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  Velleman K8020 high-end tube preamplifier. 
  Feb 23, 2006, 08:30am EST 

Powering it up

By: Sander Sassen

Once all parts had been mounted onto the printed circuit board I proceeded with mounting the PCB into the chassis and mount the front and back panel. All in all it took me about an afternoon to put this preamplifier together. After a thorough check of all connections and a proper placement of all components, take particular care to check capacitor polarity, I plugged in the power cord and flipped the switch. The preamplifier welcomed me with a friendly glow of the vacuum tubes and after the mute relay disengaged the CD I just started was sounding through the loudspeakers. Allright, now that weve finished construction and have a working preamplifier it was time for a closer look at the schematics.

Reed relais

Reed relais are used to switch between inputs of the preamplifier.

What is immediately clear is that the double triode ECC82 vacuum tube that is used is run rather conservatively with very little current flowing through it and sees a heavy load at the output in the form of a 5K6 resistor. This not only reduces its linear behavior but also causes for more distortion. Obviously this had to be changed; hence I started with calculating the proper current through both triodes and implementing the changes on the PCB. I also removed a number of components that served no real purpose, or only served to further complicate the schematic. The updated schematic has all the changes that I made and results in a compromise between lowest distortion and gain. It can be configured for even lower distortion, but the gain would rise considerably, which is not desirable.

High grade potentiometer

A high grade Alps stereo potentiometer is used for volume control.

All in all this new schematic resulted in a far better sounding preamplifier, with the heavy load at the output it was previously configured with it sounded straining, now its open and airy with a very neutral balance from the bottom octave to the top one. The overall gain is slightly higher than originally, but that cannot be avoided, a higher current will mean a higher gain, which can only be reduced by increasing distortion, tubes are funny that way.

1. Vacuum tubes
2. Powering it up
3. Some numbers
4. Conclusions

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