Negative Feedback Amplifier with embedded Positive Feedback Booster - diyAudio
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Old 14th June 2015, 03:21 PM   #1
Koonw is offline Koonw  Malaysia
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Talking Negative Feedback Amplifier with embedded Positive Feedback Booster

Dear DIYer, Friends,

I wish to share this OTL amp with you all. It uses low gain tubes and embedded positive feedback.
For more theory you can read this article, this amp can fit in the mechanism descript here:
Ref: POSITIVE FEEDBACK GAIN-ENHANCEMENT TECHNIQUES FOR AMPLIFIER DESIGN


It is possible to use many different types of low gain tubes include Triode connection Pentode which I have not tried. As for now my best listening result is 5687 and 6H9C/6SN7 and 8*6C19p output tube. It 's also possible to build smaller version as headphones amp with reduced driver voltages. 2 output tubes. There are too many tasks for one person to do this is why I want to share it instead. If you know SPICE, you input the information to see what happened, many would perhaps like to know more than what I already being observed. If you wanted more info about the driver PCB I will send the one that I use and sound clip if possible.



Let me know what you think, thank you in advance.





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Old 14th June 2015, 05:20 PM   #2
Koonw is offline Koonw  Malaysia
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Default RE:Negative Feedback Amplifier with embedded Positive Feedback Booster

This is the PDF document I want to attach earlier, hope it is successfully uploaded this time...
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Old 14th June 2015, 06:09 PM   #3
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It is a useful and effective technique. I have used it in solid state amps with success.
There is one thing you need to be aware of. The effective feedback is still negative, otherwise it would not be stable. But depending on your circuit, you can get the situation that when the amp clips, the nfb becomes nonoperative, while the pos feedback still works. In such a case the amp hangs on to one of the power rails and that's the end of your speakers (and possibly the amp).

So you must make sure that even when clipping, the effective fb remains negative.

Jan
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Old 14th June 2015, 08:01 PM   #4
Koonw is offline Koonw  Malaysia
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Dear Jan,

Thank you very much for the good comment:clipping is one thing I did not mention in my presentation. But I just do a quick test just now the clipping is just gradual squaring of top and bottom waveform, nothing else happened. I have used the amp for about 3 years it still going...as to what you said, it is certainly true to some extend.
As I mentioned even with total loss of negative feedback (by simply removed the NFB link), the amp would just went into oscillation (motor boating, square wave) and survive many times. I have speaker protection circuit in the output, response time about 1ms. The DC you mentioned is not really a big problem, one way or other you can have DC in the output though, especially serious when direct coupled and solid state. It CAN be protected by good design as you pointed out, thank again.
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Old 16th June 2015, 03:23 PM   #5
Koonw is offline Koonw  Malaysia
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I found this 1953 patent US2652458 yesterday, it was filed in 1949.
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File Type: pdf US2652458.pdf (626.3 KB, 38 views)
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Old 16th June 2015, 06:59 PM   #6
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Thanks for posting this patent and the other links and info. The positive feedback with phase correcting to match the negative feedback is most interesting.

There is a later Wolcott patent or two, 3328711 and 3111630, with a similar motif. A thread on it here:
Wollcott cross coupled circuit

The negative impedance (current sensing output tap) feature in your design can also be applied to the primary side of a conventional OT'd amp to remove the fixed winding resistance.
Attached Images
File Type: bmp Miller_Pos_Fdbk1.bmp (128.5 KB, 25 views)
File Type: gif Wollcott-1.gif (21.4 KB, 303 views)

Last edited by smoking-amp; 16th June 2015 at 07:03 PM.
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Old 16th June 2015, 07:19 PM   #7
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Here's Miller's full article, from March 1950, Electronics:

http://linearaudio.nl/sites/linearau...rch%201950.pdf

Jan
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Old 16th June 2015, 08:00 PM   #8
Ketje is online now Ketje  Belgium
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PFB is used in many amplifiers, also in OTL.
Some examples,
Mona
Attached Images
File Type: jpg Ph-AG9014.jpg (107.8 KB, 290 views)
File Type: jpg HARMAN-KARDON-HK20.jpg (146.0 KB, 290 views)
File Type: jpg PH-HF309.jpg (130.8 KB, 289 views)
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Old 16th June 2015, 08:25 PM   #9
Koonw is offline Koonw  Malaysia
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Yes, but my OTL is asymmetrical totem pole, do you think the PF in my design also provides additional function balance equalization of top and bottom tube. I did say something in that respect in my presentation, but I hope to hear some clear explanation, remember there were no bootstrap or any other mechanism involved.

(I have speaker protection in output, it hardly got activated except critical PSU failure like +ve or -v rail blown fuse, the fuse is 1A, it is 25W hardly blown. Little or no DC at full load, as well observed from speaker cone. With the same gain and NFB only in another design , it it quite different story.)
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Old 16th June 2015, 08:45 PM   #10
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Looks like the driver circuitry in your design is providing equal drive voltages to both the output tube grids. Usually there is a feedback bootstrap (from the output) to one of the V2 driver plates, or cathodes (to the load resistors) to compensate the drive levels for the cathode follower and grounded cathode output tube drives.

The VR4 adjustment looks like it would allow adjusting the relative output impedances of the top versus bottom output tubes. So that could compensate some, but would depend on the load Z. The overall NFB of course tries to equalize the output swings. I would guess that VR4 needs to be re-adjusted if the speaker Z is changed. How high is the 2nd harmonic for this thing?

OTL Bootstrap driver from Tubecad site:
Attached Images
File Type: gif OTL_C.gif (6.9 KB, 270 views)

Last edited by smoking-amp; 16th June 2015 at 09:15 PM.
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