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Old 10th February 2013, 12:28 PM   #2591
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Originally Posted by JPV View Post
High gain stage requires high impedance node.
You could generate overall high gain by cascading but then you have stability problems.
It seems that there are many trade offs like noise and distortion
I have studied the AD797 that uses the elevated impedance approach. Nice on an IC where stray capacitance can be minimized but I worry for discrete circuitry.
Rather that raise the collector load impedance I think to lower the emitter impedance - it is the ratio that matters of course. Potential for lower noise and better stability at the same time. Need to increase the LTP current, so the trade-offs become with thermal problems. Maybe paralleled LTP transistors to spread the heat and lower Rbb at the same time. Lower the feedback network too. Burns a little power but is not too complex or expensive, uses what discrete circuitry does best. Sound reasonable?

Best wishes
David
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Old 10th February 2013, 12:36 PM   #2592
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Edmond Stuart's TIS ...does not eliminate the intrinsic logarithmic transfer curve distortion. Although in this case the output stage generates most of the distortion
Multi-tanh input could be another addition to Bob's second edition (or second volume!) but there may be more productive topics to cover.
Has anyone ever used this in an audio amplifier?

Best wishes
David

Last edited by Dave Zan; 10th February 2013 at 12:44 PM.
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Old 10th February 2013, 04:26 PM   #2593
mortron is offline mortron  Canada
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This book good for beginners? Can someone suggest a book for someone just starting? I have some experience with a Bottlehead Preamp, but nothing with high voltages or dealing with amplification. I understand parts of concepts, and am planning an F5 Turbo but am not near the level of comprehension that I could build it with just a schematic and no hand holding. I might be able to... But I'd never try. I am planning to try maybe an LM4780 Kit or PCB and BOM to throw one of those together a little cheaper and use that one to learn before ripping into an F5. I just feel I need to up my chops, and rad more.

Is this more of an advanced book? Does the level of comprehension really make or break this book, or can a beginner pick it up, tough through it and learn something properly, or should one be taking baby steps?
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Old 10th February 2013, 04:41 PM   #2594
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I would not teach electronics with this book. It's more about theory and design, and it really helps to have basic electronic understanding from the beginning. I don't think this book will help with what you're trying to do, but it might once you start with more advanced designs. If you want to learn electronics, you might try the free online course here:

https://www.edx.org/
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Old 10th February 2013, 04:47 PM   #2595
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Oh yeah, the multi-tanh! It does not correct Vbe distortion though, it just combines and you get a weird wavy transfer curve.

This seems like such a great idea but if you talk to audio guys about it their eyes glaze over and they start repeating "no... no... no...". I suppose I should have ignored them at the time. It would introduce it's own special kind of distortion, I know that much. But the total distortion would be less. Sounds fun.
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Old 13th February 2013, 05:41 PM   #2596
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Originally Posted by mortron View Post
This book good for beginners? Can someone suggest a book for someone just starting? I have some experience with a Bottlehead Preamp, but nothing with high voltages or dealing with amplification. I understand parts of concepts, and am planning an F5 Turbo but am not near the level of comprehension that I could build it with just a schematic and no hand holding. I might be able to... But I'd never try. I am planning to try maybe an LM4780 Kit or PCB and BOM to throw one of those together a little cheaper and use that one to learn before ripping into an F5. I just feel I need to up my chops, and rad more.

Is this more of an advanced book? Does the level of comprehension really make or break this book, or can a beginner pick it up, tough through it and learn something properly, or should one be taking baby steps?
Hi mortron,

I would not recommend it for a true beginner, although it depends a bit on the definition of a beginner. I tried very hard to start off easy and then go deep in the book, but of course there is a big compromise in that when one is limited to about 600 pages. There are a lot of books out there that could precursor reading this one. If you know about and have build some transistor circuits and understand schematics and devices like transistors, capacitors diodes and resistors you have a fighting chance. You definitely do not have to have an EE degree to get a lot out of this book and enjoy amplifier-building as a great hobby. I deliberately tried to keep the math to a reasonable level. You certainly don't need to know the Laplace transform and the like for this. I do in the early chapters try to give a person an easily digestible idea of things like poles and zeros and their relationship to frequency response. Similarly, I try to explain in straightforward terms negative feedback and stability.

