MOTM

> Maybe this is getting a little too off-topic, but good you
> (or someone else) explain me this Hi-Z/Lo-Z thing?

There are really two issues here Lo-Z/Hi-Z and Balanced Line/Unbalanced
Line.

It is very difficult to give a really good explanation of impedance (the Z
thing) without getting into phasors and complex math and other EE stuff.
I'll see what I can do.

First off, the output impedance of a device is largely a measure of it's
ability to deliver a signal over a wire.  The lower the output impedance,
the more current it can deliver.  A low output impedance is a ggod thing.
The theoretical output impedance of an op-amp is 0.  Paul designs his
modules to have an output impedance of 1K if I remember correctly.  The
reason is circuit protection.  If the output impedance were very low
(approaching zero) and you accidentally plugged two outputs together,
WHAMMO!  You'd have little op-amp fires.  The final stage op-amp of the
module would try to drive the load which would be 0 ohms, so it would try to
deliver infinite current. With a 1K output impedance, if you plug two
outputs together no destruction should occur.  This is similar to plugging a
2 ohm speaker into an amplifier designed for 16 ohms.  The amp will try to
deliver 8 times the current it is designed for.

Secondly the input impedance of a device is largely a measure of how much
load it puts on the device driving it (or how much current it requires.)  A
high input impedance is a good thing.  The theoretical input impedance of an
opamp is infinite, which means it requires no current to drive it.  Paul
designs his modules to have an input impedance of 100K (again if I remember
correctly.)  You can in theory plug a module output into many, many module
inputs without signal loss.

Some devices have a really high output impedance.  A typical example would
be a piezo pickup in an acoustic guitar.  These typically have an output
impedance in the megohms.  This is why there has to be a preamp with an even
higher input impedance (like an FET based design) very near.

Now this is all really just considering the resistive componant of the
impedance.  All devices and cables also have a reactive componant which can
be capacitive or inductive.  Lets assume that the two conductors in a cable
are like parallel plates in a capacitor.  This cable/capacitor is going to
act just like a lowpass filter.  It will attenuate and phase smear the upper
harmonics.  The frequency where this happens is determined by the
quality/construction of the cable, the length of the cable and the output
impedance of the device driving it.  With a high impedance output the cable
must be kept short to keep the cutoff frequency of the signal above the
audio range.  This is why preamps for piezo pickups are usually located
within inches of the pickup.  This is also why most mics are Lo-Z.  They can
drive a signal through a cable and keep the lowpass effect above the audible
range.  In some cases the capacitance in a cable can cause a device to break
into oscillation.  I have seen this happen with very cheap high power
amplifiers and cheap speaker cables.  Uually the oscillation is ultrasonic.
It can burn up a tweeter and fry the amp as well, not to mention a truly
horrible intermodulation distortion.

Anyway, direct boxes usually have very high input impedances and convert the
signal into a balanced signal at a low output imedance.

The primary reason behind using balanced lines is to prevent hum and noise
pickup in the cable.  Any long cable will act like an antenna, in addition
to acting like a capacitor.  With an unbalanced line (1 conductor plus a
ground) any signal picked up by the cable by induction will be amplified
along with the signal the cable is carrying.  With a balanced line (2
conductors plus a ground)  the hum is induced into both the + and - lines.
The amplifier will amplify the difference between those two signals,
cancelling out any induced hum or noise.

I believe this question came up because of the discussion of whether the
input impedance of a preamp, converter or amplifier would color the sound of
the MOTM.  I don't know the answer to that, but in theory if the cable is
long enough or the input impedance of the device is low it could attenuate
some of the high end and somewhat smear the upper harmonics.  The best thing
to do is to either lower the output impedance of the MOTM by using a direct
box, or use a preamp, converter or whatever known to have a high input
impedance.  If the distance is over 15 or 20 feet using a direct box and a
balanced line is a good idea.

I assume Paul will be addressing this issue with a dedicated output module
in the not to distant future.  I would anticipate it having balanced line
outputs and a low output impedance eliminating the need for a direct box.

Hope this helps.

It's a fine point, but op amps have output current limiting built in.  
 From the ST Micro TL072 data sheet:

4. The output may be shorted to ground or to either supply. Temperature 
and/or supply voltages must be limited to ensure that the dissipation rating
is not exceeded.

If you stay within the power supply voltage limits and temperature 
ratings, shorting an output does no harm.  I remember this because I 
once fixed a Serge module that had an op amp output shorted to ground by 
a faulty etch.  The signal from that baby was everywhere, seeming to 
come from every jack!  It was driving the ground up to its current 
limit.  Cutting the faulty etch fixed it.  The op amp was not damaged.

1K series resistors on outputs gives a standard output impedance and 
allows such outputs to be mixed together by patching to a multiple, 
though not many people do this.  The Serge used 330 ohm output 
resistors.  I have no idea why.

An op amp is more likely to be damaged by leaving one of its *inputs* 
unconnected.  Because of the extremely high input impedance, a bit of 
static electricity discharged into that pin could exceed the power 
supply voltage by a thousand volts!

-Richard Brewster

>The theoretical output impedance of an op-amp is 0.  Paul designs his
>modules to have an output impedance of 1K if I remember correctly.  The
>reason is circuit protection.  If the output impedance were very low
>(approaching zero) and you accidentally plugged two outputs together,
>WHAMMO!  You'd have little op-amp fires.  The final stage op-amp of the
>module would try to drive the load which would be 0 ohms, so it would try to
>deliver infinite current.
>

On Monday 09 February 2004 19:16, Paul Haneberg wrote:

> Hope this helps.

A LOT! :)

Thanks for that deep explanation. I think I understood the 
whole story. And I could even combine it with another 
explanation Larry once gave me about impedance and 
multiples.

What a great list this is.

- Robert