Conversion of mV to dB


Hello,
My phono premamp has gain values expressed in terms of mV. Is there a way to convert those values into dB? They are 0.3, 0.4, 0.6, 0.9, 1.4, 2.5, and 5 mV. Thank you.
actusreus
KAB has a calculator for the conversion:
http://www.kabusa.com/pregain.htm
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.
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.
Almarg, Actusreus,
OK, thanx for the clarification.
wow, I have not come across a phono stage outputing a 2Vrms signal! But yeah, the link Almarg sent surely says that...
Looks like this RCM Sensor Prelude phono preamp should be set to the 1.4mV setting so that it provides 63dB of gain for MC & should be set to 5mV so that it provides 52dB gain for MM. This seems to jive with Al's statement "...the mv marking of the gain setting that is used should be significantly higher than the rated cartridge output."
So, really only 2 settings of all those provided really make sense & are really useful. One could play around w/ the other gain settings but one would get the preamp volume control out of the 12 o'clock - 2 o'clock sweet-spot zone.
Jmcgrogan2, it's been many years since I played with LPs, fonocartridges, and fonostages, but I do recall something about 5cm/s (centimeters per second) and 3.54cm/s...being that they're the same!

5cm/s is the standard horizontal groove modulation for a mono signal, and 3.54cm/s is THE SAME goove modulation measured perpendicular to the groove wall and then expressed as a stereo signal, PER CHANNEL.

...I think.
.
Jeffreybehr, as far as I know, it has to do with which test record the cartridge was measured on, JVC (3.54 cm/s) or CBS (5.0 cm/s). I don't know about the mono versus stereo numbers. I do know that you can translate between the two numbers by using basic math.

As I mentioned, the KAB calculator assumes the 5 cm/s standard, so if your cartridge is measured using the CBS standard 5 cm/s then you can just plug that value in. However, if your cartridge's output was measured using the JVC standard (3.54 cm/s) you can divide 5 by 3.54 and multply the outcome (1.4) by the output to translate the output voltage into the CBS standard of 5 cm/s.

So a cartridge that has a rated output of 0.24 mV @ 3.54 cm/s will put out the same amount of voltage as a cartridge with a rated output of 0.34 mV @ 5 cm/s.
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
Thanks for chiming in here Al, I was waiting for you to help us out. I don't really understand it that much myself, but Mehran, of Sorasound, who distributes ZYX cartridges which are rated at 3.54 cm/s is the one who explained to me about the output voltage conversion to 5 cm/s rating. It seemed to make sense to me....but I'm not an EE, though I did stay at a Holiday Inn Expres last night. :)
This is a bit of an old topic but I just bought the same phono pre, the RCM Sensor Prelude. My cart is a Lyra Dorian.

Output voltage : 0.6mV (5.0cm/sec., zero to peak, 45 degrees)

Right now I have set the RCM to 0.6mV. Is this correct?
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
@Almarg,

Thank you I will try this out.