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Old 27th August 2012, 05:36 AM   #1191
lhquam is offline lhquam  United States
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Quote:
Originally Posted by Nelson Pass View Post
Another thing, your thd vs frequency plot tells me that your C4 and C15
values are low.

OK. My capacitor selection is somewhat limited, but I have some electrolytics scavenged from a PC power supply. These are extreme values: 4700uF! They seem to work well but the THD vs Freq plot is about the same as the right hand plot in post #1178. I have not managed to trigger any oscillation. With an RC of 47 seconds, it takes a long time get reach bias voltages (like 5 minutes). I will obtain and try some other cap values like 100uf and 200uf.
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Old 27th August 2012, 06:42 AM   #1192
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Quote:
Originally Posted by lhquam View Post
I just re-verified that the circuit built to this schematic will oscillate about about .5Hz when stimulated by about .5V at .5Hz.

Correction: The input cap is not shorted in the circuit tested.
why are C4 C5 different (22μ vs44μ)?
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Old 27th August 2012, 07:55 AM   #1193
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lhquam,
  • imho the real reason for the effect might be that
  • you use a bootstrap for the top bias
  • the bias of the top is C4, 44μF connected to the output (and I just note that value differs from C15).
  • sonically this is an electrolytic in the signal path!!
  • an alternate method would be to connect the bias to the output and insert a big bootstrap in the middle of the R20 (1k4+1k4) (and treat it similar with SAME cap in a split up R21).
Q:
if you disconnect the transformer from one of the outputs does it still oscillate/motorboat?
- just for instance the top one gets no modulation, it will be a BA-1 output stage.

I like a method of bias with just a MP cap!
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Old 27th August 2012, 01:11 PM   #1194
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Should be (if well understood)
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Old 27th August 2012, 01:34 PM   #1195
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Quote:
Originally Posted by lhquam View Post
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?
Your finding that a CLG = 1 gives a stable amplifier is an eye opener, but is counter to that commonly known. Why is it not oscillating? Please post your refined schematic after implementing the suggestions by Mr. Pass.
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Old 27th August 2012, 01:44 PM   #1196
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Originally Posted by bobodioulasso View Post
Should be (if well understood)
Your schematic is pleasantly simple. Does the underlined mean after implementing [or not] the suggestions of Mr. Pass in your original schematic?
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Old 27th August 2012, 01:47 PM   #1197
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Yes, after implementing the suggestions of Mr. Pass

Just trying to abstract all datas.
Bigger caps and no cap across zenners.
Not different from Ihquam schematic but drawn differently.

Last edited by bobodioulasso; 27th August 2012 at 01:58 PM.
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Old 27th August 2012, 02:32 PM   #1198
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Quote:
Originally Posted by lhquam View Post
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?
lhquam: Recall that kasey197 has used a step up transformer [other than Jensen] to further increase OLG without an additional gain stage in the front end.
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Old 27th August 2012, 03:07 PM   #1199
lhquam is offline lhquam  United States
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Quote:
Originally Posted by Antoinel View Post
lhquam: Recall that kasey197 has used a step up transformer [other than Jensen] to further increase OLG without an additional gain stage in the front end.
Nelson explained that he made a typo in post #1182 and was not suggesting a closed-loop gain of 1. I will proceed under the assumption of a gain of around 6.
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Old 27th August 2012, 03:18 PM   #1200
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Quote:
Originally Posted by triode_al View Post
lhquam,
  • imho the real reason for the effect might be that
  • you use a bootstrap for the top bias
  • the bias of the top is C4, 44μF connected to the output (and I just note that value differs from C15).
  • sonically this is an electrolytic in the signal path!!
  • an alternate method would be to connect the bias to the output and insert a big bootstrap in the middle of the R20 (1k4+1k4) (and treat it similar with SAME cap in a split up R21).
Q:
if you disconnect the transformer from one of the outputs does it still oscillate/motorboat?
- just for instance the top one gets no modulation, it will be a BA-1 output stage.

I like a method of bias with just a MP cap!
Does the underlined mean metalized polypropylene? Are you still working with your schematic in post#1023.
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