>Are there any modules or kits, in addition to the Ken Stone CGS one,
>that'll convert MOTM audio levels down to guitar pedal levels (and
>impedances) then back up again? Not that there's anything wrong with
>the CGS kit, I'm just curious what else is out there.
Short Answer:
Use a cable with a 1/4" jack on each end. Plug one end into the MOTM
and the other end into the pedal.
Long answer:
Over the years, I've seen a number of posts asking how can I connect
MOTM to some other thing. So I'm going to explain how.
The most important thing to consider when using line-level signals
such as MOTM is that the output impedance must be lower than the
input impedance. You do not "match" impedances when using a modular
synth.
Why is this so important?? The output needs enough current to
"drive" the input at the output voltage. In order to do this, the
output impedance must be higher than the input impedance. The
impedances do not have to be close to each other. Nor do they need
to differ by a massive amount.
Fortunately, with the exception of microphone pre-amps, it's
extremely rare to find two things that you would want to connect to
each other, where the output impedance of is higher than input
impedance. Mic pre's can have a very low input impedance. So if for
some reason you wanted to connect something other than a microphone
to the input of a mic pre it could result in serious damage.
(Why do many mic pre's have such low input impedances?? It has to do
running a low voltage signal over a balanced connection. Lowering
the impedance increases CMRR, and reduces high frequency loss and
slew limiting from long cable runs. If the output impedance is the
same as the input impedance there is 6dB of attenuation. So the
input impedance should be at least five times the output impedance,
but not so high that the microphone isn't "working". Since a
balanced microphone can have an output impedance anywhere from 20 to
600 Ohms, a mic pre can have input impedance as low as 100 Ohms,
although higher values are much more common.)
In practice, I have connected headphone outputs with an output
impedance of a few ohms or less to instrument inputs with an input
impedance of 1M (~50,000 times greater), and I have connected gear
with an output impedance of 2K and an input impedance of 20K (only
ten times as much), without problems.
How is this possible?? Just like MOTM modules, almost all modern
effect pedals, rack effects, mixers, digital audio interfaces, etc.
-- that do not use tubes -- have op-amp inputs and op-amp outputs.
Op-amps have a high input impedance and a low output impedance. So
connecting the op-amp output of one device to the op-amp input of
another device works very well.
Following Ohm's Law, going from the low output impedance to the high
input impedance typical of most line-level devices, the resulting
signal loss is negligible. For example, an output impedance of 100
Ohms and an input impedance of 20K results in -.04dB -- a few
hundredths of a Decibel. If instead it were a loss of a few tenths
or a few thousandths, it wouldn't make any audible difference.
(Yes, differences in input impedance can effect the sound of
mechanical sources such as guitars and microphones that can "see" a
load. These impedances can be frequency dependent within the audible
range. This is one reason why mic-pre's and fancy DI boxes often
have variable input impedance.)
People often ask about "standards" such as "-10dB" or "+4dB". In
practice, "+4dB" is pro line level, and "-10dB" is consume/musician
line level, although no specific standards exist.
Within a studio, having equipment that is "+4dB" is important. It
prevents having to attenuate and boost the signal unnecessarily when
connecting devices to each other. Which would increase noise and
reduce dynamic range. In other words, consistently maintaining a
signal level of around 20Vpp provides the best sound quality.
So what if you want to use a piece of "-10db" equipment, such as
keyboard or effect pedal, with your "+4dB" studio gear?? As long as
the output impedance is lower than the input impedance, which is
almost always the case, you can connect one to the other.
What about the signal level?? When going from "+4db" to "-10db" you
might have to attenuate the signal, and when going from "-10db" to
"+4db" you might have to boost the signal. Most devices have input
or output level controls, or both, so you do not need to use a third
piece of equipment. If you are going from a higher level to a lower
level it is very good idea to turn down the levels first. If it's
too low, turn it up. If it's too high, turn it down.
You only need to "match levels" if there are no level controls. For
example, I have a Drawmer 1960 for sale on Eb∗y that supports both
"standards". It has two sets of insert points -- "+4dB" (for pro
rack gear) and "-10dB" (for guitar pedals). Why?? Since there is no
gain control at the send and return points, it uses its own internal
"match box".
