Understanding Audio Signal Flow: Guitar & Bass

Let's try to demystify the voodoo that happens under your gear’s hood and equip you with confidence to experiment on your own unique sound

By Peter D.KayContributing Author

Article photo - Understanding Audio Signal Flow: Guitar & Bass

Understanding how the audio signal travels through equipment can arm the user with the mandatory knowledge to explore his/her options in depth. But anything non superficial comes usually at the cost of added complexity. This series of articles aims to demystify any voodoo that happens under your gear’s hood and equip you with confidence to experiment on your own unique sound.

 

1. The Source

Article photo - Understanding Audio Signal Flow: Guitar & Bass Well the real source of the signal is your brain...sending signals to your fingers and arms , them in turn striking or fretting guitar strings in millions of different ways. But for simplicity’s sake we will skip any neuroscience or psychology topics and jump to the instrument and it’s onboard equipment.

 

The guitar and bass pickups

Article photo - Understanding Audio Signal Flow: Guitar & Bass Pickups are the first point of electrical signal generation. It happens because the movement of the strings alters the electromagnetic field of the coiled wire inside the pickup. Single coil pickups and humbuckers share some characteristics and also sport some differences. The attributes of tonality and responsiveness are all about taste so we won’t touch on those but cheap pickups , you will find , tend to be noisy. I cannot stress how important it is to get your hands on low noise pickups. Read through the article and you will find out why. 

 

2. On board circuits

There are some electric guitars out there that will pass the pickup signals straight through the output jack but generally you will find volume and tone controls as well as a selector switch. Instruments without batteries in them are equipped with passive circuits and instruments that use batteries are equipped with active circuits. Think of it like this, passive means that there is a whole pie and you can only take out parts of it . Active circuits in the other hand can cut from the pie (using their passive part of the circuit) but also add to it. In essence a passive equaliser can only cut frequencies , very much like how a tone control on your guitar will cut high frequency content . An active equaliser will be able to also add to your frequency spectrum. For that reason knobs on active circuits are at their neutral position at 12 o’clock.(This is also a good way to identify an active circuit.) 

 

3. The Obligatory (or not) Pedal Chain

While purists will insist that a good instrument and a tube amp is all you need , there are a plethora of ways to stray from the beaten path. Pedals are small versions of signal processing units. They range from the classic dynamic and frequency processors (compressors , equalizers , tone manipulation) to the usual suspect of overdrive and distortion boxes (still dynamic processing to a degree) to more exotic stompboxes like harmonic content generators , envelope filters or combinations of the above.

Article photo - Understanding Audio Signal Flow: Guitar & Bass

 

4. The Amplifier

Amps are very important in the chain of the electric guitarist/bassist. In practicality they amplify your signal by a huge margin turning it from the minuscule guitar level signal to power level signal strong enough to move a speaker. The majority of amps have at least two stages of amplification. First your guitar signal hits the pre-amplification circuit getting a small boost and turning your weak guitar signal into a stronger version to be processed by the rest of the amp. Pre-amps all sound different in their own way. Relatively new modeling technology gives you the option to emulate different preamps in one amp , switching from a Fender twin reverb to a Vox AC30 in a simple twist of a knob.


After that stage optionally your signal goes through an equalization circuit and they also vary from brand to brand and model to model. As an example a usual equalization scheme would be an active three band equalizer sporting a bass , mid and treble potentiometer. The three bands are fixed in cutoff frequency and in Q value which controls how much of the surrounding frequencies will be affected relative to the cutoff frequency. A weirder example , in fact the most peculiar one I happened to come across is the paragraphic equalization scheme. You still get three knobs to twist but what happens here is that when you boost or cut one frequency the cutoff frequency of the other knobs changes value! The result is a less extreme and more balanced tone whatever you try to dial in.


Article photo - Understanding Audio Signal Flow: Guitar & Bass Now it’s time for the big guns , the power amp comes into play and turns your boosted yet still comparatively weak signal into a monster capable of moving speaker cones. Power amps have less of an imprint in your sound as their job is not to color but just boost the signal to get the speaker moving , still it would be rare to find a completely neutral sounding power amp in a guitar amplifier. You would expect that the only way forward is the speaker cone but speaker cones vibrating will surely annoy your neighbors and can sometimes prove to be a hassle because they have to be mic'd up and that’s a whole different subject that includes many challenges to overcome. Still you can chose to go on and move the speaker cones as you are traditionally supposed to but you can also go for a load balancer box which emulates your speaker / mic /room setup in a digital chip inside specialized gear (Uad OX) and exports that signal in a balanced XLR output. It should be noted that speaker cones , microphones (as well as their placement) ,rooms and their emulations by extent do alter your tonal imprint in a serious way and they should not be neglected.

[About noise : the amplifier does what it’s name suggests but really goes ham at it so if your pickups or your cable or anything in your chain until that point carries any noise or hiss the amp will...well amplify it. Make sure you get your signal there as intact as possible using low noise-floor pickups , shorter than 7 meters of shielded TS cable and quality low noise pedals. Distortion and overdrive pedals have a best friend called the noise gate]

 

5. The Mixing Desk or Audio Interface

Your speakers (either real or virtualized) move and that movement pushes air in your room. A well placed microphone is there to pick up that movement with its very sensitive diaphragm and carry that signal to the mixing desk or to your audio interface depending on your setup. But guess what , that signal is weak. And so it hits another preamp , a microphone preamp this time. This preamp will get your signal ready to be recorded on your favorite media, be it tape (if you’re rich) or hard disk. In the case of the latter there is another step to go through and that is the all important Analog to Digital converter.Article photo - Understanding Audio Signal Flow: Guitar & Bass These chips are included in most audio interfaces but also come as dedicated units. They transform your electrical signal into a digitized version so that it can be stored and manipulated in your computer environment. Converters try to clone the electrical signal in a one to one ratio and even though they cannot ever completely succeed our perception proves to be more incompetent. To make a long story short the converter produces a chopped version of the analog counterpart but those chops are so tiny and precise that our brains just can’t tell the difference. 
 

...the end justifies the means. Don’t hesitate to break rules.


Conclusions

There are a few points I would like to make ending this story.  First up, a chain is as strong as its weakest link. This means that If your amp costs a fortune but that room is a non treated basement and your mic came from a toy shop chances are your amp’s value will go down dramatically.


Secondly, the end justifies the means. Don’t hesitate to break rules. Knowing that your power amp should only power speakers or offload that current in a box, like Uad’s OX, you will use that knowledge to not blow up your gear or start a fire. But say you want to record a part that’s very dark , lacking high frequencies. Well put your mic behind your open back cabinet and you’ll pretty much get a much darker tone than a traditional mic position on the front of the amp.


Last but not least. Your pedals sound how they sound as a part of a chain. But change that amp or even the speaker of the same amp and your pedals sound different. Never forget there are a ton of variables when it comes to tone and sound. Your fingers , your pick (or lack of one), your strings, the pickups, the pedals, your amp, your cab, your speakers, the ROOM(!!!), the mics, their placement and we’re not even touching the oh so important cup of coffee you didn’t take this morning because you woke up late and you had to rush. Yes my friends before jumping to conclusions like “this distortion box sounds crap” give it a second chance with a different setup and you will be amazed how different the results are.

About Peter D.Kay

Peter is a producer and audio engineer operating in Thessaloniki (Greece). He boasts a background as a session guitarist and a degree in audio engineering. His works with independent artists equipped him with truly invaluable experiences resulting in an unusual yet effective set of skills and techniques.

Contact Peter D.Kay at peter.d.kay@musicngear.com

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