| A 11 year long quest [message #94518] |
Wed, 29 September 2021 21:51  |
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gofar99
Messages: 1902
Registered: May 2010
Location: Southern Arizona
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Illuminati (5th Degree) |
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Hi Everyone, Ever since the first tube based phono preamp I have wanted them to be quieter. I'll admit that the first few were nice sounding, but hummy and noisy. Later ones were really quiet but I felt that S/N of -75dbv was not the limit. Gradual improvements along the way brought it close to -80. So now I revised the design and completely over hauled the power supply as it seemed to be the thing holding me back. It was. Now with a different transformer, a medical grade SMPS for the heaters (neither over $20US, final three stages of filtering with poly caps and a capacitive multiplier in place of the regulators it is really, really quiet. I have reached the level where the tubes themselves are the biggest source of noise. They are selected EH7025s. With the better pair I show on the scope -93dbv, the worse pair -90. (see the image). The peak in the image is an artifact of the scope. Response is within +0.5db/-0.5db from 20HZ to 20K. The preamp has the same lovely sound as all the previous ones as that part is not changed. I'll post a schematic and photo of the guts in a day or so. In my system the listening level (on the preamps digital display) is right at -40db. When I increase it to 00 I need to have my ear within 6 inches of the speakers to hear anything. 40dbv is a ratio or 100 to 1. That is a lot of headroom!
Good Listening
Bruce
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