STA1 Stereo Tube Pre-Amp Low-Gain Conversion

 

Our original STA1 kit has a gain of 60, which is unsuitable for audio input signals of more than 500mV. Modern CD players, Tuners and Tape Decks have output voltages of 1000mV and more. Such high input signal will overdrive the pre-amp stages and cause distortion. This inspired us to go back to the drawing board and re-design this circuit. The outcome is an excellent sounding pre-amp with only 15dB (or 5.6 times) gain. Frequency response is flat from 25Hz-50kHz +/-1.5dB We changed only a few components and the input tubes.
Summary of changes, right channel shown. Left channel components in brackets.
All tubes V1, V2 and V3 are now 12AU7
Cathode resistor R4 (R24) of stage 1 changed to 1k Ohm.
Cathode resistor R12 (R32) of stage 2 changed to 1.2k Ohm.
Cathode follower resistors R16 (R36)and R18 (R38) of stage 3 changed to 1.8k and 10k Ohm respectively.
Plate resistors R3 (R23) and R13 (R33) of stage 1 and stage 2 changed to 27k Ohm.
Control grid resistors R17 (R37) of stage 3 changed to 470k Ohm.

Coupling resistor R7 (R27) changed to 82k.
1st stage power supply series resistor R14 (R34) changed to 15k Ohm.
2nd stage cathode by-pass capacitor C8 (C28) is switchable as "Loudness" or omitted.
Coupling capacitor C6 (C26) changed to 100pF.
Coupling capacitor C10 (C30) eliminated.
Volume control potentiometer P3 moved to the front end.
Plate supply voltage lowered to 215 VDC.
Below is the new, low gain STA1-LG circuit.

If you own an STA1 pre-amp and want to convert it to the low gain version, you need the following parts:
Resistors
1k Ohm 2pc.
1.2k Ohm 2pc.
1.8k Ohm 2pc.
10k Ohm 2pc.
27k 0.5W Ohm 4pc.
82k Ohm 2pc.
470k Ohm 2pc.
15k 0.5W Ohm 2pc.
3.3k Ohm 3-5Watt power resistor 1pc. (see note below)
Capacitors
100pF/50V film or ceramic radial cap. 2pc.
470uF/16V electrolytic cap. axial 2pc. (see note below)
DPST miniature switch. (see note below)
12AU7 vacuum tubes 2pc.
1 conductor shielded audio cable

Note:
3.3k series resistor is connected between STA1 and the SAPS1 power supply, in order to lower the plate voltage to 215VDC. You may need a different resistor value if your power supply provides different plate voltage than +275V. 470uF axial capacitors and DPST switch are needed only if you choose to have a "Loudness" switch. That switch will increase the amp's gain by 1-2 dB.
Conversion procedure:
Remove all changed components as mentioned above, including C2 (C22), C10 (C30). Install new components. The volume potentiometer must be rewired. Using shielded wire, connect the amp's input to the CW tap (cw=clock wise) of the volume pot. Next, run a shielded wire from the pot's center tap to the PCB input "R In". The CCW tap is connected to ground. Ground the wire shields here too. Now, a jumper must be installed, where the volume pot was previously connected, terminal 37 and 38. Since the C10 was removed, connect the jumper between terminal 37 and the point where R15 and R17 join. Do the same for the other channel. Drill two holes as indicated below, for the shield grounds. See diagram below:

 


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