Re: jack's output level

["Bruce A. Wahler" <bwahler@tiac.net>]

Jean-Charles Debonneville wrote:
> 
> I've seen about the Blue Tube Preamp:
> >The Blue Tube has both line and instrument output level jacks:
> 
> but what are these levels (in dB ??) : line , instrument, mic

Instrument level is usually in the neighborhood of 100mV for full 
output, or -20dB/V (decibels relative to one volt output).  Line level 
is in the range of 1-2V for full output, or 0 to +6dBV.  Often I've seen 
this as "-20dB" and "0dB" ratings on mixers and effects.  Output ratings 
are also calculated in dBm, or decibels relative to 1 milliwatt into a 
"standard" load, which I believe is 600 ohms. (Anybody else???)

Power = Voltage**2 / Resistance --

        V = SQRT (P * R) = .775V = 0dBm

To convert from dBV to dBm -- 

        dB = 20 log (1/.775) = 2.2 dB

add 2 dB to the dBV reading to get the dBm equivalent (approx.)

-BW