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52 Posts
Discussion Starter · #1 ·
You know, too often I see lots of guys that have the misconception that gain=volume, when this is simply not the case. You will spot these guys as they tend to set the volume of each channel with the gain pots instead of the channel faders.

Gain in a live PA is signal strength. A microphone converts sound pressure into electrical signals that correspond with the frequencies/sounds picked up by the microphone element. These signals are sent through a mic cable into the mixer board. However, these signals are so weak in amplitude(again signal strength) that they need some sort of gain to increase the amplitude of the signal so that it can drive the gear down the signal chain. If the signal flow down the signal chain is not strong enough, the gear it runs through will be less effective and it will not be able to process the signal to its full capacity. This is because the weaker the gain in the signal, the less musical content it will contain for the gear to process.

With gain also comes higher signal to noise ratio. Signal to noise ratio(commonly referred to as S/N Ratio) is the ratio between signal content and noise content. All gear produces a small amount of "noise", which is the common "hiss" noise you hear in systems with no signal going through. The better the gear, the lower the amount of produced noise in the system. A signal to noise ratio of 1:1(1 part signal to 1 part noise) would mean that the level of signal and the level of noise are at an equal, which is definitely not desirable as it will "cloud up" the signal. However, the more you turn up the gain, the stronger the signal becomes and the higher the S/N ratio. A 90dB-100dB or higher S/N ratio(90dB-100dB of signal to 1dB of noise) is a much more desirable S/N ratio for optimum performance and superb clarity.

Also, with more gain comes more voltage "swing", or dynamic range. This is where the gain structure comes into play. On a mixer board, you'll see that all the faders are labeled in negative dBs, until you get about 3/4 of the way up on the fader travel, which will be a 0dB or a "U" mark. Mackie's tend to use the "U" mark, which stands for Unity Gain. In the law of Unity Gain, this law states that the only place in the signal path that the signal should receive ANY kind of gain is from the "Gain" or "Trim" control on the channel. If you push the fader past the 0 or U mark, the signal will then be receiving gain from the fader itself, which can overdrive/clip the main mix buss inside the board. On all mixers, with the channel faders set at the 0 or U mark, and with the Main Mix fader at the 0 or U mark, and with no boost from the channel EQ, whatever signal level is coming from the Gain or Trim control is what will be seen at the mixer's output and at all various points in the mixer signal flow path. With maximum possible gain coming from the trim control itself, this will feed each channel EQ with maximum signal, making the channel EQs MUCH more effective to where you won't need to boost at all from the channel EQ.

As a matter of fact, when I run sound, I take whichever bands I want to stand out more and leave them completely flat, then cut everything else around it. With max gain coming from the Gain or Trim control, your EQ cuts become MUCH more effective. For instance, on drum channels I typically leave the lows and highs flat. I then put in about 3-6dB of mid cut(depending on the drum), and as I'm listening to the drum I'm EQing, I sweep the frequency control until that drum sounds nice and punchy with plenty of attack definition and clarity(usually around 200-400Hz...each drum is different). With only EQ cut put into the EQ, I can now punch some more gain from the Gain or Trim control through the channel, allowing for an even more powerful signal.

Most gear down the line from the mixer will feature input and output controls. Most of these controls have a 0 or U setting. Set these at 0 or U and that same signal coming from the mixer will flow through each piece of gear from input to output while maintaining the same signal level it had coming from the Gain or Trim stage in the mixer.

The crossover, however, is a little different. The low band, since it typically only passes through frequencies from 40Hz to 100Hz, will have less signal energy than the high band as the high band is passing through a lot more frequencies than the low band, so you may need to punch up the low gain on the crossover so that the low band will clip at exactly the same time as the high band. On my crossover, the input and high bands are at 0dB and they both clip at the same time, but the low band required about 5dB of gain in order to clip at the same time with the high band and the input.

