Do Hi-Fi Amplifiers Sound Alike?
Note: This is a long read. Scroll down for a TL;DR at the end. Links to sources are also at the end. If you have time, please read it all.
What Does An Amplifier Do To The Sound?
What difference does your hi-fi amplifier make to the sound? If you’re into music and home entertainment like I am it’s something that can be difficult to get your head around. You could spend many hours reading reviews and listening for those differences and trying to carefully match your amp to your source, speakers and even the cables. It could be more simple than that if the amplifier makes no difference to the sound, only making it louder. Can an amplifier effectively just amplify? Knowing the answer could save you a lot of money and complication.
A hi-fi amplifier (amp) increases the power of a signal and provides a current so that it can be heard. An ideal amp should do this without altering the sound in any way that can be heard. They are specified with an RMS power rating or THD+N rating. Below this level the distortion will be below a certain level of total harmonic distortion (THD). In a hi-fi amp this will be typically 0.05% or lower. This level of distortion is well below what can be heard by the human ear.
In audio systems, lower distortion means the components in a loudspeaker, amplifier or microphone or other equipment produce a more accurate reproduction of an audio recording.
If you find yourself wondering about my hearing, in A/B listening tests with two pairs of speakers using the two speaker outputs on my amplifier I can easily tell the difference between speakers. Different speakers all sound clearly different to me. Even the same speakers sound different as you move them in a room or toe them in or out or as I move around a room. In terms of discerning ears I’m better than average.
I Believed Amps Sounded Different To Each Other
Years ago my Kenwood KA-3020SE (above) developed an intermittent fault and needed to be replaced. I bought a NAD 312 (below) to replace it. Before I sold the old Kenwood for spares or repair I decided to compare the sound. It wasn’t that easy, as to switch between them meant switching off, unplugging and then re-connecting leads and switching back on before sitting back down to listen. Long enough to forget what the sound was like? Maybe. Reading Hi-fi reviews, it’s written that these two amplifiers have different sounds but when I listened to both amps back to back, all I could say for sure was that the NAD didn’t go as loud before it started to distort. I expected this as it had a lower power rating. I just settled with the fact that it sounded good to my ears below a certain volume, which was still loud enough.
I had two sound systems in my living room. A stereo set-up for music and surround sound for the TV. A pair of stereo hi-fi speakers, five satellite speakers and a subwoofer. That’s a lot of speakers and wires. Plus two big amplifiers, one stereo amp and an AV reciever. I was happy with this and it sounded good to my ears.
The Amp/Speaker Switch
I read a read a forum post about a switch (Beresford TC-7220 on Amazon.com) that can allow you to connect two amplifiers to the same pair of speakers. (Beresford TC-7220; My experience. on AV Forums). The idea is that your stereo amp (having the best sound for music) and your AV amp (having the best sound for movies) can share a pair of speakers. Then you can remove one pair of speakers and your AV amp can benefit from the quality of your hi-fi speakers. This assumes that the two amps sound different. I was happy to believe that my hi-fi amp sounded better.
I ordered the switch and wired it up with many short lengths of speaker cable. It does mean lots of extra wires behind to connect the two amps to the switch. This set-up worked for me and the sound from my AV amp was much improved by the big stereo speakers replacing two of the small surround speakers. Any TV or film with music was much improved. Floor-standing speakers sound better with music than satellite speakers. No surprise there. I’m not sure my wife understood what the switch was for though!
The Mini Amp
Some months later I started planning a hi-fi system for the bedroom. I wanted a little amp I could put anywhere and I read a forum thread (Mini-T 2020 20W Class T Amplifier on AV Forums) about TA2020 tripath amps.
“The T-Amp is lovely. It’s powering JBL Control Ones, which are less sensitive than people say you need for a T-Amp, but I never need to take the volume past 9 o’clock to get a level I’m happy with. It doesn’t match the bass of the Nytech, but in this room size (2.5m x 2.7m) it does the job brilliantly and I enjoy the slightly brighter sound. I’ve also compared it to a NAD 3020 in a larger room with AR18 speakers and couldn’t tell the difference between them.”
On the basis of that and other glowing reviews I bought one from www.amptastic.com. It’s tiny size compared to a normal hi-fi would lead you to presume there is no way it could sound any good. It’s power rating is low (THD+N < 0.03% @ 10W 4Ω) which does fit the size, so it can’t go as loud as a big amp without distortion. It’s also a class D, whereas conventional hi-fi amps are class A. Surely another class of amp and type of circuit will sound quite different? I decided to do some A/B tests using my Beresford switch.
