- Joined
- Jan 28, 2017
- Messages
- 688
- Likes
- 887
a nice problem to have! thank you for contributing your expertise to this forum. I placed an order this morning
-
Welcome to ASR. There are many reviews of audio hardware and expert members to help answer your questions. Click here to have your audio equipment measured for free!
Review and Measurements of Benchmark AHB2 Amp
- Thread starter amirm
- Start date
I don't understand what you meant with this. I don't remember seeing any significant non-harmonic distortion componenets with single tone distortion AB class amp tests. Not with any AB class tested by ASR nor anywhere else.
I used the 2.6 number because this is the minimum impedance of the speakers that that we keep on hand as examples of hard-to-drive 8-Ohm speakers. Part of the logic in this choice is that 4 Ohm speakers will often have minimum impedances that are lower than 2.6 Ohms. So, these speakers fit in the middle ground between 8-Ohm and 4-Ohm speakers, making them a good example for a general discussion.
Change the 2.6 Ohm number in the spreadsheet to a number that matches the minimum impedance of your speakers.
The zero crossings are a function of all frequencies being reproduced. Music is not a single tone. The zero-crossing distortion causes IMD when playing more than one tone.
Aerith Gainsborough
Addicted to Fun and Learning
- Joined
- May 4, 2020
- Messages
- 970
- Likes
- 1,444
You know what would be cool to have?
Sound examples of the different distortion types, so we could try to listen for it in our gear. I can create harmonic distortion in REW but I have no idea how to create the other types.
That for sure, but I thought we were still discussing audilibity of the single tone 1kHz.
Btw, as you noticed IMD is specced with the same 0.03% figure as THD but no detail figures for lower power.
That also makes sense, but THX is also probably right with disqualifing speakers which impedance drops below 3.5 Ohm as kind of a "bad" design. Personally I agree with them as with modern drivers this can be avoided.
2.6 isn't overly conservative either. My old Polk LSi9's dip down to 2.5 ohms and my JBL S/2600 dips down to 1.8 ohms!
I also agree with THX. There are some very good engineers that have worked for THX.
What do you make of the THX certification process for HT AVRs/AVPs? The Onkyo preamp that was tested here is THX UItra2 certified, yet didn't do as well as you'd expect for the price. I think there's a large community of people who want both 2-ch and surround sound hifi. We're seeing how the THX AAA topology is working well, and it'd be wonderful to have a new generation THX Ultra 3 standard where we don't have to wait for Amir to test things.
At low power, the amplifier may produce more IMD than the speaker. But the main issue is that the IMD produced by amplifier crossover distortion has many high order products. This means that it will not sound the same as the IMD produced by a speaker.
Coupla' comments, bearing in mind I am no expert on audio amps:
1. Many amplifiers exhibit low-level distortion at low power but they are not typically tested there. Looking through a few old Stereophile and Audio test results, crossover distortion is fairly obvious at 1 W when they do they test at low power (rare). I tend to believe (but do not know, no expert) this may be exacerbated by the trend for multiple amplifiers in one chassis and subsequently lower bias current. Crossover distortion is pretty ugly; I am not sure I would call it IMD though that is a by-product. It can generate a series of spurs based on the waveform that has both harmonnic and non-harmonc content. For a data converter designer, it is akin to sampling glitches from a T/H or at the output of a DAC.
I am going to have to rethink my assumption that the reduction in SINAD (THD+N) at low power levels is usually noise-dominated. It looks like that may not be true these days.
I don't know if the hysteresis of a class D amp contributes in the same way as crossover distortion; IME the error waveform (looking at it extracted from the output with the fundamental notched out) is somewhat similar but frequency distribution quite different and moved above the audio band where it is more innocuous. Of course class D has other challenges... As does any other class.
2. Last I checked the THX spec min load was 3.2 ohms, though my memory may be faulty. The trend for a number of years now has been lower impedances and there are quite a few speakers besides ESLs (which often go below 2 ohms) that are in the 2-3 ohm range. Heck, even my old Maggies measured something like 2.7 ohms at HF where the ribbon took over.
FWIWFM - Don
Edit: Crossed posts with John, who is an expert, and at least my comments don't appear completely out-of-touch.
