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Old 3rd March 2005, 03:24 AM   #1
MC is offline MC  United States
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Arrow I guess I'm obviously doing something wrong here...

I'm trying to wire up an OPA1632 differential opamp sort of like so:


A-_____|--------\-------out -
A+_____| opamp/-------out +



Anyways.. It's powered by a 9 volt battery and I've got an unbalanced microphone hooked up to the A- and A+ terminals. However, when I plug the mic in, there's apparently current leaving inputs because I can hear a static and sometimes a popping sound comming *FROM* the microphone.

Am I powering this incorrectly by using a 9v batteries 9v and Ground terminals?

Thanks
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Old 3rd March 2005, 04:32 AM   #2
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better schematic attached..
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File Type: jpg schematic.jpg (16.7 KB, 369 views)
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Old 4th March 2005, 01:38 AM   #3
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I have no experiance with circuits like this at all....but...

Why not a power kill switch...kill the voltage to the opamp, and power up after it's plugged in.....?
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Old 4th March 2005, 04:08 AM   #4
MC is offline MC  United States
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I'm not sure what a power kill switch would do...

Anyways, I've got it hooked up again, this time with a 0v ground, however now it will amplify a line-level source but it will introduce a hum on the output AND input. If I plug a mic into the input, the popping is gone but I can hear the same hum through the microphone as if it were acting like a speaker. What could be causing this?

Regards,
Matt

ps heres an attached schematic of the *current* design
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Old 4th March 2005, 07:23 AM   #5
jaycee is offline jaycee  United Kingdom
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Try bypassing the supply pins with 100nf capacitors
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Old 4th March 2005, 07:42 AM   #6
Jennice is offline Jennice  Denmark
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Are you using a dynamic or capacitor-based mic, and does it have a built-in buffer op-amp like many PC microphones?

Jennice
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Old 4th March 2005, 11:04 AM   #7
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I would say the dynamic mic is upsetting the DC conditions around the opamp, causing current to flow through the mic. I would suggest a coupling capacitor in line with the mic, say 10uF non-polarised.

Cheers
Graeme
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Old 4th March 2005, 01:26 PM   #8
MC is offline MC  United States
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Quote:
Originally posted by Jennice
Are you using a dynamic or capacitor-based mic, and does it have a built-in buffer op-amp like many PC microphones?

Jennice
Nope, it's an old-ish dynamic "professional" recording mic. It's got a transformer in it, and that's about it.

I'll try adding some caps as has been suggested.
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Old 4th March 2005, 04:55 PM   #9
Jennice is offline Jennice  Denmark
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Hmm... The application/data sheet has these 1nF, possibly because of a gain/stability issue. Does your circuit work OK with other sources?

Maybe you have to consider feeding a fair of "normal" op-amps, one as unity gain, and one as inverter, to end up with your differntial signal.

Jennice
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Old 4th March 2005, 05:15 PM   #10
JohnW is offline JohnW  Hong Kong
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MC,

Have you grounded the OPA1632 VCOM pin (pin 2) this will insure the outputs are at 0V (but you will have to operate the device with +/- DC source) - this will insure the outputs swing about 0V & therefore 0V DC on the input resistors.

If this does not help, I would try a unity gain buffer on each input - this will also gain you high input impendance.

I recently used the OPA1632 to drive a ADC front-end, the device is VERY sensative - proved horrible to use.

I had to put 2 resistors to ground on each input (Microphone end) to insure the device would work correctly with true floating inputs - I used 2x 3K3 to ground - but depends on your required input impedance - remember the load presented by the FB resistors.

Also - I had to add 470pf directly across the opamp inputs to insure stablity - even with TI published FB values. Was not happy about adding this Capacitor - but without the Cap the distortion was about 0.003% - and dropped to about 0.0002% with Cap. Could find no sign of instablity (HF oscillations) using a spectrum Analyser - however something was going on....

Notice how TI are pulling a fast one - the OPA1632 seems to be a re-badge of the TI THS4130 / THS4131 - but cheaper - even packaged at same plant....

Can the Mike drive the low input impendance presented by the Opamp feedback resistors? ( I don't know much about mikes)

Good Luck,

John
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