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First amp ever, symmetrical and with EC
First amp ever, symmetrical and with EC
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Old 17th July 2017, 07:53 AM   #11
bimo is offline bimo  Indonesia
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Quote:
Originally Posted by heinrichs View Post
Well that is certainly a good point So I am back to the drawing board, either cutting away the symmetrical par of the amp, or designing another VAS.

And thank you, this was exactly the reason i made this thread
Wally is right. It is on the Randy Sloane's book, but it worked intermittent and it corrected on Bob Cordell's book.
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Old 17th July 2017, 08:07 AM   #12
heinrichs is offline heinrichs  Denmark
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Originally Posted by mchambin View Post
As a student, you will learn much better what is involved in a audio amp with a simple design based on a op amp based amplifier. This is basically an output stage ( a current booster ) with an op amp as a front end with massive feedbacks.That is the architecture of most audio amplifier made here where the op amp part is re invented made of discrete component.
IMO it is a waste of time for a student to reinvent this part in a project where the objective is a power amplifier with low distortion.
In case you want to go the way, all discrete including the front end part, go simple with P3A from Ron Elliot ESP.
I disagree, a few weeks ago i designed and build a mostly discreet buck converter. With a discreet ramp oscillator, diff amp and driving circuit. I used opamps for the PID regulator. (so... this is not exactly my first amp, just my first amp for audio....) however, i could have used opamps for the osc, diff amp, and a big part of the driver. But then i wouldnt have learned nearly as much, my efficiency were +93%. (5v 1A output, asynchronous, and could have been higher with a more suited diode and mosfet)

I see this project the same way. In my view there is much more to gain, knowledge wise, by going discrete. However, right now i am trying to decide which way to go from here, regarding the complexity.
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Old 17th July 2017, 08:08 AM   #13
mchambin is offline mchambin  France
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Going right away into details, which is the wrong way to go in early design.....:
The Zobel circuit is on the wrong side of the inductor.
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Old 17th July 2017, 08:23 AM   #14
heinrichs is offline heinrichs  Denmark
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Originally Posted by mchambin View Post
Going right away into details, which is the wrong way to go in early design.....:
The Zobel circuit is on the wrong side of the inductor.
So, are you saying that it would make more sense to use opamps were i can. And then remove them one by one as i design new discrete blocks? (as i really want a discrete design in the end) (or just saying that i should design and amp using op amps, and then the second amp could be more discrete)

Last edited by heinrichs; 17th July 2017 at 08:25 AM.
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Old 17th July 2017, 08:56 AM   #15
mchambin is offline mchambin  France
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This was my advice as a teacher.
For a project where the objective is an op amp design I would recommend designing one using discrete components.
In a project where the objective is a low distortion power amplifier re inventing the op amp is not the first way to go.
BTW What is your objective in terms of: Output power, distortion, thermal stability, efficiency, stability in term of phase and gain margin?
Anyway, that is your amp.
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Last edited by mchambin; 17th July 2017 at 09:01 AM.
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Old 17th July 2017, 09:20 AM   #16
mchambin is offline mchambin  France
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Yes, firstly design with one op amp and an output stage.
You can get right away distortion lower than 0.01% at 20Khz in spice.
This will hopefully end with a good low distortion well biased output stage and mastering negative feedback to go at the best performances.
Unfortunately, spice is no direct help about thermal stability.
Then, mastering this first step, you can go at replacing the op amp for a all discrete version where you'll have to tackle higher performance op amp design in the very high Gainbandwith and slew rate with very low distortion and noise.
The same issues as the original project, you would say, but...without the need of high output power and driving weird loads, speakers are far from 8 ohm resistors.
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Old 17th July 2017, 09:53 AM   #17
heinrichs is offline heinrichs  Denmark
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Quote:
Originally Posted by mchambin View Post
This was my advice as a teacher.
For a project where the objective is an op amp design I would recommend designing one using discrete components.
In a project where the objective where the objective is a low distortion power amplifier re inventing the op amp is not the first way to go.
BTW What is your objective in terms of: Output power, distortion, thermal stability, efficiency, stability in term of phase and gain margin?
Anyway, that is your amp.
That makes sense. And when i think about it, it was the same thing my professor said when i worked on the buck converter, so I will proberly go that way.

the only specifications i have at the moment are.
50W 8ohm, 200W 2ohm (I know the current output stage cant really do this)

THD below 0.01% at 20kHz, 50W, I dont know anything about other forms of distortion, but I am waiting on self's book on power amps, and cordell's as well. Also... I dont know if the goal is reasonable, but time will show.

I dont have an exact goal for efficiency, but since i will need 6 channels, i want it to be pretty good, and that is the reason for using the hawksford EC. I am hoping that it will help me to reduce bias currents while keeping a reasonable performance. (this might be quite a misunderstanding... but time will show)

In terms of thermal stabillity, if we are talking about linearity across a temperatur range, then i havent really thought about it untill now. if we are talking risk of thermal runaway, then it is something i think about, but i dont know how to specify it.

about phase and gain margin. I dont really know what is reasonable for the gain margin. But i would like my phase margin to be between 40-60 degrees. (I had actually already forgotten the importance of this... even though it is less then 3 months since i learned about it... )

and thanks for the questions and advice, was exactly what i needed.
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Old 17th July 2017, 11:14 AM   #18
mchambin is offline mchambin  France
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50W 8 ohm is a good start, mostly typicall.
200W 2 ohm is involved, look closely at SOA and Beta drooping.
Efficiency:
Class A: Terrible efficiency, easy very low distortion.
Class B: Efficiency depends of quiescent current ( aka bias ). Choices are 2 EF output stage versus CFP output stage. Low qiescent current, optimal ??, hight qiescent current.
Thermal stability:
The issue has to do from the BJT's Vbe tempco. it's decrease with temperature makes it difficult to get a stable bias and is prone to induce thermal runaway. See Rod Elliot ESP discussions about this and CFP versus 2 EF. Getting a stable qiescent current over BJT temperature variations with output power is a challenge in Class B amplifiers.
Phase and gain margin are of a paramount importance. This will account for the stability of the looped system( stability, here means no oscillation ).
Unfortunately there are other causes of oscillations, but you will have enough of those to fight, you do not want to oversee looped linear system theory to go in a hopeless fight on this one.
I would aim at Phase margin 30 and gain margin 20dB.
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Last edited by mchambin; 17th July 2017 at 11:17 AM.
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Old 17th July 2017, 12:44 PM   #19
heinrichs is offline heinrichs  Denmark
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You have certainly given me something to think about, and a good place to start. thank you.

I am now in the proces of designing a simpler circuit, and reading up on some theory
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Old 17th July 2017, 04:58 PM   #20
Mark Johnson is offline Mark Johnson  United States
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First amp ever, symmetrical and with EC
Quote:
Originally Posted by mchambin View Post
I would aim at Phase margin 30 and gain margin 20dB.
I've attached a pdf below, which discusses phase margin and other characteristics of second order linear systems (which are good approximations of power amplifiers & voltage regulators).

My own personal preference to design circuits with phase margin of 65 degrees or more. This reduces overshoot to an acceptably low level, and keeps gain peaking modest.

_
Attached Images
File Type: png phase_margin_vs_damping_zeta.png (11.7 KB, 87 views)
File Type: png phase_margin_vs_gain_peaking.png (17.8 KB, 87 views)
File Type: png phase_margin_vs_percent_overshoot.png (20.3 KB, 86 views)
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File Type: pdf Phase_Margin_and_Overshoot_Daly.pdf (614.9 KB, 19 views)
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