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Username: tcube

Post Number: 99
Registered: 8-2007
Posted on Thursday, December 22, 2011 - 2:41 pm:   Edit Post

I use an F1X preamp and a QSC power amp to drive my speaker cabinet(s). How do you all set the pre and power amp gains? Two possibilities:

1. Max the pre amp gain and control instrument level w power amp gain.
2. Max the power amp's gain and control instrument level w pre amp gain.

Senior Member
Username: elwoodblue

Post Number: 1321
Registered: 6-2002
Posted on Thursday, December 22, 2011 - 4:15 pm:   Edit Post

option 2 gives you more headroom in the amp,
It will be easier on your speakers also,they don't like to see the square waves of a clipped signal.
Some times I know given the size of the room my power amp will be clean at levels lower than max.
Senior Member
Username: sonicus

Post Number: 2199
Registered: 5-2009
Posted on Thursday, December 22, 2011 - 11:37 pm:   Edit Post

Basic rules of gain structure for lowest distortion levels would be your option 2.
Senior Member
Username: jacko

Post Number: 3068
Registered: 10-2002
Posted on Friday, December 23, 2011 - 5:29 am:   Edit Post

I go for option 3. neither.

i set my QSC around 22 on channel 1 - (high frequencies) and 20 on channel 2 (low frequencies). Current pre is a fender tbp-1 but I can switch to an ampeg SVp or an F1-X depending on my mood. I set the input gain to just under clipping and control my stage volume with the pre-amp master gain. Volume on the bass is set to around 80% to give me a little room for surreptitious tweaking.
I find on the stages I play that if I max out the power amp, the pre will then be set so low it loses some 'drive' and starts to sound weedy.

Senior Member
Username: jazzyvee

Post Number: 2653
Registered: 6-2002
Posted on Friday, December 23, 2011 - 9:20 am:   Edit Post

I have often wondered the best way to use the F1x and QSC to get the best sound quality at any volume. My concern with option 2 would be, is there more chance of blowing the speakers if you dig in than the other options? Is there a limiter in the circuit somewhere?

Username: tmimichael

Post Number: 31
Registered: 3-2010
Posted on Friday, December 30, 2011 - 2:09 pm:   Edit Post

Here's an excerpt from a DBX manual (which is written concerning PA use), but it applies to all amplifiers.

"One thing that is critical to system setup is maximizing gain structure. Gain structure refers
to aligning the gain of each device so that they all clip at the same point, and the noise
floor of the entire system is at its absolute minimum. Quite often PA systems are setup with
the amplifier input controls turned all the way up in the incorrect assumption that this is the
only way to get the maximum output level. Amplifiers are fixed gain devices, turning down
the amplifier input attenuators does not change the potential output of the amplifier; it only
requires more input voltage to get full output power. Many amplifiers will clip with an input
level greater than +6 dBu when the input attenuators are turned all the way up. Most mixing
consoles can deliver over +18 dBu of output level before clipping. This means that with your
amps tuned all the way up you are sacrificing 12 dB of headroom, resulting in poorer noise
performance and the potential of system clipping. By adjusting the amplifier controls properly,
you can maximize your system performance."

SO, I run the controls on my CREST CA9 at about half way up...
Senior Member
Username: edwin

Post Number: 1092
Registered: 5-2002
Posted on Saturday, December 31, 2011 - 12:44 pm:   Edit Post

I try to set my gains so my preamp (Eden Navigator most of the time) is just about reaching clipping when when I am as loud as I possibly want to be. For the Crown XTi4000, that means that the gain on the power amp is usually from 11 o'clock to 1 o'clock. That usually gets the peak output of the amp at around -5 to -3.

A lot of preamps don't have an output level indicator, so it can be a question of trial and error when it comes to finding the right balance, but you don't really want to clip the input of the amp, as the dbx excerpt states. When using my F2B, it's a similar situation.
Senior Member
Username: sonicus

Post Number: 2221
Registered: 5-2009
Posted on Saturday, December 31, 2011 - 1:31 pm:   Edit Post

In finite engineering terms it comes down to user awareness of the output voltage of the final stage of your signal chain going to the input stage of the power amplifier and user awareness of the input sensitivity of the power amplifier. These parameters will of course differ with different equipment manufacturers and model designation. The latter is true from device to device in the signal chain . Basic Gain structure awareness from device to device will keep your distortion down.
Intermediate Member
Username: wideload

Post Number: 182
Registered: 6-2003
Posted on Wednesday, January 04, 2012 - 4:15 pm:   Edit Post

Get the sound you want from the pre, and the volume you need from the power amp. And you always NEED more speakers!
Senior Member
Username: byoung

Post Number: 1324
Registered: 12-2004
Posted on Thursday, January 05, 2012 - 2:27 pm:   Edit Post

More speakers = more comb filtering, mostly.
Intermediate Member
Username: wideload

Post Number: 183
Registered: 6-2003
Posted on Friday, January 06, 2012 - 2:47 pm:   Edit Post

Keep the speakers in a vertical array and you're fine.

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