Recommendations for Modifications

Improvements

There are some recommendations how to improve the sound quality of the preamplifier, however without any warranty for success as I did not perform any tests:

• You may improve the dust, scratch and EMI sensitivity of the phono preamplifier by adding a lowpass of about 50 kHz to the first amplifier stage. This is done by bridging $R_{8}$ with a capacitor.
• The input noise of $U_{5}$ is fully added to the audio signal. As $U_{5}$ amplifies only DC it is possible to reduce the amplification within the audio band to 0 dB. This is done by bridging $RVAR_{1}$ and $R_{18}$ with a capacitor. Its capacitance is calculated according to the equation
  
  $$C=\frac{1}{2\pi R_{16}f_{u}}$$
  
  
  to a value of about $C=68\mu F$ at $f_{u}=21Hz$^A.1.
• Improvements of the power supply:
  • Additional filtering or regulation of the supply voltage of the first stage of the phono preamplifier.
  • Separate lines transformers and filtering for each channel.
  However, as the PSRR^A.2 values of the operational amplifiers used are very good we cannot expect much improvement; this is much more importent for discrete circuits.

Simplifications

You can also simplify the circuits to get a cheaper preamplifier but on the expense of sound quality loss:

• Elimination of the offset compensation of the phono preamplifer:

  Just chose an alternative solution to the DC offset problem as mentioned in 2.1.4 (see page). I would chose solution 3.

• Elimination of the class A output stages of phono- and/or line level preamplifier:

  Just elimnate the FET and its source resistor (leading to the negative supply voltage). This will result in a sterile, metallic and somewhat boring sound; and in case of the line level stage you can no longer drive a headphone. It is advisory to use an operational amplifier with low output resistance and high short circuit current (e.g. OPA604), and to increase the output resistor from 50 $\Omega$ to 100..200 $\Omega$.

• Use a cheaper potentiometer.
• Reductions in the power supply:
  • Common supply regulation for both phono preamplifier and line level stage.
  • Common supply regulation for both channels.
  • Use cheaper voltage regulators (7815/7915).
  • Use a smaller and cheaper lines transformer.
  • Use smaller and cheaper reservoir capacitors.

Other Modifications

You may also do the following modifications which may lead to a different sound--let's call this the individual fine adjustment:

• Chose a different quiescent current of the class A output stage of phono and line level preamplifier by changing the resistance of the source resistors. Increasing quiescent current reduces the harmonic distortion, although this distortion is quite small anyway due to negative feedback.
• Replace the FETs by small bipolar power transistors (e.g. BD135).
• Replace the class A single ended output stage by a class A push pull output stage.
• Replace the class A single ended output stage by a class AB push pull output stage. Instead of a discrete circuit you may also use an integrated buffer (e.g. LT1010, BUF04, LH0033 o.ä.). This is especially interesting for the line level stage if you must drive a low impedance headphone but do not want the necessarily high quiescent current of a class A output stage.
• Using other operational amplifier:
  • Regarding line level stage and the second stage of the phono preamplifer any operational amplifier may be used, in theory. However, for the line level stage you should use an operational amplifier with FET inputs due to its very low input offset current. In the phono preamplifier I successfully used a OP37 in the second stage. In the line level stage you must use an operational amplifier which is unity gain stable or may be compensated for unity gain. An expensive alternative is the OPA627, cheaper is the OPA604.
  • Regarding the first stage of the phono preamplifier I could only think of the AD797 which has a similarly low input offset noise voltage. However, the input offset current of the AD797 is much higher which means that the MC pickup must be able to stand the higher input current. I succesfully used an OP37 and an AD846 (a current feedback amplifier!) but with increased noise, although the noise can be heard only if the pickup is lifted off as the noise of the records is yet higher than that of the preamplifier. An expensive alternative is the OPA637, but it still is more noisy.