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500mV (0dB) on the voltmeter?

I am tuning up my Nakamichi CR-3A cassette deck and am trying to understand the meaning of a dB value that follows a mV value. In relation to measuring output with a 400Hz input, the service manual refers to obtaining "500mV -2dB on the AC voltmeter" from the output jacks. In another spot it refers to obtaining "500mV (0dB) on the AC voltmeter" with a 19kHz input. I am using a Brymen 867 multimeter. I can measure 500mV from the output jacks easily enough but am left with two questions: 1) what do these dB values refer to? 2) is there any difference between the dB value within or without parentheses?
brymul
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almarg
9,670 posts
 
I took a look at the service manual, at hifiengine.com. It appears that they are somewhat arbitrarily defining 0 dB as corresponding to a voltage of 500 mv, with other voltages that are expressed in dB undoubtedly being related to that 500 mv level in accordance with the standard formula for expressing the ratio of two voltages in dB:

dB = 20 x log(V1/V2)

where "log" is the base-10 logarithm.

So for example the reference in the procedure to a voltage level of -20 dB would correspond to a voltage of 50 mv, since:

20 x log(50/500) = -20 db

And the reference you stated to 500 mv -2 dB would mean 2 dB less than 500 mv, which I calculate to be 397.1 mv:

20 x log(397.1/500) = -2.00 db

Hope that helps. Regards,
-- Al

minorl
1,014 posts
 
I love this.

I tell my track and field athletes

"Math is hard, so is track, deal with it"

brymul OP
1 posts
 
Thanks Al. Can’t understand why they would "arbitrarily" refer to something in the service manual. Also, why not just refer directly to 397 mV?
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