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Redesign Elektor gigant2000 Amplifier


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Hi

I Love Accuphase Amplifier topology very much and it is long time I am trying
to find a good schematic.
I found Elektor gigant2000 design very similar to Accuphase. But I Need a Lower power
Design.
Can Anybody tell me how is possible to redesign it for 50 to 60Watt.

I Know some what to do, But I affraid to Miss something and can't get the result.

Here a link to German site which contains the schematic
http://www.corradodriver.de/HTML/El...000/Gigant1.htm


ThanX
Shahriar

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The 2kW amplifier uses many extra parts to handle the high voltages and high currents.
For only 60W it is best to design or copy a circuit for only 60W.

There are many excellent medium-power amplifier ICs available. St Micro and Philips make over 100 of them.

The TDA7294 is 70W and has Mosfets in its outputs. It is 11 years old and there are newer and better ones that I have not seen. The datasheet has a schematic for an amplifier, its parts list and a pcb layout.

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But All of the famouse Brands are biased in Class A with Big Heat Sink to avoid Distortion (Denon, Accuphase and ...) . Class AB is used in Usual Systems like JVC, SONY, LG and ...

Hi Shahriar,
Nobody can hear the very low distortion from a good class-AB amplifier. Some class-A amplifiers have lower distortion and some have higher distortion.

The famous brands make class-A amplifiers TO MAKE MORE PROFIT!
They have bigger transformers and heatsinks. They have more output transistors. They cost much more. People think they are better and buy them.

I would never heat my home with a class-A audio amplifier.
Some audio fanatics making headphones amplifiers have added a resistor to the output of an excellent class-AB opamp to make it operate in class-A for reduced distortion. It is the OPA2134 opamp that originally has only 0.00008% distortion.

The 2000W amplifier article is written in a foreign language so I can't read it. I noticed that its output transistors have a low bias current so that they operate in class-AB.
Too many parts would be wasted if it is modified for a lower power output.
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noticed that its output transistors have a low bias current so that they operate in class-AB.

90mA quiescent current for each Transistor is not Low.

Nobody can hear the very low distortion from a good class-AB amplifier


It is said that when your Amplifier is playing, Try to Increase Bias current to convert it from Class AB to Class A, Then You will Understand the difference in Details!

I have just heard it, Not tested. But I think It is really depending on Your Speakers. Without good Speaker, It is just wast of time.


Shahriar
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90mA quiescent current for each Transistor is not Low.

Class-AB usually has a current of 50mA in each output transistor. This one has 90mA and has 7 pairs of output transistors. Its supply is 140VDC so the quiescent power dissipation is only 88.2W.

If its ouput can create 130Vp-p across 8 ohms then its peak current is 65V/8 ohms= 8.1A. For class-A its quiescent current must be 8.1A which is a dissipation of 140V x 8.1A= 1134W. Rediculous.
This class-AB amplifier has a quiescent current of nearly 1/100th of the quiescent current of a same power class-A amplifier.

The max power of this amplifier into an 8 ohm speaker is "only" 264W RMS.
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ThanK you for your reply and your calculation.
I Think It is because of our different explanations of Class A.

Can You help me to Lower the output of this Power amplifier.

Tell me If I am right:

I want to decrease The 78V regulated supply to 30V and 70V Unregulated to 25V.
(I have a 3-way  cross over circuit and want to use this Amp for it)

1- I will reduce R22, R23 (3K3) to 680ohm.
2- I will reduce R19, R21 (10K) to 2K2

But I have No Idea about Which Transistor to remove among T15-T26 and T29-T34 If I use Just Two Output transistor (i.e T35 & T39)

ThanX in advance
Shahriar:rolleyes:

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You don't just remove transistors and reduce the power supply voltages to reduce the power of an audio amplifier.
With reduced supply voltage then many resistor values will be too high to supply enough current and each resistor will need to be analysed and a value selected for it to perform its function properly. It would be easier to design a reduced power amplifier from the beginning.

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With reduced supply voltage then many resistor values will be too high to supply enough current and each resistor will need to be analysed and a value selected for it to perform its function properly.


Can You give few examples of resistors you believe to be calculated again except those I Said.

ThanX
Shahriar
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  • 2 months later...

Nearly all excellent audio amplifiers are class-AB. Class-B has crossover distortion, class-AB doesn't. A few very hot amplifiers are class-A.


This is technically not correct.
Class-B is by far the most popular mode of operation, and probably more than 99% of the amplifiers currently made are of this type.


Class AB is less linear than either A or B, and in my view its is only legitimate use is as a fallback mode to allow Class-A amplifiers to continue working reasonably when faced with a low load impedance.

For outputs below a certain level both output devices conduct, and operation is Class-A. At higher levels, one device will be turned completely off as the other provides more current, and the distortion jumps upward at this point as AB action begins.

Class-AB amplifier will have more, not less distortion than a properly designed Class-B amplifier
Reference: Douglas Self - Audio Power Amplifier Design Handbook, 3rd edition
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Hi AN920,
I was quoted but I said only the 1st quote.

Douglas Self has an article "Distortion In Power Amplifiers" on the web at
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm

In it he frequently says "class-B" by mistake when he is talking about "class-AB".
He shows the schematic of a "generic" class-AB amplifier and says, "... representing something like 98% of the amplifiers ever built. It is the obvious starting point for amplifier investigation."
He frequently calls it a class-B amplifier.

