Gain Staging

[DunedinDragon]

It's my impression from several of the posts on this forum that many of the members here are unclear about the use of gain and what's referred to as "gain staging", which has lead to some bad experiences in connecting their PODs to either amps, FRFR speakers, and/or PA's.  So just to add some clarity here's a rundown of what gain staging is about.

 

In the case of all three situations guitar amps, FRFR speakers and PAs, the raw signal coming in goes through a pre-amp stage to get the signal up to the level where it can be amplified.  It's probably easier to understand the process looking at it from a traditional mixing board perspective.

 

At the top of each channel on a mixing board there is a knob typically labeled "gain".  During sound checks the sound engineer uses this knob along with a button on the channel sometimes labelled PFL for pre-fader level/listen, to maximize the signal strength of that signal so that it's at a maximum setting without overdriving it which would cause it to potentially clip or distort.  With the PFL button engaged they watch the signal lights on the meter to get it up as high as they can while leaving plenty of headroom for temporary spikes in the intensity of the signal (such as when a singer hits a high note with lots of power).  This is separate and apart from adjusting the ultimate volume output each channel on the PA which is controlled by the faders on each channel.

 

Guitar amps do the same thing using the Gain knob or Drive knob.  However, guitar amps don't have the PFL function so you have to pretty much do it by ear by listening to the output to make sure you have a very clean signal with no breakup or distortion.  For some guitar amps this is with the gain set to 0.  Others have a bit more play to them.  The volume knob on the amp acts just like the fader on a mixing board to increase/decrease the overall volume output measured in SPL or Sound Pressure Levels.

 

Typically most, but not all, modern FRFR speakers come from the factory with their gain levels preset to appropriate levels.  But they do make the assumption that the line input level on unbalanced signals will be at a pre-specified level of -10dBV, which is not the case on guitar amplifiers.  That's why it's important to set the unbalanced output line level switch to either line or amp depending on the output device.  The same would be true of going into an audio recording device.

 

What's important in all of these cases as it pertains to the POD is that gain and volume are two very different things, and if gain is out of whack in your final signal chain you're not going to get the results you're looking for from the models and effects on the POD.  You have to bear in mind the POD is MODELING distortion, not actually creating distortion by way of overdriving a preamp as on a guitar amp.  Therefore if gain is not correct you'll likely be adding clipping or distortion on top of the modeled distortion coming from the POD, and your ears won't like it.

 

I know this can be a very broad subject from a technical perspective, but I've tried to keep it as simple as possible so as not to confuse anyone but provide an accurate perspective in how to connect their PODS in various situations.  If i've missed something feel free to add on.

 

 

[BillBee]

Pretty nice my man. I wonder if this is why some folks run into issues going into the front end of an amp.

 

All hail the "master volume" knob!

 

-B

[Old-Rocker]

Thanks,DunedinDragon,as always very informative & easily understandable.

[BBD_123]

A commenter on another thread was having volume and distortion problems with the JC-120 / Jazz Rivet which may have stemmed from gain staging.

 

Just as an example:

 

The Jazz Rivet does seem to be prone to input clipping, even with not-silly-hot humbuckers. Setting input impedance to 22K stopped the clipping, but with Drive at 30% and Channel Vol at 45% (to prevent any post-amp EQs in the chain from clipping) the patch was quiet. So I added gain with a Vetta Juice in front of the Mixer block and maxed the Mixer gain. This produced a very clean tone that was loud enough to be used alongside high gain patches in a setlist.

 

I've posted the example on Custom Tone:

 

http://line6.com/customtone/tone/1811263/

[DunedinDragon]

I've used the Jazz Rivet on a number of my patches and have never experienced any clipping even with the model volume up at 75%.  It's actually been my go to model for jazz and clean BB King style blues.  If you're referring to the global Inputs parameter for Guitar In-Z I have mine set for Auto which should let the POD manage the input impedence correctly.  But then again I'm not using super hot pickups which could make a difference I suppose.

 

I've made no changes to anything on the POD when using the Jazz Rivet other than to configure the amp model EQ, Drive, and Volume.  It's output is not terribly different than many of the other models on the POD and is easily brought up with either the mixer block volumes or a Studio EQ toward the end of the chain.

[BBD_123]

>But then again I'm not using super hot pickups which could make a difference I suppose.

 

Nor am I. Using the patch linked above, 22K with amp Drive at 30% is clean and defined while Auto is harsh and beginning to clip.

[pfsmith0]

Just a point of clarification: setting the input to impedance to AUTO does not let the POD manage input impedance "correctly". It just sets the impedance to that of whatever is in first FX block. So as you change patches with different 1st FX you may get different tonal characteristics. It introduces another set of perhaps unintended variability. If someone is having a hard time dialing in their tone they may benefit from setting the input impedance to a constant value. Higher value = harder/brighter.Lower value = more polite/darker.