Next comes the setting of the amp's input attenuators. Another common misconception of these controls is that they somehow control the OUTPUT of the amp and if you don't max them out then you won't get the full power output from the amps. This is not the case at all. These controls simply control the input sensitivity of the amp. The input sensitivity rating specifies how much voltage is required at the input of the amp to get the output to swing to full clean power before clip. Most amps are rated anywhere from 1VRMS to 2VRMS with the attenuators maxed out. Well, most +4dBu mixers with the gains maxed out and all faders at unity usually put out around 9-10VRMS of signal. With the amp's input attenuators maxed out, you're gonna hit clip long before the mixer puts out its full rated output. Once the amp hits clip, you can't go anymore...PERIOD! So in effect you actually end up sacrificing a good 8 7-8 volts of signal strength from your mixer, which results in about an 18dB-20dB loss in dynamic range/headroom, which amounts to much less voltage swing from the amps. This is where the amp's input attenuators come in. By turning them down to the point where the amp will clip at exactly the same time as the mixer/house graphic/crossover, you can now get the full 9.8 volts of signal from your mixer down the signal chain and into the amps, resulting in max dynamic range/headroom and a much larger voltage swing from the amps, which results in a much more powerful, cleaner sounding system with lots of punch and clarity. By dialing down the input attenuators, the amps will STILL put out full will just take a much larger signal to get it to put out full rated power, which is what you want. A big high gain signal results in a signal with much more musical content for the gear to work with, making each piece of gear MUCH more effective, allowing you to get the most performance out of your gear.

Another one I see a lot of is guys that max their trims out, but then set the main mix fader at unity and control the overall mix volume with the channel faders. Again, you're limiting your system this way plus making it harder on yourself to control an overall mix volume. Typically what I do is I subgroup vocals to groupd 1/2 (1-Left 2-Right) while subgrouping drums to groups 3/4(3-Left 4-Right). I then set all vocal faders plus the vocal subgroup to the 0 or U mark while maxing out the gain on all vocal channels, which will give me max level from the vocals. This way if the vocals are getting drowned out, I know to pull the instrument channel faders down to better balance with the vocals. All drum channels I set to the 0 or U mark with max Gain/Trim, except the snare and overhead, which will almost always be too loud with the fader up that high, so I start with the snare and overhead faders halfway between full down and the 0 or U mark. I then set the overall volume of the drum mix with groups 3/4, which almost always end up about -5 to -6dB below the vocal subgroups. I then set the guitar and bass faders about halfway up. With max level coming from all channels, I can now control the overall mix volume with just one fader, the Main Mix fader, while maintaining the same balance as the volume changes throughout the night. This fader setup technique always starts me out pretty close to a good mix. Then as the band kicks off the night I fine tune the balance/overall volume, slight EQing of the vocals(almost always Low/Low Mid cut is needed for the vocals to cut through, then dial in the effects last. Usually I have the band dialed in by the end of the first song one shot go.

To Summarize:

Gain does not equal volume

The amp's input attenuators DO NOT control the amp's output level

Always max out all gain controls for the most clean and clear signal from the source

Set the loudest instruments in the mix's faders to 0 or U on the channel faders, then use the remaining faders to balance the rest of the instruments to those instruments

Control your overall mix volume with the Main Mix fader, not the channel faders

52 Posts
Discussion Starter · #3 ·
A few simple definition for tube amp........

Overdrive: Used to be called Distortion but that isn't descriptive enough. It is the idea of using one small amplifier (preamp) to push another one very hard, causing it to overdrive and distort. When done properly, it is controllable to the limit...

Distortion: The difference between what goes into an electronic device and what comes out.

Gain: The amount of voltage amplification in the preamp section of an amp. This voltage amplification ultimately drives the power tubes which do not add any more gain. The power tubes add current (power).

Saturation: A condition in which maximum current is reached and no more current can possibly flow.

Premium Member
1,911 Posts
superleadfixer said:
A few simple definition for tube amp........

Gain: The amount of voltage amplification in the preamp section of an amp. This voltage amplification ultimately drives the power tubes which do not add any more gain. The power tubes add current (power).
Some folks do refer to the power amp as a current gain stage, where a relatively small swing in input voltage (and current) results in a large current change through the power tube. As the input voltage increases, the current throught the power tube will decrease. If the signal is large enough, the output tube will eventually go into cut-off with no current flow. As the input signal decreases, the current through the power tube will increase up to the point where a maximum (saturation) point is met.

