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 Al. Let me just add that the designer himself stated to me that the setting of 1.4 mV corresponds to approximately 70 dB of gain, and 0.6 mV to approximately 68 dB, which essentially matches to the values obtained by using the formula.

Also, the assertion that "[a] phono preamp cannot output more than a few 100s of mV as it will overload the linestage pre" surely cannot be true. The Parasound JC-3 has a nominal output of 1V, and it will probably output way more than that in peaks. The Sensor is listed to have the max output of a whopping 8V, which I don't quite understand, but certainly the designer would not design it this way if it was likely to be incompatible with most of the line preamps on the market. I certainly didn't experience any incompatibility with my line preamp, which btw outputs 1.5V whereas my power amps are listed as having input sensitivity of 1V, and they are all from the same manufacturer. Needles to say, they work very well together. Sounds like the issue is more complicated than just pure numbers on paper. I also apologize for not providing more information to begin with, which is never a good way to ask for advice. However, I simply and honestly did not know what information was pertinent to the answer.

As a follow up, I looked up specifications for some phono and line preamps out of curiosity. My Rogue preamp has "rated" output of 1.5V, but "maximum" output of 30V PP ("peak power"?)!!! With the power amps listed as having input sensitivity of 1V RMS, it sounds like a bad match, on paper, but definitely not in reality.

Also, the Rogue Ares phono preamp, is listed as having rated output of 1V and maximum output of 8V RMS @ 1KHz. Based on this, and in light of our conversation, I would venture an uneducated guess that the max values are not truly relevant in real life conditions, and even the nominal value in case of the phono pre would depend on either the cartridge's output or the gain setting. So Bombaywalla's assertion might as well be true in that under real life conditions a phono pre will not output 2 or more V's. Otherwise, these figures wouldn't make much sense...

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

Ahh, the attenuator on the line preamp! Sometimes the obvious avoids us (at least me :).

Yes, the first figure is a mistake; it was supposed to state "0.4 mV" not "1.4 mV." Hazards of posting late. These figures are a few dB off from what is obtained using the formula, but he did say "approximately." Thank you for pointing it out, Al.