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Old 2nd February 2013, 08:18 AM   #1141
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Quote:
Originally Posted by terranigma View Post
...
Hi
it looks like oscillations but the great potential:
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File Type: png ac.png (13.8 KB, 373 views)
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Old 2nd February 2013, 07:16 PM   #1142
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Judging by that output phase it's local oscillation of one of the output CFP's. One solution I've seen in this thread is to put a several nF cap across the B-E of one of the outputs. However C6 could simply be too large. The phase-lead cap should never be larger than it needs to be unless it has a series resistor.
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Old 3rd February 2013, 02:52 PM   #1143
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Quote:
Originally Posted by padamiecki View Post
Hi
it looks like oscillations but the great potential:
It would be useful to see the circuit and conditions generating that waveform
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Old 3rd February 2013, 04:22 PM   #1144
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Quote:
Originally Posted by Elvee View Post
It would be useful to see the circuit and conditions generating that waveform
it is a curcuit from here: http://www.diyaudio.com/forums/attac...darlington.asc
I only changed
step time to 1ns
start time 5ms
stop time 11ms
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Old 3rd February 2013, 05:17 PM   #1145
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Quote:
Originally Posted by padamiecki View Post
it is a curcuit from here: http://www.diyaudio.com/forums/attac...darlington.asc
I only changed
step time to 1ns
start time 5ms
stop time 11ms
The parameters of the darlingtons differ from the CFP, which destabilizes the common mode loop.
Problem can be fixed by adding small compensation caps to each OP transistor.
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Old 21st February 2013, 12:08 AM   #1146
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When I select 1K (instead of 1K2) for R6 and R7 in original schematic, THD is getting o bit lower in simulation. Can you clarify that how is it safe to use 1K (or lower) with 32V rails?
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Old 21st February 2013, 06:57 AM   #1147
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R6 and R7 steal some current from the bases of the VAS. If they become too low, the residual base current multiplied by the Hfe will become too low to drive fully the OP compounds.

The actual point at which it will effectively happen depends on a number of factors, mainly the Hfe of the transistors involved. Also, if the current becomes too low, this will be noticed by the common mode servo, and it will try to correct it by increasing the tail current of the LTP.
This means you can probably go rather low without the amp ceasing to function, but some characteristics might be degraded.
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Old 21st February 2013, 10:31 AM   #1148
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Quote:
Originally Posted by Elvee View Post
you can probably go rather low without the amp ceasing to function, but some characteristics might be degraded.
I never thought that their function is to steal some current, I actually thought that current direction is opposite. I'll stay put with 1K2's in case of whatever simulation shows.

Another thing is R values in compensations. Are their value mostly depends on the current where they suited rather than altering phase behaviour like corresponding C's? In short, we can alter the compensations without altering the R's, is it right?
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Old 21st February 2013, 04:26 PM   #1149
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Quote:
Originally Posted by terranigma View Post

Another thing is R values in compensations. Are their value mostly depends on the current where they suited rather than altering phase behaviour like corresponding C's? In short, we can alter the compensations without altering the R's, is it right?
Not very clear. Anyway to summarize, the R associated with the C creates an inflection point where the C action stops.
You are free to experiment other values, there is some tolerance on each side of the optimum, and even if you go too far, you will normally not destroy anything
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Old 22nd February 2013, 06:53 PM   #1150
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I'm still very interested to replace the Zeners with a transistor, except not the same as last time that made a kink in the treble as if the cap parallel to the feedback resistor quit working (weird and painful ear pressure--high treble went off the beam--feedback didn't work right). This time, I don't want to plug that transistor into the feedback loop. Is there a way?
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