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Modifiying a commercial amplifier for Hi-Fi
Modifiying a commercial amplifier for Hi-Fi
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Old 6th May 2021, 06:59 AM   #11
Rick PA Stadel is offline Rick PA Stadel  United States
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Join Date: Apr 2017
Location: greater Kansas City
Maybe it's just me -- and apologies to the designer if she/he's reading this -- but it looks like there was a kind of *symmetry seduction* going on. That the notion of complete symmetry was so compelling, the designer just couldn't let go of it! Even as more and more kludges and Band-Aids were required to garner a modicum of performance / stability -- way past the point where the circuit sections function as originally imagined or intended.

As it is, Q7 and Q8 are pretty much just a CCS. Q3 and Q4 have no appreciable differential function. The note about a C10 between their Emitters is a good one -- maybe 1 or 2,2uF in series with 68 to 150R. THEN it would make a differential contribution.

But, there is already a C10 between the off-board Volume pot and the '072 output.

I also think the same treatment would be worth trying in place of C13. As it is, Q5 / Q6's dynamic range is frightfully limited, and they have no ability at all to control at DC. Plus, there's only so much they can do with 404uA to 425uA, which is the full range available for rail-to-rail output swing.

R27 is a whopper! I must be reading that wrong: It's going to want 27A through one or the other 1N4148's as the output swings to + or -68V !?

Having a hard time figuring why you'd want to omit the speaker fuse. Is it somehow damaging sound quality?

Just my 2 ..


Last edited by Rick PA Stadel; 6th May 2021 at 07:07 AM.
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Old 6th May 2021, 08:03 PM   #12
PRR is offline PRR  United States
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Join Date: Jun 2003
Location: Maine USA
Modifiying a commercial amplifier for Hi-Fi
I would not change a thing on any BiAmp gear I could use. Very clean elegant designs.

Yes, the schematic has error(s). Get over it. It was not computer-checked; also sometimes details are wrong to confound copycats or even unauthorized repair.
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