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Review and Measurements of Benchmark AHB2 Amp

RichB

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Sure we do. But question here is if we would be able to hear 2nd and 3rd harmonic distortion component of the 1kHz or 3 kHz tone which is 40 or 50 dB lower than the base tone
Yes. I detected that when my Salon1 midrange began to fail. It was detectable at my normal listening levels (<80 dB) with female vocals.
Here is the measurement of 97 dB that was used to identify which driver was failing:
SalonRightSpectrum.jpg
SalonRight Dimple.JPG


I did not measure it a 80 dB so I cannot say what level the distortion was occurring at that level but it was surely less.

Again, detected at low levels listing to music in stereo. Something was clearly wrong with the right speaker. I replaced both midranges which I later discovered had a know flaw with the glue (as told to me by Harman tech support). Harman provided the midrange in the Voice1 for free but Salon1s were well out of warranty.

- Rich
 

John_Siau

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1) @John_Siau , without naming the amplifier can you at least say something like the MSRP or approximate weight? In the automotive world, people are happy to say that they benchmarked their car against the Porsche 911 or “insert competitor here”. It would be nice to understand what is going on.

2) I think what you are saying is that there are audible differences between amplifiers, and that the science of it is a) crossover distortion b) damping factor. But with all of the caveats you speak of.
The other amplifier is rated at 0.05% THD+N into 8-Ohms at 150 watts.

These are better than average specifications, and yet defects are clearly audible at low power levels when playing test tones.
 

QMuse

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Yes. I detected that when my Salon1 midrange began to fail. It was detectable at my normal listening levels (<80 dB) with female vocals.
Here is the measurement of 97 dB that was used to identify which driver was failing: View attachment 67327 View attachment 67328

I did not measure it a 80 dB so I cannot say what level the distortion was occurring at that level but it was surely less.

Again, detected at low levels listing to music in stereo. Something was clearly wrong with the right speaker. I replaced both midranges which I later discovered had a know flaw with the glue (as told to me by Harman tech support). Harman provided the midrange in the Voice1 for free but Salon1s were well out of warranty.

- Rich
Distortion you're showing is app 2% (several components at -33dB). That is nowhere close to the distortion of -73dB which is claimed to be audible in the "First watt" paper.
 

QMuse

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The other amplifier is rated at 0.05% THD+N into 8-Ohms at 150 watts.

These are better than average specifications, and yet defects are clearly audible at low power levels when playing test tones.
My amplifier is Rotel RMB-1075, THX certified. It has app the same power when 2 channels are played. Here are the specs:

THD (20-20,000Hz)
cont. rated power 0.03%, maximum
one-half rated power 0.03%, maximum
one watt per channel 0.05%, maximum
IM Distortion (at rated ower, 60Hz:7kHz = 4:1) 0.03%, maximum
Damping Factor (8 ohms) 180

So, if I turn the amp to 0.01W of power it will play quietly, somewhere between 60-65dB with speakers of 87dB/1m sensitivity. How am I supposed to hear the distortion of 0.05% which corresponds to -66dB when it is lower than the SPL of the test tone?
 

Bombadil

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The problem with traditional class AB amplifiers is the thermal stability of the bais circuit. If the biasing of the class-AB output stage is ideal, the crossover distortion can be nulled to a relatively low level, but it is very difficult to maintain this over temperature range and over time. This nulling is also impacted by the load impedance and phase angles. Remember, most tests are performed into a resistive load.

When crossover distortion does occur, it raises havoc with the amplifier's feedback network. The transient steps produced by the zero crossing, get extended in time by the slow response of the global feedback network.

The ABH2 avoids this by using high-bandwidth feed-forward error correction combined with a minimal amount of feedback. This is part of the reason why the AHB2 can have a bandwidth that exceeds 200 kHz, while being stable into any phase angle within the audible band. Feed-forward correction is inherently stable.
Can you share your insight on when the backlog of orders for this amp will be resolved?
 

John_Siau

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Can you share your insight on when the backlog of orders for this amp will be resolved?
The issue has been heatsinks. These have a very long lead time and we underestimated the demand. We have been setting sales records over the last 2 months and we have been aggressively purchasing raw parts since January. We received a large shipment of heatsinks yesterday and we will be shipping many AHB2 amplifiers next week. We should be shipping from stock by June 12, if we don't get overwhelmed with new orders.
 