He shows three traces of crossover distortion with a class-AB amplifier's
output transistors under biased, optimal and over biased.
Here is the very low distortion trace of the optimally biased class-AB amplifier:

He says, "All three traces were averaged 64 times to reduce noise.
The distortion in 14b (optimally biased) is normally invisible in a 80 kHz measurement bandwidth.
The RMS THD reading for Fig 14a was 0.00151%, for 14b 0.00103%,
and for 14c 0.00153%."

This is extremely low distortion for a power amplifier. 

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I don't think "Self" is calling it Class-B by mistake as he states in the foreword of his book

"most of the book is devoted to Class-B design, enough said".

The problem comes that most people see a biased Class-B amplifier and think it is a Class-AB. "Self" sees the same amplifier from his Engineering analysis as a Class-B.

Also from the book:
To summarise:
Class-AB is best avoided. Use pure Class-A or B, as AB will always have more distortion than either.
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Hi AN920,
I don't have the book by Douglas Self that you have. I have only his article about Distortion In Power Amplifiers on the web.

He shows a "blameless" amplifier designed by him and he claims it has 0.002% distortion from 10Hz to 20kHz when it is biased as class-A. But none of his distortion measurement graphs are anywhere near as good because his test equipment can't measure it.

He under-biased his amplifier so that the crossover distortion is higher than the noise level so that he could see how much the crossover distortion amount increased with reduced level.
But when the amplifier is biased optimally (it is biased so it must be class-AB) then its distortion is only 0.001% at 1kHz at higher levels where noise doesn't mask the distortion.

How can a class-B amplifier have less distortion than a class-AB amplifier??

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He talkes about a biased Class-B but says this is not a Class-AB. The distortion caused by gain-doubling in Class-AB seems to be more of a problem to him than to deal with the crossover distortion in Class-B amplifiers. He also states that most amplifier designs suffer from so much distortion in the small signal amplifier stages that it swamps the distortion caused by the output stage.

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Hi AN920,
We are talking about only 0.001% crossover distortion at 1kHz when the bias is optimal. It is masked by many other distortions and by noise.

I know that crossover distortion increases at low levels but then the noise is also a higher percentage so there is more masking of the distortion.
I also know that the gain of the amplifier decreases at higher frequencies which reduces the amount of negative feedback so that crossover distortion is more.

In his article on the web, Douglas Self shows a class-AB amplifier that is grossly over-biased so that it was in class-A one-quarter of the time. The 'scope picture shows the edges as the output doubles its gm as it shifts into class-A. The output level was high and the distortion measured only 0.00153%.

I might be able to hear 0.1% distortion at lower frequencies. I cannot hear distortion as high as 50% at any frequency higher than 10kHz because the harmonics are at frequencies higher than 20kHz.

Maybe very high frequencies intermodulate each other and create new lower difference frequencies that can be heard if the amplifier has high distortion at high frequencies.
It reminds me about Transient-Intermodulation-Distortion (TIM).

I can easily hear the crossover distortion "buzz" from an LM358 or LM324 opamp. They also have a high level of TIM.

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The jury is still out on TIM. Some people believe this distortion to be caused by a combination of other well known distortion types.

One of the misconceptions is the square wave output testing into a capacitive load, measuring the overshoot-ringing to evaluate transient response/distortion of the amplifier. It was proved to show nothing about the performance of the amplifier, but only the resonating of the Q-damped output inductor with the load.

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Hi AN920,
I agree that an amplifier should be designed for low distortion before a modest amount of negative feedback is applied.
The OPA134 opamp has a very low distortion of 0.00008% at 1kHz with a gain of 1, and 10 times more with a gain of 10. Its open-loop gain at 1kHz is 10,000 times so its open-loop distortion might be about 0.8% which is pretty good.

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Another problem with feedback is that you need to increase the slew rate BW of the front end gain stages including the feedback network by the factor of feedback (40 to 50 times) to prevent/minimize dynamic intermod distortion.

This was the reason so many transistor amplifiers sounded so bad in the early 70's. This situation improved after it was discovered and pointed out by Dr. Otala in his published papers. He suggested that minimum slew rates for power amps with a 30kHz BW should be 100V/us !

Also adding negative feedback to single-ended, differential pair or push-pull output stages using FET's can be very surprizing, as it actually may boosts higher order intermod distortion over the full spectrum.

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Here is a good explanation about the subtle differences between biased Class-B and AB

One of the most common forms of audio amplifier design is the class-B. The final stage of amplification in this class of amplification is split into two halves so that at any given moment only one half is producing sound. This results in an efficient method of amplification since no one transistor or output tube need do all the work by itself. However, the two halves, in a class-B amplifier, must be rigidly controlled by other circuits and must be very carefully matched in characteristics or considerable distortion can result.

In ordinary class-B amplifier design enough overlap presents itself at the cross-over region that there is no cross-over distortion under steady state tests. However, under transient conditions, the cross-over region breaks down to display a kind of distortion similar to cross-over distortion now termed transient intermodulation distortion.

In the theoretical sense, an ideal class-B amplifier amplifies only one half of a cycle at a time , the quiescent dc current, is zero. However, the presence of loading current, through each amplifier half causes heating.

In a practical sense, cross-over distortion is always present and a residual current, is allowed to flow. The practical result is where the positive waveform overlaps with the negative waveform to eliminate cross-over distortion. The operation cycle of each amplifier half is actually greater than 1/2 cycle.

The cross-over region overlap determines the heating of the power transistor. In class-AB operation, the heating is very high and excessive overlap is used to reduce transient intermodulation distortion.




From the works of "Wang & Chien"

Biased Class-B is not Class-AB

It is important to realize this. Many people and even most engineering text books get this part wrong or don't understand the difference.
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