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Old 26th February 2021, 06:29 AM   #751
Phloodpants is offline Phloodpants
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Actually, a slight tweak makes it easier to get parts in the exact value, or within tolerance for hand-picking.
75K - Widely available
10.9K - Not available, but on the low side of 1% tolerance 11K resistor.
29.2nF - Not available, but well within 5% tolerance 30nF
10nF - Widely available.
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Old 26th February 2021, 12:58 PM   #752
rjm is offline rjm  Japan
Richard Murdey
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You could split the gain difference by reducing R3,4 (in your figure above) from 2.2k to 1k ohms, which increases the gain of the second stage 6dB, leaving just 8 to make up for in the first stage.
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Old 26th February 2021, 06:04 PM   #753
Phloodpants is offline Phloodpants
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Thanks! Yeah, I'm trying to avoid loading up the output too much. The OP27 will swing +/-10V into 600 ohms, but I want to be nicer to it than that. The knee of the curve for output swing vs. load is around 1K. Distortion remains low anyway.

So here's a happy medium, 1.21K.

Click the image to open in full size.

And this is the preamp itself. I built the power supply into the boards themselves, and it's picking up a little hum, which I think will go away with the new feedback values.

Click the image to open in full size.
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Old 27th February 2021, 10:22 AM   #754
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Hi Chris,

While it's a compact an efficient layout, the close proximity means there is a risk of noise coupling between the AC wires and the audio signals, as well as from the magnetic fields of the power transformer.

I hope you can manage to get the hum down to manageable levels.

RM
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Old 27th February 2021, 05:10 PM   #755
Phloodpants is offline Phloodpants
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Yeah, the plan is to move the power supply to another chassis if it still hums, but I wanted to give it a shot without needing a separate chassis.

The hum isn't too bad now, actually.
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