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School me on power amp/pre-amp setups

#21
actually, not backpedalling at all. and if i were, so what? the truth is that i agreed with some of your points and i continue to disagree with some others.

several truths remain:

* dialing down the amp does indeed limit the amp's output -- it's why people do it
* closed loop gain may never change, but
* input attenuators do change the available voltage out by decreasing the amount of signal swing available for amplification
* if you never open up a power amp, then you've bought more amp than you need

> "An attenuator does nothing to the available voltage out."

I guess that statement would be true if you were talking about a load attenuation device, but in the context of this thread haven't we all been talking about input level attenuation?
 
#22
That's right. And, it depends on the preamp in question. The idea is to get tone and CORRECT volume. I understand that the power amp wants to run full out. But if the pre requires that the power amp be the volume control so be it.
 
#23
Downunderwonder said:
Sensitivity controls are rare.
Click to expand...

Although controls that are actually labelled as "sensitivity" may be quite rare, I'm thinking that the reality is that sensitivity controls are pretty common --

any amp that has an "input level" control or an "input attenuation" control is really a "sensitivity" control. the way that the "input level" control actually works is to progressively reduce the amplifier's input sensitivity as they're adjusted from their "full-on" (fully clockwise) position to their "full-off" (fully counterclockwise) position.

How, and at what level, the input level controls are set depends entirely upon how someone has configured their individual system. If a mixing console or an instrument preamp is quiet, then it makes sense to set the input level controls full-on, and to use the master gain controls at the console or preamp to vary the level of the drive signal. doing this is especially important if the system uses a crossover with variable output level controls for biamped hi/low frequency feeds. Doing this helps maintain the best signal-noise performance for the system. When the preamp or the associated signal chain is noisy then it makes sense to adjust the master gain control and adjust the amp's input level attenuators until the noise isn't objectionable. Finally, the input level controls can be used to balance levels if the system doesn't have separate output controls for different feeds to different amp/speaker combinations.

it all depends on how you set up your individual system.
 
#25
My Ampeg SVL-CL preamp likes to have the input gain and master up almost all the way to get a good rock tone so for my pre/power setup I leave the pre almost maxed and use the poweramp control to adjust volume . When I had my RBI preamp I did just the opposite , poweramp dimed and preamp master for stage volume . Here's a pic of my Ampeg pre , first and last knobs are input gain and master volume . You can see my setting for stage volume on the Crown . Using it this way also helps the FOH mix because it remains constant for the sound man while I can adjust the Crown for my cab without screwing him up .

011.jpg
 
#26
May be getting into a bit of vocabulary semantics here.....

An amplifier is a voltage multiplier. It takes whatever's coming in the front, multiplies it "X" times, and sends it out the back, no matter what.

If you want less going out the back, you reduce how much is coming in the front.

To the musician, that's another volume knob, regardless of the specifics of how it's controlling the volume.

It is important to understand though to have good knowledge of what is happening throughout your signal chain.
 
#27
DI turned off but split in stereo >

(1) Eden preamp (shapes tone) to 200 watt Eden power amp (Nemesis combo, the pre amp provides eq and gain, sends to the power amp to pump up to 200 watts, sends power to speakers and blasts your insides with vibration, all one amp/speaker fusion)>

(2) Orange Terror 500 watts of hybrid power. The preamp has a tube to sculp the tone and gain structure while then sent to the SS 500watts power amp side.>

So anything can be a preamp but not anything can be a power amp. Preamps are cheap these days just by using plugins on the computer. Power amps will always cost lots of money if you want 1000 watt custom cabinet 18 inch speakers and so forth.
 
#28
[/URL]
Downunderwonder said:
You're the one who wanted to distinguish between volume control and sensitivity control. An attenuator is an attenuator. It reduces the level of the signal going into the amp, that is all it does afaik.
Click to expand...

Not exactly:

Link Removed

That's a Crest CA series block diagram, most pro sound power amps are roughly the same.

QSC PLX2 owner's manual said:
Note! The GAIN controls do not adjust the power of the amplifier. They adjust its sensitivity to input signals.
Click to expand...

[Invalid or Expired Link Removed] has a nice tutorial on gain staging and how to set your power amp gain controls, with links to a few other interesting articles.
 
#29
This is how I ran my setup 6 years ago.
Maybe this might give you ideas!

