Presence Circuit and Shunt Resistor

[FourT6and2]

I'd like to discuss the pros/cons and theory behind each of these three presence circuits. Amp is a 50-watt JCM800. But focusing just on presence.
I'm asking because I'm familiar with Arrangement 1 and 2. In #1, the 5K presence pot acts as the end of the PI tail. In #2, a 25K pot is used and the presence cap is in series to ground, rather than parallel. And a 4K7 resistor is used to end the tail. But what about Arrangement #3? At first glance it appears to be a mistake. As though someone wired it up by accidentally combining the first two. But... I want to give the amp builder the benefit of the doubt and assume maybe I'm just not understanding something and that the builder did it intentionally.

Arrangement 1




Arrangement 2




Arrangement 3

[LOUDthud]

#3 reduces the amount of feedback, and thus the amount of Presence when the pot is dimed. It increases the closed loop gain. The pot will still be scratchy. You could get some of the Presence back by changing to the 16 Ohm tap.

[FourT6and2]

#3 reduces the amount of feedback, and thus the amount of Presence when the pot is dimed. It increases the closed loop gain. The pot will still be scratchy. You could get some of the Presence back by changing to the 16 Ohm tap.

Ok so essentially, it's simply reducing the overall range of the presence control from 5k to 4k7?

[LOUDthud]

Ok so essentially, it's simply reducing the overall range of the presence control from 5k to 4k7?

It's more like using a 2.5K pot.

[FourT6and2]

Alright thanks!

Seems like a weird choice then. Just stick with Arrangement 1 with 5K pot and you get the same result, but with more range if you want to dial it in?

But maybe the builder chose to do that to prevent the end user from making amp too bright and complaining.

[FourT6and2]

Actually, wait... as you said, it would reduce NFB too

[roberto]

I discussed the argument in another forum.
I hope to be ok to link it here to give some more points of discussion here, otherwise please moderators remove the link: http://www.slocloneforums.com/viewtopic ... 24&t=14225

[FourT6and2]

I discussed the argument in another forum.
I hope to be ok to link it here to give some more points of discussion here, otherwise please moderators remove the link: http://www.slocloneforums.com/viewtopic ... 24&t=14225

Thanks, I saw that. I'm having trouble visualizing which plots go with which actual circuit. I need a schematic to go with each one because I don't know what these mean:

- new 4k7 // 25k+100n: I guess this is Arrangement 2 (Marshall's 25K pot with 4k7 in parallel?)
- my 4k7 // 10k+100n; Same but with 10k pot instead of 25k?
- unused 4k7 // 5k+100n. What's this one mean? Unused 4k7?

[roberto]

I report the files here for other users too.

The 25k plus 100n presence pot that is in almost all post-80s amps works only on the very last part of the rotation.
I use 10k instead of 25k because it has a wider useful range (but I still prefer the old scratchy one).

[roberto]

Some more plots:

Old scratchy ones:
- 5k lin pot fully CW with sweep on 100n 220n 330n 470n 680n 1u;
- 5k lin pot with 680n;

Non-scratchy ones:
- 4k7 plus 5k lin pot with 680n;
- 4k7 plus 10k lin pot with 680n;
- 4k7 plus 25k lin pot with 680n;

[martin manning]

The 25k plus 100n presence pot that is in almost all post-80s amps works only on the very last part of the rotation.
I use 10k instead of 25k because it has a wider useful range (but I still prefer the old scratchy one).

I prefer the old scratchy version too. Besides the improved sweep, and the ability to eliminate the bypass capacitor completely, the older circuit brings the high frequency boost in from the high end, rather than across the range.

[pdf64]

I seem to prefer the original DC on pot version with NMV amps such as JTM45, whereas with an overdrive preamp 2204 based build, that just didn’t sound right, the later version with a 5k pot sounded more natural at every setting and had the best range of control.
Thanks for the plots demonstrating why that might be the case, roberto

[FourT6and2]

How are those all wired up? Cap in series or parallel?

[martin manning]

Old Scratchy is your Type 1, non-scratchy is your Type 2.