Need mircrophone preamp circuit

[George]

I have a generic computer microphone that works fine on my desktop
computer for voice, which is all I need. It's a standard electret
condenser mic. Actually, it's a genuine QuickShot (not some cheap
imitation).

It also works fine on my laptop, but only if I'm on battery power.
When I run on the A/C adapter, I get really bad hum - so loud that
it isn't usable that way.

But if I turn off the mic amplification in Volume Control, and feed
in any higher-level line input, the hum drops way down.

In addition, I have an external mic input on my Canon camcorder
which has the instruction, "Use commercially available consender
microphones with their own power supply", and I've confirmed that
there's no DC supplied through that jack.

So, I think it's time for a microphone preamp. It needs to be
battery powered, hopefully no more than 3V. I have on hand a TI
TLC27M2CP CMOS dual opamp which the data sheet says will run on a
minimum of 3V, and I have a variety of bipolar transistors on hand.
I would appreciate it if someone could point me to a relatively
simple circuit that they like that would operate on 3V. It just
needs to be mono voice, but should do that well. If it matters, I
would like to be able to use a 15-foot cable run when I use it with
the camera.

Thanks for any suggestions.

 

[Vladimir Vassilevsky]

I have a generic computer microphone that works fine on my desktop
computer for voice, which is all I need. It's a standard electret
condenser mic. Actually, it's a genuine QuickShot (not some cheap
imitation).

It also works fine on my laptop, but only if I'm on battery power.
When I run on the A/C adapter, I get really bad hum - so loud that
it isn't usable that way.

This tells me of a ground loop with the high frequency AC modulated
noise generated by the AC adapter. This noise gets rectified by the mic.
circuit, so there is bad hum.

First, try to put a big ferrite on the AC power cord. Make ~10 turns of
wire around the core and see if it makes any difference.

So, I think it's time for a microphone preamp.

May be, may be not. First, you need to find out the cause of the problem.

It needs to be
battery powered, hopefully no more than 3V. I have on hand a TI
TLC27M2CP CMOS dual opamp which the data sheet says will run on a
minimum of 3V,

LMV722 is an inexpensive audio opamp specified at 2.7V.

I would appreciate it if someone could point me to a relatively
simple circuit that they like that would operate on 3V.

???? If you don't know how to make the basic amplifier, there is no
reason to do that. It won't work.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

 

[Eeyore]

This tells me of a ground loop

A ground loop requires 2 ground connections. I don't see that in this instance.

Graham

 

[Vladimir Vassilevsky]

Vladimir Vassilevsky wrote:




A ground loop requires 2 ground connections. I don't see that in this instance.

The power supply creates the common mode high frequency noise modulated
by AC power. So the chassis of the laptop is at noise potential over the
earth. There is a capacitive coupling from the earth to the input; the
mic. cable acts like antenna; so the noise gets to the input.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

 

[Eeyore]

The power supply creates the common mode high frequency noise modulated
by AC power. So the chassis of the laptop is at noise potential over the
earth. There is a capacitive coupling from the earth to the input; the
mic. cable acts like antenna; so the noise gets to the input.

Do you not see the error in your 'explanation' ?

Graham

 

[Vladimir Vassilevsky]

Do you not see the error in your 'explanation' ?

You mean that the noise is applied as the common mode to the input,
right? Unfortunately, it is not so simple. There is a high frequency
coupling directly to the electret mike, and it is real bad due to the
high internal impedance. I had to deal with very similar problems before.

VLV

 

[Eeyore]

You mean that the noise is applied as the common mode to the input,
right? Unfortunately, it is not so simple. There is a high frequency
coupling directly to the electret mike, and it is real bad due to the
high internal impedance. I had to deal with very similar problems before.

Your explanation makes no sense to me. The ground of the laptop has some small
signal on it referred to 'true' ground, however since the mic and its input
anren't connected to the true ground (they're connected to the laptop's ground)
there will be no pickup. There is quite simply no relevant mechanism.

Graham

 

[Vladimir Vassilevsky]

Eeyore wrote:

Your explanation makes no sense to me. The ground of the laptop has some small
signal on it referred to 'true' ground,

This signal is no small. Depending on how bad is the SMPS, it can be at
the order of tens of volts easily. Just take a scope and put it between
the primary and secondary sides of a cheap SMPS.

VLV

 

[Eeyore]

This signal is no small. Depending on how bad is the SMPS, it can be at
the order of tens of volts easily. Just take a scope and put it between
the primary and secondary sides of a cheap SMPS.

Even so, it still won't 'couple' into the mic input since that too is 'riding' on
top of this signal.

Graham

 

[Rich Grise]

.

So, I think it's time for a microphone preamp. It needs to be
battery powered, hopefully no more than 3V. I have on hand a TI
TLC27M2CP CMOS dual opamp which the data sheet says will run on a
minimum of 3V, and I have a variety of bipolar transistors on hand.
I would appreciate it if someone could point me to a relatively
simple circuit that they like that would operate on 3V. It just
needs to be mono voice, but should do that well. If it matters, I
would like to be able to use a 15-foot cable run when I use it with
the camera.

