did something wrong with opamps?

Guest
Heya,

I'm trying to set up an OPA1632 differential op amp. I've got an
unbalanced dynamic microphone hooked up to the inputs, however when I
power the chip by a 9v battery (9v+ terminal to V, ground terminal to
-V), apparently noise is getting sent into the microphone, as I can
hear static comming from the element and it'll pop when I plug the mic
into the jack.

Is there something I'm obviously doing wrong? or would you need more
information to figgure out why this is producing such odd results...

Regards,
Matt
 
mattcarpenter@gmail.com wrote:
Heya,

I'm trying to set up an OPA1632 differential op amp. I've got an
unbalanced dynamic microphone hooked up to the inputs, however when I
power the chip by a 9v battery (9v+ terminal to V, ground terminal to
-V), apparently noise is getting sent into the microphone, as I can
hear static comming from the element and it'll pop when I plug the mic
into the jack.

Is there something I'm obviously doing wrong? or would you need more
information to figgure out why this is producing such odd results...

What are you doing with the OCM input to the amp? What are you using
Click to expand...
for input and feedback resistors? Do you have feedback resistors at all?

So I guess the answer is yes, we need more information. Answering the
questions above will be a help, posting a schematic of what you're
doing, either here in ascii art, on your website with a link, or on
alt.binaries.schematics.electronic will be a big help.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
 
Here's a schematic I drew up in paint. I've got nothing on the OCM
input, though I've patched it to the Vcom pin on my analog to digital
converter (PCM1804) and had the rest all hooked up, and the same thing
happened.

http://www.pixelwrench.com/schematic.jpg
 
On 2 Mar 2005 21:28:27 -0800, mattcarpenter@gmail.com wrote:

Here's a schematic I drew up in paint. I've got nothing on the OCM
input, though I've patched it to the Vcom pin on my analog to digital
converter (PCM1804) and had the rest all hooked up, and the same thing
happened.

http://www.pixelwrench.com/schematic.jpg
Click to expand...
What are you using it for that you need a diff output/input? I'd
ground/common one mic input, so there goes the single batt. Right
now, your mic/line is probably a big antenna picking up everything
you see when you touch your scope probe, if you have a scope. Go
beyond the first page of the spec sheet and note the filter caps on
the supply and the output.

That "pop" is usually caused by not having a ground path for the
inputs. The input caps which you don't show, but those feedback caps
are probably the culprit, should have a path to ground/common on the
mic side.

--
Best Regards,
Mike
 
On 2 Mar 2005 20:34:18 -0800, in sci.electronics.design
mattcarpenter@gmail.com wrote:

Heya,

I'm trying to set up an OPA1632 differential op amp. I've got an
unbalanced dynamic microphone hooked up to the inputs, however when I
power the chip by a 9v battery (9v+ terminal to V, ground terminal to
-V), apparently noise is getting sent into the microphone, as I can
hear static comming from the element and it'll pop when I plug the mic
into the jack.

Is there something I'm obviously doing wrong? or would you need more
information to figgure out why this is producing such odd results...

Regards,
Matt
Why are using it as a mic amp?
Click to expand...

wrong choice AFAICS, they only spec it up a gain of 10 or so, whats
the o/p level of mic, maybe 5mVpeak

a dynamic mic has a low source impedance, so you are really degrading
the signal/noise by having those1Kohms in series.

Go for the SSM 2017 architecture, works fine, second sourced by TI and
AD and Thatcorp( i furget the part nums)

martin

"An eye for an eye makes the whole world blind"
Gandhi
 
I've changed the schematic a bit.. added a 0v ground, however im still
getting a pretty loud popping/static-ish noise comming from the mic
see http://www.pixelwrench.com/schematic2.jpg
 
On 3 Mar 2005 19:32:14 -0800, mattcarpenter@gmail.com wroth:

I've changed the schematic a bit.. added a 0v ground, however im still
getting a pretty loud popping/static-ish noise comming from the mic
see http://www.pixelwrench.com/schematic2.jpg
Click to expand...
It may be perfectly normal. A dynamic mike can be a very sensitive
"speaker".

Try a couple of tests and report back.

Test 1, With the mike disconnected, use an ordinary multimeter to
measure the voltage across the amplifier's input connections.

Test 2, Use a single 1.5 volt flashlight battery in series with a
resistor to test the mike. Start with 10K or so for the resistor and keep
increasing its value until the mike makes the same volume of "static" noise when
you connect it to the battery that it does when it's connected to the amp.

Test 3, If you have access to an oscilloscope, measure various points on
the amplifier to see if it is oscillating at some frequency much higher than
ordinary audio.

Jim
 
On 3 Mar 2005 19:32:14 -0800, in sci.electronics.design
mattcarpenter@gmail.com wrote:

I've changed the schematic a bit.. added a 0v ground, however im still
getting a pretty loud popping/static-ish noise comming from the mic
see http://www.pixelwrench.com/schematic2.jpg
Click to expand...
Why did you choose the this opamp?

martin

"An eye for an eye makes the whole world blind"
Gandhi
 
Because it was suggested by Texas Instruments for use with my
particular ADC (PCM1804). I've put in a samples order for the parts
you've suggested however.

@ James Meyer: I'll work on those tests once I get home and report back
=)
 
On 4 Mar 2005 06:29:31 -0800, in sci.electronics.design
mattcarpenter@gmail.com wrote:

Because it was suggested by Texas Instruments for use with my
particular ADC (PCM1804). I've put in a samples order for the parts
you've suggested however.

@ James Meyer: I'll work on those tests once I get home and report back
=)
I'd start off with the circuit shown in fig 44 of the 1804 data sheet,
Click to expand...
its pretty standard . ( see the AKM and Cirrus datasheets)
You could put lower THD opamp in , say AD797, but I doubt if you would
hear any difference.
You would still need a mic preamp to get the sensistivty ie 30 to 60dB
of gain.


martin

"An eye for an eye makes the whole world blind"
Gandhi
 
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