MOTM

I'll take a stab at explaining "ground".  But this IS April 1st, so if I 
royally screw this up I'm gonna cop-out and say it was a "joke", OK?

"Ground" is very confusing because everyone has heard the term in differing 
contexts.  In each context it means something different.  Here are some 
examples:

1. (From an electrician)  "Ground" is that wire attached to your cold-water 
pipe as it enters your house.  (If a Chicago area electrician he would say 
"yaw, dat ting over dare".)  That wire connecting to the cold-water pipe goes 
to your breaker panel and then connects all of the 3rd lugs on your AC 
outlets.  If you don't have one of them on your pipe, you'll have a six or 
eight foot ground rod pounded into the dirt just outside your foundation.  
That wire connects to that ground rod.  It is a safety thing to help you from 
getting electrocuted when a toaster or a drill shorts out.

2. (From your car mechanic)  "Ground" is the negative terminal of your car's 
battery.  It connects through a real heavy cable to the chassis of the car.  
Usually to somewhere on your engine, too.

3. (From a father)  "Grounded" is what his 15-year old daughter is when he 
finds out she's been dating a 27-year old guy she met at the rock concert 
last month.

4. (Context of analog synthesizers)  "Ground" is nothing more than a common 
reference point.  All like "ground" symbols on a schematic are simply 
connections tied together on the physical circuit.  It is sort of a shorthand 
used in drawing schematics.  Usually the +15V and -15V connections are also 
shown in a shorthand format.  They rarely show a connection to the power 
supply itself.  Anyway, "ground" is the "common" connection to a power 
supply.  Since it is THE reference point for everything else, it will always 
have a voltage of zero... it is THE reference.  So when someone says to 
measure the voltage at the end of R57, they mean that the black wire of your 
voltmeter should connect to "ground" and the red wire of the voltmeter should 
be probing R57.  The middle two pins of the MTA-156 connector on all MOTM 
modules is "ground".  So is the shield portion of the coaxial cables that 
connects the inputs/outputs on the PC board to the 1/4" Switchcraft jacks 
affixed to the front panels.

Think of a MOTM 900 power supply as if it were two batteries connected in 
series.  The positive terminal of battery A is the +15V volt terminal of the 
power supply.  The negative terminal of battery B is the -15V terminal of the 
power supply.  The negative terminal of battery A -and- the positive terminal 
of battery B are connected together.  This junction of the negative terminal 
of battery A -and- the positive terminal of battery B shall be known as 
"Ground" (signal ground, actually).  When you touch the common terminal of 
your voltmeter (usually with a black wire) to "ground", you are arbitrarily 
establishing a reference point.  If the black wire on your voltmeter connects 
to the junction of the negative terminal of battery A -and- the positive 
terminal of battery B (which we are now calling "signal ground") you will 
measure +15V on the positive terminal of battery A and -15V on the negative 
terminal of battery B.  That is the ordinary convention for most circuits.  
BUT, you could connect you voltmeter's black wire to the negative terminal of 
battery B.  When you then touch the meter's positive lead (usually, red) to 
the junction we labeled "ground", you will read +15V.  Leaving the black lead 
of the voltmeter on the negative terminal of battery B, touch the positive 
terminal of battery A: you will read +30V.  Did that surprise you?

Attached is a .PDF document "ground.pdf".  It is a little helper to go along 
with the explanation I just attempted.  Perhaps one picture is worth a 
thousand words.  Or if you write software, one picture is worth 1024 words :o)

Anyway, there are two identical schematics drawn.  One uses the conventional 
"shorthand" using ground and voltage supply terminals.  The other schematic 
connects everything with wires (no "shorthand").  Hope this at least starts 
in the right direction.  (Yes, I know I misspelled "connection" on the PDF.  
Too late.)  Be brave and prototype those circuits!  If you want, you could 
use two 9V batteries wired as shown, in place of the MOTM 900 power supply to 
actually make the amplifier shown.  Yes, it will work.  And if you use 
batteries, you can rest easy knowing that playing around with circuits won't 
kill off your synthesizer if something goes wrong.  After a couple of 
circuits you'll buy a pair of 9V wall-warts instead of batteries to save 
money.  Then you'll wonder why a so-called 9V wall-wart puts out 14 volts!  
But that is another story.

Cheers!
Tom Farrand