| Re: RIAA preamp project [message #76987 is a reply to message #67165] |
Fri, 07 June 2013 22:29  |
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gofar99
Messages: 1947
Registered: May 2010
Location: Southern Arizona
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Illuminati (5th Degree) |
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Hi, I recently found a good way to test phono preamps. You need to build a reverse RIAA filter (not to difficult and only about 6 parts) and then the way I do it is use a DSO that has a built in signal generator and can do direct Bode plots of the results. With a good RIAA inverse filter the result should be a straight line. Mine is accordingly within +/- 0.2 db from 20HZ to just past 22KHZ. Ruler flat in the 100-10K range. The S/N is right on - 89dbv with common JJ ECC803S and ECC81S tubes. Low noise ones can bring it down about another 3 db. I stuck with the passive filtering as it seemed to sound better. Hard to define how, just better.
BTW you are quite correct that good grounding is extremely important. Layout is equally important. This is the type of project that can not be just thrown together and expected to work well.
Good Listening
Bruce
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