I really appreciate your original post...I printed it out for the next time I get suckered into doing sound for somebody.:D

52 Posts
Discussion Starter · #6 ·
My pleasure if this can help, I'm happy......

so to conclude on a more practical matter, setting proper gain structure:


One of the most overlooked yet most important things in a typical band's PA system is gain structure. Gain structure is the practice of matching the levels throughout your system so that:

1) Each device in the chain clips at exactly the same time

2) The only place the signal receives ANY kind of gain is at the trim(channel input gain) control

3) The signal level stays exactly the same as it is coming out of the trim control all the way to the power amps

In other words, if you have a +22dBu signal coming out of the trim control, a +22dBu signal will be seen at the amp's inputs. The ONLY place the signal should receive ANY kind of gain is from the trim control.

Again, another example...a +22dBu signal comes off the trim control. It should maintain this value through the channel EQ, the channel fader, subgroup fader, main mix fader, house EQ, crossover, and right to the amps. When the gain stays the same throughout the whole system, this is known as a "unity gain" system.

On each fader on most mixers, there will be a "0" or a "U" mark(most Mackie boards use the "U"). Setting the channel, subgroup and master faders to this mark will ensure unity gain through the mixer. In other words, with the fader set to the unity gain mark, whatever is coming from the trim control will appear at the outputs of the mixer.

On all PA gear, you have gear that is either +4dBu or -10dBv compatible. Some pieces of gear have a switch where you can switch it to either +4 or -10. This matches the inputs and outputs to the type of mixer you have. Most "consumer" gear is -10dBv while "pro" gear is +4dBu. In this tutorial we will be working with +4dBu gear.

Most +4dBu gear has an input and output rating of +22dBu. 0dBu=0.775 Volts, so +22dBu is roughly about 9.756 Volts. Mackie mixers actually have a +28dBu output rating, however that's just to give you another +6dBu of headroom to keep from clipping the mixer's output stages. The max output from each channel's trim control is actually +22dBu.

Power amplifiers, however, have an input sensitivity rating that is much lower than that. The input sensitivity rating on power amps is the minimum voltage required on the input to cause the amp's output to put out full clean power. Most power amps have an input sensitivity rating of about +3dBu-+8dBu. This is why power amplifiers typically have input attenuator controls on them, which are the volume knobs on the front panel. A VERY common misconception is that if you don't set the input attenuators all the way up that you won't get full output from your amps. However, power amps are fixed gain devices...they will put out full power no matter where the input attenuators are set, however, depending on where they're set, it will just take a much larger signal to push the amp to full power. Judging by this statement, we now know that all the input attenuators do is raise and lower the amp's input sensitivity. The input sens rating that is advertised in the power amp's user manual is the sensitivity of the amp's input with the attenuators fully clockwise(all the way up). This is why they're there. I mean, for example, let's say that your mixer is capable of putting out +22dBu of signal, yet your most sensitive amp clips at +4dBu at it's input with the attenuators fully up. Well, now you're sacrificing +18dBu of gain/headroom you could be getting out of your mixer because once your amp is at it's clip point...that's all you get. This is why the input attenuators are that you can match your amp's input sensitivity to your mixer/crossover.

The reason proper gain structure is important is so that we get max gain from the mixer, giving us more dynamic range(headroom) and a lower noise floor. Max gain from the mixer will increase your signal to noise ratio, giving you better seperation between instruments. It also makes your channel EQs MUCH more effective. I see a LOT of people use WAY too much boost from the EQ itself than from the gain control. If you flatten the EQ, then maximize the gain from the trim control, your EQ will be MUCH more effective to where on some instruments you may have to use cut more than boost, which enables you to get even more gain from the channel's trim control. Too much boost from the EQ rather than from the trim will cause phase distortion, which will cloud up, or muddy up your signal to where it almost sounds buried in the mix.

