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Fet 6Gk5 ECC88 mc phono stage

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#21
RIAA Values

I'm curious about your chosen RIAA values. My ability to calculate values is about nil, so I went to the kabusa calculator that I used in building my own phono pre. It indicates a very close match with your chosen values IF your R5 is changed from 56k to 73k. Obviously I'm missing something. Perhaps you can enlighten me concerning the difference. Thx
 
#22
Hi

Regarding the RIAA values, your value of 73k for R5, assumes a zero source impedance, and an infinate termination impedance.

It does not take into account the output impedance of the cascode stage (23K5) or the input impedance of the following stage (R7, 1M)

Cheers
 
#23
Bruce Anderson said:
I'm curious about your chosen RIAA values. My ability to calculate values is about nil, so I went to the kabusa calculator that I used in building my own phono pre. It indicates a very close match with your chosen values IF your R5 is changed from 56k to 73k. Obviously I'm missing something. Perhaps you can enlighten me concerning the difference. Thx
Click to expand...

What did you assume for rp of the 6GK5 cascode when you used the kabusa calculator? (Not 0 right?) I would assume that the effective rp of the cascode is relatively high, likely something on the order of 90K-100K//23.5K so with the plate load resistor shown which would put the combined source impedance around 17 - 18K and the effective value of R5 + source impedance shunted by 1M in the ballpark you calculated.

FWIW: It looks like an elegant design, and I would suspect it sounds quite good.

Edit: Velvetsunrise I see you beat me to the punch.. 😀 Nice design btw..
 
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