AW: To earth, or not to earth...

[media at mail1.nai.net]

This whole shielding thing is giving me a headache ;)  This is a long post,
so I'm not going to try to keep straight who said what so far.

What are our options??

1) One conductor unshielded cables with banana plugs
   (commonly used with test equipment)
2) Two conductor shielded cables with one end tied
   (emergency procedure to evade ground loops)
3) Two conductor shielded cables with both end tied
   (typical unbalanced audio, used on most synths)
4) Three conductor shielded cables with one end tied
   (professional way to avoid ground loops at line level, floating at bay)
5) Three conductor shielded cables with both ends tied
   (necessary for low voltages and long runs, such as with microphones)

>-Please declare if you belong to the "banana" or the unsymmetrical
>shielded party or whatever.

I think I'm in the "balanced 600 ohm party" but I think I'm voting for #3
in this local election.  Candidate #1 is too soft on noise, imho.
Candidates  #2 and #4 will be impossible to follow once they're in office.
Candidate #5 might be able to balance the signal but won't be able to
balance the budget.


>   The earth "dustbin approach is also quite usefull (use an op-amp
>   unity gain buffer with the non-inverting input connected to ground,
>   the output is a goood solid ground (limited by the outputs
>   compliance).

>(I've never heard it called a "dustbin" before!)

Well, I'm not even sure what it is!!

>It also suffers from other opamp problems; noise, rising output
>impedance with frequency, offset voltage, etc.  'Sounds like it makes
>any grounding problem several times worse!

Yes, it sounds unnecessarily complicated.   So I take this to mean he is
not referring to active balancing transmission lines, but making the entire
ground a product of an op-amp, such that the chassis and shielding are both
floated from the mains??  Perhaps you are avoiding noise from HVAC and
fluorescent lights, but it sounds dangerous.  You are trading a planet for
an IC.

If the ground from the mains is too dirty or isn't "low" enough, try
running a wire to a cold water pipe (they even make specific clamps for
this) or a four foot copper spike driven into the earth.

>Other problems with banana plugs:
>  They're a pain to interface to external pieces of equipment.
>  They can't do input or output normalizing.  (Or shorting an unused
>    input to ground.)
>  They can't accommodate balanced or stereo signals.
>  You can't borrow spare cables from your guitar player.

I think these are all very good points.

>>This is true.  You are trading off sheilding for a simpler ground path.
>....I could be wrong, but if shielding is an issue, shouldn't the shield be
>connected at one end only of the cable?

Not necessarily.  Most cables are generally tied at both ends.  This is
certainly the case when Don borrows cables from his gtr player :)  When you
start using cables that are tied at only one end you then have to keep
track of which end that is (using silver for one and black for the other
perhaps).  It still means having a pile of special cables.

>>My understanding of banana patching systems is that they use voltages and
>>input impedances that are high enough that RF isn't much of a problem.
>>This is what Serge told me about their system.
>.....actually it is a matter of keeping the impedances low, so any intercepted
>stray signals can't develop much of a voltage across the inputs

I'm confused here.  If the input impedances are low and the voltages are
high, wouldn't that mean we are talking massive power??   What is the
average input impedance of a commercial synth module??   I would think that
most circuits with op-amp inputs would draw very little current.

>You said it yourself: To many people there is no real difference between
>audio and modulation signals.

>>Perhaps a good compromise would be to have unsheilded stackable banana
>>plugs for control voltages and sheilded  1/4" or 1/8" cables for audio
>>signals
>........PLEASE DON"T!  The beauty of modular systems is that there is no
>intrinsic difference between control and audio signals, hence posibility of
>ring mod inputs from DC to 30KHz, FM by audio etc....having a two-teir
>system would make interesting patches more difficult and less obvious.

OK, OK, OK!!   Use only one type of connector!!  But wait, if you use 1/4"
couldn't you use balanced and unbalanced at the same time??  See, you could
use a balanced TRS output for audio and still stick it into a TS control
input.  I stick unbalanced effects such as foot pedals into my balanced
patchbay all the time (this is why I don't use TT).

>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.

As you know, balancing increases the system's ability to reject the noise
of the cable -- this will only help if the hum is caused by some
transmission entering the cable (like from a video monitor).  RF
interference typically manifests itself as smog that is far less tonal than
a hum and much more like thermal noise.

>The greatest problems I see is
>(1) Input Attenuation and
>(2) Output drivers.
>... see below.

>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.

>(spelling: is this "balanced" as well, or "symmetrical"?)

Balanced, symmetrical, and differential all mean the same thing in this
context.

>>If the modules are star eathed i.e. a separate earth lead from
>>each module to the central earth point, there should be no
>>increase in crosstalk.
>
>Yes, that should do it in most cases. An exception might be crosstalk
>between several signals to / from the same module. When would this
>hurt? Not often, I admit. Maybe a ring modulator might become slightly
>unbalanced when one input bleeds to the other one over a shared GND
>connection. But I admit it's an extreme example, and wouldn't hurt much
>either.

True, crosstalk within a module might not have any effect, although if it
did its inherent periodicity might be more irksome than cable noise.  Then
again how quiet can do you want to build this thing??

>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??

>Overkill - maybe. But no one says you need to build studio quality
>symmetrical microphone inputs for your CVs. Most demanding
>problem of symmetrical inputs in general is their CMRR at higher
>frequencies. But you wouldn't need this for many CV inputs. Should
>just have a decent CMRR at 50Hz. One opamp with four matched resistors
>would do here. (This is for typical CV inputs.)

OK :)  However, isn't having balanced outputs more critical than balanced
inputs??   I'm thinking this would make building LFO's and envelope
generators more complicated.

>>  Secondly, you would have to balance the outputs of each of your
>> modules requiring either transformers (!!!) or active balancing
>> with additional op-amps.

>Yes, this is really a problem, if not *the* problem.

Good, we agree then :)

>If you would really use balanced cables exclusively, you could simply
>build an additional inverting opamp stage to feed the ring of the
>stereo jack. Problems start when you plug in a mono plug, however.
>Even with current limiting resistors there is still much current injected
>into the gnd/shield path to cause bad crosstalk.

>A truely balanced output stage (like this SSM chip - forgot the number)
>is too expensive (and try to build *this* from discrete opamps - no!).

I know that chip!!  How expensive off the street is it??

It is commonly used in commercial equipment as it far more cost effective
than using transformers.

>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!!

>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.

>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.

>My vote is for using *cheap* symmetrical circuits, even with bad CMRR.
>They are not much more expensive than unsymmetrical circuits, often
>you get them for free (Moog Ladder ...), and they still improove SNR.

OK :)

>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.

Can I get a witness??

(Sorry, I've been listening to Gospel :)

>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?!?

>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.

>Long post - hope it was of some interest.

Well at least I read it  :)

PEACE OUT :)
MARK


--
"No one gets into Heaven without a glowstick!"
                                      -- Homer Simpson