As said before, after finishing GainPre and while working on
PhonoClone I decided that I would need to build another Opamp based phono equaliser,
one that would be comparable with tube amps in sound (if at all possible) yet
be simple like the phonocube design.
Having built several tube preamps I initially thought to achieve
that goal best by using a passive RIAA correction network similar to Loekie
and Scratch.
However, experience with the Phonoclone told me to minimize
components, especially resistors, in order to keep the noise floor down. And
also, since opamps need a feedback anyway there was probably something to gain
by using an active RIAA network.
Also, reading about the Phonocube, the Dynavector PA-200 and
other material on the Internet I already planned to use an inverted design for
the amp, and use a transimpedance stage for the input.
Goals
The goals for this project are again slightly more ambitious
and it will be difficult for me to get the desired results I'm sure. Starting
point is the PhonoClone. Here we go:
Get the best sound possible, therefore use even better components
with higher precision (but not too fancy).
Better noise values for MC amplification (this time I do
want to use OpAmps only and get the same noise as the internal Pre in my Marantz
PM14KI). No, I take that back: I want less noise as my Marantz internal Phono
preamp.
Prepare for battery power supply (and some sort of charger
circuit)
Use blue LED's for power-on (in any case, use low-power LED's
because of the battery power)
Look at inverted mode for 1st and 2nd stage amplification
Well, that's about it folks. Additional design criteria will
follow on page 2.
Why
Why make yet another phono preamp? Because I feel like it. Only
after building 5 of them I start to get a feeling about what I like and do not
like about certain phono preamps.
And I'm personally convinced "there is always room for
improvement".
Inverted
Looking at the last design goal above, having an inverted amp
design, there are some direct advantage for doing so:
We can save components, especially resistors and this has
a positive effect on the noise floor.
In an inverted setup, the amplification factor is defined
by the impedance of the feedback loop divided over the impedance of the negative
input. That is a major advantage over the non-inverted design: If we connect
the cartridge directly to the negative input of the Opamp and ground the other
input, then the amp will transform in a current-to-voltage amp (transimpedance
amp) so not the DC resistance of the cartridge determines the amplification,
but the current from the cartridge. For (relatively) low-Z MC cartridges,
the amp behaves more like a current amplifier than a voltage amp. This would
make it suitable for AD797 and other low-noise op amps.
But more important, whatever RIAA filter is used in the feedback
loop, it is independent from the cartridge. Should we change cartridges, then
only the gain changes (a little), no changes to the filter are necessary.
