#1
I was at my mate's house the other day, he has a TSL and we were messing around with it, namely the 'Virtual Power Reduction' or VPR button. According to marshall this emulates a 25W amp, allowing you to get a cranked sound at lower volumes.

It was nearly midnight so we couldn't crank the amp, but VPR certainly made it sound quieter, and also warmer. Now being a guitar geek that I am I decided to look into how Marshall are doing this, and there's very little available. In the manual it says it achieves this between the phase inverter and the power valves, and in the schematic, http://www.drtube.com/schematics/marshall/tl10-60-02.pdf, there are some 680K resistors which can be switched in and out using what is described as 'CON12', which I am guessing goes to a switch for VBR. So, it reduces the level of the signal going to the valves - how does this achieve a more saturated sound?!

Another thing I was wondering about is the LED/LDR combination in the schematics. What does this do exactly, some form of compression?
#2
It's basically a crude version of a post phase inverter master volume. Basically, it lowers the amplitude of the two antiphase signals that come from the phase inverter. The signal that reaches the power tubes is lower, so the overall volume is lower. However, the signal that's going into the phase inverter is still high amplitude, so some of the characteristic PI distortion is kept at a lower volume. You don't get a perfect emulation of power tube distortion, but by overdriving the PI you get one part of it at a lower volume.
Edit:
The LDR/LED combos are probably vactrols. On that schematic, LDR3 looks like its controlling volume- when the LED turns on, the resistance drops, shunting the signal from V1A to ground, reducing or muting it. The signal that controls that LED comes from pin2, CON10- It's driven by a buffered darlington pair. Problem is, I can't see what controls it as the other schematics are discontinous and don't follow the same naming convention. Chances are it's used to redcuce the volume for one channel when you switch to another.
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Last edited by Wyld Stallyn at Jul 2, 2007,
#3
Ah I see, so for the most part it's a gimmick really.

Thank you kindly.