Furman Power Conditioner
Sansamp RBI pre amp
dbx 215 dual 15 band graphic equalizer. 2 channel
dbx 266xl dual compressor/gate. 2 channel
QSC PLX 1602 dual power amp. 2 channel

This was my favorite when playing live!
I split the signal from the Sansamp.
Affected signal was sent to the left side of my rack which had drive/dirt and was feeding the top cab mids and treble into (Avatar 4x10).
Unaffected signal was sent to the right side of the rack which was low end bottom frequencies and was feeding the bottom cab (Avatar 4x10).

Each cab was getting 500 watts each.


I swapped all of this to save my back. Got tired of lugging 300lbs worth of gear.
I realize now I could have done without the power conditioner and compressor.
 

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#30
Passinwind said:
[/URL]

Not exactly:

Link Removed

That's a Crest CA series block diagram, most pro sound power amps are roughly the same.



[Invalid or Expired Link Removed] has a nice tutorial on gain staging and how to set your power amp gain controls, with links to a few other interesting articles.
Click to expand...
It looks like A+ A- is helping to sum the balanced signal, attenuator works on the summed signal, attenuator attenuates, what am I missing?
 
#31
As I understand it the power amp "volume" control is an attenuator or sensitivity adjustment.

I don't agree at all that unless you dime the power amp you have bought more amp than you need. As Will33 says, all the power amp does is multiply the input signal by X, it is also designed to produce it's maximum output when it receives a given input voltage, so it is pointless giving it more than it needs, all you will do is overdrive it.

If your preamp is capable of providing a much hotter clean signal than the amp needs to produce it's max output, then decreasing the sensitivity on the power amp to trim the incoming signal to it's max required means that the amp will still be 'dimed' regardless of how far the pot is round it's travel.

For my purposes, I set the attenuator on my Crown XLS to around 2 'o' clock. This means that when the gain on my pre is set for whichever bass I'm using I can dime the output on the pre and that's it...the maximum volume that this rig is capable of, increasing the poweramp sensitivity would just overdrive it.
 
#32
Downunderwonder said:
It looks like A+ A- is helping to sum the balanced signal, attenuator works on the summed signal, attenuator attenuates, what am I missing?
Click to expand...

There's a preamp stage in the power amp before the attenuator. You could turn the attenuator all the way off and still clip that preamp stage. So the attenuator is actually attenuating at an internediate stage of the power amp, not right at the input. ;)
 
#35
Thanks for all the very informative posts. Thanks for the posts that are so informative I cannot actually make sense out of them. :D

I think you guys have covered it, one more question for you guys though, I intend to build a rack to mount this stuff in to. My Carvin power amp is one rack space (1U?) should I make a rack that is two spaces or three? I only want to have a pre amp with my power amp, nothing else is in the plan at this moment, I want it to remain as portable as possible. Are there any 2 space pre-amps? Are they common enough to worry about?
 
#36
DiabolusInMusic said:
Thanks for all the very informative posts. Thanks for the posts that are so informative I cannot actually make sense out of them. :D

I think you guys have covered it, one more question for you guys though, I intend to build a rack to mount this stuff in to. My Carvin power amp is one rack space (1U?) should I make a rack that is two spaces or three? I only want to have a pre amp with my power amp, nothing else is in the plan at this moment, I want it to remain as portable as possible. Are there any 2 space pre-amps? Are they common enough to worry about?
Click to expand...

There are a few really nice 2RU units, like the Monique and JGR ones. Also, I've found 2RU rack cases to be a bit of a pain in that many of them don't balance all that well when placed on their side during load-in and load-out.
 
#39
DiabolusInMusic said:
Really? Well that is good to know. I assume most pre's are this same size then. I should make a 4U rack then.

Are there plans to make a rack on TB or should I just go over to the fEARful forum and pester somebody? I know some guys over there have built them.
Click to expand...

*Most* preamps are single space, but there are a few that are 2U.

I would go for a 4 space at the minimum.

You can try the other forum for plans, or maybe start a new thread here to attract the builders.
 
#40
A 4U will give you room for most any poweramp and most any preamp. If you happen to have 1U left over....fine. That could for future changes (like a 2U pre, or a compressor, eq, etc.) or just for better airflow/cooling. Or as Passinwind says, so it won't fall over when set on it's side.
 

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