-
Recently I searched for an inexpensive transistor preamp to use with a pair of vintage condenser microphones. I had used the ART tube preamps with good sucess and spotted this model in their lineup. It appeared rugged, clean and well built and had good reviews. I found it quiet and accurate and had no issues with the performance...but I love modifying equipment and decided to see what could be upgraded.
To open the unit pull the knobs off and unscrew the speed nuts on the Gain knobs. Then remove all the phillips head screws. Don't worry about anything falling apart, the fasteners just hold the faceplate on. Do the same on the back. At this point the circuit boards will slide out of the housing from either side. Wiggle gently to get them out. Note how the cardboard battery housing is situated so it makes sense going back in.
You'll notice that there are two circuit boards held together by a pin connector down the center. The pins were gold plated on mine which shows the quality of this unit. The two boards pry apart easily.I was unable to find a schematic for this so I decided to keep it simple and spot obvious things to be done. Following the signal...the left board handles the input from the microphones and contains the power supply so this is a good place to start. The right board contains several chips and the USB circuit. The photo below is after I modified the input board.
MODS
ART did an excellent job building these boards which contain a large number of surface mount components. This of course limits what you can change...the capacitors being pretty much the only item to modify. My choice for electrolytic capacitors in the signal path is the Elna Silmic II. To my ear...it has a smooth, natural sound with no glare or hyper-detail. The harmonic distortion spec of "Third high frequency distortion -120dB or less" makes them suitable for a low-level circuit designed for the human voice and music.
There are 42 polarized capacitors of an unknown brand scattered through the circuit. The caps closest to the inputs (marked in red) were all 47uf 63V to couple the 48V of phantom power. I replaced as many caps as I could with identical value Silmics. The two most rearward were too tight so I left them alone.
NOTE: the signal traces are tiny and plated though so take special care not to overheat and lift them. The second circuit board that handles the USB/ICs I decided was too complex to mess with not knowing the schematic so I left it alone. Finally, I used some contact enhancer on the pin connector before reassembly. To finish up simply put it back together in the reverse order. The final result is nothing monumental and not a redesign of the circuit...but an enhancement using better parts. A fun project.
All mods are illustrative only, perform at your own risk.
Datsun is a registered trademark of Nissan®