Conversion of mV to dB

Note when using the KAB calculator that the output is based @ 5 cm/s. If your cartridge is rated @ 3.54 cm/s, you will need to multiply it by 1.41 to equate to 5 cm/s.
Just to make sure you are comparing apples to apples.

As for your phono preamp, it would help to know what speed was run to determine the mV values. For example, if running at 5 cm/s, then 0.3 mV will equate to 61 db of gain on the KAB scale. However, if using the 3.54 cm/s speed rating, then you 0.3 mV would equate to 58 db of gain. I would think it best if you ask the manufacturer this question, since we don't know what recording speed they based their numbers on.

Thank you both. I agree the KAB calculator needs to be used with caution as it does not take into consideration all of the parameters that might come into play when trying to convert different values. It appears that an accurate calculation can be done using the formula of 20 log (V1/V2) where V1 is the preamp's nominal output in terms of V rms and V2 is the sensitivity setting that is being converted. The preamp's nominal output is listed as 2V rms so let's say for the 1.4 mV setting, the gain would be 63 dB. This perfectly reflects the range provided by the manufacturer of 52-76 db from the lowest to the highest setting.

I owe credit for this to, and would like to thank, Al (Almarg), whose advice has been absolutely invaluable to me since becoming an Audiogon member. Thanks Al!

06-30-12: Actusreus
Thank you both. I agree the KAB calculator needs to be used with caution as it does not take into consideration all of the parameters that might come into play when trying to convert different values. It appears that an accurate calculation can be done using the formula of 20 log (V1/V2) where V1 is the preamp's nominal output in terms of V rms and V2 is the sensitivity setting that is being converted. The preamp's nominal output is listed as 2V rms so let's say for the 1.4 mV setting, the gain would be 63 dB. This perfectly reflects the range provided by the manufacturer of 52-76 db from the lowest to the highest setting.

No, I'm afraid that you did not get this right. From my experience a 1.4mV output cartridge does not need 63dB of gain. It needs much less - more like 45-47dB. A 1.4mV output cartridge in not exactly hi-output MC (which is usually around 2.5mV) but neither is it low output MC (which is sub 1mV). MM cartridges are often in the 4-6mV which need 40dB of phono preamp gain.
You could use a phono preamp that has 63dB of gain for a 1.4mV cartridge at your peril as it will trend towards overloading the linestage that follows the phono preamp & will most likely give you high hiss.
The assumption that's wrong in your calc is that a phono preamp outputs 2Vrms. Reading that it appears that you are thinking that a phono pre connects directly to the power amp. It does not. A phone preamp usually outputs a voltage that is in the 100s of mV. This output is fed into a linestage preamp & further amplified before feeding into the power amp. A phono preamp cannot output more than a few 100s of mV as it will overload the linestage pre.
For example, My power amp has an input sensitivity of 1V. Imagine if a preamp output 2Vrms - I'd have 300W into 8Ohms & I would blow my eardrums & fry my speakers! And, I know that this does not happen.
I'm quite sure that your preamp is set for 60dB gain at the 0.3mV setting & for 40dB gain at the 5mV setting. I did calculate the ratios of 0.4/0.3, 0.6/0.4, 0.9/0.6 & they are all close to sqrt(2). The ratio 1.4/0.9, 2.5/1.4 are slightly higher than sqrt(2) & of course, 5/2.5 = 2.
So, I would say:
0.3mV would be 60dB gain
0.4mV would be 57dB gain
0.6mV would be 54dB gain
0.9mV would be 51dB gain
1.4mV would be 47dB gain
2.5mV would be 42dB gain
5mV would be 36dB gain.
Of course all of this at 5cm/s.

I owe credit for this to, and would like to thank, Al (Almarg), whose advice has been absolutely invaluable to me since becoming an Audiogon member. Thanks Al!

yeah, I agree Almarg is a very nice fellow who genuinely wants to help fellow Audiogoners & is very knowledgeable. He's kept me honest a few times! ;-)

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.