Impedance - an explanation for guitarists

so here I am surrounded by lots of guitar amps I've got little transistor apps here all the way up to sort of bigger valve amps with multiple speakers etc I was going to talk today about impedance what's impedance well it's about matching speakers to your amplifier and how you get the best possible result from your combination now we have combos here lots of them so they are actually designed with that all in mind but sometimes you'll get speaker cabinets where you have to plug your valve amp into and they might be different cabinets with different speakers different impedances now there are four things to consider the output of an amplifier is a voltage signal now in order to get sound out of your speaker your speaker is an impedance of resistance and if you apply a voltage across your resistance a current will flow through that resistance speaker then the voltage multiplied by the current gives you your power so these four things need to be thought through very carefully if we start with the transistor up here this is the h and h vs musician it's a 100 watt amplifier into 4 ohms there we go so 4 ohms is your impedance that's your speaker now if we liken this for example to a van where the amplifier is the engine and the van weighs 4 tonnes so it drives along very nicely thank you very much now we go to a quarry and pick up four tons of rubble stick in the back of the van so you've now got eight tons of course you're going to get a reduction in your power available because the engines going to struggle a bit that's what would happen with this we would go from a hundred watts at four ohms down to sixty watts or so at eight ohms notice that it's not a distinct halving of power because of things like heat loss and general efficiency of your amplifier but you'd still get a lower power i've connects 16 ohms up to this the power drops further still if i connect a pair of headphones up to this which are much much higher resistance the power will drop even further if i then short out the output of the if i goo a short-circuit of the output of the amplifier essentially i'd get zero ohms which is not good because voltage divided by resistance gives you the current so if i have zero resistance i'd end up with infinite current which is quite tricky that results in flames and meltdown and new amplifier so never short out your amp that's transistors let's move on to valves now we've got this whole nerve vendetta 30 watt valve them quite rare these now so this is a bit different now a valve amplifier has an ALP a very high output impedance that is there's lots of voltage from your power amp but with very little current associated with it so how are we going to match that to a speaker well the answer is you have to output that valve amplifier signal through an output transformer which then feeds your speaker what's the transformer well it's something that steps a voltage either up or down but like the pylons that cross the countryside they got massive voltages and then you reduce the voltages to household current via a transformer so we have the output of this maybe 50 ohm output impedance of this valve member going through this transformer we've got to turn it into something that we can drive a 15 ohm impedance which is the speaker that's inside this so that's fine we do that using the transformer so what happens is your voltage decreases on the output of your trans but the current increases that's fine so what happens then if you go to a gig with an amplifier maybe your valve head and you've got a valid cabinet there that's got a different impedance to your amplifier well it's not necessarily a problem and in fact you if you've got lots of different cabinets in your studio you can get quite creative with this so if I have an 8 ohm output of my valve amp and I supply a 16 ohm cabinet actually what happens is in practical terms you get a slight increase in your mid-range so you can get yeah you can get your mids up there if you however supply a lower impedance like a 4 ohm speaker you'll get a drop in your mid-range so it's not necessarily an issue that you have a mismatch of your power amp to your speaker but it can have a undesired effect as well and also if you go too far one way so let's say you've got a 16 ohm output of your amplifier and you're connected to a 4 ohm load you are inviting problems now if you don't connect a mode to your amplifier what you end up with is something quite nasty going on in your output transformer you end up with lots of transient spikes because the speaker actually dumps it it's moving and generating the sound but it's also damping the system quite nicely so that everything that there are no excessive spikes in voltage so if we take the speaker away a lots of spikes which will kill your output transformer your output valves and probably the power supply so you don't want to do that anyway there is a slight there is a bit of an explanation on impedances and amplifiers now if you've got multiple speakers let's say with this Marshall here it's a 50-watt amplifier supplying 2 speakers how are you going to connect those speakers up well there are two methods or three methods in fact there's a series arrangement parallel arrangement and a combination of the two series-parallel if you've gots four speakers for example or multiple e8 speakers so in here this Marshall amplifier there are two eight ohm speakers and they're wired in series now that means you the amplifier goes into 180 ohm speaker and then into the other one and then back to complete the circuit that your current flows through to 8 ohm speakers in series it's easy you just add those together you have 16 that's fine parallel it's a bit different if you've got your 8 ohm speaker from your power amp and then you daisy chain or sorry rather piggyback another 8 ohm speaker onto that you end up with something where the formula is a little bit more complicated you have to say speaker a and speaker B multiply those together so it's 8 times 8 64 divided by a plus B which is 8 plus 8 which is 16 so that's 64 over the 16 which is 4 so 2 8 ohm speakers in parallel will give you 4 ohms so you can use that formula to calculate if you have an 8 ohm speaker and a 4 ohm speaker for example as 32 over 12 which is 2.8 or something so there's your way of calculating your speaker impedances now this amplifier and the Marshall and the vox ac30 have speaker impedance switches which are more designed to cope with extra cabinets rather than tweaking a mid range but the mid range thing is is a byproduct of that change in your impedance so there are your variable ones this cabinet up here this is the blunt instrument vs 315 which is a 15 watt amplifier supplying an 8 ohm speaker that is fixed to a tones likewise with the Fender 40 watt up there that's also an 8 ohm setup so lots and lots of different sounds lots of possibilities and lots of pitfalls not to fall into you

Channel: Dan Baker

Views: 2,118

Rating: 4.8048782 out of 5

Keywords: Guitar amplifier, Speaker impedance, Voltage, Current, Power rating, Dan Baker, Musician

Id: bzKfXQZgZCA

Length: 8min 55sec (535 seconds)

Published: Sat Oct 14 2017