Feedback Takeoff Point ?

guitar89 · 13th September 2011, 10:39 AM

Quote:

From douglas book :
The exact layout of the feedback takeoff point is criticial for proper
operation. Usually the output stage has an output rail that connects the
emitter power resistors together. This carries the full output current and
must be substantial. Take a tee from this track for the output connection,
and attach the feedback takeoff point to somewhere along this tee.
Do not attach it to the track joining the emitter resistors.

I have attached a DIY Amplifier layout from DX (nice amplifier they design, i like it)
But i have some problem to maximize the layout potential, minimizing distortion problem.
The quote from respectable douglas, using his knowledge to do the best

So the problem here is that i don't know how to identify the feedback takeoff point which HE mention.
Just to confirm, so i should take a tee from the junction between the R7 & R8 (0R33) which in short the point "A" on the right, to the feedback takeoff point ?

DF96 · 13th September 2011, 11:17 AM

Put a tee somewhere on the line from R7/R8 to C9/C3/R17.

jan.didden · 13th September 2011, 11:56 AM

I would take it from the pad that has the connection to speaker post.
The idea is to take the feedback from the point that you want to be as linear and low distortion as possible. That normally is the speaker connection.

jan didden

DF96 · 13th September 2011, 12:07 PM

This amp has two feedbacks. One comes from the speaker terminal. The other includes DC and comes from before the output coupling cap.

guitar89 · 13th September 2011, 12:52 PM

DF96 : haha, thanks for the info, but i really have no idea how to identify those feedback path, anyway to let me learn about it ?

jan didden : what do you mean by 'pad' ? So the tee mention is from speaker post to where ?

So what should i emphasis on douglas' word ? can't really understand fully ^^

DF96 · 13th September 2011, 01:35 PM

Read Doug Self's book?

digits · 13th September 2011, 01:43 PM

The two A's are connected as the first, and I think the second is that R18, right at the bottom.

guitar89 · 13th September 2011, 01:55 PM

thanks for pointing out, digits.

DF96, i only know and get hold of "Douglas Self - Audio Power Amplifier Design Handbook", but its more on amplifier building/designing and not much about those little thingy (he mentioned that in the book foreword too)

DF96 · 13th September 2011, 02:13 PM

Maybe you need to find a book which starts further back. This topic comes up regularly on here so do a forum search.

selim · 13th September 2011, 08:35 PM

Hi guitar89,

If I understand correctly,what Mr. Self means about the feedback takeoff
point is that feedback tries to make an amplified replica of the signal at the
physical point where it is taken.In your case, it would be the (+) terminal
of the output capacitor (6800 uf).Thus, the cleanly amplified signal would
appear at the (+) terminal of the output cap, which is what you want.

Hope this helps.

Cheers
Selim

13th September 2011, 11:56 AM	#3
jan.didden diyAudio Member Join Date: May 2002 Location: Great City of Turnhout, Belgium Blog Entries: 6	I would take it from the pad that has the connection to speaker post. The idea is to take the feedback from the point that you want to be as linear and low distortion as possible. That normally is the speaker connection. jan didden __________________ It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts - Sherlock Holmes Check out Linear Audio Vol 5!

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13th September 2011, 11:17 AM	#2
DF96 diyAudio Member Join Date: May 2007	Put a tee somewhere on the line from R7/R8 to C9/C3/R17.

13th September 2011, 12:07 PM	#4
DF96 diyAudio Member Join Date: May 2007	This amp has two feedbacks. One comes from the speaker terminal. The other includes DC and comes from before the output coupling cap.

13th September 2011, 12:52 PM	#5
guitar89 diyAudio Member Join Date: Dec 2010 Location: Malaysia, Selangor	DF96 : haha, thanks for the info, but i really have no idea how to identify those feedback path, anyway to let me learn about it ? jan didden : what do you mean by 'pad' ? So the tee mention is from speaker post to where ? So what should i emphasis on douglas' word ? can't really understand fully ^^

13th September 2011, 01:35 PM	#6
DF96 diyAudio Member Join Date: May 2007	Read Doug Self's book?

13th September 2011, 01:43 PM	#7
digits diyAudio Member Join Date: Apr 2011	The two A's are connected as the first, and I think the second is that R18, right at the bottom.

13th September 2011, 01:55 PM	#8
guitar89 diyAudio Member Join Date: Dec 2010 Location: Malaysia, Selangor	thanks for pointing out, digits. DF96, i only know and get hold of "Douglas Self - Audio Power Amplifier Design Handbook", but its more on amplifier building/designing and not much about those little thingy (he mentioned that in the book foreword too)

13th September 2011, 02:13 PM	#9
DF96 diyAudio Member Join Date: May 2007	Maybe you need to find a book which starts further back. This topic comes up regularly on here so do a forum search.

13th September 2011, 08:35 PM	#10
selim diyAudio Member Join Date: Apr 2003 Location: izmir turkey	Hi guitar89, If I understand correctly,what Mr. Self means about the feedback takeoff point is that feedback tries to make an amplified replica of the signal at the physical point where it is taken.In your case, it would be the (+) terminal of the output capacitor (6800 uf).Thus, the cleanly amplified signal would appear at the (+) terminal of the output cap, which is what you want. Hope this helps. Cheers Selim