MOTM

At 5:50 PM -0600 03/25/01, Paul Schreiber wrote:
>
>A dBu is a voltage measurement, referenced to 0.7746 volts. This *magic*
>number is the RMS voltage across a 600 ohm resistor, dissapating 1
>milliwatt.
>For a sine, 1V RMS is 2.828V pk-pk.
>So, a negative dBu 'reading' means the voltage is *less* than 0.7746Vrms.
>The equation is
>dBu = 20 log (Vrms/0.7746)
>where Vrms is what you are wanting to measure in dBu's.
>To 'go backwards'
>Vrms = (.776) (10^dBu/20)

Thank you, I found similar information in the books I checked, but I was
unsure how much depended on the impedances involved.  As you know, modern
line level inputs are rarely 600 ohm, and I didn't want to use an equation
taken out of context.

>So, -4dBu is 1.38V pk-pk and +22dBu is 27.6V pk-pk! That's about what a
>+-15V powered op amp can drive out 'rail-to'rail', so to speak.
>MOTM's 10V pk-pk signal translates to +13.2dBu

This is where I get confused.  Your earlier statement that, "the signal
generators in MOTM use 10V pk-pk, or around +22dBu" seemed more in line
with what some of the other people on the list are saying.

For example, Larry seems to be using a Mackie, and Mackie claims a MIL of
+22dBu, yet:

At 5:18 PM -0600 03/25/01, J. Larry Hendry wrote:
>
>I have found MOTM levels to be right of the edge of being too hot into a
>mixer like this.  Of couse, for a MOTM output going to a mixer, if would be
>simple enough to just divide the voltage coming out of the MOTM with a
>couple of resistors priot to going into the mixer.

So how is +13dBu anywhere near the edge of being too hot??  This doesn't
make sense to me.  I'm trying to contradict anyone's observations, it's
just that I think I must be missing something.

Also, when I enter 20 * log (10/.7746) into my calculator I get 22.22.
Working the other way, (.7746) * 10^(22/20) = 9.75.  Would someone please
double check that for me??  If humanity relied on my half-assed math skills
we'd still be living in caves :)

I am also very hesitant to add a voltage divider to the outputs of the
motm.  This is not because I am so concerned about the effects of the motm
outputs looking into a high impedance voltage divider, but rather putting a
high impedance at the AC coupled inputs of the mixer.  Wouldn't that create
a filter, or rather move the existing "high-pass filter" into the audible
range??  Furthermore, people rarely ever use passive voltage dividers to
reduce audio levels, and I'm thinking there must be a reason why.  With
line levels one goes from a low output impedance to a high input impedance.