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In Reply to: RE: Thanks Kurt but i use the KAB compute for RIAA value posted by lovetube on February 26, 2011 at 20:42:31
Here's some tips: Start trying it with R1 = 220,000 ohms. Then many typical values will be close for you.
Correction to my other post: 0.01 uF / 2.916 = 0.0034 uF
= close to 0.33 uF.
Lipschitz' formula here expects a zero output impedance from the op amp. 220,000 ohms is the total source impedance including Rp and Rl in parallel at first tube's plate. (Rp || Rl) + R1 = 220K.
If you direct couple this, expect a weaker bass, as V2 grid current will affect the small signal's DC effect toward it.
If you choose to AC couple this, the bass becomes more accurate and can even make it sound better. The grid current then goes out of play. And the coupling cap quality becomes very critical.
If you choose to AC couple and need a grid grounding resistor at V2, then make it as high as possible because this calculator expects the load there to be infinite. This is where very high grid resistor values and grid leak biasing can get real close to that on V2.
You will never achieve a perfect RIAA. :-)
-Kurt
Follow Ups:
Hi kurt.
I did that already last night with the R1 of 220k and come up with r2= 31.989K and the C2 =0.0034uf and c1 = 0.009940uf.
so i'm not sure why your value is different ?
I'm AC coupled the first to the second stage /. however as to my drawing I think is might be best to parallel the 12AT7 for lower impedance .
what you rekon ?
Cheers
LT
Your results are correct from the calculator.
Where I went wrong was thinking C2/C1 = 2.916, and still missed; But in reality, C1/C2 = 2.916,
or C2 = C1 / 2.916.
C1 = 0.01 uF; therefore C2 = 0.01 uF / 2.916 = 0.0034 uF. That's the correct answer. Thanks for catching that.
-Kurt
Cheers
LT
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