My First A/B Amplifier Listening Test
So, with both amps playing the same source at the same time I switched between them. Once I had adjusted the volume levels to the same loudness I listened carefully and it was really hard to tell them apart. This couldn’t be right, could it? I thought they couldn’t sound the same. The only thing I could tell was that the NAD could go louder. I headed to the forums and started a thread asking for advice (Sound testing/demo-ing amps. How to tell the differences? on AV Forums). This started off a bit of a storm on those forums and I learned a lot from it. There are people out there that believe that you can’t tell the difference in sound between amplifiers. One person added a link to the Richard Clark $10,000 Amplifier Challenge, where all comers have been challenged to prove they can hear the difference between amplifiers.
When compared evenly, the sonic differences between amplifiers operated below clipping are below the audible threshold of human hearing.
One person made a switching box and conducted a very careful test to compare the mini amplifier and a high end Quad amplifier and found no difference in sound.
I have now compared the Amptastic Mini-T amplifier and the Quad 405-2 amplifier using the recently constructed relay switch box.
Using music and speech signals from a compact disc source, no audible difference whatsoever was detected, provided that the 17 V(peak to peak) limit set by the Mini-T amplifier was not exceeded.
Using a pink noise signal, sourced from a Denon test CD, it was also not possible to detect any audible difference. Indeed there was no audible indication that any switching had taken place.
This near instantaneously switched Pink Noise test is probably the most sensitive indicator of an audible difference. There is no requirement for any significant sound memory capability on the part of the listener.
Amps Really Sound The Same
It seems that amps really do sound the same. I resolved to do more testing myself. I got a pair of phono splitters (above) and used identical interconnects, so I could ensure that both amplifiers were getting precisely the same signal. Once again I adjusted the volume levels to sound the same loudness and once again the sound was the same. At louder levels with the mini amp the higher sounds such as cymbals sound harsher and it generally is more tiring to listen to. This was when it was driven beyond its’ low power rating causing audible distortion, but at lower levels I could tell no difference.
I did some calculations and measurements  and the 10 watts per channel of the mini amp can power my speakers to around 80db without audible distortion. Measuring with my phone that was about 10 o’clock on the volume control and fairly loud but not as loud as I sometimes like to listen. The ability of an amp to play loud without distortion is something that I can hear. Is true RMS power at 0.03% THD the only difference you can tell between amps by listening with the human ear? By my calculations, to enjoy undistorted music at 90db (very loud but not concert loud), in an average setup, you need at least 100 watts per channel RMS rated amp. I went back to the forums to find out more.
The next interesting link was Matrix HiFi –> Blind testing high end full equipments. In that ‘blind A/B/X’ test, an audience could not say which system sounded better, a very high end hi-fi setup or a budget setup with poor cabling and a very powerful budget amplifier (Behringer A500 1% THD = 125 watts). The Behringer is a good amp but there is more to it than that.
The result speaks by itself:
How can it be possible that a basic system with such a price difference against the “reference” one, poorly placed, using the cheapest signal cables found, couldn’t be distinguished from the more expensive one?
And, most of it all, how come the cheap system was chosen by so many people as the best sounding of the two?
Shouldn’t the differences be so evident that it’d be a child’s game to pick the best?
Well, we think that each can reach to its own conclusion…
The graph shows how an amplifier’s noise varies with volume. The clipping threshold is about where the amp’s RMS power is rated. You want to only use your amp below this level. This is why the RMS power of an amplifier at a low level of THD + Noise is important.
Can An AV Receiver Sound Good With Music?
I’ve previously been led to believe that AV receiver (AVR) amplifiers were not good for music. It’s supposed to be clear that an average stereo hi-fi amp will sound better for music than an average AVR amp. I’ve had that idea blown out of my head and I decided to find out for myself. I did some A/B listening, using the Beresford switch, with my NAD 312 vs my Onkyo TX-SR508 AVR and it’s hard to tell any difference at all. The AVR in pure audio mode does sound pretty good to me. It seemed to me that the NAD amp is just a waste of space in my rack and my AVR amp could do music as well as TV. I decided to do some more careful testing later. Listening that hard gets tiring.
