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Old 11th April 2011, 12:46 PM   #1
Bill*B is offline Bill*B  United States
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Default Positive feedback for amplifier input stage

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I just saw an amplifier schematic where the input stage had about 2 dB of POSITIVE feedback from the driver, within a global NEGATIVE feedback loop of around 20 dB.

The designer made much of this as contributing to a "powerful sound". It appears to me that he simply has 18 dB of negative feedback, but am I missing something?

Regards, Bill.
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Old 11th April 2011, 01:22 PM   #2
Yvesm is offline Yvesm  France
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According the way the positive feedback is applied to an input stage, f.e. at the anode of a triode, it may increase its drive capabilities while reducing distortion.
An ancester of the CCS load also know as bootstraping.

Yves, Worst guru.
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Old 11th April 2011, 01:51 PM   #3
Bill*B is offline Bill*B  United States
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Thanks, Yves. It was a 12AX7, so increasing the drive capabilities does make sense. Bill.
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Old 11th April 2011, 02:24 PM   #4
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Dave Hafler uses it in his amps. For example, check the phonostage of PAS3, where the positive feedback is between the cathodes.
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Old 11th April 2011, 04:35 PM   #5
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by revintage View Post
Dave Hafler uses it in his amps. For example, check the phonostage of PAS3, where the positive feedback is between the cathodes.
Deliberately applied in order to boost the open loop gain - a juggling act that if carried to the extreme results in an oscillator, and also significantly increases distortion IMLE. IIRC in the case of the PAS 3 the increase in open loop gain was something like a few dB. There are number of issues with the design of the PAS 3 phono stage, probably the most critical of which is the extreme loading on the second stage. See: Spice and the art of preamplifier design, Part 1
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Old 11th April 2011, 08:54 PM   #6
Bill*B is offline Bill*B  United States
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Thanks, guys. Lots of good data, as always. Here's a link to the schematic that got me interested: it loads a little slowly! Regards, Bill.
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Old 11th April 2011, 09:08 PM   #7
GordonW is offline GordonW  United States
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IIRC, Stuart Hegeman, then working for Harman-Kardon (or just before he went there?), either pioneered, or was an early adopter of this practice, in the late 1950s. My HK Award A300 has this circuit design... 12AX7 driver, 12AU7 phase splitter, 6V6 PP outputs. I'm about to build an elaborate integrated amp, using the Harman HK250 circuit design (PP 6L6s)... it uses a 6CG7/6SN7 type tube for the input driver and phase splitter, with the positive feedback increasing the open-loop gain... allowing a decent amount of feedback, while still having enough gain for a good preamp stage to be able to drive it, easily...

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Gordon.
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Last edited by GordonW; 11th April 2011 at 09:14 PM.
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Old 12th April 2011, 08:29 AM   #8
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The QUAD II also uses positive feedback so it must work, then.
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Old 12th April 2011, 10:25 PM   #9
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Didn't Dynaco use that trick in their power amps?
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