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Gain Staging

by DAR (Forum Member)

This page gives you insight on what Gain Staging is and how to use it.

 


"I decided to do this in another topic to sort of separate it from the philosophical take on gain staging.
There are several definitions that we need to start with in order to truly understand the topic of gain staging and what we're trying to accomplish when we do this.

1. Signal to Noise Ratio. That's the first step. Signal to Noise Ratio is, simply, the level of the signal with respect to the level of noise. It's also expressed as S/N.

2.The --dynamic range--; how wide a range of signals is the ear is capable of distinguishing of the human ear is 120dB from the lowest volume you can hear to the highest volume you can put up with at the threshold of pain.

3. The ambient environment, in a quiet studio is about 40dB SPL*. Most gear, because it's analog, has an S/N of 85dB, or higher, usually in a single piece of gear. That said, you're already left w/ a 45dB S/N as a result of the environment and a piece of gear IF you mic doesn't add any noise. Better audio equipment will run something like 92-95dB of S/N giving you a bit more S/N, overall.

*dB SPL is not directly related to dB in your hardware. dB SPL is the magnitude/power of the signal in the air, relative to a standard. dB in your hardware is the magnitude of an electrical voltage signal relative to a reference level. Like dBV usually, being dB relative to a 1 Volt signal. So -6dBV is 1/2 a volt, and -12 dBV is 1/4 a volt and so on. So if I scale my recording hardware (mic, preamp, etc) so that the 40 dB SPL noise floor in the studio corresponds to the noise floor of my hardware, I can still take advantage of the full SNR of my hardware, and maintain the full 85-100 dB that my gear is capable of. This is just another place where you can use gain scaling to increase your SNR. Of course then, you really have to (get to??) crank up the amp to get better SNR. If it weren't so, there would be no reason to use recorders with low SNR.

S/N is an AVERAGE measurement. That means we take the signal power and average it over some period of time. That "levels" the peaks and "raises" the values to some relative value that represents the changes over time.

4. The next thing to understand is the concept of headroom. The maximum gain a device can provide to you, before distortion, is what we refer to as "peak power handling" or "peak power output". It is during these periods that the actual peak power, with respect to noise, is MUCH greater than the rated S/N (a good thing).

picture below shows the relationship between dynamic range, SNR and headroom:

The difference between the signal power average (S/N) and the peak power handing is called "headroom".

The more headroom you have (i.e. the signal power average, or S/N) the lower your S/N. The less headroom you have the less headroom you have and the more you have to control your overall signal levels.

Hopefully, that explanation is slightly clearer than mud!

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So... moving on, then. The idea of gain staging is to start with your most critical component in the signal chain and get it adjusted so that you have the best tradeoff between S/N and headroom. If you're playing classical music you have to create more headroom than S/N. If you're dealing with rock, country and modern music you can lean more toward S/N than headroom. By the time a CD reaches your ears on a car stereo there's less then 5dB of dynamic range left on it... 5dB doesn't require much meter travel.

Gain staging is, simply, this: The creation of the proper balance between S/N and headroom such that all musical peaks can be handled without changing the character of the original signal while maintaining the maximum S/N allowed by the equipment.

Additionally, the settings of each parameter of each device in the signal chain is set such the the original tone is altered only by that device and not at any other point in the chain, when the device is enabled or disabled. (unless, of course, that device is designed to modify the behavior of other devices in the chain - i.e. the use of a Tube Screamer to provide extra input gain but not distortion). To explain, when you turn on your compressor there should be no apparent level change (especially if measured) it should just smooth the peaks and bring up the volume of the lows. When you add distortion (via Ratt, or such) you add distortion, not volume... when you add Delay you don't increase the overall low-end level, or high-end level beyond what the amp is currently producing, you tone those down and "balance" the level of the delay, etc.

To achieve this on your Line 6 amp (orginally posted POD XT).

1) Choose an amp/cab/mic/room sound that you're going to want to work with. Set the channel volume, output volume, gain and tone controls such that when you play the guitar the loudest you're going to play it doesn't get overly compressed or "saggy"... and when you play the softest that you'll end up playing you still have plenty of good clean tone coming out of the thing. Tweak... tweak... tweak at this point because this becomes the fundamental building block of the entire tone you're creating.

