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Old 27th August 2012, 01:37 AM   #1181
lhquam is offline lhquam  United States
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Quote:
Originally Posted by buzzforb View Post
I got mine 90% done. Started out wrong and did input stage like F5, but with signal coming out after 10ohm Rs of Jfets. Got the pot for offset trim. I used BA-1 style bias circuit, dropping teh resistor for Jfet ccs feeding 3 led string. This has parallel 10uF cap and feeds a 5K pot for gate bias. basically looks like Bodu's circuit. I am using laterals instead of SS. Have to reduce feedback resistor since i dont think Laterals will give em much gain to work with. Thinking about overall gain of about 10.
Holy cow! What are those huge caps?
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Old 27th August 2012, 01:40 AM   #1182
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I suggest you strip this down a bit.

Delete C3, C6, and C7

Set R1=0, R12=10k, R19=10k

and let's see what happens.

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Old 27th August 2012, 01:44 AM   #1183
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Another thing, your thd vs frequency plot tells me that your C4 and C15
values are low.

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Old 27th August 2012, 01:50 AM   #1184
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Originally Posted by lhquam View Post
Holy cow! What are those huge caps?
10uF Solens. All I had. Hopefully it will make bigger smoke.
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Old 27th August 2012, 01:52 AM   #1185
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Why isn't there a suitable Zobel at the output node Vo? Its presence usually calms the amp.
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Old 27th August 2012, 01:55 AM   #1186
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Not at .5 Hz it doesn't.

Me, I don't use Zobels (like .1uf and 5 ohms at output), and I sleep like
a baby.

A fussy baby...

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Old 27th August 2012, 01:55 AM   #1187
lhquam is offline lhquam  United States
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Quote:
Originally Posted by Nelson Pass View Post
I have a difficult time believing that the input cap is the culprit. The input impedance of those Jfets ought to be
way high compared to the input network.

You are right, why should that affect the stability of the feedback loop?

A 1uf, 100K RC filter will have 30 degrees phase shift at 2.6Hz.

But, as I mentioned in the earlier post, my AC Bode plot might incorrect since it is the In to Out response of the amplifier, not that of the feedback loop itself. I think I am doing the phase margin tests incorrectly. But that does not explain why the actual build oscillates under some condition.

I am using Tenma 32V, 3A switching regulated bench supplies for my testing. When the oscillation hits the rails, the supplies limit the current. With the 1 watt signal levels, the amplifier should never hit the rails.
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Old 27th August 2012, 02:08 AM   #1188
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Quote:
Originally Posted by Nelson Pass View Post
Not at .5 Hz it doesn't.

Me, I don't use Zobels (like .1uf and 5 ohms at output), and I sleep like
a baby.

A fussy baby...

Your comment eliminates a design variable. Fussy is usually good; it generates focused attention and respect. Flip side, how would you enable your stable F6 circuit to oscillate at 0.5Hz?
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Old 27th August 2012, 02:26 AM   #1189
lhquam is offline lhquam  United States
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Quote:
Originally Posted by Nelson Pass View Post
I suggest you strip this down a bit.

Delete C3, C6, and C7

Set R1=0, R12=10k, R19=10k

and let's see what happens.

Wow!

With gain=1, the Bode plot phase margin will be 180 degrees. All of the open-loop gain is going to negative feedback. This will not oscillate.
Obviously a voltage gain stage is needed ahead of the input stage if it to be driven "ordinary" audio sources.

What closed loop gain do you think is appropriate for this amplifier?
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Old 27th August 2012, 04:09 AM   #1190
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Typo. R11=0

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