Simple Op Amp Audio Preamps

A Ciphers By Ritter Page

Terry Ritter

2006 April 21

Here are tested designs for simple, low-noise, audio preamps built from modern low-voltage opamps, and designed to satisfy various goals:

Op Amps and Specs

The main issues are voltage, current, noise, and input offset:

These are worst-case specs that all marked components should meet; sometimes the actual values will be better or even much better.

             E (V)    I (mA)   nV/SQRT(Hz)   offset (uV)
  TLC272     3..16     3.2        25           10000
  LMC6482    3..15     1.9        37           38000
  LT1884     3..40     2.8         9.5            80
  OP27       8..36    ~6           4.5           100

Feedback Noise

When we use op amps, we add thermal noise from resistors. But the added noise will not matter if we keep it below, say, a third of the op amp noise. Thermal noise varies as the square-root of the product of resistance and bandwidth.

  E(nV/SQRT(Hz)) ~ 0.129 SQRT(R)
  R ~ SQR( E / 0.129 )

     Ohms    nV/SQRT(Hz)   uV RMS (10k BW)   uV RMS (21k BW)
       1M      129.x           12.9              18.9
     300k       70.7            7.07             10.3
     100k       40.8            4.08              5.96
      30k       22.3            2.23              3.27
      10k       12.9            1.29              1.89
       3k        7.07           0.707             1.03
       1k        4.08           0.408             0.596
      300        2.23           0.223             0.327
      100        1.29           0.129             0.189
       30        0.707          0.071             0.103
       10        0.408          0.041             0.060
        3        0.223          0.022             0.033
        1        0.129          0.013             0.019

To get the expected RMS noise, multiply the nV/SQRT(Hz) value by the square root of the measurement bandwidth. Here, "10k" means 10,000Hz (and sqrt(10k) = 100); "21k" means 21,355Hz (and sqrt(21355) = 146).

LMC6482 and TLC272 Preamps

The National LMC6482 and TI TLC272 are dual CMOS low voltage op amps, with somewhat high input noise.

[LMC6482 Preamp Schematic]

LT1884 Preamp

The LT1884 is a dual bipolar low voltage op amp with lower noise than many CMOS op amps.

[LT1884 Preamp Schematic]

OP27 Preamp

The OP27 is a bipolar op amp with even lower noise. Unfortunately, it also requires more supply voltage, more supply current, and a higher input bias voltage. Fortunately it has higher gain than the other op amps, so that a single stage in the package can produce 30dB of amplification. Unfortunately, it is also obsolete. [OP27 Preamp Schematic]

Terry Ritter, his current address, and his top page and electronics home.