View Single Post
  #25   Report Post  
Posted to rec.audio.pro
Phil Allison[_4_] Phil Allison[_4_] is offline
external usenet poster
 
Posts: 499
Default overvoltage on audio circuits

wrote:

--------------------


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.

https://www.analog.com/media/en/tech...016SSM2017.pdf

Note the fairly high supply current.


...... Phil