Class A/AB vs D -- Is it Audible in 2021?

[MrPeabody]

@MrPeabody , the impedance graph is measured on the same speaker that was tested as a load. And it corresponds to JBL datasheet. Re your surprise, please read the phase plot as well, not only the magnitude. You will see that the speaker complex impedance is capacitive between 3 and 8 kHz. This capacitance is added to filter capacitance and must change the frequency response. This is just physics, electrical engineering circuit analysis. And this must count with both magnitude and phase of the impedance. Magnitude plot is only a half of the story. Sorry for being technical, but it is needed for an explanation.

That the impedance is capacitive in that range is apparent in that this type of impedance plot shows capacitive reactance with negative slope and inductive reactance with positive slope. I'm not suggesting that there is anything at all wrong with you graph, only that it hasn't been adequately explained. For you to say something like, "This is just physics, electrical engineering circuit analysis" is not the least bit informative, and frankly a tad bit annoying. I suppose I must have misinterpreted your response graph. For some reason I had assumed that it was obtained by measuring RMS voltage at the speaker terminals. The reason I assumed this is that it seemed to suggest that other than the response variations brought on the effect under consideration, the response of the speaker is ruler flat in the truest possible sense. I'm now thinking that maybe your response curve was obtained by taking the difference between the speaker's acoustic response when connected to an amplifier with extremely low and uniform output vs. the speaker's acoustic response when driver by this class D amplifier. But I'm still only guessing.

 

[MakeMineVinyl]

I would bet that the hum had nothing to do with the difference in mains frequency and that you'd get the same hum if you brought it to the USA and plugged it in here.

Amplifiers of that generation weren't exactly known for their extremely low noise/hum - it was all about power to run the inefficient speakers of the time (I'm looking at you, Ohm).

 

[mhardy6647]

Buzz in my SET amp was a problem which I had to overcome. The buzz is from the single ended B+ power supply, if the amp has AC powering of the filaments, or inadequate filtering of DC used for the filaments. I used a inductor between the two B+ capacitors, doubled the capacitance of those caps, and run the filaments from an independent-per-channel regulated DC supply. There is still a slight buzz, but I have to stick my ear into the HF horn to hear it.

Are you using an SET with DHTs (direct heated triodes) being used as the power amplifier tubes (or as voltage amplifiers)? More to the point, are you using DHTs with AC filaments?

 

[MakeMineVinyl]

Are you using an SET with DHTs (direct heated triodes) being used as the power amplifier tubes (or as voltage amplifiers)? More to the point, are you using DHTs with AC filaments?

The output tubes are 2A3, and as I mentioned previously I built a two channel independently regulated power supply (one for each side) for the filaments of these tubes. I don't think any SET amplifiers would seriously consider AC on the filaments of DHT tubes, but I've seen instances of amps just using a bridge rectifier and capacitor, which is totally unregulated.

 

[Helicopter]

Buzz in my SET amp was a problem which I had to overcome. The buzz is from the single ended B+ power supply, if the amp has AC powering of the filaments, or inadequate filtering of DC used for the filaments. I used a inductor between the two B+ capacitors, doubled the capacitance of those caps, and run the filaments from an independent-per-channel regulated DC supply. There is still a slight buzz, but I have to stick my ear into the HF horn to hear it.

I didn't even know DC heater current was an option. I assume you designed the power supply. Edit: now I see you did.

From their Q&A, via Google translate, it sounds like the high voltage is DC:

2Q: Why doesn't this tube rectifier use tube rectifier? Why not all scaffold welding?

Answer: Our 845 amplifier is a high-power amplifier among household amplifiers. It is impossible for such a large power to be supplied by bile tube rectification. Even with eight rectifier tubes, high power output cannot be achieved, so only transistor rectification can be used. Our machine uses scaffolding and PCB board mixed welding, instead of all scaffolding welding, because the whole machine uses a lot of components, using nearly 300 components, which are more than the ordinary amplifiers on the market. Much. In order to save space, the main parts of the sound channel are welded by scaffolding, and the auxiliary parts (such as the protection circuit and the power supply part) are welded on the PCB board, so the hybrid welding method is adopted .

 

[pozz]

wimpsy amp

This is definitely a distinct type

 

[watchnerd]

BTW, what is Class G?

The new JBL SA750 uses it.

 

[MakeMineVinyl]

BTW, what is Class G?

The new JBL SA750 uses it.

Switched rails.

 

[MakeMineVinyl]

View attachment 104620
I didn't even know DC heater current was an option. I assume you designed the power supply. Edit: now I see you did.

From their Q&A, via Google translate, it sounds like the high voltage is DC:

2Q: Why doesn't this tube rectifier use tube rectifier? Why not all scaffold welding?

