I don't know how the headphone portition of the Bellari works, but a .8mv cartridge with a 30db gain phono stage only has an output of 25mv, which is WAY too low for using a normal preamp in a typical system. Maybe the headphone stage uses a different set of values?
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There is an article on the Absolute Sound website which gives the formula for required gain as NdB = 20 Log V1/V2, where NdB is the required gain, V1 is the voltage needed to drive your earphones, and V2 is the output voltage of the cartridge. You can play around with an Excel spreadsheet (or similar) to solve the equation for your values. The article, which is worth reading to understand how the formula works, is found here: required gain.
Why not use a linestage after the phono stage and plug your headphones into the linestage, assuming it will add enough additional gain to drive them?Unless it has a specifically designated headphone output, a line stage preamp will usually not be able to drive headphones properly. Quality dynamic headphones typically have impedances in the range of 32 to 100 ohms, which is much too low in relation to typical line level output impedances.
Also, dynamic headphone sensitivities tend to be very roughly in the area of 100db for 1 mw (milliwatt) in, the 1 mw representing 179 mv (millivolts) if the impedance is 32 ohms. 179/0.8 represents a gain of 47db. Since that is in the same general ballpark as the gain required to drive a typical line level preamp input using that cartridge, I suspect that the gain to the headphone output of the phono stage is the same as the gain to its line output (in this case 30db), the two outputs probably differing only in that the headphone output provides a buffer stage with low output impedance.
Bottom line: The phono stage is simply not suitable for a 0.8mv cartridge.
let me add some mustard from my own experience. (BTW I do agree with Al's statement.) Why?
Practically speaking, I use a 3.0mV Nagaoka MP-50 with a 60dB! phono-stage setting and unity gain with my pre-amp which is just fine at about 12 - 1 o'clock.
Now think of 1/3 (actually less) the cart output -- with yet 1/2 the phono-pre gain!!!
See what I mean - no way.
PS: Look at some 3.0mV to 4.0mV cart output at least, and you be about OK. Conclusion?
Get some top MM or MI such as e.g. a Nagaoka MP-50 (~$450) or Ortfon M20FL super(~$150) and it will work.