MOTM

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.


Secondly, has anyone tapped the INTCLK signal to add an internal
clock output??  I'm thinking of connecting the switched lug on the
EXT CLOCK jack to a new jack with a 1K resistor, but I'm wondering if
there will be enough current.

The MOTM-101 internal clock uses a 555 as an astable multivibrator, a
proven circuit that is over 30 years old.  However, I noticed that
Paul added a diode in parallel with the timing resistor.  Why did he
do that??  I'm guessing it's to keep knob twisting from glitching the
clock.

Anyway, the circuit uses a TLC555, a more practical low-power version
that can source only 10ma at output.  A regular 555 can source 200ma.
It's a flamethrower that can literally ring doorbells and drive
speakers.  Since they are pin for pin replacements, I'm thinking of
replacing the TLC555 with a regular 555 to drive the extra output.
Any ideas??  Should I increase the value of C19 (a .1uF bypass cap)
to handle the larger transient??

Other people have mentioned that there are two unused op-amp halves
(and I have no idea why) that can be hacked as output buffers, but
I'd rather not do that.


Speaking of low output impedance, if your system uses 5V for 500/600
power, there is a new op-amp for cooking bratwurst and driving blue
LED's (to keep planes from flying into your modular) :
http://focus.ti.com/docs/prod/productfolder.jhtml?genericPartNumber=OPA569


Finally, what is the difference between the MOTM-100 and MOTM-101??
Based on the "Electrical Theory of Operation" I'm guessing that a
jumper for vibrato input was replaced with a panel switch.  Will the
MOTM-102 have a clock output?? ;)

__
"For ye may all prophesy one by one, that all may learn"
                            -- 1 Corinthians 14:31

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.

> 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??


Because it is designed to 'warble' VCO and VCF inputs *ever so slightly*, like maybe 1% or so.
Starting with a "small" voltage at first makes it easier to control the effect.

Paul S.

At 3:55 PM -0600 4/23/03, Scott Juskiw wrote:
>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.

Doh!! It would have been easier if I wasn't trying to use the formula 
for a non-inverting amplifier!!

The only 27K I could find is this 1/2W carbon resistor that is too 
big for the hole spacing (from an assortment box I bought from Radio 
Shack eons ago).  While I'm sure a 20K would work, after reading 
Paul's explanation, I think I'll choose a compromise value, like 47K. 
I have those :)

So on to the external clock mod.  Luckily most MOTM modules have high 
input impedances.  The average output impedance seems about 1K (eg. 
320, 800).  One source said that a TLC555 could output 10ma, another 
15ma.  The output voltage is around 14V, which is too hot.   If I put 
one 1K from the EXT CLOCK, and another 1K from the new clock out jack 
to ground, that should be 7V at 7ma, which should work.

Of course, given my recent luck with math, that's probably wrong :)

The other option would be to replace the TLC555 with a regular 555 
that can flash lights and ignite blasting caps, and increase the size 
of its bypass cap.

> The other option would be to replace the TLC555 with a regular 555
that can flash lights and ignite blasting caps...

... and probably put noise on the power supply. The 555 seriously
crowbars its supply when ti goes over.

> and increase the size of its bypass cap.

It'll probably help, but it won't completely keep the crap out of the
supplies. Maybe use a little series resistance in the power line too??

Tony

At 5:47 PM +0100 4/24/03, Tony Allgood wrote:
>  > The other option would be to replace the TLC555 with a regular 555
>that can flash lights and ignite blasting caps...
>
>... and probably put noise on the power supply. The 555 seriously
>crowbars its supply when ti goes over.

Absolutely.  There is no reason to use a regular 555 unless you need
the extra current, or that's what you have on hand :)  So I assembled the
module with a socket.  I figure if the TLC555 doesn't work, I could
easily toss in a 555 that can power solenoids and small motors.

>  > and increase the size of its bypass cap.
>
>It'll probably help, but it won't completely keep the crap out of the
>supplies. Maybe use a little series resistance in the power line too??

I didn't want to hack the PCB, which is why I decided not to use a
spare half of TL072 as a buffer.  As it turns out the TLC555 works
fine....