Pre-amp & power amp adjustments

[tcube]

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.
OR
2. Max the power amp's gain and control instrument level w pre amp gain.
 
Thanks!

[elwoodblue]

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.

[sonicus]

Basic rules of gain structure for lowest distortion levels would be your option 2.

[jacko]

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.
 
Graeme

[jazzyvee]

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?
 
Jazzyvee

[tmimichael]

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...

[edwin]

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.

[sonicus]

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.  
 
http://www.sengpielaudio.com/calculator-db-volt.htm

[wideload]

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

[Bradley Young]

More speakers = more comb filtering, mostly.

[wideload]

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

[jazzyvee]

I have always set my basses with the internal trim pots set to max, (series and non series), and never have the master volumes on full. Would this over load the input to my F1-x even if the input gain on it was set low and thus feed an overdriven sound into the power amp section?

I have three bass heads, a Mesa boogie walkabout, PHB M-300 and F1-x; sf-2; and Crown XLS1502 rack. My cabs are Barefaced Big Twin II (1800w & 4ohms) and barefaced Big Baby 2 (800w & 8ohms) and what is happening with each rig and cabs in any combination or indivudually is the speaker starts farting out when I need more volume and bottom end.

These cabs give me much more bottom end than my previous powerhouse cabs. Anyway I contacted the manufacturer he is suggesting that my power amp could be distorting at low levels or something else in my signal chain is distorting, so I'm trying to work out where to start. I can imagine that being a the situation with my mesa boogie walkabout head as that is supposed to start breaking into a classic sound up after the 12 o'clock position on the input gain. However I wonder why that would happen with the PJB which I've had for about 18 months and is renown for being a high quality great clean head. and my rack system which is a clean power amp and alembic pre's.

BF also suggested that I should try the Crown amp in Bridge mode, if the distortion stops then it means that in standard mode the power amp is going into distortion/overload. If it continues to appear then it means there is a problem with the cabs.

Any views/suggestions?

[hieronymous]

Cool thread! I just have a lowly GK 1X12 combo but still good to hear the different viewpoints.

[tx196059]

Hey Jazzy, I had the same issue awhile back. On my Series instruments, I run the bridge through a Crown amp, and the neck through a QSC RMX (the really heavy one). The Crown is very clear, but with the digital switching, it somehow does not bring in the low end warmth that the QSC brought to the mix. As far as the preamps are concerned, I use to have them all the way up, and turned the volume down to. I then read somewhere, that the gain mid position is the starting point. Full up was best for headphone output, mid was best active output, and low was to match a passive output. So, I backed them off to about 60%, and the distortion went away. Maybe using the volume control just makes the distorted signal quieter, if that makes any sense. Hopefully some of the pros here can give you a better answer, but this worked for me. Cheers!

Joe

[rv_bass]

I set the internal gains on all my basses to 3/4 and have no distortion issues.  The exception are two of my basses (one series and one non-series) that seem to have lower overall output, I set the internal preamps on those to max and they do not distort.