The “Universal Gain Stage” preamplifier was published in 2006 at HCFR and DIYAudio. The reports and pictures on the forums were very promising and the designer and his circuit earned an excellent reputation. The plans and data clearly showed that this design was developed to a very high level.


It was very tempting and I knew I would enjoy making one…


The main semiconductors for the amplifier circuit are the rare 2SK389 BL and 2SJ109 BL dual monolithic junction FET's. The amplifier is DC-coupled and the semiconductors needs thermal coupling for a low offset. An Atmel Atmega 64 micro-controller operates the ladder volume control and all switching. This requires a great many relays. The menu offers a lot of settings and adjustments. There are four inputs and two outputs, all RCA and XLR. Each channel has its own regulated power supply. The micro-controller board and relay board have their own transformer and all power supplies are in a separate enclosure.


Once I managed to obtain the Gerber files, schematic, wiring plan and software for the micro-processor I started to build this preamp from scratch. I started with the production of the circuit boards. I did not want to etch them all, but wanted to mill some of the boards. It was really interesting to find out how to make CNC-machining data from Gerber files. I never worked with SMD parts before and it was my first time I programmed a micro-controller. Thus I encountered several serious difficulties, but the whole construction process was quite rewarding. The enclosure parts were made from 4 and 10 mm thick aluminum, black and silver anodized. For the top I used rather dark tinted acrylic glass. The preamp sounds great and I enjoy the final product a lot.

Comments

• Wardsweb
• 30 January 2012 at 10:42AM
• That is truly incredible. From PCB to CNC chassis,this is a remarkable project. Kudos!