Meanwhile in the AV Forums thread someone brought up an amplifier challenge by Alan Shaw of Harbeth UK, a renowned speaker manufacturer. He really believes that you can’t hear the differences between amplifiers with his speakers. He put his money where his mouth is as well, offering an expensive pair of speakers. There were 45,000 viewing hits at various audio forums and only one proposal, which was rejected.
Here is a challenge then.
If, in a controlled experiment with all variables accounted for (incl. differences in frequency response and within the power range appropriate to the amps) under instantaneous A-B relay switchover, driving any Harbeth speakers, if you can positively identify an amplifier by sound alone, I will give you, FOC, a pair of brand new Harbeth speakers, up to and including a pair of M40.1 in any veneer you fancy.
I am quite confident that under controlled conditions, these fabled amplifier differences disappear and that I will never be parting with my money!
Another helpful person pointed me towards Ethan Winer’s Audio Myths Workshop video on YouTube. This points out some common myths a fallacies about audio and hi-fi and how you might think you can hear differences in audio equipment that aren’t there. Room acoustics play a big part in how your music sounds.
Further on, someone else pointed to the ‘Carver/Stereophile challenge back in the ’80s’. 
Carver caused a stir in the industry in the mid 1980s when he challenged two high-end audio magazines to give him any audio amplifier at any price, and he’d duplicate its sound in one of his lower cost (and usually much more powerful) designs.
After learning all of this, I’ve got an image in my mind of a hi-fi magazine review team sitting down after listening to a group of amps and saying to each other “Well, they all sound the same again….. OK, so what are we going to write about them?”. There is an interesting article in ‘The Audio Critic’ about this: 
Why should the magazines want to perpetuate such sloppy testing? The answer has two parts: (1) The highend audio industry depends on convincing consumers that large differences in components exist and that expensive components are clearly better than cheaper ones. Buff books expounding this philosophy have increased the number of their advertising pages from both dealers and manufacturers. (2) Controlled listening tests may be difficult to set up, and doing multiple listening trials represents real work. Most of the writers in consumer magazines are having too much fun doing open-loop listening tests to want to change to a methodology that requires real work.
I can confirm that setting up an A/B test, carefully matching volume levels and listening intently for any difference is genuinely tiring and such listening can only be done for a short amount of time before fatigue sets in. I can see why most so called hi-fi reviewers don’t bother to do it. Proper testing won’t make a reviewer any friends amongst the sellers of expensive equipment either.
I decided to do a final test to compare my NAD stereo ‘music’ amp against my Onkyo AVR ‘not for music’ amp in ‘Pure Audio’ stereo mode. I connected both amps to phono lead y-splitters using identical cables. I then connected the splitters to my source. Both amps were getting exactly the same signal from my Squeezebox Touch. I really can’t tell the two amps apart in sound. I listened to Art of Noise, Jean Michelle Jarre – Aero and Emeli Sande – Our Version of Events. On the Emeli Sande album, the tracks Clown, Hope, Read all About it (pt III) are just her singing and a piano so that covers solo piano and solo voice in one. At reasonable volume levels both amps sound the same to me. I listened to some solo piano (Rachmaninov: Piano Concerto No. 3, Leif Ove Andsnes) and some Metallica, at very high levels, as high as I dare. While I was very impressed by how much punch and impact my speakers have, I also noticed the poor quality of the piano recording. I could clearly hear the hiss of the master tape. I’ve noticed the quality of the Metallica recording (24bit 96khz HD FLAC), hearing great dynamics and clarity. I didn’t notice any distortion from either amp or any difference. I switched between amps many times and once the level is matched it was as if I did nothing. Both amps sound the same to me. Not nearly the same but identical. I can’t tell them apart.
I wondered, seeing as I know I must be running the NAD into distortion why I can’t hear it. It’s actually a very impressive little amp for it’s rating of 20wpc compared to the 90wpc (0.7% THD, continuous into 2 channels 8 ohms) of my AV amp. I worked out that if I’m drawing 50wpc from a 20wpc, 0.07 THD amp it would have about 7% distortion. It seems it’s not possible to hear that much distortion. Apparently, even the best speakers give much more distortion than that so perhaps good speakers is where you should spend your money.
