just the midband up to 6db.) It's effect is to bring the singer's voice apparently forward up to 1/2 the distance between the listener and the background sound source. I suppose the corner frequency and the Q of the circuit could be modified to work in a guitar amp, after knowing what that frequency might be.

2/12/2000 12:54 AM
moocow	I looked at one of my old PSpice reissue Bassman simulation runs, and with the presence control set to '12', it shows a total boost of 7.9 dB for the presence control. The +3 dB frequency is 500 Hz, and the treble response flattens out at about 4 kHz.

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