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Original Message

RIAA design dilemma, need your opinions.

Posted by JLH on February 19, 2002 at 10:46:57:
Inmates,
I think I sort of know what the answer to my question is, but want to get some reassurance from the group. I have done a lot of research on both composite and split RIAA networks and have settled on a composite network based on my personal preferences and prejudices ;0) . I have read everything I could get my hands on (i.e. Morgan Jones, Allen Wright, Simon Shilton, et al) but did not get a sense of the impact the first two resistors (R1 & R0) make on sound. Please see below link of the circuit for clarification. The trade off I am wrestling with is gain versus grid leak resistance of following tube. If I make R1 larger relative to R0, I will get less gain because R1 & R0 act as a voltage divider. However, the grid leak resistance for the following tube will be good. I always try to keep grid leak resistances to a minimum. On the other hand, if I make R1 and R0 equal to each other, but twice that of R1â€™, gain will be better, but R0 will more than likely be over 500K. The tube that will be driving the RIAA network is a constant current source loaded 6GK5 with Vp=160V, Ip=16mA, and Vg= -1V. At these operating points the 6GK5â€™s Gm=16.8mA/V, Mu=80, and Rp=4.8K. Therefore, as you can see it will not have any problems driving the RIAA network, but Iâ€™m worried about grid leak resistance of the following stage. So, what do you Inmates feel like would do more damage, sacrificing gain that may necessitate another gain stage, or try living with a very high grid leak resistance? All your comments are welcomed and appreciated.
JLH
http://www.users.globalnet.co.uk/~valveamp/RIAAStageDesign.htm