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Old 30th January 2011, 01:20 PM   #1461
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Quote:
Originally Posted by Telstar View Post
Why an electrolytic and not a film cap?
Hi Telstar,

The impedance of the zener diode is fairly low, so the amount of capacitance required to reduce its noise in the audio band is typically larger than what a reasonable size film capacitor would deliver. Moreover, in an application like bypassing a zener voltage reference, I don't believe that the use of an electrolytic degrades the sonics.

Cheers,
Bob
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Old 30th January 2011, 10:27 PM   #1462
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Originally Posted by Olivier73 View Post
Hi Bob,
I noticed in your book that you don't use/show capacitors e.g across zener references or any other similar use. Also the current source of 2.10f is not shown with a capacitor to somewhat filter rail variations.
Are they just left out for simplified lecture or do you think this is up to the reader to decide. If the latter is the case I presume there might be reason not to use capacitors there. Is that so? Or is it rather 'more than desireable but not really a must'?
Referencing the 2.10f to Randy's drawings he uses to split the Q2 resistor in half and places a capacitor from between the resistors to the power rail.
Would there be a reason not to place them? I will not use top notch capacitors there so maybe none is better than plain standard electrolytic capacitors?
Cheers,
Olivier
Hi Olivier,

I basically left out the capacitors in the discussion to keep it simple. However, I should have devoted some space to the need for filtering and some approaches to doing that.

Cheers,
Bob
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Old 31st January 2011, 04:33 AM   #1463
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Originally Posted by Bob Cordell View Post
Hi Olivier,

I basically left out the capacitors in the discussion to keep it simple. However, I should have devoted some space to the need for filtering and some approaches to doing that.

Cheers,
Bob
Added filtering does have it's benefits and drawbacks. With the book example ("bobamp", fig. 3.8) , adding nothing will work and there will be no turn-on thump. If you add 10uF to 47uF as C5 (below), PSRR will increase 6-8db at the input pair and just a "hair" at the OP. 10uF makes for almost NO thump , it is even hard to see on the CRO. The worst case scenario is like the fig. 3.8 amp , with it's 2 separate CCS's , individually decoupled. One or both will stabilize independent of each other or the OPS , giving a "tick" or "thump" at turn-on. (I had this "problem" )

OS
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Old 1st February 2011, 12:16 PM   #1464
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Originally Posted by darkfenriz View Post
I've started the book from class D section.
Bob, you could have done better here really, there are already tons of similar popular crash courses everywhere.
You still have my need admiration of your contribution to linear amplifiers.
Hi darkfenriz,

I'm sorry that you are disappointed in the class D section in my book. It's a little hard to tell from your message if you have read all four chapters or have just started the first chapter.

You're definitely right: I could have done a better job if I had the space and that was a bigger priority for the book. I'll probably expand it in the second edition. I did say in the introduction to the class D section that I was just scratching the surface, and that is really true. The class D section is not as comprehensive as the rest of the book. Bear in mind that many readers of my book do not have a great deal of experience and knowledge with class D, so that material must start out at a fairly basic level.

You're also right about there being a great deal of information on class D out there on the net. However, it was my experience in researching the chapters that the information is spread out all over the place and very fragmented. One of the jobs of an author is to gather information on a subject and weave it into a coherent presentation, not necessarily to generate new material.

Class D is a complex and different technology. A comprehensive treatment of it would itself take a 600-page book.

You can help me in a couple of ways. First and foremost, if I have gotten anything wrong, please let me know so that I can correct it.

Second, please let me know what material you would like to see added in the second edition.

Finally, if there is a single internet site out there, or a single book out there on class D that covers the whole subject better, please let me know so that I can review it and add it to my references in my second edition.

Thank you for the feedback.

Cheers,
Bob
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Old 1st February 2011, 01:58 PM   #1465
forr is offline forr  France
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Thanks for your feedback about zeners and CCSs.

