In addition to our well known P-12 pre-amplifier (which only accepts signals at line level) a new phono pre-amp was needed.
The real audio and/or music lover will us an MC (Moving Coil) cartridge when playing his favorite records. Such a cartridge offers a nicer sound quality then its' MM colleague, but it has the disadvantage of a very low output voltage of around 200 - 300 µVolt. An amplifier should amplify 10 - 20 times more when compared with MM. But then the input noise will become noticeable. To avoid this many people use a so-called "step up transformer". That transformer shows no extra noise or hum but, as any transformer, it offers a limited bandwidth.
In technique there's no stand-still and engineers at the semiconductor laboratories succeeded in the development of a series of "new" op amps with far better specifications then ever before. Now we can use a "bipolar" op amp which shows very low noise levels when connected to a low impedance. Doing this we constructed a "hybrid"' pre-amp, combining this new op amp with tubes.
Now have a look at the schematics. For a better look click the picture.
At the input there is the op amp, loaded with a current mirror. This will heat up the chip a bit so the behaviour becomes more "class-A like".
At the input there also are a few extra resistors so the circuit can adopt MM-cartridges as well. The amplification factor is 20x. This seems a lot, but this way the noise level of the next tube will be at an acceptable level.
After the input circuit a single triode amplifies a bit and the output connects to the passive network. This network (with selected polystyreen capacitors) takes care of the essential RIAA curve.
Thereafter the output consists of a simple SRPP circuit. So the output impedance is relatively low and longer interlinks will have no influence on the sound quality.
Then there's the power supply.
At the top there's the high voltage supply for the tubes. Each channel has its own stabilizer and after that a shunt regulator, again one for each channel. The capacitor at the output is NOT an electrolytic one but a MKP type!
The power supply for the op amp also has double stabilisation. First a series regulator and after that for each channel a separate shunt regulator. These shunts stabilize at + and -17 Volt. So the input op amp has ample headroom in order to handle any possible peaks in the input signal.
TNow have a look at the PCB (printed circuit board). All components, apart from the power transformers, are placed on that board. So the wiring is quite simple.
The dimension of the board is 17 x 30 cm.
The circuit will only be delivered as an "electronic kit" without the cabinet. All components, including the transformers, fit in a standard cabinet of 42 x 25 x 13 cm (b x d x h).
The kit now comes with a protractor, without the cabinet for 900 Euro.
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