Phono preamp gain setting
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A good test of the 54db setting would be to listen to a recording that contains high volume cymbal crashes. Those sounds probably represent a worst case in terms of high frequency energy. Listen for distortion or clipping on those notes. It would probably make sense to do your initial trials at a bit less than normal volume. Whether or not overloading occurs will be independent of volume setting, since the phono stage is ahead of the volume control. JA's measurements for the LP5.3 indicate overloading occurs, for the 40 and 60db gain settings, at levels that are about 20db less at 20kHz than at mid-frequencies. The manual for the LP5.3 indicates an input overload point for the 54db setting of 11mv, which probably means that the 20kHz overload point will be about 1.1mv. Also, the 20db difference may be suggestive that the overload occurs prior to riaa de-emphasis, meaning that the relevant signal level will contain about 20db more energy at 20kHz than the music actually contains. All of which leads me to feel that you'll probably be ok with most music, but I'm uncertain if clipping or distortion might occur under worst case conditions of high frequency energy. I also note that JA's measurements of the Calypso indicated a large amount of what he called "rf breakthrough" (perhaps he really means a low level oscillation) at exactly 519 kHz (which is just below the AM broadcast band). I'm wondering if that might somehow be related to the hiss you hear, although his description seems to indicate that its amplitude is not volume control sensitive. Perhaps it's getting on the grounds and somehow beating down to audible frequencies. If you have a portable am radio that can tune to that frequency, it would be interesting to see if it detects that when brought near the preamp. As would trying Herman's suggestion of seeing if the noise is still present with shorting plugs connected to the inputs of the Calypso in place of the phono stage. Regards, -- Al |