Ostripper, would changing the cap in the circuit you show by a 9.1V zener with a 5 mA current through it would be equal or better for PSSR and without creating any thump at all?
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Old 2nd February 2011, 05:30 PM   #1466
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Quote:
Originally Posted by ostripper View Post
Added filtering does have it's benefits and drawbacks. With the book example ("bobamp", fig. 3.8) , adding nothing will work and there will be no turn-on thump. If you add 10uF to 47uF as C5 (below), PSRR will increase 6-8db at the input pair and just a "hair" at the OP. 10uF makes for almost NO thump , it is even hard to see on the CRO. The worst case scenario is like the fig. 3.8 amp , with it's 2 separate CCS's , individually decoupled. One or both will stabilize independent of each other or the OPS , giving a "tick" or "thump" at turn-on. (I had this "problem" )

OS
Hi OS,

This is a really good point about decoupling and turn-on thump. Most of us have experienced annoying turn-on thump in some designs, sometimes resorting to/relying on a delayed turn-on speaker relay to kill it. Thanks.

Cheers,
Bob
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Old 2nd February 2011, 07:59 PM   #1467
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Default Phase Margin & Gain Margin

Hi Bob,

I am a bit confused with what I measure in my simulations.

According to the setup in your book on p88 figure 4.7 you place a large inductor to make it possible to break the loop in order to measure the gain & phase margin of the loop itself (I hope to be right till here)... I added the AC analyses of this method in this reply under AC Analysis2.pdf
It is clearly visible that the GM=28dB and the PM=90

If I do the AC analyses with the amplifier schematic back to normal (removed inductor, input signal back to the input side,...) still measured on the output and still without any input LP filter (shunt capacitor) the results show me an unstable amplifier because PM and GM are totaly inadequate. See AC Analyses1.pdf ...

If this is important, there is no OPS yet implemented in schematic.

I must be missing something. Could you tell me a bit more about how I should interpretate my results compared to your book instructions? Is this amp stable or not?

Thanks in Advance

Olivier
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File Type: pdf AC Analysis1.pdf (37.8 KB, 42 views)
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Old 3rd February 2011, 11:12 AM   #1468
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Hello Oliver,

From first measurement, amplifier seems ok.

Second measurements shows input/output transfert function? ok. So how do you conclude something is unstable? No peaking in response, that seems ok.

What I read is that this amplifier have a nice 200k-300kHz bandwidth (-3dB). Phase is smoothly moving above 10kHz up to 180 at 7MHz.
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Old 4th February 2011, 06:26 PM   #1469
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Quote:
Originally Posted by Olivier73 View Post
Hi Bob,

I am a bit confused with what I measure in my simulations.

According to the setup in your book on p88 figure 4.7 you place a large inductor to make it possible to break the loop in order to measure the gain & phase margin of the loop itself (I hope to be right till here)... I added the AC analyses of this method in this reply under AC Analysis2.pdf
It is clearly visible that the GM=28dB and the PM=90

If I do the AC analyses with the amplifier schematic back to normal (removed inductor, input signal back to the input side,...) still measured on the output and still without any input LP filter (shunt capacitor) the results show me an unstable amplifier because PM and GM are totaly inadequate. See AC Analyses1.pdf ...

If this is important, there is no OPS yet implemented in schematic.

I must be missing something. Could you tell me a bit more about how I should interpretate my results compared to your book instructions? Is this amp stable or not?

Thanks in Advance

Olivier
Hi Olivier,

I'm sorry that I've been tardy in getting back to you. Macleod's comments are right. The amp is fine. The only thing I see (which I may be misinterpreting) is that the closed loop gain on your graph looks to be about 36 dB, where I believe the expected gain for the amplifier arrangement of that figure should be 26 dB.

The thing to bear in mind is that you should not be trying to measure phase and gain margin from the closed loop gain curve.

Cheers,
Bob
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Old 4th February 2011, 08:50 PM   #1470
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Default Gain & Phase Margin

Hi Bob, Hi Macleod,

I understood that CL phase and gain measurement are pretty useless. So even if the CL gain goes throug zero at 10MHz and the phase at that point is already way beyond 180 lagging, that is not a direct concern... right?
Indeed measuring openloop (while breaking the loop with a large inductor) the result seem promising. If this figure defines the PM and GM I am happy.
FYI, for now, the compensation capacitor over the VAS is a simple miller cap, however stretching over a darlington and cascoded vas...

Bob, according to me the 36dB is normal. Where do you deduct from that the gain should be 26dB?

I will now try to incorporate TMC where I follow Edmonds view. But anyway according to your book and the many forum topics here the TMC camp seems to be more convincing than the TPC camp. My main reason is that I don't like the peaking in the bode plot...

Olivier
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