This article/ any following articles are intended as guides, for people looking to learn more about effects/amplifiers/ signal analysis with reference mostly to guitars. It is by no means intended as a complete guide to everything, but more of a starting point to initiate discussion/ get people going in designing their own guitar gadgets, most likely pedals/ pickups/amplifiers. I will try not to drown you in equations and if I'm going too slow/ too fast then let me know.
Given that in this field a certain amount of trust is required, my name is Andrew I'm in my mid 20's and I have a Masters degree in Electronics from Sheffield University. My main hobby is guitar but, alas, in reality I'm not particularly amazing at it. Until recently I never really combined my hobby with my livelihood but I figure it'd be an adventure to find out about it.
So you go and buy an effects pedal, you are curious as to what happens when you put an effects pedal in your chain but it's off? The when it's on bit is easy, you can play it a bit in the shop and see that it makes a cool sound, but when it's off, well that's a different matter.
You can encounter whole heaps of terms mostly including but not limited to, true bypass, (hardware/hardwire) bypass, (buffered, FET) bypass and digital bypass and here I will attempt to briefly explain the difference between them/ what's actually going on.
The True Bypass:
So, is it the holy grail of pedal manufacturing? Well I probably wouldn't go on any quests/ kill any dragons to achieve it, why? Well because it's exceedingly simple to achieve. Essentially true bypass means that nothing is interfering with the signal of your guitar, your pedal is just becoming a means of joining two cables together when it's off, it should have no discernible effect on the sound of your signal chain.
All that's required to make a simple true bypass is two dual switches (it can also be done with relay's but that's a little more complicated) which disconnect the signal from the input AND the output of the effects pedal's inner circuitry.
The Hardware Bypass:
There are all kind of names for this kind of bypass, for the most part from what I can ascertain they are marketing spiel. This kind of bypass seems the "laziest" as it were because it switches the effects circuitry off only on the input. So, why is this lazy you ask? Well what it means is there is still electronics connected to your signal even though it's not actively doing anything.
Electrical current will attempt to flow anywhere it can; it does this proportionally based on the resistance it faces. Anyone who read my previous article may remember V=IR being the equation we discussed. Well, everywhere a current goes has an inherent resistance; your guitar cable, an effects circuit, the air even (lightning? Yeah thats electric current; it tries to find the shortest resistance path to ground which is why it mostly hits tall things- less air to travel through as air has a really high resistance). So when the current gets to the output of your guitar pedal and see's the route back into your effects pedal, well depending on the output resistance of the circuit some of it will go in there (some credit to the manufacturers they keep the output impedance pretty high to limit the amount of current that does this) and some small amount of signal degradation is unavoidable.
The Digital Bypass:
Some people are scared off by digital pedals feeling that "analogue" provides a better sound, well bypass wise, with a reasonable sampling rate the pedal should create a pretty damn good approximation and I would doubt that any difference between the sound with and without it would be audible in a reasonable digital pedal (If your happy enough to record your guitars digitally then really it's only the same thing).
The Buffered Bypass:
This is the kind of bypass you see in most Boss pedals and it's a useful one. What they do is stick a mini amplifier (with no gain, it doesn't make the signal louder) called a buffer on the input/output of your pedal.
So above we discussed how all things have resistance, well even your nice guitar cables have resistance; the thing with resistance is... It add's up. If your guitar cable has a resistance of 2 Ohms and you have two of them (say, one either side of your shiny pedal) then the total resistance is 4 Ohms. Looking back at V=IR this means for a set voltage you get half the current... Half! Clearly the cables your using and the amount of them can become a serious problem degrading your original guitar sound, a really long effects loop can dramatically affect your signal.
So buffered bypass works by replenishing the current of your guitar signal as it goes through the pedal. However given that again your signal is going through circuitry it will, as always have some effect on your signal.
How can I tell what bypass I have?
You probably can't, if it works with the power off its either true or hardware, if it doesn't its buffered or digital. Beyond that you probably need a screwdriver and a volt meter.
What should I take from this and further questions?
It really depends on how detailed you care about your sound. But if you have a long effects chain with all true bypass, your signal will have to have gone a long way without any replenishment. If you have all buffered then your signal may be un-duly affected by the amount of electronics it has to go through. If you go all hardware bypass... well... you get the worst of both worlds. So just have a play with your effects loop take some pedals in or out, which ones have an effect? Which don't? Do any bypasses make your signal sound better? Do none of them have any noticeable effect at all? Let me know below.