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[SOLVED] Design precaution of 78xx and 79xx series regulators

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120W of heat!? Then you are making a class-A home heater, not a modern audio amplifier that has a small class-AB idle current and lots of negative feedback for extremely low distortion. The negative feedback and wide bandwidth does the error correction like most modern hifi amps.

Crossover distortion is a totally different animal. It is constant and is independent to the input signal. That is it affect the program the most when the signal is low. Another miss conception in THD spec that is tested in large signal. Crossover distortion usually contain higher harmonics, You cannot just use GNFB to lower crossover distortion as loopgain ( or they called feedback gain in audio world) is very low above 10KHz. For example, even if you have BW of 200KHz, at 20KHz, loopgain is only 20dB. You don't cancel much with GNFB.

I studied hundreds of schematics of all different kind of hifi amps, from cheamp Yamaha to Krell, Mark Levingson, Bryston etc. None has very high loop gain. They are cut corners in the designs like I said before. The more expensive ones just use more output transistor pairs to cover up the short comings ( and it really works, that's the key for lowering crossover distortion and lower output impedance). In fact, it is not a good practice at all to rely on GNFB to lower distortion. GNFB should be used as the final touch.


I use 1A of idle current to get a bigger class A region. Higher idle current shift the crossover region to higher signal level......that is signal to THD ratio is much bigger. for 1A, the amp remains in class A until the peak signal is 2A X 8ohm = 16V for 8ohm speaker or W= 1/2 X 16V X 2A = 16W. At this level, crossover distortion is drown out.

I don't know of any other way around this except designing a TRUE error correction local feedback around the OPS. That's my next design.

The only other design has some hope is class H that has a small class A amp as the main amp, then modulate the two rails according to the input signal to keep the voltage across the middle amp low to reduce heat. BUT this has a big short coming both in implementation and theory. In theory, modulation of the rail will introduce distortion to the main amp as the collector output impedance is NOT infinite. That's it has finite Early Voltage. Secondary and more problematic is with collectors of output transistors not bypassed solidly to ground, you invite parasitic oscillation.

So many amps have instability, I really don't think people appreciate the difficulty of stabilizing these audio amps. Most just use an inductor to separate the output that drive the speaker from the output of the power amp. This is really cheating and has big consequence in THD. Another miss conception in THD spec that using a linear resistor load. The inductor does NOT increase THD measure BUT become a problem when driving a real speaker................... I ranting on to much already.
 

Alan, you mentioned crossover distortion at 20kHz. Can you hear high frequency harmonics of 20kHz? I bet you cannot hear 60kHz and higher harmonics of 20kHz.
Can you hear 0.1% distortion of medium frequencies even if the fundamental frequency is blasting loud? 0.1% is -60dB. Doesn't the fundamental frequency cover up the harmonics?

You mentioned that crossover distortion affects the program the most when the signal is low. But that is when a class-AB operates in class-A with no crossover distortion. The negative feedback is very low in a class-AB amplifier and at low levels you can't hear it anyway because it is less that the sensitivity of your hearing.
 

Alan, you mentioned crossover distortion at 20kHz. Can you hear high frequency harmonics of 20kHz? I bet you cannot hear 60kHz and higher harmonics of 20kHz.
Can you hear 0.1% distortion of medium frequencies even if the fundamental frequency is blasting loud? 0.1% is -60dB. Doesn't the fundamental frequency cover up the harmonics?

You mentioned that crossover distortion affects the program the most when the signal is low. But that is when a class-AB operates in class-A with no crossover distortion. The negative feedback is very low in a class-AB amplifier and at low levels you can't hear it anyway because it is less that the sensitivity of your hearing.


But remember 3rd harmonics of 5KHz is 15KHz. At 15KHz, loopgain is still low, there's no other way about it. Even when I push 350KHz BW, at 15KHz, it's only a little over 26dB loop gain. So crossover distortion can start a low frequency and the effects are still in audible range.

About class AB, not all class AB are the same. You can calculate the class A region. Like I did in the last post, take 2 times the idle current multiply by the impedance of the speaker to get the peak voltage. Then calculate the class A power. The lower the idle, the smaller the class A region. Like if you run 200mA idle, your peak class A current is 0.4A. Peak class A voltage for 8ohm is 3.2V or peak class A power is 1/2 X 0.4A X 3.2V = 0.64W!!!! This is not much of class A. You run class B most of the time. You will have crossover distortion as low as 1W signal power.

Also, this is well studied, called the Oliver's condition that the lowest crossover distortion is when the r'e of the transistor equals to the value of the external emitter resistor Re used for the output transistor. The higher the idle current, the lower the emitter resistor you can use. The lower the idle current.....say 50mA per output pair, emitter resistor used is 0.5ohm. This govern the output impedance of the output stage OPS. ( forget the GNFB, it's not the way to go to lower output impedance). So you want to run higher current per stage to get the max benefit by lowering the emitter resistor.

Also it is well studied that double the output pairs literally half the crossover distortion. All these shows more pairs with higher bias current is the key......unless you run class A to avoid crossover distortion.

I have been struggling on this for two years, there is no way out of this. If you have some other idea, I would love to hear it. I am going the error correction route in my next design. Right now, I can only design lower power amp using +/-34V rails. I get about 56W with 8ohm speaker. But my amps are happy with lower impedance speakers. with 4ohm, it's 100W, with 2ohm, it's 200W. The lower the better. As is, the chassis alone is $250USD each from China. My bigger amp use a bigger chassis ( Krell KSA50 clone chassis) that is $350USD to use 44V rails to get 100, 200 and 400W resp. Most of the money for a high quality amp is on the chassis, transformer and the filter caps, the rest are not much.


