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One of the Top Solid-State CFA amp design
One of the Top Solid-State CFA amp design
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Old 26th November 2017, 01:24 PM   #1
AndriyOL is offline AndriyOL  Ukraine
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Default One of the Top Solid-State CFA amp design

I started this thread because I would like all best knowledge, experience and practice on a matter to be gathered in one place: the most common reasons of oscillation in different types of solid state audio amplifiers, as well as the best ways to defeat it.

All are welcomed to share their succesfull experience of oscillation solutions.

Technicaly advanced members are highly expected to participate.

As for the current oscillation problem I'm interested in is a following CFP audio amplifier project.

Current layout without silkscreen errors:
SSA.JPG - Google Drive

Current Schematic:
SSA-HP-BAL.jpg - Google Drive
Attached Images
File Type: jpg HP.JPG (767.7 KB, 1977 views)
File Type: jpg SSAHP.jpg (411.6 KB, 1934 views)
File Type: jpg 20171122_220129.jpg (738.6 KB, 1907 views)
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Last edited by AndriyOL; 30th December 2017 at 07:49 PM.
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Old 26th November 2017, 01:30 PM   #2
BesPav is offline BesPav  Russian Federation
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Quote:
Originally Posted by AndriyOL View Post
the most common reasons of oscillation in different types of solid state audio amplifiers
Improper understanding of Niquist and Bode dissertations, mr. Middlebrook and Tian articles.


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Originally Posted by AndriyOL View Post
the best ways to defeat it.
Proper simulating with good gain tuning and sophisticated PCB routing.
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Old 26th November 2017, 03:18 PM   #3
AndriyOL is offline AndriyOL  Ukraine
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Could you please be more specific about "sophisticated PCB routing" solution?
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Old 26th November 2017, 04:13 PM   #4
DouglasSelf is offline DouglasSelf  United Kingdom
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I assume we are talking about power amplifiers.

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Improper understanding of Niquist and Bode dissertations, mr. Middlebrook and Tian articles.
Study of feedback theory is actually of very little use. Much of the data you need is unknown, and I am thinking here of Class-B output stages. Their frequency response varies with the operating conditions, which change wildly over a cycle. Very little has been published on this important issue, but see Chapter 3 of my book for discussion of the implications:

The Audio Power Amplifier Design Handbook

This is why amplifiers are often seen to be unstable for only part of the output sinewave. Assuming dominant-pole compensation the design process is not much more sophisticated than increasing the compensation until the oscillation stops, then adding a bit more as a safety margin. Lower load impedances (4 Ohm, 2 Ohm) are usually more difficult to stabilise probably because the higher device currents increase local transconductances.

On another level altogether are local oscillations, typically in the output stage, which to a first approximation are not affected by the feedback loop compensation. A simple example is the tendency of an emitter-follower to act as a Colpitts oscillator under the right conditions. This sort of thing is not amenable to calculation nor simulation.
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Proper simulating with good gain tuning and sophisticated PCB routing.
I am not sure what gain tuning is supposed to be. I can say that I have never known PCB routing have anything to do with HF stability.
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Old 26th November 2017, 04:50 PM   #5
davada is offline davada  Canada
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"A simple example is the tendency of an emitter-follower to act as a Colpitts oscillator under the right conditions"

I think this needs to be looked at first rather than assuming compensation issues.

Increasing compensation may solves this problem but at the expense of high frequency distortion performance.

Try to identify local oscillation first. Check for RF ingress as well. Source of RF include CFL. Even LED lights replacing fluorescent tubes is suspect. Cable modems, ADSL, WIFI and now Moca networks etc.
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Old 26th November 2017, 05:08 PM   #6
greierasul is offline greierasul  Romania
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Maybe ferrite beads in the bases of the TTC5200 and TTA1943 may cure the oscillation tendency ?
Why not to rise up the emitter resistors of the TTC and TTA from 0.1 ohm to 0.33 ohm ?

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Old 26th November 2017, 05:11 PM   #7
AndriyOL is offline AndriyOL  Ukraine
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I haven't tried that. Maybe higher value of base limiter resistors?

Mr. Douglas, Davada would you please advise, what could a sophisticated solution in my case?

I have Wi-Fi adapter 1m close
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Old 26th November 2017, 05:14 PM   #8
greierasul is offline greierasul  Romania
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Or maybe a bigger quiesscent current of the output stage to stabilise the B-E capacitance.
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Old 26th November 2017, 05:42 PM   #9
DouglasSelf is offline DouglasSelf  United Kingdom
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Quote:
Originally Posted by greierasul View Post
Maybe ferrite beads in the bases of the TTC5200 and TTA1943 may cure the oscillation tendency ?
Not in my experience. Have never known ferrite beads help anything.

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Why not to rise up the emitter resistors of the TTC and TTA from 0.1 ohm to 0.33 ohm ?
On the other hand, why? Do you think that will increase stability? If this was an emitter-follower output stage that Re change would greatly increase the crossover distortion. That may not be true for this triple output stage, but the point needs checking carefully.
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Old 26th November 2017, 05:44 PM   #10
DouglasSelf is offline DouglasSelf  United Kingdom
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Originally Posted by greierasul View Post
Or maybe a bigger quiescent current of the output stage to stabilise the B-E capacitance.
I think you're just guessing.

In a Class-B output stage the quiescent current is critical and has one optimum value for minimal distortion.
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