Following is a step-by-step procedure for setting proper gain structure. You will need a test CD with a pink noise track to do this. Pink noise is every frequency within the human hearing range played simultaneously(at the same time). You can download a full blown test CD featuring all kinds of test signals, including pink noise, frequency sweeps, left/right test(to make sure you wired the system so that left is left and right is right).

Test CD Download

Test CD Info/Track List Download

For this procedure, set all channel EQs flat, all channel Low Cut filters off and all faders all the way down:

1) Play the pink noise track through two channels. Pan one channel hard left and the other hard right.

2) Rotate the left channel's trim control clockwise until you see the channel's "Clip" or "Overload" indicator start to come on, then back off until the indicator JUST turns off. You are now at the max gain setting on the trim.

3) Repeat step 2 for the right channel.

4) Set both the left and the right channel's faders to the "0" or "U" mark. This puts the channel at unity gain and sends the full signal from the trim control to the main mix buss.

5) Make sure both the left and right channel's are assigned straight to the main mix buss.

6) Set the main mix fader at the "0" or "U" mark. Now the trim control's full signal will appear at the mixer's outputs. Run this signal through your crossover. Set your crossover's input and output controls to the point where the input/output clip indicators JUST start to come on, then back off until they JUST go out. If your crossover's max input rating is +22dBu(it should be), then it like the mixer should be JUST going into clip.

7) With the speakers disconnected and the amp's input attenuators fully counterclockwise, turn your mains/subs amps on.

Rotate each input attenuator clockwise until the amp's clip lights are JUST starting to come on.

*Note: Most amps that are bridged will only use Channel 1's input attenuator and Channel 2's will not be active.

9) Turn on your crossover's built in limiter and set the threshold to it's highest point. Then slowly lower the threshold to the point at which the amp's clip indicators JUST go out.

Your gain structure is now properly set, and your limiter will protect your amps from going into clip.

10) Turn off your amps and reconnect the speakers, then turn them back on.

11) Now play a music CD of your choice through the system, and listen. Almost always I find that when the mains amp is set to clip at exactly the same time as the subs amp, the mains are almost always too loud, so you'll have to turn the mains amp input attenuators down more to balance the mains to the subs. This will give you even more headroom on the mains amp.

Now when you sound check your signal sources (i.e. instruments)...have each performer play the loudest that they'll play during the performance. Making sure each channel's EQ is flat at this point, rotate the trim on each instrument channel until the clip light JUST starts to come on, then back off a hair. GET AS MUCH GAIN AS YOU POSSIBLY CAN FROM THE TRIM CONTROL!!! THIS IS VERY IMPORTANT!!! Bring the channel's fader up to 0 or U, then slowly bring the master fader up to performance volume. Listen to the instrument and apply EQ if necessary, then reset your trim as stated above. Do this for all instruments.

Set all channel/subgroup faders to the 0 or U mark, then slowly bring the main mix up to performance levels. Listen to the overall mix, if you need more of a certain instrument, bring the other instruments down around it. It's always helpful to subgroup drums and vocals since they take up more channels than guitars usually(especially drums). ALWAYS keep the lead vocal channel and the vocal subgroup faders at 0 or U to get the max signal from your vocal mix. Balance your backup vocalists to the lead vocalist. The drum subgroup faders will typically be about 5-10dB lower than vocals, depending on personal taste. Then bring the guitar and the bass channel faders up to balance with the drums and vocals.

Basically the key is to start with all channel/subgroup faders at 0 or U, then listen to the live mix, and bring down the faders on channels with instruments that stand out too much. Drums and lead vocal channels, with the exception of snare, hi hat and overhead channels will almost always stay at 0 or U, then balance snare, hi hat and overheads to the kit. Then slowly bring the guitars and bass into the mix until they blend with the drums and vocals well. You may have to EQ them slightly to make this happen.

I hope you guys find the above information useful.

Premium Member
79 Posts
Thanks Superlead for the post! Great one, I agree.

And by the way, I've just recived the NOS tubes and took off the JJ and all the modern crap. Man, it's another step further for the never ending quest for tone. I will never get back on those modern tubes again. Great improvement on my Komet. Thanks for everything your the best.

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