Will the real presence circuit please stand up!

[roknroll]

Hi guys
I have collected a bunch of presence circuits from the Amp Garage and am totally confused. Which one is correct? Considering 50 non hrm or similar.
Regards
Jack[/img]

[heisthl]

Top Right

[ic-racer]

Bottom left also.

Confounding factors producing variability in the schematics are 4ohm OT vs. multi tap OT (which tap has the NFB?) and 50w OT vs 100w OT.

[roknroll]

Bottom left also.

Confounding factors producing variability in the schematics are 4ohm OT vs. multi tap OT (which tap has the NFB?) and 50w OT vs 100w OT.

Thanks for the replies.
On the pencil drawing I indicated 50w on most and I think there is one 100w. I always build with multi tap OT. On a multi tap, which tap should have the NFB? The Ceriatone Overtone Special has a multi tap and taps off of 4 ohms and uses 4.7k nfb resistor on a 50w amp. Other well documented schems have 8.2k, 6.?k, 4k7 for 50w amps.
Regards

[schorsch]

each one of the above circuits works, but which one you choose depends on the OT you use, the speaker you use and finally what your ears love the most. I would suggest, choose the version with the 390R/1uF combination at the 4 Ohm tap of the OT as suggested and then using a pot with for example 10k in place of the NFB-Resistor and trim it for your needs. Let your ears decide!

kind regards
George

[Structo]

I've read on here that some guys like the NFB to go to the 8 ohm tap.

So I guess it's one of those adjust to taste type thing.

The idea of subbing a 10K pot for the 4.7K resistor sounds good to dial in what you like.

[GatesofDawn67]

Also the upper left and the one directly to the right of it are identical from an electrical point of view. The one on the right is the best practice. Connecting the wiper to the other tap prevents an open circuit if the wiper fails.

The upper right and the lower left only differ in the type of cap used. Tantalum's are the most notorious for distortion. Since they are used in the Tube Screamer's tone control, perhaps it is good distortion.

The Music Man and the Danble do not show the series resistor so they are hard to compare.

Since the 1k is 2.5 x the 390, the 8k is just under 2 x 4.7k, and the 1uF is 2 x 0.47, they all will sound similar. Assuming of course other components connected to this circuit are not affected by the impedance differences or load this ckt down.

[talbany]

FWIW.. Dumble amps wired presence pot acts as a variable resistor.. (Like a bass control on Fender).. just varies the internal resistance of the pot.. (Top to wiper)....The series feedback resistor, in conjunction with the resistor to ground, determines the amount of voltage being fed back.... If you want to feed back more voltage, you make the series resistor smaller, or the shunt resistor larger, or you use a higher impedance tap on the output transformer......You can get the same attenuation from a 10K/500ohm pair as you would from a 100K/5K pair....if you were using a 100K/5K attenuator running from the 16 ohm tap, you would get roughly the same amount of feedback if you used a 47K/5K attenuator running from the 4 ohm tap.

Tony

[CHIP]

Bottom left also.

Confounding factors producing variability in the schematics are 4ohm OT vs. multi tap OT (which tap has the NFB?) and 50w OT vs 100w OT.

Thanks for the replies.
On the pencil drawing I indicated 50w on most and I think there is one 100w. I always build with multi tap OT. On a multi tap, which tap should have the NFB? The Ceriatone Overtone Special has a multi tap and taps off of 4 ohms and uses 4.7k nfb resistor on a 50w amp. Other well documented schems have 8.2k, 6.?k, 4k7 for 50w amps.
Regards

So if I hook up the NFB to the 4 ohm tap on a multi tap OT and then switch the multi tap switch to the 16 ohm tap, (since I have a 16 ohm speaker), it's OK?

[crbowman]

Bottom left also.

Confounding factors producing variability in the schematics are 4ohm OT vs. multi tap OT (which tap has the NFB?) and 50w OT vs 100w OT.

Thanks for the replies.
On the pencil drawing I indicated 50w on most and I think there is one 100w. I always build with multi tap OT. On a multi tap, which tap should have the NFB? The Ceriatone Overtone Special has a multi tap and taps off of 4 ohms and uses 4.7k nfb resistor on a 50w amp. Other well documented schems have 8.2k, 6.?k, 4k7 for 50w amps.
Regards

So if I hook up the NFB to the 4 ohm tap on a multi tap OT and then switch the multi tap switch to the 16 ohm tap, (since I have a 16 ohm speaker), it's OK?

Correct.

[Structo]

Yes, no matter what you have the impedance switch set to or what speaker you have connected, the NFB is still connected to the 4 ohm tap.