Do amplifier specs tell us anything useful?

[curiosity_saved_the_cat]

Great advice, it seems to apply to many other situations apart from choosing an amplifier as well.

[cristi]

I sent this somewhat similar question a couple of months ago. Would have liked to hear your take on damping factor, S/N ratios, THD measurements, i/o impedance, and most of the specs you list on your products.

[InsideOfMyOwnMind]

Paul, I know people want you to just get to the meat of the subject but I like when you take a few seconds to describe the function and purpose of the spot you are sitting at. What is that bench used for? Who uses it? That kind of thing. I think it brings people closer to the fold.

[audiofun]

There are some speakers that are very hard to drive: when you look at their measurement it may show impedance drops from 8 Ohm to 4 Ohm in high and/or low frequencies. Example: KEF LS50. When choosing an amp for such speakers one necessary spec to look at is its power output into 8 and 4 Ohms. The suitable amp will produce 2x power into 4 Ohms comparing to 8 Ohms. If it's less than 2x – avoid, otherwise those demanding speakers will sound lean in the bass and constricted in the high frequencies.

[AudioMaverickcom]

I consider spec-sheets like a resume flagging which ones to call in for an interview.

[rakeshmahindrakar]

Here are some parameters to look for,
More Wattage is always good idea.
Linear frequency response.
Ultra wide bandwidth.
Lower distortion. (THD)
Powerful transformer.
Gain ratio
Build quality.
One might need multiple analog and digital inputs and preouts depending on their requirements.!
And best parameter are your ears, Let them audition well before you decide.

[maxheadrom]

A note about harmonic distortion. First, learn about the Fourier Theorem - which states that any signal can be represented as an infinite sum of sines and cosines. So, you enter with your signal into the amp - this signal has a Fourier representation - and when it comes out it has a different Fourier representation with some of the sines and cosines with different amplitudes - therefore, harmonic distortion.

Why "Harmonic"? It comes from the harmonic series that is the fundamental (the 440Hz of the center A in the piano) and its harmonics (2*440, 3*440 and so on).¹ The fundamental is called the first harmonic and the 880Hz the second and so on ... Now lets check a piano keyboard: 880Hz is one octave above the center A and the 4th harmonic is 2 octaves above the center A - and that sounds good and was even used by piano roll arrangers to get more sound out of the short strings those machines had. On the other hand, there are the higher harmonics like the 13th, 15th and so on which aren't even in tune with any note!²

So, to tell if the harmonic distortion is high or low, knowledge about the technology used is important: a tube amp with high THD will sound very good even in bad loudspeakers but a BJT amp³ with high THD will sound worse the better the loudspeakers are.

(1) Check the harmonic series in math. The amount of each harmonic was used as the first definition of "Timbre" by Helmholtz (if I'm not mistaken). Nowadays, the definition of timbre has changes and it has been proven that only the harmonics can't account the whole of it's qualities.
(2) Check how modern tuning is done and what is "temperament". You will also learn why higher harmonics will not match any note.
(3) I'm not sure if the BJT always behaves like that specially after watching Arnie Nudell speaking on one of PS Audio's videos - there's more to the art than meets the eye. Nonetheless, classic BJT amps will have horrible sounding distortion.

Paul is absolutely right: specs are not what we hear. I wish someone would do a works, like Beranek did for Concert Halls, and create a set of specs that could give us more information about how an amp sounds - specially in this e-commerce era when we buy stuff without listening to them.

Note: did you know PS Audio is mentioned on one episode of MST3K? Yep!

[Geerladenlad]

What are the specs to pay attention to for bad sound quality? Can the specs differ from class and design?

[ThinkingBetter]

Simple specifications and measurements cover simple matters. Complex and more advanced specifications and measurements, that usually aren’t as widely known, can do much more and can be of tremendous value. Ears alone can not be relied on but of course ultimately equipment need to pass critical listening tests. Make no mistake, equipment that fails certain tests also fails in critical listening tests. And equipment that fails a critical listening test likely also measure poorly but it’s quite possible that those particular measurements were not done.

[tapemaster]

I remember a video you did on tube amps and the spec to pay attention to was output impedance, that applies to solid state as well, the majority of consumer brands output impedance is 600ohms, a brand that worked hard on there product will list that spec because they are proud of it, 400ohms and below will sound really good and it’s really expensive, I follow up on knowledge you present

[andershammer]

I read once that the only spec that mattered was the shipping weight. I agree.

[tornadotj]

This reminds me of when I was in the audio industry. (this was in the 80's, yes I'm old!) We had a Bose rep come in and talk to us about their speakers. We asked for specs and he said "We don't provide any specs for any of our speakers, specs don't matter, it's all in the sound."

[johnsweda]

What do you think Paul of feedback on mismatched power transistors like fets, there is a benefit. Can't remember I'm not sure what it is I think it's some type of oscillation the lack of it, if their balance you're more likely to get an isolation problem, I'm really talking about tubes but I'm sure it applies the same with power transistors tried etc. How do you manage your feedback circuit.

[henrikl]

So true, My old Dynaco ST-30 don'ät have so great specs from a modern point of view..But it sounds great with the right speakers.

[krikand]

Weight. BIG transformers and the need for BIG Heatsinks are more important than WATTS..

[MegaBri]

How an item sounds to the individual, is far more important than any other statistic...

[benkrake]

I have noticed especially with cheaper multi channel A/V receivers, that quite often the power ratings in specs are only for 2 channels driven and not at all channels driven, which I think is a little dishonest. If you run your receiver with all channels, it will most of the time struggle to put out that same power that it’s supposed to be rated at.

[kevin-vtdw]

I have infinity quantum 2 speakers what amp should I use I only have 2 grand to spend does any one have good ideas

[jimocampo]

Dear Paul, I have seen equipment for DYI Power Cables and Speaker Cables, but I never see any equipment for DYI Balanced Cables. Why?

[andershammer]

I used to be able to tell how an amp is going to sound by looking at the parts used.