When I needed a mic preamp, I just built a plain vanilla, bog-standard,
textbook common emitter amp; this basic circuit:
http://www.ecircuitcenter.com/Circuits/trce/trce.htm

And did a little DC analysis to figure out resistor values; I ran it
on two "C" cells, and used a 10K pot for Rl.

But, I was using a 600 ohm dynamic RS mic - for an electret, you'll
have to figure out how to power it.

Have fun!
Rich

 

[Jamie]

Vladimir Vassilevsky wrote:




A ground loop requires 2 ground connections. I don't see that in this instance.

Graham

connections do not need to be direct, capacitive, surface ..
modulated RF can travel all over. most reasons why you see
little common mode chokes in the supply line. with out those, you can
introduce some modulated RF with 50/60 hz on it spreading all
around inside your electronics.

 

[Eeyore]

connections do not need to be direct, capacitive, surface ..
modulated RF can travel all over. most reasons why you see
little common mode chokes in the supply line. with out those, you can
introduce some modulated RF with 50/60 hz on it spreading all
around inside your electronics.

You fancy making an idiot of yourself again do you ?

Graham

 

[Jamie]

Jamie wrote:




You fancy making an idiot of yourself again do you ?

Graham

Typical British idiot. I feel sorry for those in your country
that actually know something. Your image over here isn't helping them
at all.

Mr. Expert. Get your head out of your ass and stick it back in the
text books that you obviously think is below you. Take my word for it,
they're not.

There's no shame in referencing basic 101 material from time to
time. Oh wait, I forgot, you have no shame! How mislead
I must be.

 

[ehsjr]

When I needed a mic preamp, I just built a plain vanilla, bog-standard,
textbook common emitter amp; this basic circuit:
http://www.ecircuitcenter.com/Circuits/trce/trce.htm

And did a little DC analysis to figure out resistor values; I ran it
on two "C" cells, and used a 10K pot for Rl.

But, I was using a 600 ohm dynamic RS mic - for an electret, you'll
have to figure out how to power it.

Have fun!
Rich

He can power the mike by adding a 10K from Vcc to the
+ terminal on the mike with the circuit you posted.

Ed

 

[George]

When I needed a mic preamp, I just built a plain
vanilla, bog-standard, textbook common emitter amp; this
basic circuit:

And did a little DC analysis to figure out resistor
values; I ran it on two "C" cells, and used a 10K pot
for Rl.

But, I was using a 600 ohm dynamic RS mic - for an
electret, you'll have to figure out how to power it.

Thanks very much. That looks like it should work fine.

 

[Phil Allison]

"Eeyore"

Even so, it still won't 'couple' into the mic input since that too is
'riding' on
top of this signal.

** But it could easily couple into the * electret * capsule of the mic -
wherever the ES shielding is not 100% effective (it never is).

The laptop's ground is likely floating at circa 50 % of the AC supply (ie
60 volts rms in the USA) above local earth - due to the use of Y caps in
the SMPS.

See this article, in particular see figures 3 & 12.

http://sound.westhost.com/articles/external-psu.htm

If this is the real cause of the hum in the mic signal, then the OP only
needs to GROUND his laptop when using the external supply.



........ Phil

 

[Eeyore]

"Eeyore"


** But it could easily couple into the * electret * capsule of the mic -
wherever the ES shielding is not 100% effective (it never is).

The laptop's ground is likely floating at circa 50 % of the AC supply (ie
60 volts rms in the USA) above local earth - due to the use of Y caps in
the SMPS.

See this article, in particular see figures 3 & 12.

http://sound.westhost.com/articles/external-psu.htm

If this is the real cause of the hum in the mic signal, then the OP only
needs to GROUND his laptop when using the external supply.

That much is certainly true.

Graham

 

[George]

** But it could easily couple into the * electret *
capsule of the mic - wherever the ES shielding is not
100% effective (it never is).

The laptop's ground is likely floating at circa 50 % of
the AC supply (ie 60 volts rms in the USA) above local
earth - due to the use of Y caps in the SMPS.

See this article, in particular see figures 3 & 12.

If this is the real cause of the hum in the mic signal,
then the OP only needs to GROUND his laptop when using
the external supply.

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.

 

[Phil Allison]

"George"

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.

** Essentially - that is a correct diagnosis.

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.

** Just try connecting the laptop to ground.

Use a length of bare wire to some metal plumbing if you must.

Ground the damn thing.




........ Phil

 

[Eeyore]

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,

That's entirely normal.

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.

It might conceivably be because the laptop mains PSU has some hum on the DC
output and that's getting into the audio via the DC power connection an electret
mic needs.

It's very unusual though.


Graaham