Selasa, 13 Agustus 2013

Discrete MM Phono Pre, Salas Simplistic nJfet RIAA

Having a bit of free time lately it's time for tinkering on one of my DIY to do list, a discrete MM phono pre.
I have built a MC gain stage here and combining this to the new MM pre is gives you flexibility of using (..ahem..) two tonearms and cartridge, MM and MC, another dream and to do in the list :).

The circuit is not far from what I always like, single ended Simplistic njfet RIAA from Salas, ditto for supply.
Utilizing a pair of nice small trough hole pcb (thank you mr. Liarto), the MM pre is made point to point soldering, as usual using components at hand. Toshiba 2SK170nJfet are 2 sets of dual match pair, a pair of not so matched for buffer. IDSS are about 8 to 8.5 mA as the 'doctor Salas' suggested.

First iteration below, matching of RIAA filter component for both channel was a pain, nonetheless I 'enjoyed' the process of selecting a pair of 15.33nF capacitors....that's why buying at least 4 pcs of each cap is important. Salas shunt regulator in the background there.


A close-up, Cout and interstage coupling is done to be easily changeable, so does the cartridge Rload and source resistors soldered on risers for easy tweaking.


Before putting in the 'box' some noise & hum was measured, quite a fight but finally the hum was beaten. Key is using shielded, preferably coaxial hookup leads and metallic box, in my case a quick and dirty biscuit tin can :)

Here the mighty hum..

Hum tamed..red trace is base noise of my laptop



in the box and first listening...bye bye Azur A540 :), those compressed sound of opamp finally gone.



satisfied? not yet...must set cartridge loading to match the whole system.

2 pots for adjusting Rload, Cload is blue siemens 33pF C0G


using headphone to set channel balance ..turns out needing to re setting cartridge VTF, azymuth, anti skate and VTA...sweating here..


Finally Rload is R22k by hearing for Sumiko Black Pearl (standard was R47k) and setting the B+ around 33-35V and adjust some Rsource to get that kick..


Now, it's time to hit those miles long groove..:), thanks for reading.


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