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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 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 to 100..200
.
- 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.
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.
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