RichB

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My amplifier is Rotel RMB-1075, THX certified. It has app the same power when 2 channels are played. Here are the specs:

THD (20-20,000Hz)
cont. rated power 0.03%, maximum
one-half rated power 0.03%, maximum
one watt per channel 0.05%, maximum
IM Distortion (at rated ower, 60Hz:7kHz = 4:1) 0.03%, maximum
Damping Factor (8 ohms) 180

So, if I turn the amp to 0.01W of power it will play quietly, somewhere between 60-65dB with speakers of 87dB/1m sensitivity. How am I supposed to hear the distortion of 0.05% which corresponds to -66dB when it is lower than the SPL of the test tone?
From Benchmark's ABX test:
https://benchmarkmedia.com/blogs/ap...of-the-first-watt?_pos=1&_sid=8a6f4a42a&_ss=r

At a 1 watt output, this class-AB amplifier produces a distortion waveform that measures 70 dB below the output level of the 1 watt test tone (see graph at the bottom of this application note). This means that the power produced by the THD+N is 70 dB below 1 watt. If we drop the level of the test tone by 20 dB, the output power is 0.01 W. At this output level the amplifier was still producing distortion at a level of 73 dB below 1 watt. At 0.01 watt the distortion waveforms look virtually identical to the 1 watt waveforms.
I suppose the audibility in this particular test is dependent on the performance of your amplifier at 0.01 watt that is not included in the posted specification. Assuming it is also attenuated may or may not be correct.

- Rich
 
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QMuse

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From Benchmark's ABX test:
https://benchmarkmedia.com/blogs/ap...of-the-first-watt?_pos=1&_sid=8a6f4a42a&_ss=r



I suppose the audibility in this particular test is dependent on the performance of your amplifier at 0.01 watt that is not included in the posted specification. Assuming, it is also attenuated may or may not be correct.

- Rich
Amps are not tested at 0.01W per THX certification test, but as is the case with AB class amp that @John_Siau tested it is reasonable to assume that my amp also has the same distorion there.

Btw, don't get me wrong here, personnaly I think AHB2 is SOTA amp, except for that small housing which can house only tiny heatsinks.
 
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John_Siau

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My amplifier is Rotel RMB-1075, THX certified. It has app the same power when 2 channels are played. Here are the specs:

THD (20-20,000Hz)
cont. rated power 0.03%, maximum
one-half rated power 0.03%, maximum
one watt per channel 0.05%, maximum
IM Distortion (at rated ower, 60Hz:7kHz = 4:1) 0.03%, maximum
Damping Factor (8 ohms) 180

So, if I turn the amp to 0.01W of power it will play quietly, somewhere between 60-65dB with speakers of 87dB/1m sensitivity. How am I supposed to hear the distortion of 0.05% which corresponds to -66dB when it is lower than the SPL of the test tone?
In answer to your first question, the fundamental does not fully mask the harmonics that are, almost always, at significantly lower levels. If they were masked, all musical instruments would sound like tea kettles (producing what would sound like pure sine waves). Harmonics are audible because they are separated by frequency.

In further answer to your question, the THD+N produced by your amplifier will not continue to go down at lower levels.

This can be seen from the fact that the one watt THD+N is higher than the half-power THD+N. This means that the 0.01 W THD will be much higher than 0.05%. Here are some calculations:

At 1 W, 0.05% is -66 dB relative to 1 W.

At 0.1 W, the THD+N will still be about -66 dB relative to 1 W or -56 dB relative to 0.1 W. THD+N will measure 0.16% at 0.1 W from your amplifier.

At 0.1 W, the THD+N will still reproduced at the same acoustic level (87 dB - 66 dB = 21 dB SPL).

At 0.01 W, the THD+N will still be about -66 dB relative to 1 W and the acoustic level of the THD+N will still be 21 dB SPL.

At 0.01 W the THD+N will measure -46 dB which is 0.5 %.

What I cannot tell from your specifications is the spectrum of the THD+N at low power.

The limitation may be noise (the N portion of THD+N) or it may be THD produced by crossover distortion.

An FFT of the low-power output will show the difference between noise and THD. This is an important test, but it rarely shows up on a spec sheet.

Unfortunately, low-power THD+N is rarely limited by random noise. It is usually crossover distortion plus power supply noise (line-related hum and buzz).
 

QMuse

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In answer to your first question, the fundamental does not fully mask the harmonics that are, almost always, at significantly lower levels. If they were masked, all musical instruments would sound like tea kettles (producing what would sound like pure sine waves). Harmonics are audible because they are separated by frequency.
Well of course the harmonics are of different frequency than fundamental. :D When I mentioned masking I was referring to 2 things:

- tone masking with other tones of music material
- distorion masking by the speaker

In further answer to your question, the THD+N produced by your amplifier will not continue to go down at lower levels.

This can be seen from the fact that the one watt THD+N is higher than the half-power THD+N. This means that the 0.01 W THD will be much higher than 0.05%. Here are some calculations:

At 1 W, 0.05% is -66 dB relative to 1 W.

At 0.1 W, the THD+N will still be about -66 dB relative to 1 W or -56 dB relative to 0.1 W. THD+N will measure 0.16% at 0.1 W from your amplifier.

At 0.1 W, the THD+N will still reproduced at the same acoustic level (87 dB - 66 dB = 21 dB SPL).

At 0.01 W, the THD+N will still be about -66 dB relative to 1 W and the acoustic level of the THD+N will still be 21 dB SPL.

