Diff-Amp with unequal pull-up resistors - diyAudio
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Old 8th June 2004, 01:14 AM   #1
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Default Diff-Amp with unequal pull-up resistors

On this complementary topology, to get the correct gain in the diff stage, plus the correct idle current in the VAS stage, it seems necessary to use two different values of pull-up resistors in the diff-amp. Equal pull-ups with high enough R to create 8-10x gain in the diff stage cannot supply enough current into the JFET VAS to cleanly drive the output stage. Equal pull-ups with low enough R to generate 5-10ma in the VAS create almost no gain in the front diff-amp. What distortion or stability problems might this cause? Any better solution?
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Old 8th June 2004, 01:31 AM   #2
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Old 8th June 2004, 04:29 AM   #3
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R10 and R13 have to supply current for both the input stage and the VAS. Therefore they will have a lower value of necessity.
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Old 8th June 2004, 04:31 AM   #4
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Also the 2SJ74 is rated at only 25 volts. Since it will have to swing the full rail-to-rail range at clipping, you will be limited to +/- 12.5 volt rails in the front end.
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Old 8th June 2004, 06:24 AM   #5
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Do a search.

That front end is called a 'Lender circuit', is about 40 years old, and improves linearity through local feedback.
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Old 29th June 2004, 11:55 PM   #6
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don't forget, if the power of the two diff. transistors must be the same, (P=VI) or the DC offset may not be stable. also adding more transistors adds more noise.
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