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Old November 1st 19, 11:02 PM posted to
Phil Allison[_4_]
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Posts: 459
Default overvoltage on audio circuits



> >> Also, bipolar amps don't use large devices to reduce noise.

> >
> >
> >** Discrete op-amps use paralleled BJTs at the inputs and
> > fairly high current levels to get voltage noise down low as possible.
> >
> >Enlighten us -
> >
> >what magic trick do integrated ones use instead ?
> >


> Increasing bias currrent increases shot current noise at the collector
> and base, so low noise bipolar amps tend to be biased at lower currents.

** FYI so called "shot noise" is not relevant to input devices operating over the full audio band. High frequency noise completely dominates.

> Not sure why discreet amps would use high currents for low noise.

** Then you are simply not very familiar with audio circuitry - as I suspected.

The idea is to get the best NF impedances down to around 150ohms for use with dynamic mics.

> I designed a decent low noise op amp a few years ago, the OPA1662, 3.3nv/rtHz noise densisty. -124db distortion, total noise+distortion 0.00006%,
> 22MHz GBW, 22V/uS SR, Sig/noise 95db, voltage gain 114db, and ..... 1.5ma power draw total.

** Nice part - but with about 1uV of input noise.

With 3.3nV and 1pA per rtHz of input noise, the best impedance is 3.3kohms.

Though obsolete, this op-amp set a bench mark for low noise audio.

Note the fairly high supply current.

...... Phil