Need help modifying the amp for an instrument

Which video are you referring to and can you give us photos of the plugs you're referring to.

Oh, sorry abou that. I'm kinda new to this whole forum thing. The video I'm referring to is the usb-c microphone video.

The amp is highly optimised (designed) for the specific application. It would be easier (and cheaper) to design something from scratch or even buy a cheap Chinese import that's optimised for an instrument.

@marcdraco Where should I start if I want to design such a circuit? It would probably be easier to buy a Chinese import, but I've been interested in these kinds of projects for quite some time, so I want to try and design something, despite my lack of experience.

It's not that complicated actually, designing something from scratch is rarely necessary because so many things have already been designed and put into the public domain on the Internet. (Even what I'm doing with the microphone is largely the engineering side of it, getting these circuits to do something novel, like putting a high-quality impedance converter capable of driving a line level input [vs. a mic input] right at the head.)

Here's one from Rob Robinette that would do the trick, complete with his signal path and the explanation of what the various sections do.

Rob's using a "source follower" to reduce the relatively high impedance from the pickups down to a more manageable level and then feeds that into a much-loved and very reliable amplifier, the LM386 (datasheet with examples below).

If you need more power, you could replace the 386 with a small op-amp circuit with a gain of around 40dB (x100) and then use that to drive a Class-D amplifier module which you can get from many sellers on eBay and AliExpress. Such modules (you'll also need a power supply for them but these are also modular) are preferable because someone has already done the testing and there really is no point re-inventing a relatively complicated wheel. Even the modular designs "lean on" existing integrated circuits to do the heavy lifting.

Designing an amplifier around an op amp building block is child's play (you'll find any number of worked examples on the web) although in many cases you will need a "split rail" supply; again though, there are ways around this (often shifting the "ground reference") which are all well described on the web.

If you really want to get your hand's dirty, might I suggest you invest in "The Art of Electronics" by Horowitz and Hill, it's not cheap but the amount of information the authors have compiled is astonishing and it's explained, for the most part, without dropping into wordy and complex mathematics that it often unnecessary for existing building blocks. Even when you do need some help in that department there are numerous calculators and design aids on the web including tools like LTSpice, PSpice, QSpice (and quite a few other "spices") although some are commercial and quite costly, right the way up to Altium which is a complete workshop right down to the PCB design, assuming you have a cool $10,000 (and change) to throw at them!

LM386 Low Voltage Audio Power Amplifier datasheet (Rev. C) (ti.com)

@marcdraco I'm probably going to get that book, because most of what you said required me a fair bit of googling to understand. I probably need to get more educated on electronics before I attempt a project like this. But thank you for the help

My apologies for not being clearer with that explanation. It's difficult to judge exactly where everyone is, but Horowitz and Hill have done an amazing job cramming decades of experience and untold working examples into the book. 

One thing I did note with Rob's design is the "Zobel network" at the end, which is actually a simplified version called a Boucherot Cell - intended to match the speaker impedance and stop weird distortions caused by the complex (in mathematical terms) load. Resistance typically doesn't change with frequency (except at RF where things get weird) but it does vary with "complex" loads such as capacitors and inductors - and the coil in a speaker is an inductor.

Reactance is a complex number representing the resistance of something that varies its *resistance* with frequency. The reactance (effective resistance) of a capacitor gets lower as frequency increases. Inductors work in the opposing direction. You can imagine this as a capacitor has an infinite resistance at DC (frequency = 0) whereas an inductor has a resistance of 0 at DC because it's just a coil of wire. 

Pure resistance is termed R (or uses the ohm symbol) whereas reactance is shown as X (no connection to the social networking site formerly known as Twitter).

This is a bit simplified but it's enough to get you going and quite sufficient for a lot of work. Oftentimes we select these components based on their behaviour at a specific operating frequency using a fairly simply formula:

For a capacitor X = 1/2*pi*frequency.
For a coil X = 2 * pi * frequency.

Note that capacitive "X" is the inverse of inductive "X" 

This isn't really my area but it looks superfluous for a couple of reasons. Power amps where Boucherot Cell's are found (and Zobel is easier to spell) often have a large inductor at the output and are designed for a fairly tight range of speaker impedances (or often just one, because the design is in a cabinet). Rob's suggesting a very wide range of speaker impedance from 4 - 32 ohms and Zobels are intended to balanced the load impedance as "seen" by the amplifier (I would guess this was intended for 8-16 ohms originally).

The impedance (that's the sum of the resistance plus the reactance of the capacitor) here falls with increasing frequency but if you plug the numbers in, we're into radio frequencies (>300,000 Hz) before this starts to put any significant load on the amplifier. But there are other effects here which get into the realms of transmission lines (which is where Zobel's work was based) and why I question the reasoning behind putting one in since the length of the cable to the speaker is only going to be a few meters.

You'll probably find more accurate information online, it's been a while since I did this stuff and with luck someone with better math skills than I have can help or explain this better.