MOTM

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)

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

Paul S.

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.

Mark -

As a theoretical math thing, I can't help.  Paul's much better!  But 
I can say, and as others have clearly stated in this thread, that you 
can use the 410 for MOTM inputs and anything else.  I built mine with 
the resistor option for handling MOTM inputs.  With a preamp (I also 
use the ART MP) I've been able to use the 410 on pretty near 
everything I own, including electric mandolins, lap steels, and 
acoustic guitars.  You can do amazing stuff with it.

My mixer (Tascam TM-D1000) has trim pots for the inputs.  It just 
plain works.  Don't worry, buy one, you will not be disappointed.

Mike

--- In motm@y..., mark@i... wrote:
> 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 .
.
.
.

>
> 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.

If it's not 600 ohms, the 0.77blah blah blah number changes. The new number
is left as an excercise for the reader.

>
> 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:
>
>
> 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.


Well, +13 is lower than +22 :) Mixers have either an input pad trimmer or a
knob to adjust
this. So, shouldn't be an issue coming to going.


>
> 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 :)

That's because the .7746 is *RMS*. When you plug '10' into the equation,
that
means 10Vrms or 28V pk-pk. That is where the +22 comes from.

Again, just buy a mixer that has attenuators on the insert bus.

Paul S.

In a message dated 3/26/2001 12:58:36 PM, mmarsh@... writes:

>I've been able to use the 410 on pretty near 
>everything I own, including electric mandolins, lap steels, and 
>acoustic guitars.  You can do amazing stuff with it.

Maybe even a banjo -- eh Ken?

Thanks Paul for the dBu formula.
JB

>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.

Feel free to use a 11:1 potential divider network with a 10K and 1K
resistor to ground. Your output source impedance will be low enough
(less than 1K) so as not to cause any problem. HF filtering will be
unlikely to happen unless you have excessive capacitance in your cables.
Its only capacitance to ground that really matters here. You can fit
this little network in a jack plug if you are careful.

Regards,

Tony Allgood  Penrith, Cumbria, England

Oakley Modular Synth and TB3030:
www.techrepairs.freeserve.co.uk/projects.htm
My music: www.mp3.com/taklamakan