Banana jacks and impedance

[Harry Bissell]

Awwwhh....

There are two schools of thought...
One is that all the signals in a synth should be a standard (like +-5 or
+-10V) to allow anything to be patched to anything ... use VCO for
control source etc.
This has the problems of feedthrough, and poor slew rate. Sounds do not
bleed into this... they are bled into from this !!!  There can be a
problem for (lets say) a square wave to bleed into a sine wave though.

Two is the Control Voltage vs Signal Voltage method (like PAiA old
modular stuff... 0-5V control and 500mV signal.) It is easy to get
bleedthrough onto the 500mV signal...
so PAiA used pin jacks (like bananna but smaller) and unshielded wire
for control... and 1/8" (mini) phone jacks for audio. The ARP 2600 used
the 1/8" phone for all patches.

But nobody likes the mini jacks anymore... the quality is not good.

That leaves 1/4" Phone jacks (any quality you want $$$) with shielding
and normalization if you like...

And Bananna jacks (any quality you like but less $) with no shielding
and no normalization...

Grant is dead on about the "output impedance". It does not soak up noise
!!! Lower input impedances help, because they form a high pass filter
with the cable capacitance. If the input impedance is low enough, the
filter can be above the audio range and then you can't hear the
artifacts... but it is still there waiting to bite you in the @ss if it
can find a high frequency clock or non linear stage to get rectified
in... and come back again as noise instead of bleedthrough. Arrghhh !!!

If your synth will live alone its fine to use bananna jax, unshielded
wire. If it will co-exist with other audio gear then the shielded wire
is a REAL GOOD IDEA.
I'm going with 1/4" phone for new designs.

:^) Harry

Grant Richter wrote:

> >
> > Damn, why on earth isn't _everything_ +- 10v p-p?
> >
>
> Because it radiates like a Bitch! This is a good reason to
> use shielded patch cables. Even then there's enough signal
> to capacitively couple from an input to an output jack with power
> removed entirely from the module.
>
> Here's a test - if you think you have bleedthru in your VCA,
> try removing power from the VCA and see how much signal
> still comes though just from capacitve coupling.
>
> One of the problems of banana cord systems is to get the
> signals from muted patches to go away. If you have
> a very loud TB-303ish bass screaming in one channel and a
> high-hat with mostly silence in another, even with the bass
> muted you can hear it in the high hats. Just a matter of
> capacitive coupling of crossing wires.
>
> In "theory" the low output impedance of the op-amp should
> absorb the signal - but the low output impedance is a function
> of the op-amp servo mechanism, which becomes less
> effective as frequency increases.