The noise of the four 6.81 kΩ resistors has little or no influence on the S/N ratio coming from the four input amplifiers. The noise of the feedback (2.2 kΩ in parallel with 10 Ω) is 0.41 nV/√Hz. The input of the LT1028 has 1 nV/√Hz. With a 5 Ω MC cartridge impedance this becomes 1.04 nV/√Hz. So total input noise is 1.12 nV/√Hz (1.08 nV/√Hz input shorted). At the outputs of the amplifiers the noise is 221 times larger, 247 nV/√Hz. Adding the noise of the 6.81 kΩ resistor will only increase the noise to 247.2 nV/√Hz. The total noise at the summing junction is half of this, 123.6 nV/√Hz (119.34 nV/√Hz input shorted). We ignored current noise. Noise of the MC input stage is 3.7 times higher as noise coming from the MD version (shorted). The gain however is 4.62 times higher. If we recalculate the measured MD noise with a shorted input and 270 mV output voltage S/N is 4.84 dB lower, so 83.15 dB. The calculated noise difference between MC and MD is 11.4 dB when inputs are shorted. From these calculations we can predict the S/N for the MC modification to be in the order of 72 dB.

Measurements

0.3 mV (at 1 kHz) input signal via 4.99 Ω/4.75 kΩ divider. With 100 Ω input impedance this means the generator signal is almost exactly divided by 1000. Shorted input increased signal to noise measurements hardly, maybe half a dB or so. Power supply +/-14 V (laboratory PSU).

Input impedance 100 Ω

Signal to noise >73 dB (B = 22 Hz..22 kHz)

>79 dBA

THD+N (1 kHz) 0.02 % (just noise, B = 80 kHz)

THD (1 kHz) <0.0007 % (interpreted from FFT plot)

THD (10 kHz) <0.0002 % (interpreted from FFT plot)

Output level 270 mV

Deviation from RIAA <0.1 dB (50 Hz..10 kHz)

< 1 dB (20 Hz)

<0.15 dB (20 kHz)

Again we recorded some plots.

**Plot A**(MC_AMPvsFRQ_2) shows the amplitude as a function of frequency when applying a RIAA standard equalized sine wave to input. At 20 Hz the amplitude was -0.876 dB. At 20 kHz the deviation was 0.113 dB. At 138 Hz and 3.3 kHz the amplitude rises slightly by 0.022 dB. This is exceptionally good considering all parts for the RIAA correction have a 1 % tolerance!

**Plot B**show a FFT of a 1 kHz signal from 10 Hz to 130 kHz with the fundamental suppressed (MC_FFT_1kHz). This shows if any harmonics are present (16 measurements were averaged to get a better view) and so we can conclude that since no harmonics are visible total harmonic distortion is less than 0.0007 %.

**Plot C**shows a FFT of a 10 kHz signal to make sure THD is also extremely low for higher frequencies (MC_FFT_10kHz). Only a second harmonic is visible at a level of -114 dB, which translates to 0.0002 %.

**Changes in a nutshell**:

R1, R30 = 100 Ω/1 % (look for manufacturers recommended value!)

R4,R6,R7,R9,R10,R12,R13,R15,R33,R35,R36,R38,R39,R41,R42,R44 = 10 Ω/1 %

C1,C26 are not needed but can be left in place if present

P1/R2/R3/C2/JP1 and P2/R31/R32/C27/JP2 are not needed but can be left in place if present