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Old 5th May 2012, 08:00 PM   #1
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Default Another diy MM RIAA

I've designed and build a MM RIAA preamplifier that has a good dynamic and detailed sound. The noise is very low. Although I am very pleased with it, I would like to hear if anyone has ideas on how it can be improved.
I have built several of this preamp, as several of my friends want one, because they were not satisfied with the RIAA preamps they had.
Attached Files
File Type: pdf Polyphonia RIAA MK2.pdf (44.5 KB, 196 views)
File Type: pdf Polyphonia RIAA MK2 PCB.pdf (144.4 KB, 66 views)
File Type: pdf Polyphonia RIAA MK2 placement.pdf (120.9 KB, 55 views)
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Old 7th May 2012, 09:49 PM   #2
mlloyd1 is offline mlloyd1  United States
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that's a lot of capacitance with all those paralleled, non-cascoded 2sk170 jfets.
not a good thing for a moving magnet cartridge interface.
also, the source degeneration resistors are quite large - not great for low noise.
mlloyd1
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Old 8th May 2012, 02:16 AM   #3
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Noise, not counting excess noise, in a MM RIAA stage if realistically including a MM cartridge is dominated by the combination of the inductive (and resonant in the upper audio band) cartridge source impedance and the 47K load resistor.

Designs optimized for a short circuit source impedance are mistaken, and often compromise other goals. As mlloyd1 posts above, the heroic efforts to get the device noise equivalent down to a couple dozen Ohms is then thrown away with 300 Ohms of degeneration. Why?

But the 325 Ohm equivalent device noise + 47 Ohm input filter + 150 Ohm feedback resistor is *still* unimportant because noise is determined elsewhere.

Sounds like a lot of criticism, but is intended constructively. Took me a while to figure out why my system noise factors were so wrong.

All good fortune,
Chris
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Old 8th May 2012, 04:00 PM   #4
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Thank you for your feedback. The first version i made, did not have the parallel-coupled jfets. The noise was remarkable higher. I think i will try make a complete other design.
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