One thing commonly known, the system sounds better when cranked to higher level. Only high end system can sound good at low volume. I suspect the crossover distortion play a major part of it. When you crank up the volume, crossover distortion becomes only a very small part and it's not affecting the sound quality, so a cheap system sound decent. Case in point, in the 80s, I often went to disco, the place I went used Klipsch, they sounded great. But one time I went there early before the dancing, they played in low volume. It sounded awful!!!
 
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You are paranoid about crossover distortion that cannot be heard and can barely be measured, and "high end" that sounds the same as much less costly amplifiers but looking pretty with most spec's that are barely measureable.
 

You are paranoid about crossover distortion that cannot be heard and can barely be measured, and "high end" that sounds the same as much less costly amplifiers but looking pretty with most spec's that are barely measureable.

That I don't agree. I just gave a blind test to someone that is into audiophile, actually thought I designed the Nakamichi PA-7 because it was obviously inferior only after 20sec listening. PA-7 is the Stasis almost identical ( actually better than )to Threshold S300 all designed by Nelson Pass. He guessed the worst one must be the one I designed.

There are so much fine details in power amp design, it's no where close to what people give them credit.

Also, it all depends on the quality of your speakers. I can tell you, even for some mid fi speakers like Kef, amp does make a difference. I gave my Kef floor standing and a Japanese SAE amp to my stepson. After a few years, the SAE blown, he replaced with some AV receiver. One day, I went there, something was really wrong with the sound. I chased down to the amp. I finally gave him my Marantz amp that I was not using, it was day and night difference, not quite as good as the SAE, but at least it's not insulting to the Kef. You can't tell me amp does not make a difference.
 
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Amplifiers sound the same unless they are old or defective. Speakers sound different but EQ can be added to make them sound almost the same.
 

Amplifiers sound the same unless they are old or defective. Speakers sound different but EQ can be added to make them sound almost the same.

You design power amp? There's a whole community in DIYaudio that begs the different. It's no point in talking about this. It becomes non technical and subjective. I guess there's a whole community of people that own Krell and other high end amps don't know what they are listening.
 

Many audiophools have more money than audiophiles.

I once interviewed with a company that makes high-end audio gear. Their chief engineer told me they design amps with .001% distortion so that rich people can brag about their specs. He admitted that it was a bunch of baloney.
 

...........................


Never mind.
 
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I bought inductors for the crossover networks of some speakers I put together from a local high end speaker manufacturer. They were the wrong inductance so I took an audio spectrum analyser to the manufacturer and showed them that their high end speakers used those wrong inductors and had the frequency response screwed up. They were not upset about their mistake, they only care if a speaker does not look perfect when it has a scratch on it.

My son has a high end car. He "brags" about its power by burning out the tires whenever he accelerates it. Most of the Chinese kids at the local university drive VERY expensive high end cars.
 

I bought inductors for the crossover networks of some speakers I put together from a local high end speaker manufacturer. They were the wrong inductance so I took an audio spectrum analyser to the manufacturer and showed them that their high end speakers used those wrong inductors and had the frequency response screwed up. They were not upset about their mistake, they only care if a speaker does not look perfect when it has a scratch on it.

My son has a high end car. He "brags" about its power by burning out the tires whenever he accelerates it. Most of the Chinese kids at the local university drive VERY expensive high end cars.

What speaker company?

As a Chinese, I know how they feel about cars. I own two MB, I know very well they are no where as reliable as the Japanese cars, not even as some American cars. AND it's very expansive to fix because of the stupid engineering. But I will buy another one again. Why? Nothing to do with it's better, it has everything to do with the status. It's purely the name and I am willing to pay for it.

BTW, the German cars DO perform better. If you disagree, you don't know what you are talking about. It's the combination of handling, agility and comfort. You test drive a Lexus LS, you quickly find out it feels like a ship instead of a car. Also, particular MB, their safety is at the top. The roof strength is the highest. The ML/GLE tested like 5 times the body weight ( 25,000lbs) in the crush test. More than ANY of the Japanese cars. For an SUV, this is mighty important as they tend to roll over. So don't say there is no reason.....Status, performance and safety.


But hifi is a totally different thing. I was from Hong Kong. Lots of people are into audiophile. We talk about sound quality first, not just the brand. For me, brand means nothing to me in hifi. Car does. Like I prefer Rolex watch over Seiko even though I know Seiko out last Rolex any time of the day. I go in with eyes wide open.

Bottom line, you have to be well informed.

What speakers and amps you guy own? Design power amps? I ask because there is also law of diminish return. It takes 10% effort to get 90%, but it takes 90% effort to get the last 10%. More accurately, 89% effort to get the last 2%!!! So it really depends on how high your aim.

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I can't speak for all hifi people or car people, I can only speak for myself. I go in with eyes wide open. I don't envy Krell, I intend to design and build to beat it for much less. I do hear the difference from mid fi to high end. Not all high end amps are good, I don't think much of Krell and Mark Levingson. But they did the output stage right. If I were to buy an amp, Adcom 565 would be my first choice, it's not expensive, it's the design.
 
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