Answer: Our 845 amplifier is a high-power amplifier among household amplifiers. It is impossible for such a large power to be supplied by bile tube rectification. Even with eight rectifier tubes, high power output cannot be achieved, so only transistor rectification can be used. Our machine uses scaffolding and PCB board mixed welding, instead of all scaffolding welding, because the whole machine uses a lot of components, using nearly 300 components, which are more than the ordinary amplifiers on the market. Much. In order to save space, the main parts of the sound channel are welded by scaffolding, and the auxiliary parts (such as the protection circuit and the power supply part) are welded on the PCB board, so the hybrid welding method is adopted .

I've never heard of scaffold welding. Any idea what they mean in this context?

 

[Doodski]

BTW, what is Class G?

The new JBL SA750 uses it.

Using multiple rail voltages. The amp operates on the lower rail voltage except for peaks when the higher rail voltage is also run into the load.
https://en.wikipedia.org/wiki/Power_amplifier_classes#Classes_G_and_H
https://sound-au.com/articles/class-g.htm

 

[Helicopter]

I've never heard of scaffold welding. Any idea what they mean in this context?

I think that means point-to-point wiring.

 

[Helicopter]

I think that means point-to-point wireing.

Bile is from gallbladder and refers to the glass ribbon envelope of the vacuum tube.

 

[MakeMineVinyl]

I think that means point-to-point wireing.

I've never heard that term, other than what I have to rent soon to work on the side of my house. I'll have to use the name 'scaffold wiring' to confuse and confound my co-workers.

 

[mhardy6647]

I've never heard of scaffold welding. Any idea what they mean in this context?

It means automated translation from Chinese to Amurrican/English.

BTW, what is Class G?

The new JBL SA750 uses it.

Switched rails.

Yup, it was a thing back in the day (late 1970s). Hitachi was big into it.
HitachiSR2004 by Mark Hardy, on Flickr

and, as @Doodski mentioned, "Class H" (embraced by Soundscraftsmen in roughly the same timeframe) was kinda, sorta similar.

source: https://worldradiohistory.com/hd2/I...io-1986-Equipment-Directory-OCR-Page-0078.pdf

 

[watchnerd]

It means automated translation from Chinese to Amurrican/English.



Yup, it was a thing back in the day (late 1970s). Hitachi was big into it.
HitachiSR2004 by Mark Hardy, on Flickr

and, as @Doodski mentioned, "Class H" (embraced by Soundscraftsmen in roughly the same timeframe) was kinda, sorta similar.

View attachment 104681
source: https://worldradiohistory.com/hd2/I...io-1986-Equipment-Directory-OCR-Page-0078.pdf

What are the alleged benefits?

And why did it never really take off?

 

[MakeMineVinyl]

What are the alleged benefits?

And why did it never really take off?

The benefit is reduced dissipation across the output devices (and into the heatsink) at times where the output signal level does not warrant higher rail voltages. Something like +- 20VDC dissipates a lot less heat than +-60VDC. The tangible benefit is that heatsinks can be smaller, and thus less costly/bulky/heavy, and of course less energy is wasted as useless heat.

This technique has never gone away, and is probably going to become more prevalent as awareness of energy conservation becomes more of a factor. It is especially beneficial in multi-channel home theater amplifiers where power density is high. I believe the Emotiva amplifiers use this technique.

One gotcha about class H is that it can be subject to the same type of 'glitches' as one would see in regular crossover distortion, but in this case it occurs at much higher wattages, as the rails are switching, and presumably is 'covered up' by the audio.

 

[Geert]

Class G is being used by Creek audio for example, also in their latest model https://www.creekaudio.com/products/integrated-amplifiers/voyage-i20/:

"The i20 biases its output stage with a more efficient form of Class AB, called Class G. Class G allows the powerful i20 circuitry to fit inside a slim case yet run cool, even with a relatively small heats ink. A Class G amplifier operates with two voltage levels. The i20 tracks the input and output signals and a clever MOSFET circuit switches momentarily to higher voltage if required. To identify to the user when the amplifier is operating in Class G mode a letter G is displayed on the OLED display".

Also Arcam uses it to improve power efficiency.

 

[Hrodulf]

Fellas, you don't listen to an amp class, you listen to a real-world realization via specific circuit materialized with actual parts. It would be a perfect scenario to hear the limitations of an amplification class.

I've heard stellar and horrid amps of every class both tube and sand. Class-D is the one poised to prevail mostly because that's where the money is going.

 

[Helicopter]

My Harman Kardon HK3770 stereo receivers use 'lightweight digital power supply' with class AB amplification. I am guessing that means class H. They are light and powerful, but they get hot and go into protection mode, so I put thermostat regulared fans in the cabinet above.

Edit: definitely have no special sound. Actually it is probably class G considering how the new SA750 appears to be almost identical except for streaming, a nice retro case, and the 7x higher street price. I wonder if SA750 also overheats and goes into protection mode.

 

[DonH56]

Emotiva Gen3 amps use class G IIRC. The Benchmark AHB2 uses class H. Both are essentially class-AB amplifiers with varying (switched for class G, tracking for class H) power rails to reduce heat and power during normal operation.