I decided that was enough of listening to my NAD amp, seeing as it does music no better than my Onkyo AV amp I don’t need both. The NAD 312 got sold and I’m happy with my AV Receiver for music. A little A/B testing and a forum thread saved me hundreds of pounds!
My conclusion is that all amplifiers, that are working correctly, below their rated RMS power output, sound the same at the same volume level. Modern hi-fi amps are so well made, with such low distortion, that we can’t hear any differences between them. That’s awesome. It means we can buy Hi-Fi amps based on RMS power output at 0.03%THD, features such as inputs, how it looks, build quality, warranty/after-sales support and how it’s going to fit into our space.
There is no need to read reviews that say an amp is more ‘forward’ or more ‘relaxed’ or any of the words that hi-fi writers have struggled to find to earn their pay and to please the manufacturers. How can an amp sound ‘forward’? What does that even mean? How can a device designed to reproduce sound faithfully and succeeding at it (they do succeed) change the ‘pacing’ of the music? It doesn’t. A tube amp can sound ‘warm’ and ‘fuzzy’ by rolling off the high frequencies and adding pleasing distortion and a bad amp can add noise and clipping distortion and just sound awful but that’s about it. I think Hi-Fi reviewers should focus on the specification and how accurate that is by testing electronically. Clean power output is important, so is it reported correctly? Features, looks, build quality etc. are all important. Trying to find words to describe how an amp sounds is just not needed though unless you are trying to say the sound is distorted due to a problem with the amp.
More from the forums:
Quite. Not as much fun for the advertisers and manufacturers and hi-fi shops who will now have to find new ways to get inside your wallet.
I definitely agree that “psycho acoustics” can and DO play a big part in the sale of Hi-fi equipment… And I also agree that there is a lot of very questionable products which try to capitalise on this…. It is very easy to waste a lot of money if you do not research things first…
At this point, I am starting to believe that pretty much all hi-fi electronics do sound the same even though I may have thought I heard differences in the past. Which may have been more due to differences in volume levels and my position in the listening room. Not tapes or turntables though, which I believe can be very dependent on the characteristics of the cartridge used and are very different to digital sources.
I know this would seem hard to believe and I find it difficult after all I’ve read in hi-fi magazines about CD players offering ‘more solid bass’ compared to another or being bright sounding. All CD players have a very even frequency response in the audible range. That’s the point of digital audio. A review might say a player is ‘glassy sounding’. How can it sound glassy? Glass has a ringing sound and there is no way a digital source that is working correctly would add a ringing sound to music.
Wrapping It Up
I now believe the human ear can’t hear the difference in sound between amplifiers working below their rated RMS power at 1% THD+N. I have saved money and time choosing and buying hi-fi amplifiers and it will save me more in the future. I’ve got a new perspective on hi-fi reviews and how subjective the sound of a product can be. You can believe me or not. You can choose to do your own listening test or you can keep knowing what you know. Above all, please remember to enjoy the music, because that’s what it’s all about. If you’re enjoying the sound, it doesn’t matter what’s amplifying it! This knowledge has saved me spending lots of money already and it could save you time, cash and worry as well.
- You can enjoy hi-fi music without spending a fortune. You don’t need to spend £1000+ on an amplifier to enjoy high fidelity music.
- Hi-fi amplifiers do sound the same to human listeners with the exception of tube based amps.
- Tube based amplifiers do have audible distortion that some people like. I like my music without distortion and colouration from the amplifier so tubes are not high fidelity to me.
- Power output is important to avoid distortion at loud volume. You need 100 watts per channel RMS @ 0.03%THD into average hi-fi speakers to fill a modest living room with loud music without clipping distortion.
- When buying an amp and speakers, you don’t need to match the sound of the amp to the sound of the speakers. You only need to look at impedance specification and sensitivity when matching speakers to amplifiers. IE If your speakers are 2 ohm, can your amp drive a 2 ohm load and how much power is needed to play music loud enough for you with these speakers?
- AV Amps do sound good with music in stereo direct mode. As good as a stereo hi-fi amp.
- Class D amps sound just as good as Class A analog amps and they use much less power from the wall socket.
- Choose your amp based on inputs, specification, features, looks, warranty and build quality. Ignore talk of tonality, musicality, pacing, richness, or vividness etc..
- Speakers make much more difference to the sound. Spend more on your speakers than your amp.
Links, Sources And Further Reading
 Carver/Stereophile challenge (Wikipedia)