MOTM

hi,

noticed some 'weird' behaviour:

i'd really like to understand what happens in the following patches:

1/
vco to mult
plus
vcf or lag to mult
equals:
mult output reduced!!

(almost no sound comes out of mult)

what happens here, electronically seen?
can this damage the modules?

2/
shortening audio in of the 820

while there's cv applied to up/down

gives very strange noises at audio output.

have i created some kind of feedback path with this patch?
can this damage the module?

3/ self oscillating 420 produces sine with randomly changing
amplitude? especially noticeable with q pot set to trip point.

thanks for your feedback!

gregory!

1/ multiple outputs to mults

While this may not damage the modules, it is certainly not recommended.
The outputs are low impedance stages and are not meant to be
crosscoupled. The mults are for taking a single output to multiple
inputs.

2/ audio to 820 Lag

You can only create a feedback path by feeding the output of a module
back into a stage prior to it in the patching chain. It sounds like you
haven't done that here. It might be the result of the Lag responding
only to positive voltage swings, but we need more information here.

3/ 420 astable oscillation

I haven't tried this, but this may be natural since that filter is
designed to have some nonlinear characteristics. Do you have audio
inputs to the filter? This can effect the oscillation near the
oscillation trip point.


John Loffink
The Wavemakers Synthesizer Site
http://www.wavemakers-synth.com



-----Original Message-----

noticed some 'weird' behaviour:

i'd really like to understand what happens in the following patches:

1/
vco to mult
plus
vcf or lag to mult
equals:
mult output reduced!!

(almost no sound comes out of mult)

what happens here, electronically seen?
can this damage the modules?

2/
shortening audio in of the 820

while there's cv applied to up/down

gives very strange noises at audio output.

have i created some kind of feedback path with this patch?
can this damage the module?

3/ self oscillating 420 produces sine with randomly changing
amplitude? especially noticeable with q pot set to trip point.

> 1/ multiple outputs to mults
>
> While this may not damage the modules, it is certainly not recommended.
> The outputs are low impedance stages and are not meant to be
> crosscoupled. The mults are for taking a single output to multiple
> inputs.
>

Actually if the outputs go through 1K resistors (MOTM-300, MOTM-410,
MOTM-440, etc.) there is no harm in connecting these together by a multiple.
They will mix fine. In fact this is one of the design reasons for using 1K
resistors on the outputs, as noted in Electronotes. The signals mutually
attenuate each other, so that the combined peaks will not exceed the normal
output level (usually +5 to -5 V).

But not all modules have a 1K output resistor. In particular the MOTM-820
has low impedance outputs for driving longer cables. Something similar is
true of the Oakley Multimix. So these outputs may not mix well in a
multiple.

It is hard to damage an op amp by connecting its output to another output or
a ground. You don't have to worry that shorting an output to ground when
plugging patch cords will do any damage. I once repaired a pre-built Serge
module where one of the op amp outputs was directly grounded by a short in
the circuit board etch. The symptom was that no matter where I patched into
the Serge, I always got the signal output of that module. It was driving
the ground! After I cut the offending etch, it all worked fine again.

-Richard Brewster

thanks for replying, john! :)

> 1/ multiple outputs to mults
>
> While this may not damage the modules, it is certainly not

recommended.

> The outputs are low impedance stages and are not meant to be
> crosscoupled. The mults are for taking a single output to multiple
> inputs.

i 'd better get a mixer module, then.

> 2/ audio to 820 Lag
> It might be the result of the Lag responding
> only to positive voltage swings, but we need more information here.

i will check and post extensive information asap.

i discovered this behaviour by accidently shorting ring and tip from
a patchcord plugged into the 820's 'audio in' with my fingers.

> 3/ 420 astable oscillation
>
> I haven't tried this, but this may be natural since that filter is
> designed to have some nonlinear characteristics. Do you have audio
> inputs to the filter? This can effect the oscillation near the
> oscillation trip point.

no inputs at all, just the 420 oscillating near trip point.

i noticed just slightly delayed reaction from the freq and q pots
too, btw, which makes it annoying to set the trip point at an exact
position around self-oscillation.

perhaps this happens when the module is still 'warming up'. i'll
check for that too.

greetings,
gregory!

hi richard,
thank you for this extensive explanation!
i should perhaps buy a mixer module - i use the mult for merging the
820 output with other modules all the time.
greetz,
gregory

--- In motm@yahoogroups.com, "Richard Brewster" <pugix@n...> wrote:

1) Yes, you need mixers! Several of the modules have built-in mixers since they are so often used (like the signal and CV inputs to the VCFs). But you'll need some more for miscellaneous things.

2) If you have the Alan Strange book, I'm sure he mentions (somewhere) a technique called "non-linear" mixing which *is* mixing in multiples. When two (or more) audio siganls are connected together without the isolation provided by a mixer, you get all kinds of wierd modulation effects (IIRC they are the same as AM but have a different amplitude on the sum and difference tones). Get the Strange book!

JB

In a message dated 8/9/2003 10:15:34 AM Pacific Daylight Time, pugix@... writes:

Actually if the outputs go through 1K resistors (MOTM-300, MOTM-410,
MOTM-440, etc.) there is no harm in connecting these together by a multiple.
They will mix fine. In fact this is one of the design reasons for using 1K
resistors on the outputs, as noted in Electronotes. The signals mutually
attenuate each other, so that the combined peaks will not exceed the normal
output level (usually +5 to -5 V).

But not all modules have a 1K output resistor. In particular the MOTM-820
has low impedance outputs for driving longer cables. Something similar is
true of the Oakley Multimix. So these outputs may not mix well in a
multiple.

It is hard to damage an op amp by connecting its output to another output or
a ground. You don't have to worry that shorting an output to ground when
plugging patch cords will do any damage.

hi john,

thanks for yet another patient and helpfull reply to my newbie non-EE
questions :)

i 've been going through a lot of problems to get a copy of the allen
strange book since you first suggested that i should get it - i've
been in touch with several people at the publisher and with the
author himself, who has been very friendly.
i will post accurate, extensive information on how to order the book
once i have figured out the way to get it.
(i know tentochi posted on this subject before, but i'm not in the US
so his method won't work for me).

the book will probably cost me around 60 £ including shipping -
unfortunately it's only a photocopy made by the publisher and i've
been told the photo's are are unclear because of this.

original second editions (second hand ones) are going crazy prices -
i've seen one at 250 $.

it's easy to get an original first edition at an affordable price
(30 - 40 $), but the first edition is only 160 pages (instead of the
second one's 270 pages).

can anyone please comment on the additional differences between the
two editions of this book?

greetings,
gregory!

--- In motm@yahoogroups.com, jwbarlow@a... wrote:
> 1) Yes, you need mixers! Several of the modules have built-in
mixers since
> they are so often used (like the signal and CV inputs to the VCFs).
But you'll
> need some more for miscellaneous things.
>
> 2) If you have the Alan Strange book, I'm sure he mentions
(somewhere) a
> technique called "non-linear" mixing which *is* mixing in
multiples. When two (or
> more) audio siganls are connected together without the isolation
provided by a
> mixer, you get all kinds of wierd modulation effects (IIRC they are
the same
> as AM but have a different amplitude on the sum and difference
tones). Get the
> Strange book!
>
> JB