Need mircrophone preamp circuit

[John Popelish]

I'm the OP, and I've been trying to follow this. The mic
works fine, with no hum at all, on my desktop computer,
which uses your standard ATX SMPS, and also on the laptop
if it's on battery power. So I assumed the problem was the
dinky Toshiba AC/DC adapter that came with the laptop.
Well, I just figured the PS output was not well filtered,
and was just noisy - noisy relative to it's own ground,
whereever that may be. There is no ground prong on the
adapter's plug, and it isn't even polarized, so I don't know
how I would ground it to earth ground, and don't understand
why that would make any difference.

But here's something curious. On both computers - if I plug
a plain cable into the mic jack, a cable that just has male
mini-plugs on both ends, I only get the hum if I touch the
connector at the other end, and I become an antenna. Since
both computers have this effect with a plain cable, it's not
clear why the microphone produces the hum only on the
laptop, and only when the laptop is on the A/C adapter.

I did try another microphone, but that produced no change on
either computer.

The "ground" side of the mic jack is not necessarily at zero
volts, compared to the surroundings. But as long as the
signal line inside that cable has the identical voltage
waveform as the "ground" side, the amplified difference
between these two nodes is zero (no hum). When you touch
the tip of the cable with your finger, you are adding
capacitance to the surroundings (Earth, pipes, wiring, the
universe at large, etc.) altering its voltage, compared to
the moving potential on the "ground" side.

A desktop box normally has a 3 pin plug, so that the case
has some sort of connection to Earth, so that there is at
least some attempt to keep that "ground" side of the mic
jack near Earth potential. If you are sitting in a room
strung with AC wiring, there is still a capacitance between
that wiring and your body, that will provide a hum signal
for the mic jack, with respect to the earthed ground side.

But the lap top supply generally will have only two pins,
and no safety ground (Earth) connection. So the case and
the "ground" side of all signal connectors are actually
isolated from any direct connection to Earth potential, and
just float at whatever voltage the various capacitive
couplings between the power supply primary side and all
other capacitances to the surroundings produce. So, in
addition to the cable picking up hum signals that pass
through you, from the surrounding wiring that are injected
into the signal side of the jack, the capacitance back
through the supply and all the other capacitances to the
electric fields near the case, inject different hum signals
into the ground side of the jack. You hear the difference
between them when you touch the tip of the wire.

When you plug the mic into the desk top system, its shield
and most of the internal metal structure is also connected
to something very nearly Earth potential through the ground
pin on the supply plug, so the only hum signal coupled into
the mic are those generated by the room wiring electric
fields coupling to the tiny bits of the mic signal line that
are not inside the grounded cable and mic structure (i.e.
the solder joint where the cable connects to the mic capsule).

But with the lap top computer, the entire "grounded"
structure of the mic cable and capsule are not nearly
"motionless" with respect to the Earth and all the other
conductive stuff that is essentially are ground potential,
but is bouncing all around with whatever voltage the supply
capacitance back to the hot line is producing. So not only
does the tiny capacitance from the room wiring to that tiny
exposed part of the signal line inject hum current into the
mic input, capacitance between that bit and anything that is
actually grounded also injects hum current, because the
"ground" reference for that input is humming along with
respect to all those truly grounded surfaces.

(whew!)

You might measure the actual ground voltage between the
laptop mic jack "ground" and the room wiring ground or a
water pipe. Or you can greatly reduce that voltage by
adding a real ground connection between the laptop case and
the Earth.

 

[George]

But with the lap top computer, the entire "grounded"
structure of the mic cable and capsule are not nearly
"motionless" with respect to the Earth and all the other
conductive stuff that is essentially are ground
potential, but is bouncing all around with whatever
voltage the supply capacitance back to the hot line is
producing. So not only does the tiny capacitance from
the room wiring to that tiny exposed part of the signal
line inject hum current into the mic input, capacitance
between that bit and anything that is actually grounded
also injects hum current, because the "ground" reference
for that input is humming along with respect to all
those truly grounded surfaces.

Thanks very much for the explanation. However, that
suggests that ALL laptops should produce bad hum when a mic
is plugged into them. Is that the case? If not, then what
might be wrong with mine specifically? I still think it's
either the AC adapter being noisy, or the design of the mic
input being goofy in some way.

But everyone seems to agree that the first thing to do is
ground the sucker. So I'll try that.

 

[John Popelish]

Thanks very much for the explanation. However, that
suggests that ALL laptops should produce bad hum when a mic
is plugged into them. Is that the case?

They vary all over the place, depending on exactly how their
power supplies capacitively couple their outputs to each
side of the line and what harmonics they generate and inject
through these capacitances.

If not, then what
might be wrong with mine specifically? I still think it's
either the AC adapter being noisy, or the design of the mic
input being goofy in some way.

I would suspect the AC supply, first.

But everyone seems to agree that the first thing to do is
ground the sucker. So I'll try that.

It doesn't take long to do.

 

[Jamie]

Thanks very much for the explanation. However, that
suggests that ALL laptops should produce bad hum when a mic
is plugged into them. Is that the case? If not, then what
might be wrong with mine specifically? I still think it's
either the AC adapter being noisy, or the design of the mic
input being goofy in some way.

But everyone seems to agree that the first thing to do is
ground the sucker. So I'll try that.

Laptop power supplies work differently over a simply wall
wart.

 

[Eeyore]

Laptop power supplies work differently over a simply wall
wart.

Come on Jamie. Tell us all about it. I'd love to hear this one.

Graham