High and Low Gain Inputs in an Amp: How to Use Them?

A lot of amps these days only have one input jack but some of the most famous amps have two. 

Most people always plug into the first one and never think twice about it. 

But there is a difference between the two even if the instrument’s control panel doesn’t explicitly say so.

Amplifiers with two inputs offer a normal input and an input with lower gain to offer more headroom for “hot” input signals if you have high gain pickups or are using pedals. Although you can plug in two instruments, it isn’t common as the results are usually lackluster.

For the purpose of this article, I am not referring to amps with stereo inputs that have two separate inputs corresponding to separate channels (if there is a vibrato channel it also has two inputs of its own). 

Rather, the amps in question have two inputs both feeding into the same channel. So what can you do with them?

Why do guitar amps have different kinds of inputs?

When I think of dual inputs I usually think of vintage and vintage-reissue Fender amps. 

Having two jacks allows you to drive the preamp in different ways to get slight tone differences and it looks cool for marketing I guess. 

But I think that the real reason for adding a low gain input is related to the music scene when these amps were first made. 

Basically when early guitar amplifiers were being designed, playing with overdrive or distortion wasn’t a “thing” yet. 

That didn’t come until later as players purposefully caused their amps to break up and the amp companies then went on to embrace the new style. 

So the high impedance / low gain option made more sense back when (Fender thought) only clean tones were sought after.

What is the difference between low and high gain inputs in an amp?

On most amps such as Fenders, resistors are used to pad the input of the second jack (Instrument 2 / Low Gain / High Impedance). 

The first jack was a regular one and the second reduced the input by six decibels. If both input jacks are used at once, I believe the 6dB padding is split between the two signals. 

There are also some other variations like Marshalls where resistors were not used but the second input jack skipped the first gain stage of the preamp. 

The exact drop in decibels would depend on the amp’s design in this case but the idea is the same; the preamp will not distort as easily with the second input. 

Schematics for the input sections of a Fender Blues Jr. (one input) and a Fender ‘65 Deluxe Reverb (two inputs) are shown below if you’re curious.

When should you use the low gain input on an amp?

The low gain input is useful if you have hot, active, or even just humbucker pickups and you want to maintain a clean tone. 

It is also for hot signals coming from effects boards or pedals or whatever else you are brave enough to plug into it. 

While Fender labeled the jacks as Instrument 1 and 2 most people don’t plug two instruments into their amp at the same time

To clear this up once and for all, here is what the user manual for the Fender ‘65 Deluxe Reverb says:

“INPUT 1 – Plug-in connection for instruments. INPUT 2 – Plug-in connection for higher output instruments (see NOTES). ——– Both INPUTS “2” (Normal & Vibrato) provide LESS gain (-6dB) than INPUTS “1” (useful for high output and pre-amplified instruments).”

So that’s the official explanation these days and I’m not going to argue with Fender.

When should you use the high gain input on an amp?

Most players always use the high gain jack on their amp, especially for “normal” humbuckers and single-coil pickups, which don’t output as much as humbuckers. 

These days players usually like to break up the amp at least a little so there’s not much reason to use the low gain jack.

What happens if you use the wrong input on an amp?

Well, if you plug a guitar with weak single-coil pickups into the -6dB jack you may not get the juicy tone you want but there’s certainly no reason that you can’t do it. 

But if you are running active pickups through a lot of gain pedals or an effects unit and then going into the normal input jack you could experience some clipping or unwanted distortion that will ruin your experience. 

This is where the padded input jack might be useful, so give it some love.

Can you damage an amp if you use the wrong input?

A guitar amp should be able to handle any instrument-level signal that you throw at it, regardless of whether you are using the high or low impedance input. 

Within reason… that doesn’t mean the speaker will play nicely if you plug a bass guitar into an amp designed for guitar. 

And there is a big difference between instrument-level and line-level signals so just because you have a padded input doesn’t give you a license to start plugging in line-level outputs. 

I’m not saying it can’t be done, provided the volume of the signal is low enough, but you have to understand the risk and accept the responsibility if you smoke something in your preamp.

Why do modern amps only have one input?

These days most modern amps only offer one input unless they are trying to mimic the classic amps of the past for nostalgia reasons.

I mean if most people only use the first jack anyway why not save a little money on an extra jack and a resistor, plus some space on the control panel. 

Really, there are a lot of ways to temper your guitar’s output in the signal path before it gets to your amp: an equalizer pedal, a volume pedal, the master level controls on pedals and effects processors, and of course the volume pots on your guitar. 

Yeah, two inputs with different gain levels might be a cool idea but you can basically achieve the same thing by other methods.

Are there any benefits of an amp having separate high and low gain inputs?

So I think it’s safe to say that two inputs aren’t necessary for amplifiers since we can adjust the instrument level in other ways. 

And are you and a friend really going to play through the same amp? Two guitar signals into the same amp channel tend to fight each other for frequencies and nobody wins. 

There is something cool that you can do with that other input though that I’ve saved for last. 

If you like to mess around at home (and pretend you’re playing to a sold-out crowd or something) you can have your guitar plugged into one input and plug a microphone into the other. 

You will need a special adapter that converts mic XLR to ¼” and does a little magic to the impedance output. One of these can be purchased for under $20. 

Of course, you will need a dynamic microphone as well because it has low output and doesn’t require phantom power like a higher output condenser mic. 

A Shure SM58 is the classic choice. 

Now you can play and sing through your dual-input amp, on the clean channel of course. 

I’m not saying it sounds fantastic or anything but I’ve done it when I had a big combo amp with two inputs and I had some fun with it. 

Besides if you’re the only person in the audience and you’re having a good time, who cares what anyone else thinks?