To briefly recap, the RIAA preamplifier was presented for those who have a modern sound system.
More often than not these days, modern systems don't include a "phono"
input, which precludes the use of a turntable to listen to records (as distinct
The preamp not only provides the necessary gain for a magnetic
cartridge, it also "tailors" the signal to the RIAA standard, allowing you to
plug into any "aux" or line-level input.
The idea of our preamplifier was to make it fully
self-contained, with its own 240V to ±15V DC supply built on the PC board. To be
honest, this didn't work out quite as well as we had hoped due to noise being
induced into the preamp from the mains supply. But we were one step ahead on
this one (just in case!), making the two sections of the PC board separable.
Fig.1: each channel of the headphone amplifier uses an op amp driving a complementary pair of transistors which are within the overall negative feedback loop.
The 240V to power the whole shebang is, of course, already
available from the turntable supply. The ±15V, 10VA DC supply was in fact a bit
of overkill, being capable of much more than demanded by the preamp. But then,
we always had this little project in the back of our minds.
About the only change we'd make from the project published in
March would be the use of larger U-shaped heatsinks to cope with increased
current (there's tonnes of room on the PC board for them) and the use of
PC-board mounting screw-in terminal blocks on at least the power supply board
(where there is plenty of room) and possibly the RIAA board - where space might
be a tad tight.
Of course, there's nothing to stop you using this project as a
general-purpose headphone amplifier, as long as it is fed with "line level"
signals - such as the output of a CD/DVD player, tuner, etc.
Performance of Prototype
||90mΩ (max) into 8W headphones
||0.5dB down at 30Hz and 20kHz (see Fig.2)
||0.83V RMS for full power
||see Fig.3 & Fig.4
||-95dB unweighted (20Hz to 20kHz) with respect to 500mV input signal
|Separation between channels
||-50dB between 20Hz and 10kHz