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A Simplified Example RIAA Stage > Valve Noise Summary - Pg. 607

Chapter | 7 The Pre-Amplifier 607 better to present the input stage with a healthy signal, rather than hope to amplify a weak one cleanly. Valve Noise Summary Despite all the previous caveats, qualifications and provisos, we can make some useful generalisations that will avoid unnecessary calculations when designing for low noise: G G G Pentodes are significantly noisier than triodes, and JFETs can be even quieter. Valve sample variation can be large. (1/f noise is largely determined by the cleanliness of the room in which the valve was assembled, so although a given manufacturer tends to be consistent, there can be differ- ences between manufacturers À or more accurately, their assembly rooms.) To render the noise of R L insignificant, there must be no feedback at the cathode, since this reduces the shunting effect of r a . This is also true for a -follower, even though omitting C k has no discernible effect on gain. The cascode has r a % N, so the noise from R L must be considered.