I was in the mood to build something different and stumbled across a design for an RIAA preamp based on Germanium transistors. The schematic calls for P28 and MP38A transistors but I used MP13B and MP10B instead. MP13B is supposed to be low noise. The only change was an electrolytic coupling cap on the output because one was already used for the input. For a regulator I picked the NCV47551 because it was cheap and had very good noise and PSRR numbers. Power is supplied with a 12v linear wall wart so under load it's supplying 14v which is enough overhead for 12 volts out. One bodge was required on the regulator, the current limit pin should be grounded. The trim pot is to adjust the voltage.
Initially it sounded harsh and I decided to see what the distortion analyzer had to say. Input was 10mv at 1khz and output was reading 1.1v so gain of just over 40db. VCC was set to 9 volts and that was giving 0.9% THD. Increased VCC to 12 and distortion dropped down to 0.4%. I have some P27 transistors still in transit so I may build another board to compare against.
Initially it sounded harsh and I decided to see what the distortion analyzer had to say. Input was 10mv at 1khz and output was reading 1.1v so gain of just over 40db. VCC was set to 9 volts and that was giving 0.9% THD. Increased VCC to 12 and distortion dropped down to 0.4%. I have some P27 transistors still in transit so I may build another board to compare against.
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I would have thought common-emitter followed by emitter-follower for the output section would make more sense...
Not sure what T2 does in that place.
Very unlikely any of these transistors could be called low noise. 10db?
Very unlikely any of these transistors could be called low noise. 10db?
Did some more testing tonight and found some interesting observations. The P28 in the original schematic has a very low Vce of 5 volts, the MP13B is rated for 15. So I started cranking up the voltage and even bypassed the regulator. Distortion is much lower now, 0.04% THD with a 35 volt supply. But the problems associated with germanium devices show up mainly noise. Noticed the numbers on the analyzer were jumping a bit and some glitches on the sinewave were visible on the scope. I looked at the measured output from the analyzer and slowed the scope time base down and the random noise was clearly visible with higher supply voltages. This isn't your typical device noise but a failure due to age or manufacturing. The MP13B has a date code of 1975 and the MP10B has a date code of 1990!
Hello, astouffer !
Your build looks very good!
I also built several phonopreamps on germanium transistors.
But I used circuits with a higher supply voltage. This is usually 22-32 volts .
The noise of germanium devices is , of course , greater than that of silicon . But I like the sound of germanium circuits.
Your build looks very good!
I also built several phonopreamps on germanium transistors.
But I used circuits with a higher supply voltage. This is usually 22-32 volts .
The noise of germanium devices is , of course , greater than that of silicon . But I like the sound of germanium circuits.
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Germanium transistors were produced in the USSR until the end of 1991 .
When assembling my phonopreamps, I made a selection of transistors based on leakage current. Usually, the minimum leakage current was for high-frequency germanium transistors - GT-308 ( ГТ-308 ) , P416 ( П-416 ) , etc.... And these transistors work well in audio devices .
When assembling my phonopreamps, I made a selection of transistors based on leakage current. Usually, the minimum leakage current was for high-frequency germanium transistors - GT-308 ( ГТ-308 ) , P416 ( П-416 ) , etc.... And these transistors work well in audio devices .