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Why do we hold amplifiers to such high standards?

Joseph Crowe

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Even mention of THD and uper highs in same sentence makes me laugh. Second harmonic is A*2 so eventually up to 10 KHz it theoretically has some sense and only theoretically. In practice to about 6 or 6.25 KHz.
Yeah, it’s not H2 per se, but rather the energy required to produce H2, which drained out from the primary tone. This is where the audibility aspect comes into play. We can’t hear H2, but we can hear how it robs the primary tone of its transient peak energy on music. It’s the same principle as a studio dynamic compressor, which produces more and more H2 with more aggressive compression.
 

ZolaIII

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Yeah, it’s not H2 per se, but rather the energy required to produce H2, which drained out from the primary tone. This is where the audibility aspect comes into play. We can’t hear H2, but we can hear how it robs the primary tone of its transient peak energy on music. It’s the same principle as a studio dynamic compressor, which produces more and more H2 with more aggressive compression.
I elaborated more elsewhere.
 

SIY

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Re: impedance peaks in the bass, that's from loudspeaker resonance (electrical and mechanical)- so the peak means that the amp will be working less hard at those frequencies to accelerate the cone. The amp merely has to provide the same voltage that it did above the frequency of the resonance, but now at a lower current, to maintain constant SPL. Don't let watts mislead you, volts is where it's at; think of the difference between speaker sensitivity and efficiency.
 

Axo1989

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So if we say that the sweep is at 2.83V, then at 15V output, there is +14.5 dB of gain. So the Topping can hit 99 dB at 70 Hz and 104.5 dB at 1 kHz.

At 1 kHz, it is pulling 56W (2.5A) and at 70 Hz, it is pulling 9.4W (0.625A).

If you “could” deliver 56W into 70 Hz, then your amp would need to push 36.6V, and that would be a 22.1 dB gain over 2.83V or 106.6 dB. Even though that is only 1.525A, the LA90 cannot hit 36.6V because this sweep shows that the max voltage is ~15V.

So the impact of the high impedance “somehow works out” is that if you have the FR response curve and maximum voltage of the amp, you can do the math.

The other way we can do it (since I based it off the 4 ohm measurements) is that 24 ohms should be 1/8th of 4 ohm and 1/4 of 8 ohm, or 7W-9W give or take, which matches my 9.5W calculation earlier because it might be easier for the amp to drive at high impedance.

The impact is the “90 dB @ 1 watt” sensitivity. Here, if you assumed the LA90 hit 46W into 6 ohms, you would see 106.6 dB predicted but you don’t really get that.

If you wanted 36.6V into 70 Hz, your amp would need to deliver 335W into 1 kHz, which is how the impedance affects the “power” of the amplifier you need when shopping, even though that is to get to 56W at 24 ohm.

But way too much maths. That's the point where I p*ss the Topping off for a joke and get an old Krell. :)
 

ZolaIII

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Re: impedance peaks in the bass, that's from loudspeaker resonance (electrical and mechanical)- so the peak means that the amp will be working less hard at those frequencies to accelerate the cone. The amp merely has to provide the same voltage that it did above the frequency of the resonance, but now at a lower current, to maintain constant SPL. Don't let watts mislead you, volts is where it's at; think of the difference between speaker sensitivity and efficiency.
It's highly recommended especially with ported speakers to use high pass (as limiter) a tad above such at Fs or at least Fb (second and first peak in importance/phase plots and after the impedance peak where they meet up). That way it ease up things for both speakers excursion and amplifier. Of course Fs is for use with sub's.
 

Joseph Crowe

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I elaborated more elsewhere.
I read that but I don't see how it relates to what I said. I think you might be talking past me.
 

Ze Frog

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To be fair a lot of the measurements are certainly beyond the hearing of most mortals, for me measurements mean more in the sense with electronics as a denotation of how well engineered they are. Obviously you don't have to get the absolute best measuring, but at the same time you likely don't want to spend a massive wodge of cash on something that is beaten by something a fraction of the price.

Speakers are somewhat different, good measurements here allow you to know what the speakers characteristics are. Even if you don't want to listen to a flat response, starting there allows far more effective EQ and tailoring of the sound. Also factors such as directivity and such allow you to get a better idea of how it will interact in a room. If a speaker has loads of peaks and troughs, you end up with something where in some cases you just won't have enough EQ available to fix, and if you did it will degrade the sound quite markedly. So best to build or buy flat and then tailor to your own liking.
 

Tell

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0.1% THD was SOTA in the late 70s in an amp. And 0.005% was late 80s, early 90s. At least for McIntosh gear.

If a manufacturer is not at least 0.005% THD today, what are they doing? To me, it's a measure of competency.
But if humans can't hear distortion over 0.1% then anything below is academic anyways? But yeah sure, if they can build better without charging to much extra for it go ahead, but personally I don't care that much at all anymore as long as I get enough power.
I mean I've had a few class D amps with the best being maybe 0.01% and I never heard any distortion from it unless it's clipping, but the one I was using started just a few weeks ago making awful high pitched noises because of a bad PSU, so instead I started using an old Marantz 2015 that I inherited from my late grandmother. The specs from Marantz says 0.9% THD and 79dB SNR, and to my ears it sounds very clean and absolutely noise free. I might maaybe hear some graininess to the sound, but that might very well be all in my head just because I know it's old and what the specs says. But then of course, just before I started using with Marantz my daughter was born so I haven't really been playing particularly loud anyways so I'm quite sure than any distortion is far below the noise floor in my room.

But even so, sometime in the not too distant future I will be building myself an TPA3255 PFFB amp with some sexy VU meters, just because it's fun and a bit of feel good :)
 

egellings

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Agree. Once distortion gets acceptably low so that it becomes inaudible, I don't see any point in striving for lower amounts outside of getting a better handle on the engineering involved in achieving that. No point in ordering a drop forge when all you need is a tack hammer. Audio amplification is a solved problem and has been for some time now.
 

rdenney

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I place a higher priority on no distortion. It is relatively easy to get plenty of power now. And I'm glad it doesn't require megabucks to get both.
Not me. Lack of power leads to clipping, which is far more audible than any harmonic distortion short of clipping.

Quiescent noise is also something I look at.

You are right that power is now easy to get and affordable, and so is low distortion. More difficult is attaining both of those objectives at near-short loads, so if I had speakers that dipped down to 2 ohms or less, I’d pay attention to that, too.

Back to the OP’s topic, lots of high end audio stuff is sold based on the star power of the designer. It’s a club for the wealthy whose concerns are only nominally about audio fidelity. We should remember that the likes of Dan D’Agostino and Nelson Pass (and, for that matter, Bob Carver) made their reputations in designing and building high-power, low-distortion amps when it really was difficult.

But good engineering doesn’t sell, and salesmen always make more money than engineers.

Bruno Putzeys—as much as anyone the reason we have inexpensive high-power, low-distortion amps—should have that star power but he does only among geeks like us on this forum.

Rick “an engineer” Denney
 
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