At 0.01 W the THD+N will measure -46 dB which is 0.5 %.

What I cannot tell from your specifications is the spectrum of the THD+N at low power.

The limitation may be noise (the N portion of THD+N) or it may be THD produced by crossover distortion.

An FFT of the low-power output will show the difference between noise and THD. This is an important test, but it rarely shows up on a spec sheet.

Unfortunately, low-power THD+N is rarely limited by random noise. It is usually crossover distortion plus power supply noise (line-related hum and buzz).
I only have this spec for noise, and that one doesnt' tell much about S/N at low power levels.

S/N Ratio (IHF A) 115dB

Even if your assumption is correct (and I can't prove otherwise as I don't have more detialed specs, except for the fact that amp is THX certified), speaker distortion also raises with low level signals so my question was actually how am I supposed to hear amp distorion ver the speaker distorion?
 

John_Siau

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I only have this spec for noise, and that one doesnt' tell much about S/N at low power levels.

S/N Ratio (IHF A) 115dB

Even if your assumption is correct (and I can't prove otherwise as I don't have more detialed specs, except for the fact that amp is THX certified), speaker distortion also raises with low level signals so my question was actually how am I supposed to hear amp distorion ver the speaker distorion?
There is more information here than you might think:

Given an SNR of 115 dB relative to a 120 W output, the SNR will be 21 dB lower relative to 1 W. This means that the output noise is 94 dB relative to 1 W. Given the sensitivity of your speakers, the noise will be inaudible (87-94 = -7 dB SPL). No audible noise.

This means that the THD+N at 1 W is almost entirely distortion. Remember, we calculated THD+N at -66 dB. This means that the distortion is 28 dB higher than the noise at a 1 W output. So the THD (no +N) is -66 dB at a power output of 1 W. This THD will be reproduced at 87 - 66 = 21 dB SPL.

If this THD is caused by crossover distortion it is really IMD and it may not be well masked.

Crossover distortion sounds much different than the harmonic distortion produced by speakers.
 

QMuse

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There is more information here than you might think:

Given an SNR of 115 dB relative to a 120 W output, the SNR will be 21 dB lower relative to 1 W. This means that the output noise is 94 dB relative to 1 W. Given the sensitivity of your speakers, the noise will be inaudible (87-94 = -7 dB SPL). No audible noise.

This means that the THD+N at 1 W is almost entirely distortion. Remember, we calculated THD+N at -66 dB. This means that the distortion is 28 dB higher than the noise at a 1 W output. So the THD (no +N) is -66 dB at a power output of 1 W. This THD will be reproduced at 87 - 66 = 21 dB SPL.

If this THD is caused by crossover distortion it is really IMD and it may not be well masked.

Crossover distortion sounds much different than the harmonic distortion produced by speakers.
I agree, this seems logical. But wouldn't it be masked by speaker's IMD which is higher than that of the amp?
 

John_Siau

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If the distance remains 10' and using bridged mono is there a gauge of wire that can make up the difference. At some point, does thicker gauge cause other issues?

- Rich
Rich, this is an excellent question and it will probably be the topic of another application note.

The problem is that the inductance of a cable is determined, in part, by the spacing between the conductors. Large conductors produce a cable where the conductors have a larger center-to-center spacing. The solution to the inductance problem is to have multiple small conductors running in parallel. The "11 AWG" cable that we sell is really a 4-conductor cable with 14-AWG conductors. Two 14-AWG conductors wired in parallel is equivalent to 11-AWG in terms of resistance, but the 4-conductor cable can have a lower inductance than a 12-AWG 2-conductor cable given the same insulation thickness. The star-quad configuration also reduces the inductance.

I would not recommend conductors large than 12 AWG.

Use the Damping Factor spread sheet to calculate the performance of your system. Remember to use 370/2 for your bridged AHB2 amplifiers when looking for the correct row on the spreadsheet.

Link to spreadsheet:

https://cdn.shopify.com/s/files/1/0321/7609/files/DampingFactorCalculator.xlsx?v=1591278482
 

Bombadil

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The issue has been heatsinks. These have a very long lead time and we underestimated the demand. We have been setting sales records over the last 2 months and we have been aggressively purchasing raw parts since January. We received a large shipment of heatsinks yesterday and we will be shipping many AHB2 amplifiers next week. We should be shipping from stock by June 12, if we don't get overwhelmed with new orders.
a nice problem to have! thank you for contributing your expertise to this forum. I placed an order this morning :)
 

QMuse

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If this THD is caused by crossover distortion it is really IMD and it may not be well masked.
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.
 

John_Siau

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Nice well written paper! I would, though suggest that you use more realistic minimum impedance of 3.5 Ohm instead of 2.6 Ohm as that is the lowest figure recommended by THX speaker standard.
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.
 

John_Siau

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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.
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.
 
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Crossover distortion sounds much different than the harmonic distortion produced by speakers.
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.
 

QMuse

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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.
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.
 

QMuse

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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.
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.
 
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