You can probably go onto Amazon and read pieces of it to get a flavor for the level of it and how well it matches your current technical level.

Hope this helps.

Cheers,
Bob
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Old 13th February 2013, 05:52 PM   #2597
jcx is offline jcx  United States
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Oh yeah, the multi-tanh! It does not correct Vbe distortion though, it just combines and you get a weird wavy transfer curve.
there is a "maximally flat" version - no "waveyness"

but it is easy to have enough global loop gain at audio that the the input error V at audio frequencies is so low that ordinary diff pair tanh distortion is sub ppm

a question is what is the practical import of RF/EMI infiltration, possible rectification, mixing in the diff pair - that is when maximally flat multi-tanh may offer improvement - from input diff gm linearity improvement for frequencies way above negative feedback "working band"

another case is for those who insist on "low feedback" - then even the audio frequency (linear) error V input at the diff pair can become large

Last edited by jcx; 13th February 2013 at 05:57 PM.
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Old 14th February 2013, 03:06 AM   #2598
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Originally Posted by mortron View Post
...
Is this more of an advanced book? Does the level of comprehension really make or break this book?...
This is the "Bob Cordell's Power amplifier book" thread.
Of course you should buy Bob Cordell's Power amplifier book.

Quote:
Originally Posted by jcx View Post
... the input error V at audio frequencies is so low that ordinary diff pair tanh distortion is sub ppm
That's what I meant when I wrote that "there may be more productive topics."
But if there are multiple input transistors to reduce noise then it is an attractive idea to also lower distortion as a bonus. Do you have any ideas or references?
So far I have the Gilbert article from IEEE JSSCircuits.
And does anyone know of an audio amp example?

Best wishes
David
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Old 14th February 2013, 06:05 AM   #2599
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Hi Bob,

What's the prefered sensing location of a VBE device, for vertical mosfets (k1530 & j201 pair), if they are matched for 5% Vgs in a typical high power application, perhaps with a +/- 75v supply...? On top of the device or heatsink...? I 've read that mounting the Vbe on heat sink would suffice in chapter 11 (fig 11.8) of your book. Does mounting the device on top of the output devices would over compensate....?
Hi Magna,

I'm sorry I took so long to get back to you on this. For a Vbe multiplier bias circuit, I often mount a TO126 device on the heat sink between the P-channel and N-channel devices. The MOSFETs are more tolerant of thermal drift than BJTs, so mounting on the H/S in close proximity to the P and N devices id fine. Note the different kinds of Vbe multiplier bias spreaders shown in the book that allow one to adjust the amount of compensation. In some cases one junction will be put on the heat sink and another may be at ambient to achieve less compensation. While vertical MOSFETs need bias spreader temperature compensation, designs with lateral MOSFETs often don't have it, since the temperature coefficient of current with temperature for fixed Vgs goes through zero at about 200mA.

Cheers,
Bob
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Old 14th February 2013, 06:19 AM   #2600
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Originally Posted by Dave Zan View Post
My personal preference is for more information on solid state, performance aspects. If practical limits on book size mean less or no Class-D and tubes then that is fine. There is always a balance between depth and breadth. In this case both topics are rather specialized and self-contained so they would be a minimal loss compared to the benefit of more space for the central focus of the book.

Best wishes
David
Hi Dave,

I'll definitely have to expand the five chapters currently devoted to class D because that really is the wave of the future and its quality is getting so much better every year.

One thing that I found at the time of the writing of the book was that there was a great deal of information out there on class-D, but it was spread out all over the place. The other thing that is different about class D is that it is not quite as DIY-friendly as conventional amplification in regard to the ability to tinker with and refine the performance of the design. Even for the various incarnations of PWM, the art is more specialized. The very high speed switching needed to achieve good performance can be quite layout-sensitive, for example. Things get even more difficult for DIY design in the realm of more advanced modulation techniques like sigma delta and DSP-enhanced designs. However, class D is still DIY user-friendly if one is talking about building with modules to create complete amplifiers.

Cheers,
Bob
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