Conversion of mV to dB

Thanks very much for the nice words, guys.

Bombaywalla, thanks for your comment, with which I essentially agree. However, note that the question was not what gain setting to use with a cartridge having a given output. The intent of the question was to determine the gain in db that corresponds to each of the gain settings provided by the phono stage, which are marked in millivolts, rather than in db.

The phono stage is an RCM Sensor Prelude, which is indeed specified to have a 2 volt nominal output. So, for example, the gain setting that is marked as 1.4 mv provides a gain of 20 x log (2000/1.4) = 63.1 db.

That interpretation can be confirmed by noting in the data sheet that the range of possible gain settings is specified as 52 to 76 db, corresponding to markings of 5 mv and 0.3 mv respectively. 20log(2000/5) = 52.04 db, and 20log(2000/0.3) = 76.48 db.

I agree that generally a 2 volt nominal phono stage output will be much too high, so the mv marking of the gain setting that is used should be significantly higher than the rated cartridge output.

Thanks again. Best regards,
-- Al

Thanks for the followups, Marek. A point to keep in mind is that the specs for maximum output, for both phono stages and line stages, refer to the maximum output that the component is capable of (without clipping or a severe rise in distortion). And in the case of a line stage, it is based on the volume control being turned all the way up, as is the specified gain of a line stage.

So that number should be much greater than the maximum output level that would ever actually be used, and it should also be much greater than the input sensitivity of the next component in the chain.

Also, "rated output" tends to not be a particularly useful figure, at least without being put into the context of the input level it is based on. The output level will be the input level factored up by the gain (expressed as a voltage ratio), and (in the case of a line stage) factored down by whatever amount of attenuation is provided by the volume control at the setting that is being used.

Finally, I believe that the reference to 70 db in the first paragraph of your post immediately following mine is a typo, and perhaps should be 60 db.

Glad the new phono stage is working out well. Best,
-- Al

John and Jeff, I believe that BOTH of your statements are correct, but there are several factors that muddle the issue.

5 cm/s lateral velocity corresponds to 3.54 cm/s at 45 degrees, meaning 3.54 cm/s in each channel, as Jeff indicated. Also, however, 5 cm/s peak in a given direction corresponds to 3.54 cm/s RMS in that same direction.

See the photos of the labels of CBS and JVC test records near the end of this thread, and also see some of the preceding posts. The 1 kHz test tones on the CBS record are clearly indicated on the label as being at 3.54 cm/s RMS at 45 degrees, which corresponds to 5 cm/s peak at 45 degrees. The JVC record is also marked as 3.54 cm/s at 45 degrees, but with no indication as to whether that is RMS or peak. Measurements cited in the thread indicate that figure is peak, implying only 2.5 cm/s RMS, which is 3 db less than on the CBS record.

So comparing apples to apples, and using peak figures at 45 degrees, John is correct that CBS is 5 cm/s and JVC is 3.54 cm/s. Which leaves open the question, however, of what a cartridge specification is referring to if it refers to "3.54 cm/s." I have no particular knowledge of that, but it wouldn't surprise me if some manufacturers are referring to 3.54 cm/s RMS per the CBS record, corresponding to 5 cm/s peak, while others are referring to 3.54 cm/s as marked on the JVC record, which is peak and therefore only 2.5 cm/s RMS.

Fortunately, though, the difference is only 3 db.

Best regards,
-- Al

Mordante, you'll probably do better with a less sensitive gain setting (i.e., a higher number of mV, such as 1.4 mV). As indicated earlier in the thread, since the mV settings on this phono stage are the input levels corresponding to an unusually high 2 volt output, if you use the 0.6 mV setting with a 0.6 mV cartridge it is likely (depending on the gains and sensitivities of the rest of your components) that you would find yourself having to set the volume control too far down in its range to be optimal. Degradation of distortion performance and/or overloading might even occur in line stage circuitry that is "ahead" of the volume control.

It might be worthwhile to also try the 0.9 and 2.5 mV settings, as well as 1.4 mV, and see which setting gives you the best sonics, the lowest background hiss, and the most preferable range of volume control positions.

Regards,
-- Al