1. Many amplifiers exhibit low-level distortion at low power but they are not typically tested there. Looking through a few old Stereophile and Audio test results, crossover distortion is fairly obvious at 1 W when they do they test at low power (rare). I tend to believe (but do not know, no expert) this may be exacerbated by the trend for multiple amplifiers in one chassis and subsequently lower bias current. Crossover distortion is pretty ugly; I am not sure I would call it IMD though that is a by-product. It can generate a series of spurs based on the waveform that has both harmonnic and non-harmonc content. For a data converter designer, it is akin to sampling glitches from a T/H or at the output of a DAC.
I am going to have to rethink my assumption that the reduction in SINAD (THD+N) at low power levels is usually noise-dominated. It looks like that may not be true these days.
I don't know if the hysteresis of a class D amp contributes in the same way as crossover distortion; IME the error waveform (looking at it extracted from the output with the fundamental notched out) is somewhat similar but frequency distribution quite different and moved above the audio band where it is more innocuous. Of course class D has other challenges... As does any other class.
2. Last I checked the THX spec min load was 3.2 ohms, though my memory may be faulty. The trend for a number of years now has been lower impedances and there are quite a few speakers besides ESLs (which often go below 2 ohms) that are in the 2-3 ohm range. Heck, even my old Maggies measured something like 2.7 ohms at HF where the ribbon took over.
FWIWFM - Don
Edit: Crossed posts with John, who is an expert, and at least my comments don't appear completely out-of-touch.
Last edited:
restorer-john
Grand Contributor
My current main speakers dip to 2.6 Ohms at 120Hz and average around 4 Ohms between 100Hz and 1Khz. That's not even what I would consider remotely a difficult speaker. They aren't a problem for competently designed amplifiers.
Drawing a line at 3.5 ohms (mentioned above) as being a "bad design" is just a joke, surely?
restorer-john
Grand Contributor
reserved
restorer-john
Grand Contributor
Cheeky.
(I reserved it for a *little project I'm doing this fine Saturday morning. *little- you know, things that always take more time and effort than you'd think...)
Cheeky.
(I reserved it for a *little project I'm doing this fine Saturday morning. *little- you know, things that always take more time and effort than you'd think...)
Story of my life. Cheeky, and projects taking at least 2x what I expect in time and 4x the effort. Might have the multipliers backwards, and probably still too low...
How is it “obvious” from the measurements?
I have high efficiency speakers and I definitely hear an improvement when running my Accuphase in pure Class A. I am in the 33-100 milliwatt range for normal listening.
One thing that I noticed about the Accuphase amps is that it doesn’t have the high THD at low power that most amp topologies have.
Coach_Kaarlo
Active Member
But the speaker cable resistance is also halved right? Stereo = 2 runs with 3m speaker cables for one amp. Mono = 1 x 3m speaker cable each amp. Seems to make a difference in the numbers I get with your spreadsheet. Slightly compensates for the reduced damping factor.
I like the tip re amp positioning - thanks!
I have been reading up on various tests and standards regarding the dynamic measurement of amplifiers - trying to get to the bottom of what I am hearing and observing when comparing amplifiers. Can you suggest any simple methods to test the dynamic behaviour I have observed (cone extension, kick drum impact, etc).
Last edited:
It is obvious in the distortion pictures; when they notch out the fundamental and superimpose the residue (error) on the fundamental waveform you see little glitches at each zero crossing. Harmonic distortion, at least low-level, is similar in shape to the fundamental but higher in frequency, and noise produces spikes/signals not correlated to (at random times compared to) the fundamental. It is not a foolproof means of finding crossover distortion but is not unreasonabe IME.
For folk who are not following the technical gibberish, this is a very rough quick sketch. Blue is the fundamental, a single tone (sine wave). When you notch out (remove) the fundamental, and amplify (greatly, typically 100x to 1000x) what is left, you can see the distortion waveform. In this picture, the amplified disortion terms are the second harmonic (2HD) in purple, and the crossover distortion in red. Notice the 2HD is the same as the fundamental but twice the frequency -- two cycles for each full cycle of the fundamental. In general it would be a little out of phase from the fundamental. The crossover distortion presents as little "glitches" that occur at each zero crossing of the fundamental. Their shape vries with the design, but usually (at least in my experience) tend to look something like this. These sharp little "spikes" produce wideband (high) frequency content with a fundamental at twice the main (fundamental) tone but, unlike regular 2HD, these narrow pulses create a "spray" of high-frequency noise that can sound quite obnoxious. Noise (not shown) would be random "grass" or "fuzz" in the residue (error) signal.
HTH - Don
Similar threads
