MOTM

The op-amp driving the Random output has a cap in the feedback loop 
which turns it into a lowpass filter (corner frequency about 10.6 Hz 
according to my calcs). With R11=100K and R12=150K the gain is 
-R12/R11 or -1.5, but only for DC and very low frequencies (below the 
cutoff point). Higher frequencies have a much lower gain. The Pink 
output that feeds the lowpass filter has 8 Vpp levels but everything 
over 10.6 Hz gets significantly removed by the filter. In order to 
get a decent Random output level R11 needs to be lowered. I put a pot 
in for R11 and watched the Random output on a scope until I was 
getting, roughly, a 10 Vpp signal on the output, then I measured the 
pot value and it was very close to 27K, so that's what I used for R11.


At 4:24 PM -0400 2003/04/23, media.nai@... wrote:
>I'm in the process of building an MOTM-101, and I'm confused about a
>few things.
>
>First, is the Random output mod.  On page 16 it says "The amplitude
>of this signal can be adjusted by lowering R11 (bigger output).  With
>the value shown it will swing 1.5Volts."
>
>That makes sense, but why was it designed to have a such a low output
>in the first place when all of the other AC outputs are 10Vpp??  Is
>there a reason why I shouldn't change it??
>
>If you view U2A as an inverting amplifier:
>
>Vout = Vin (Rin + Rf)/Rin
>
>So if Vout is 1.5V, and R11 (Vin) is 100K, and R12 (Vf) is 150K, then
>the gain of the amp is 2.5 and Vin is .6V.
>
>http://www.tellun.com/motm/mods/motm101/motm101.html
>"Changed R11 from 100K to 27K to get 10 Vpp on Random output. Random
>output level is now similar to Pink output level."
>
>So if Vin is .6V then:
>
>Vout = .6 (27 + 150)/27
>Vout = .6 (6.6)
>Vout = 4V
>
>Which works, since the Pink output is 8Vpp.  What I don't understand
>is how can the output of U2A be only .6V in the first place??  There
>is only a 1K resistor going from the output of U2A to the jack.