Advice on building a line-level preamp for an unbalanced dynamic mic?

[Mr. INTJ]

Sorry, this post is a little long - I've tried to keep it info-rich,
with pictures (schematics)...

I want to record some audio on my PC using an unbalanced dynamic mic
(see below for specs). Like most PC sound cards, the "mic in" on mine
seems to expect an electret mic. The output of a dynamic mic is too
low, and I have to max out the gain and shout into my dynamic mic to
get a reasonable sound level.

I looked for something cheap online (incl. rat shack), but most
preamps are a little too expensive, have tubes (for that warm sound, I
guess), and/or only support balanced mic input.**

So... I'm going to whip out the soldering iron and build a little
preamp for my dynamic mic, and use the "line in" input instead***. TI
was generous enough to provide me with a sample of their low-noise,
low-distortion instrumentation amplifier chip (INA217, specs below).
The spec sheet includes a sample circuit for a balanced dynamic mic
preamp:

http://www.minsmithphoto.com/mr_intj/balanced_mic_preamp.png

My mic is unbalanced and doesn't require phantom power, so I need some
advice on modifying the circuit appropriately (or put another way,
adapting my unbalanced mic input to this circuit's balanced input). My
first thought was to use an audio transformer like this:

http://www.minsmithphoto.com/mr_intj/unbalanced_to_balanced.png

....but I'd need a very specific transformer, and going through my
boxes of junk circuit boards, I found that modern electronics doesn't
seem to use them as much as they did in the old days. Also it seemed
like the transformer would introduce loss (and possibly noise from
EMI) into an already weak signal). I've read that even with perfect
impedance matching, 50% efficiency is the best you can hope for.

Then I read up on baluns:

http://www.minsmithphoto.com/mr_intj/balun.png

....A similar idea. This requires a 1:1 isolation transformer without a
center tap (boy don't I feel dumb for not thinking of this), which is
probably easier to find... but similar concerns as above. That leads
me to something like an op-amp in an inverting 1:1 configuration:

http://www.minsmithphoto.com/mr_intj/inverting_amplifier.png

This seems good, but... I'm beginning to get out of my depth here; I'm
just an old software weenie with a soldering iron and some high school
electronics (sadly, mostly on vacuum tubes).

Trying to eliminate the phantom power (which I don't need), and the op-
amp that removes any DC offset from the output, I come up with this:

http://www.minsmithphoto.com/mr_intj/balanced_mic_preamp_modified.png

Can anyone comment on this? Is there a simpler choice that I've
overlooked?

Thanks very much in advance!

-Mr. INTJ


NOTES:
------

Sound card: SoundBlaster Audigy 2 ZS

Microphone: Sony F-V620, unbalanced dynamic mic, 600 ohm impedance
(http://reviews.cnet.com/microphones/sony-unidirectional-enriched-
sound/4507-6469_7-20648859.html)

TI INA217 Low-Noise, Low-Distortion Instrumentation Amplifier:
http://www.datasheetcatalog.org/datasheet/texasinstruments/ina217.pdf

** Someone suggested this amplifier from SparkFun:
http://www.sparkfun.com/commerce/product_info.php?products_id=8704

The price is right - but I couldn't determine from the specs whether
it would fit my needs.

*** There are some schematics floating around the web for a single-
transistor booster for a dynamic mic (uses the 3-5V supplied by the
sound card). I'm not looking for studio quality, but from what I've
read, the circuit is very limited, and distortion is a problem. It
also relies on the preamp built into the sound card, which is probably
intended for voice communication only.

 

[Phil Allison]

"Mr. INTJ"

The spec sheet includes a sample circuit for a balanced dynamic mic
preamp:

http://www.minsmithphoto.com/mr_intj/balanced_mic_preamp.png

** Step 1 = remove all components that connect to pins 2 and 3 except for
R5.

Step 2 = link pin 3 to ground.

Step 3 = wire mic cable shield to ground and signal wire to pin 2.



........ Phil

 

[Mr. INTJ]

"Mr. INTJ"



** Step 1 =  remove all components that connect to pins 2 and 3 except for
R5.

 Step 2   =    link pin 3 to ground.

 Step 3  =  wire mic cable shield to ground and signal wire to pin 2.

.......   Phil

Phil, thanks for the quick response. I figured it would be something
much simpler than what I was thinking.

Here's the modified circuit - I'll go breadboard this up and try it
out tonight.

http://www.minsmithphoto.com/mr_intj/unbalanced_mic_preamp.png

The specs say that V+/V- can range from 4.5V to 18V, so I'm planning
to use a pair of 9V batteries. That way I can avoid having to deal
with filtering the noise out of a wall-wart power supply. For the same
reason, I think a battery supply will mean that I can omit the two
0.1uF capacitors.

I'll post again after some testing.

 

[Phil Allison]

"Mr. INTJ"
"Phil Allison">

** Step 1 = remove all components that connect to pins 2 and 3 except for
R5.

Step 2 = link pin 3 to ground.

Step 3 = wire mic cable shield to ground and signal wire to pin 2.

Phil, thanks for the quick response. I figured it would be something
much simpler than what I was thinking.

Here's the modified circuit - I'll go breadboard this up and try it
out tonight.

http://www.minsmithphoto.com/mr_intj/unbalanced_mic_preamp.png


** That was quick by you work too.

Battery operation should be fine.



...... Phil

 

[neon]

for something that simple use lm124 type of preamp. single rail to gnd. since you got 4 intoa pachage use them all you can use USB PORT for that the power is 5v .5amps no battery.