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Generic problem with microphones,phantom powering and op-amps?

Actually a repair problem for a low noise microphone pre-amp but seems to be a general design flaw. Pre-amp uses a Burr Brown INA103 very low noise instrumentation op-amp. In this M-Audio Omni i/o preamp and an outline design application in the Burr Brown book show much the same circuitry. The 48Volt phantom supply to the mike is protected by 6.8K limiter resistors. But to block the 48V DC to the op-amp there is a 10uF/100V electrolytic in each line directly to the inv & non-inv i/p of the op-amp .

If , as seems in this case, a balanced line microphone with a short to ground is connected to such a system then the +48V / 0V across the elecrolytic will instantneously go to 0V / -48V with -48V directly connected to the op-amp i/p powered from +-15V rails and according to the databook can be taken to only +-12V.

Blown input to this op-amp due to just the owner connecting a microphone. Anyone familiar with this, adding limiting diode pair at each input ? I see no point in replacing this 15 GBP/ 25 USD IC until this design flaw is attended to or it will happen again should a fault to ground develop in a mike or lead while the 48V power is on.

-- Diverse Devices, Southampton, England electronic hints and repair briefs

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Reply to
N Cook
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I read in sci.electronics.design that N Cook wrote (in ) about 'Generic problem with microphones,phantom powering and op-amps?', on Wed, 21 Sep 2005:

Yes, diodes to each +/-15 V rail.

Part of the price you pay for not using a transformer.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Reply to
John Woodgate

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Many thanks for the link. Not only no protection in this 300 USD as new, year 2000 made, bit of kit but no mention in Burr Brown data book book either. Previous repair job concerning problem with phantom, as it was never used , except when someone accidently engaged it, I totally disabled that function on that large mixer/amp

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N Cook

I read in sci.electronics.design that N Cook wrote (in ) about 'Generic problem with microphones,phantom powering and op-amps?', on Wed, 21 Sep 2005:

There are plans to produce an Audio Engineering Society standard on input protection, but progress is slow.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Reply to
John Woodgate

also look at the other "typical " method on P12 of TI's PGA2500

martin

Reply to
martin griffith

"N Cook"

** The use of back to back zeners to protect sensitive input devices is standard practice with phantom powered mic pre-amps. See figure 23 on this pdf for how it is commonly done:

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Hard to believe any commercial product has no protection against the common phantom accident.

............. Phil

Reply to
Phil Allison

This is classic. I suppose Burr Brown missed the 'usual method' of protecting the inputs.

Add 4R7s in series with the INA inputs and fit clamp diodes to both supplies supply at the INA inputs. Use decent size parts. It'll blown an 0805 R for sure !

I'm sure one of the other TI mic amp parts hows this on the schematic ( see PGA2500 for example ) and I'm sure the SSM parts have it too.

Graham

Reply to
Pooh Bear

Quite possibly because it requires some beefy parts to be effective.

Graham

Reply to
Pooh Bear

Ahem. "It is recommended that the Schottky diode chosen for this application be specified for at least a 10A surge current." Here Burr-Brown recommends ON Semi's monster mbra120 diode,

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But sheesh, that beefy leaky high-capacitance diode is spec'd to handle 40A for 8ms! The surge current is much less, depending on the series resistor used. For example, 2x 27 ohms (0.9nV of noise) limits the 48V surge to 2A lasting less than 1.5ms (if Cin = 47uF). A nice little SD103 can handle that with oodles of room to spare.

--
 Thanks,
    - Win
Reply to
Winfield Hill

I've seen 0805 Rs fail open in that type of protection. The component needs to have some ability to dissipate the energy. Check out the peak current too.

Graham

Reply to
Pooh Bear

** Hardly a rational explanation.

No surprise from Stevenson the Fake.

............ Phil

Reply to
Phil Allison

THAT recommends 4.7 ohms series and 1N5819 bridge rectifier...

Reply to
Fred Bloggs

** Hardly a rational explanation.

Only a total idiot would use that as an excuse for no fitting protection.

Pooh just did.

.......... Phil

Reply to
Phil Allison

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