A handy tool for any studio is a small analog mixer. I love my
Mackie 1202. You can buy one for around $175 on the used market.
It's a matchbox, a mic pre, a headphone amp, mixer, etc. Not only
can it change the level of any audio signal, it can also replace a
number of adapter cables. It's very useful for connecting "-20dB"
consumer devices such as tape decks and VCR's to your other equipment.
Pretty much everything, with the exception of tube gear, that runs
off of AC uses -15/+15 volt rails internally. MOTM is no exception.
As long as the input voltage does not exceed the rails, damage should
not occur. So you should be extra cautious when going from "+4dB"
(which is typically around 20Vpp, but can be as high as 28Vpp) to
effect pedals that run off of lower voltages such as 9V batteries.
The outputs of MOTM modules are much lower -- 10Vpp is somewhere
between "+4dB" and "-10db". I have connected MOTM to pedals from
Boss, DOD, Electro-Harmonix, Ibanez, Morley, Mutron, Pearl, Sovtek,
etc. without any problems.
Notice that the output level of most pedals is much higher than the
output level of a guitar (several volts vs. a couple tenths of a
volt), yet people offten connect pedals to each other. If you
attenuate the signal by sending the output of an MOTM module (10Vpp)
through some sort of "re-amp" or "connect to anything" device to get
an instrument level output of 200mVpp, then send that to an effect
pedal and amplify it by turning up its input or "drive" knob to get
an optimal signal level (~9Vpp), all you are doing is attenuating the
signal by around 30dB then amplifying it by around 30dB. You are not
accomplishing anything except increasing the noise and lowering the
dynamic range of your signal.
That being said, if you are some sort of guitar pedal fanatic you
might want to read the next two paragraphs. If I didn't write them,
I would probably skip over them myself.
Not all stomp boxes have op-amp inputs, or op-amp outputs. This is
especially true of "vintage" pedals. Some use emitter followers
which provide a high input impedance, but not all of them do. The
low input impedance of a few pedals has a desirable effect when used
with guitars (eg. the way a Fuzz Face loads the pickups is a large
part of its sound). While there is absolutely no standard for effect
pedals, and various manufacturers have made some awfully bizarre
designs over the years, their input impedance is rarely an issue
since guitars have a high output impedance. The issue is that some
pedals do not have a low output impedance (for the exact same reason
that guitars don't either). It can be 10K or higher. This isn't an
issue if they are connected directly to an amp, since guitar
amplifiers have an extremely high input impedance (typically 1M).
However, largely due to the use of output capacitors that form RC
filter networks, it can effect the sound when pedals are connected to
other gear, or even to other pedals.
This effect on the sound is only an issue when compared to using a
guitar and an amp. If you are using pedals with a modular, it is
never going to sound like a guitar and an amp no matter what you do.
If for some bizarre reason you need to have a vintage pedal exhibit
the same exact behavior that it would with amp, then you need an
active direct box with an input impedance of 1M. A passive box won't
do it -- using a transformer with a 10:1 ratio connected to a 20K
line level input would result in input impedance of 200K. That's
also a typical input impedance for many active DI boxes and AD
converters. For example, the instrument inputs on my Metric Halo
2882+DSP interface are 200k. While that is perfectly fine for most
applications, it still doesn't have the 1M input of a guitar amp.
However, there are active DI boxes have input impedances of 1M (and a
few active DI boxes intended for studio use, where cable noise is
less of an issue, have switchable input impedances as high as 10M).
So if you are a fanatic, you'll need one of those.
What about balanced connections?? MOTM modules do not have balanced
inputs or balanced outputs. This is a very good thing. Unless you
are in a ridiculously hostile environment, there is no reason to use
balanced connections for 10Vpp signals over distances of several
feet. If you would like to connect your MOTM modules to a device
that only has balanced connections, please see this document:
http://www.rane.com/note110.htmlDISCLAIMER: My first name isn't "Paul" and my last name isn't
"Schreiber". I'm not an EE. YMMV. If you blow something up, that's
your problem.