2) If you need to "smooth" your playing out then you'll add a compressor to the signal chain. This can either be pre-amp or post-amp/cabinet modeling. So, first, remember (or measure) the output level of the amp/cab/mic model. Add the compressor. With the same signal input level, then, adjust the compressor gain, threshold, attack, compression and release controls such that when the compressor is on, or off, you end up w/ the same average signal level. Once you've done this, you've successfully gain-staged the compressor into the circuit and it's addition, or subtraction, will not produce wide variations in level...

3) Need some distortion... grab your distortion pedal and plug it in. Leave the compressor off, at this point. Tweak the gain and tone knobs to get the overall tone out of the thing that you want. Remember, distortion adds a TON of high frequency harmonics so make sure that you A/B the sounds to be sure you're not adding too much high end to the basic tone (unless, again, that's what you want). Then... once you've the overall tone that you want tweak that distortion level to be about the same with the distortion on, as with it off.

If you're using this for lead, etc. then you can tweak the level to be about 3dB hotter than the original signal level IF you have the 3dB of headroom in your original amp setup. If you don't, you're going to have to revisit the original amp tone and tweak that down 3dB to get some headroom.

4) Add some chorus... Chorus, unless it's really used to effect the sound, is a "subtle" thing. So... 1) Tweak the chorus to get the overall "tone" you're looking for. Then tweak the "level" on the chorus so that adding it to the effects loop doesn't noticably change your levels.

At this point, double check the entire chain. Start out with the amp/cab/mic model. Then add the effects one at a time. You should notice that as you add the effects the output level should not be changing NOR should your basic tone (unless, of course, the basic tone changes by design with the addition of pedals). If things are changing dramatically go back and check each item individually till you find the one that's wreaking the sonic havoc. Once tweaked check them all, again.

5) Add some delay. Delay does two things when added, if you're not careful. It, of course, repeats a bunch of stuff. So, all those low-end noises your guitar makes when you palm-mute... well, the delay loves to repeat them over and over and over again... making your tone sound like mud. Thus, use the delay levels judiciously. It's VERY easy to run out of headroom, here, with this very useful and painful effect. You may spend a good amount of time, right here, just trying to find that right balance. You can do it! - Happy Gilmour.

6) Add reverb... ahhhhh... the last thing in the signal chain. Lush, deep, loud reverb sounds so awesome in the headphones... even with a nice wide stereo field and some good speakers, while you're playing alone. Put that in a mix and instant "washout"! So... tweak up the reverb just a hint, just so you can start to hear it without having it muck with the overall tonality... reverbs have a nice way of accentuating high-frequencies... so tone those down if you need to turn it up, a bit (remember - you can't get more level out of the thing when you turn this on)...

 

There you have it... gain staging in a nutshell. It's not too terribly hard.

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Couple of things... in the real, rather than the fairy tale world of the Line 6 Amp (original text POD XT), effects actually create a great deal of noise. Here's a quick rule of thumb.

The "louder" the input signal to a device the better the S/N will be. Conversely, the less headroom you're going to have. In noisy environments (such as the one created by vintage gear - hums, hisses, buzzes, crackles, pops and various other treats) it's best to try to crank out the most volume you can w/o changing your basic tone and compromising the basic dynamic range of the music you're trying to perform. (remember to always wear your ear plugs when doing this).

The louder things get, the more you have to drop back on the extreme settings of the tone controls, especially the bass control. Everyone loves that knob at 10... however, remember, in the XT as in the real-world, the louder things get the louder, and more pronounced, all the bad things get. Just because you have headphones, or studio monitors, available and you're listending at a cool 85dB and protecting your hearing doesn't mean that your virtual Fender Twin isn't running at 125dB and suffering all of the evils of that intense volume level.

It still works the same in the XT as it does in the real world. Thus you really have to pay attention. The louder the volume gets, the more you have to back off on the tone controls, etc. to get the amp back into a resopnsive mode where it does what it's supposed to. There are some other posts where folks describe similar issues w/ real life amps, including Bogner, Fender and others. I, also, have experienced this first hand.

Have fun. Hope that helps...

Dar
 

Used with permission: original text by Dar.

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