Express and Liverpool - Differences in the Presence Circuit

[RJ Guitars]

I've been looking at the various schematics of the presence control used in the Liverpool builds and comparing them to the Express. I'm trying to understand why so many variations and the significance of them. It might seem a bit more intuitive if the basic schematic stayed the same and the component values shifted since the output sections are different. But he actually seems to move the order of components around...

I am seeing two items of interest and wonder if anyone can help me understand them:

1) Why a 5K pot on the Express and 25K on the Liverpool?

2) I see three different locations for the 0.1uF cap:
a. Before the Presence Pot
b. Across the Presence Pot
c. After the Presence Pot

Just curious if there would be any expected differences in the results if I used the Express Presence control on the Liverpool or vise versa?

rj

[Jana]

The Express circuit is the same as the early Marshalls. This circuit is, in my opinion, the best responding circuit. It does have one inherent flaw and that is the noise when the pot is turned. The reason is that there is a voltage on the pot which is stored in the presence cap. When the pot is turned, the voltage on the wiper is changing (the pot is a voltage divider). Many complain of the "noisy" pot. The pot is fine--it is just the way the circuit is that creates that scratchy sound on the pot.

Marshall probably got tired of people complaining of the "bad pots" in their amps and went to the type of circuit used in the liverpool except with the usual 5K pot. But, because of the way that circuit is wired--the 5K pot in that configuration no longer gives the same range of adjustment as the first version.

So, they changed the pot to 25K to get back the range of control that the noisy version had. Ponder the circuits and you will see why.

Not sure what you mean by before and after the presence pot. If you mean cap--pot--ground or pot--cap--ground, it makes no difference.

Disclaimer: My observations are those of a raving lunatic and should not be construed to be rational or connected to reality--my reality is one of my own creation and serves me just fine, thank you.

[Colossal]

The Express circuit is the same as the early Marshalls. This circuit is, in my opinion, the best responding circuit. It does have one inherent flaw and that is the noise when the pot is turned. The reason is that there is a voltage on the pot which is stored in the presence cap. When the pot is turned, the voltage on the wiper is changing (the pot is a voltage divider). Many complain of the "noisy" pot. The pot is fine--it is just the way the circuit is that creates that scratchy sound on the pot.

Jana,

You bring up an interesting point: the best sounding circuit.

I have the 4k7 || 25kB presence pot->0.1uF to ground in a Marshally test circuit and I find that there is really only about 10% of the upper end of the travel of the pot that is useful. I have not tried the older 5kB ->0.1uF to ground but I have heard other reports that this is the better sounding circuit (DC on the pot aside) of the two. I have always just deferred to the Marshall 1987 "upgraded" circuit which keeps DC off the presence pot. But I really need to try the old school 5kB pot and 0.1uF to ground off the wiper.

[martin manning]

The Express circuit is the same as the early Marshalls.

You mean it's the same as the 5F6-A Fender ;^)

Marshall probably got tired of people complaining of the "bad pots" in their amps and went to the type of circuit used in the liverpool except with the usual 5K pot. But, because of the way that circuit is wired--the 5K pot in that configuration no longer gives the same range of adjustment as the first version..

I didn't know this, but you have to ask, what were they thinkin'? You have to go from bypassing the tail resistor with the cap to not bypassing it, and using a 5k pot that way would make the impedance through that branch at most the same as that through the tail resistor. A 25k pot makes it at least 5x the tail resistance for all frequencies, and therefore essentially not bypassed.

The two TW circuits are essentially identical from the standpoint that they have the same FB ratio 5k/100k, and the same 0.1uF cap, which sets the same -3dB corner point when the presence is full on. The earlier (scratchy) circuit has better adjustability though, and that might be the origin of the "better sounding" reports. However, if you use a reverse-log pot in the later one it works even better.

[RJ Guitars]

Thanks for all the insight on this... it makes sense once someone points out the simplicity of it to me... but eventually calculus made sense too once a few professors explained it to me, but who in the heck figured that out to start with?? I doubt I would ever have gotten too far down the path figuring it out on my own in either case!

I am getting ready to build the next iteration of my PCB Trainwreck's and I like to start with the simplest basic circuit then mix in some of the cooler options once I verify that I have totally "wrecked" the circuit - so to speak. I want to use the Express presence circuit because it fits into my layout quite nicely. I have added the option to add Martin's "Presonance" control once I verify that I have the rest of the amp working correctly.

Out of several attempts I only have one Express that I am pleased with so far and particularly like the dynamic range of the presence control. If it was scratchy I forgot to notice... even then that is not a control that I typically reach for in the middle of a song so I think I could live with a little scratch on those occasions when I make a change.

rj