AW[n]: AW: To earth, or not to earth...

[Haible Juergen]

	>>Yes, there's still hope (;->) - but I don't think I'll make
the JH-3
	>>Modular symmetric. Hum is very low, with the exception of one
module.
	>>So I don't really have a hum problem there. But if I connect
other
	>>instruments (with multiple cables like audio signal, arpeggio
clock,
	>>filter CV input ...) I run into problems. Can solve them with
	>>transformers or cut cable shields, but going symmetric would
just be
	>>the better solution.
	>
	>Hmmm, I don't know if balancing the lines is the solution,
especially if
	>cutting shields and adding isolation transformers is solving
the problem.
	>Hum is usually caused by ground loops or noisey power supplies.

Cutting shields etc. *sometimes* helps, but sometimes makes things
worse. (See my mail to the same topic about the CS-50 S&H)
Ok, I have some "auxiliary" tools around my Modular. Most of my cables
are Hosa 1/4" unbalanced ones (you get them in almost every length).
Then there are a few unshielded (single wire) ones (also with 1/4" plugs
of yourse). These I use for Arpeggiater clock of my OB-8 (The OB-8
is almost an integral part of my Modular - The JH-3 modular has a
dedicated input for OB-8 processing (;->) ), and for some (very few)
connections across the Modular itself, where hum would be a problem.
I also have a 600 Ohm 1:1 transformer (chirurgically removed from a
JP-6)
in a tiny box with two balanced 1/4" jacks.
But all these are just "emergency" tools.

	>>I am surprised that everybody considers the input stage a
problem.
	>>Now, that's easy. While opamp circuits often need some
"artificial"
	>>means to symmetrize the circuit, many typical synth circuits
don't.
	>>A moog ladder cries for symmetrical input. An OTA-VCA, too.
	>
	>Really??  I have absolutely no comprehension of what you mean
by that!!
	>Please, explain.

Each ladder filter, and most OTAs, have a positive and a negative input.
And you also have to divide down your typical signal levels, so these
inputs are almost perfect summing nodes. So instead of grounding one
of them, you could build a second attenuator and connect it to the
"ring"
of your input jack.

	>>This sounds interesting - never heard about this before !
	>>So you use the opamp output for the "dirty" gnd (LEDs etc),
and avoid
	>>any bypassing caps to GND around this opamp (neither from its
	>>supply pins nor from its output), so you just get the dirt on
the +/-
	>>rails but not on GND? Great idea !!
	>
	>Is that what he means??  He has three grounds??  Dirty, Signal
and Chassis??

Does this frighten you? (Almost) each jack on my Modular has its
own GND connection to the module's local star GND. And I don't
use isolated jacks, so they are connected to each other across the
aluminium
front as well - and still the individual GND wires make a difference !
(Don't ask me why.) Ok, it's overkill. I'd use only three GNDs, if
I had to do it again, as well.
The "dustbin" idea is that you save one of these GNDs. Say you have
a LED in the middle of a PCB behinf the front panel, you can either
use a separate GND to the power supply, or use an onboard opamp.

	>>One solution might be a class A differential output stage.
Take the
	>>last section of the minimoog VCA (the buffer stage). Again a
degenerate
	>>emitter differential amplifier. But use *both* legs as
outputs, not just
	>>one as in the minimoog. I used this circuit in my JH-4 module,
and it
	>>makes a great symmetrical output stage (to the mixing desk).
You can short
	>>one output, and it still works - you can even short several
outputs together
	>>and get a mix of the two signals (that's why I chose it for
the JH-4
	>>"Four Voice" concept !)
	>
	>Please, explain this concept!!

"Concept" - I just mean that there isn't a real 4-Voice "JH-4" synth
yet, only the keyboard scanner and one working protoptype of
a voice module at the moment. But as I don't want an extra mixer stage
for the (planned) 4 modules, I have implemented symmetrical outputs
that you can simply short together. I used the Minimoog VCA
circuit. The last stage is a linearized (degenerate emitter) PNP
differential amplifier with rather high current (10mA if memory serves).
So you can directly drive a balanced cable from the two collectors
of the diff amp (with an elko and a small protection resistor in each
connection, of course). It doesn't have a higher impedance than
your usual opamp stage with 1k series resistor, it's full Class A,
and you can mix several of these by simply shorting them together.) 

	>>This is an important point.  We don't have to fight many
meters of cable
	>>length. Just want to avoid small (approx. 1 or 2 meters)
ground loops.
	>
	>Length is only an issue in ground loops when the resistance due
to a longer
	>path becomes an issue.

The R, and also the L of a shared connection. (And, if you use a lot of
star grounded connections, also the M between the different GND lines.)

	>>So, a symmetrical stage with decent or bad (30dB) CMRR is
better than
	>>an unbalanced connection (which has 0dB CMRR).
	<
	>True, but how much noise gets into a six foot cable in the
first place??
	>Wouldn't raising potentials be easier than balancing the
system??   Given
	>high enough voltages, any RF interference would be inaudible or
irrelevant.

One word: Headroom.
This isn't much of an issue for internal synth signals, but becomes
a problem as soon as you want to process external signals.
BTW, even for internal signals, I would never design a synth for
+/- 10V swing with +/-15V supply voltages.

I designed JH-3 for +/-5V swing which gives me 6dB headroom.
I would rather choose an even lower level and greater headroom next 
time.

	>>Another thought: Maybe (I' only guessing here) it would even
help to use
	>>unbalanced output signals, but balanced input stages. The
outputs would drive
	>>the tip with the signal, and connect ring and shield with
their *local*
	>>GND. The inputs jacks would have the shield unconnected, and a
differential
	>>input stage would amplify the voltage between the tip
(=signal) and
	>>the ring (=local GND of output module). This way you'd loose
the
	>>advantage of radiation immunity (but we have short cables
anyway),
	>>but you'd still avoid GND loops (the ususal unbalanced
problem) and
	>>crosstalk (the potential banana problem).
	>
	<Now, you totally lost me.  If you just wanted to avoid ground
loops
	>couldn't you just float either the inputs *or* the outputs with
unbalanced
	>cables??  You would have to decide on which one and stick to it
(btw, I
	>say float the inputs).  This way you would avoid ground loops
without
	>special cables.  I see no reason why this would give you any
less RF
	>protection than tying it at both ends.

It's not RF what I want to fight in the first place, but hum and
crostalk.

	>>Now, there's one great problem that remains: Input
attenuators.
	>>Dual potentiometers often have significan difference between
both
	>>pots. You could also try one pot and 2 resistors in a ladder
	>>configuration, but then it's the limited CMRR that kills you
at
	>>low potentiometer settings.
	>
	>You wouldn't "unbalance" the signal prior to the pot?!?

This would be possible, of course. I just thought there *might* be a
more
elegant solution. (But I don't see one at the moment either.)

	>>The only cheap way I see for this - and this solves the input
differential
	>>stage problem as well: Use an OTA, and voltage control your
input
	>>attenuation. Noise is an issue here, but you might have
standardized
	>>output levels anyway, and the OTA would always see this
maximum level,
	>>so you could optimize the divider resistor for full level.
	>
	>That sounds like too much work!!  Besides, I don't think you
could keep
	>your output levels that standardized.

You may be right. I just want to point out that this (non standard
levels)
is exactly the same constraint that forbids raising your standard level
(see above).
BTW, a 3080 and a handful of resistors may be much compared to
a simple pot, but remember that the OTA does the balanced input job
as well as the attenuation. And it could be the first step to give your
module some preset or even storage functions (;->) (;->)

	JH.