Amplifier Impedance matters! Why it is important to have a low output impedance (explained)

yes this headphone amplifier has a flat frequency response that's great let me quickly connect my headphones and listen to my music what that's not flat at all how could that happen well I'll explain it in this video hey Julian Kraus here and I've tested quite a few headphone amps in the past and one thing I always measure is the impedance of the headphone output because in combination with the impedance of a pair of headphones it can have a considerable effect on the frequency response and there is a guideline you can follow to get the best performance from your amp and headphone combination which I will share in this video but let's start with a small definition of impedance impedances stated in ohms and it quantifies how much an electronical device impedes the flow of an alternating current that sounds complex but here all you need to know is that when there is a large impedance a low amount of current will flow and with a low impedance a high amount of current can flow now a headphone amp has an impedance often referred to as output impedance and you can think of the impedance from a power source as its ability to deliver current if the amp has a low impedance it can deliver a high amount of current and if it has a high impedance it can only deliver a low amount of current in my tests I've measured headphone amps with impedances ranging from as little as 0.1 ohms up to around a few hundred ohms as you may know headphones also have an impedance headphones pose a load to the amplifier and depending on the impedance they draw more or less current from the amplifier headphones with a low impedance draw a high amount of current and high impedance headphones will draw a lower amount of current headphone impedances typically reached from around 16 ohms on the low end up to around 600 ohms or so and this means that there is quite a difference in how much current will flow depending on which headphones you use okay so now you know that a headphone amp with a low output impedance can provide a lot of current and headphones with a low impedance will draw a lot of current conversely a high output impedance means that an amp is only able to provide a low amount of current and headphones with a high impedance will draw a less current than their low impedance counterparts now let's have a look at a practical example here I have an amp with a low output impedance which is currently outputting a sine wave which will be our reference level you can see the reading on-screen if this reading changes we will be able to see the amount of deviation from our reference level in decibels let's connect the load in this case a 600 ohms which will simulate a pair of headphones with a 600 ohms they would be considered as a pair of high impedance headphones when we look at our level again we can see that nothing changed this means that the amp can easily deliver the little current the 600 ohm load draws and the signal level did not change even though we connected eight load to the amp let's connect a 16 ohm load and this simulates a pair of low impedance headphones connected to the amplifier as you can see the signal dropped just slightly by about 0.2 decibels that's only a very small change in signal amplitude so even though the load draws more current compared to the 600 ohm load the amp can still provide that because of its low output impedance keeping the amplitude of our signal roughly the same now I swapped at the amp and the new one has a high output impedance it is currently unloaded outputting our reference signal let's see what happens when I connected the 600 ohm load as you could see the moment I connected the load the signal already dropped by a bit more than a decibel I think you know where this is going let me connect the 16 ohm load and watch the signal amplitude as expected at the moment I connected the load that simulates low impedance headphones the amplitude of the signal decreased significantly in this case by a whopping 16 decibels that's because of the 16 ohm load wants to draw a high amount of current which the amplifier is not capable to deliver because of its high output impedance and thus the voltage drops significantly as I showed before the limited ability of this amp to deliver current had already in effect when I connected the 600 ohm load because the voltage already to drop slightly but because the high impedance load does not draw a lot of current the amp was still doing okay despite its high output impedance but when I connected a load with a low impedance the high current draw of the load was too much for the amp and the voltage level plummeted so the takeaway of this a small experiment is that when you have an amp with a low output impedance the voltage it puts out is hardly affected by the pair of headphones you connect but when you have an amp with a high output impedance the voltage is very much dependent on the connected load and here is where the problem arises you see even though the impedance of headphones is stated as a single number like 32 ohms or 150 ohms the impedance of the headphones typically changes over the frequency range here you can see a very typical impedance graph of dynamic headphones many of these headphones have a peak in the impedance curve somewhere around 100 Hertz and the impedance also tends to rise towards the higher frequencies so depending on which frequency these headphones are playing and the impedance can change quite a bit this means that depending on the playback frequency the amount of current that the headphones will draw will vary and when your amp has a high output impedance this will affect your signal amplitude and in turn change the frequency response this is exactly what I showed in the beginning of this video here is the frequency response of an M with a high output impedance it is currently unloaded meaning that virtually no current is drawn from this amp in this case it delivers a fairly flat frequency response but when I connect a real pair of headphones and measure the frequency response again you can see that the signal amplitude is heavily influenced by the headphones impedance which leads to this wacky response you can see here one thing to mention is that this is not of the acoustical frequency response of these headphones this is the electrical response of the amp in combination with these headphones and you want this to be as flat as possible as any deviations here will change the headphones from their design acoustical frequency response in this example the connected headphones would get a bass boost of around 1.3 decibels at 95 Hertz which obviously colors the sound of these headphones as a comparison here I measured another amp with a low output impedance this is its frequency response unloaded when I connect the exact same pair of headphones I used in the last test I measure this frequency response this is of course a much flatter frequency response and you can see how much less the response of this amp is influenced by the impedance of the connected headphones so I hope you see the problem the higher the output impedance of your headphone amp the more the sound is affected by the impedance of your headphones and low impedance headphones accentuate this problem this is why a high output impedance can limit you to use headphones with a high impedance if you don't want the frequency response to change much that's why it is important that a headphone amp has a low output impedance this way you can connect pretty much any pair of headphones you don't have to worry about the impedance of the headphones and they will sound exactly like they are designed to finally I want to share a rule of thumb with you you can use to make sure your headphones and amp apply together nicely your headphones should have an impedance of at least 8 times the output impedance of your amp or conversely the output impedance of your amp should be lower than 1/8 of the impedance of your headphones for example if the output impedance of your amp is 10 ohms of the headphones should have an impedance of at least 80 ohms to keep the changes in frequency response due to the interaction of the headphone impedance and the amp output impedance to a minimum I hope you like this more in depth look at headphone amplifier impedance please give me a thumbs up if you learn something and subscribe and hit the bell I can for more videos like this I will see you all in the next one [Music]

Channel: Julian Krause

Views: 70,945

Keywords: impedance, headphone amp, frequency response

Id: SpTsWVLI-ig

Length: 9min 18sec (558 seconds)

Published: Sun Jan 26 2020