Dumb question about channel masters on MK IV?

[fastfrets]

This may be a dumb question, but here goes...

Do the individual channel masters on the MK IV have a unity point? In other words, is there a setting at which the preamp will be feeding the power section a signal level that would be the same as if the channel had no master at all? I've always assumed its the mid-point, ie "5"..but perhaps its "10" and the controls are used to decrease the signal in order to balance each channel...? The preamp section often "comes to life" for me after the midway point....

hope that makes sense... anyone venture a guess?

 

[marklyn]

Wow, I never thought of it like this at all.

I've always been working under the idea that it's all additive, but just different amounts. In other words, there is no unity gain and the master contributes to shape and futher boost the existing signal.

I think I would like there to be unity gain, however.

I have rarely used the master past 5 but I plan on trying it out.

Thanks for the idea!

 

[mr_fender]

The master volume controls on many amplifiers are passive voltage divider circuits. That means they are purely subtractive. At the low end of the pot (0) all of the signal is shunted off to ground and none passes on to the rest of the signal chain. At the high end of the pot (10... or 11 for you Spinal Tap fans), none of the signal is shunted to ground and it all passes freely on to the next part of the signal chain. The pot allows you to fade this value smoothly between off and full blast. The only way to make an additive volume control would be to add another tube stage and make it an active circuit. This would be more like a boost control or another gain stage rather than a volume control. In fact this is exactly what is going on in Boogie's cascading gain circuits - one active gain stage fed into another active gain stage. Add volume controls to each stage and you get... lead gain and lead drive (or volume I and volume II for you Mark I lovers)

On the another hand, the Master volume control on many modern Boogies is often wired in as part of the FX loop (acts as loop return level and bypassing the FX loop takes away your Solo and Output controls). Since the FX loop is actively buffered with tube gain stages, it is quite possible that the Master output control on these amps are active and could be boosting the signal at some point in the control. One would have to scour over the schematics of these beasts to know for sure. Perhaps someone here on the forum has some soldering iron experience with these amps and could shed some light on this subject.

Just my 1.5 cents worth.

 

[phantomlord2oo]

Finally, someone answered why my Marshall JCM800 is so friggin' loud!

+Mike+

 

[fastfrets]

Thanks mr fender....that helps me out. I am curious whether or not the master controls are active.... I guess that is the hardest thing about MK IV in particular; the controls are so interactive, often with many gain stages, that its difficult to methodically approach amp settings. I've had the amp 10 years, and am still figuring it out.... knowing boogies a little, I suspect they are probably active, but it would be good to know for sure...the preamp level hitting the power amp, and thus the power amp level is an important part of getting sounds out of this amp...

For low volume playing from this amp, I often crank the channel master on which ever channel I'm using, in order that most of my tone comes from the preamp. It seems to work in situations where I know I'm not going to get power tube saturation.... for high volume playing, I often crank the amp master at the power section, and work in reverse (including eq)...using the channel master to dial in tube saturation...

thats my cents worth....