My Pre
last updated: Friday, 02-Apr-2004 23:22:54 EST

Dave's Homepage audio home The Vifa X3 Preamp Car Audio Shiva! PA Speakers Hafler stuff Phase Linear LM3886 amp lm12 amp

In the summer of 1999 I decided that I wanted to build a preamp for my stereo. I wanted it to have features that were cool, like XLR I/O remote control (unimplemented) low THD+N lower IMD 5.1 in and thru VFD display (LCDs are so...1995) EQ (unimplemented) fully balanced topology XO (unimplemented) rack case digital level control rotary encoder volume knob And so on and so forth. I ran across a bunch of projcts around the net, most of them very disappointing. I toyed with the idea of building a kit from Velleman but their pre got such bad reviews I gave up on that avenue fast. Along the way it came to my attention that a preamp would make a very good senior project for my degree. And so I set about to make it happen. The actual beginning of the project was long before the fall of 2001, when my one semester of hell actually began (I opted to accelerate my senior design project, finishing in one semester instead of two.....bad idea. I left with a degreeand an award, but at the cost of my love and my sanity). Almost two years of planning and testing have gone into the design, and as of 7/2002 it's stil not done to my satisfaction. As far as fixing the problems with version one, I've obtained a new case (1U), new VFD and fixed most of the design flaws. I have parts for the remote and plans to make the surround stuff work right this time. Version two will still be some time off, since the PCB needs to be re-laid out and sent to California to be manufactured, and both of those require time and evergy that I do not currently have. The design itself is quite simple. In order to make the fully balanaced design goal possible I used Burr Brown DRV135UAs to convert all the single ended inputs to balanced lines on the left and right channels. I cheated on the surround inputs and made them straight thru, unbalanced, because of some time and PCB issues I ended up having. The switching is handled by Burr Brown MPC507Us, which are 8x1 low distortion balanced analog MUXs. Gain is handled by Crystal Semi CS3310-KSs. They're digitally controlled stereo volume controls. I use one per balanced line. -90ish to +30ish dB in 0.5dB increments. That's 256 steps for those of you playing along at home. Buffering is handled by Burr Brown OPA2132UAs. Misc. balanced to single ended conversion is handled by Burr Brown INA134UAs. I used A grade parts for everything, as they had the lowest offset and self noise. Of course, they cost 2-10x as much, but this was a cost no object design for the most part. A Basic Stamp is at the heart of the project, but it's too slow and misses things (if you rotate the volume knob too quickly for example). The plan is to use my copy of PicBasicPro and a 16F876a/20 to make things faster. I'm not going to post schematics, code or more data than this right now. When the project is finished, publishing is my goal. Not to make money (altho that'll be nice) but to spread the word in a manner other than digitally. AudioXPress (formerly Audio Amateur, previously Audio Electronics) looks like a promising place, especially if they let me do a feature. I was thinking about one of the descendents of Radio Electronics, but I'm not sure how tightly I can keep hold of the copyright on the design/code. I put 2 years of my life into this beast. It's mine dammit. Here are some pictures. Thanks to Tony, who kicked me to post them. They're Quickcam pictures, so don't be suprised when you see washed out 320x240 JPEGs that make your eyes hurt. More and better ones will come eventually, when I get them scanned and properly formatted, etc. Input wiring, there was a lot more when I was done. Inside view of my ghetto ass wiring job Font panel before paint and stuff Back Panel, also before paint Badass power supply design Dave Paton        |        dave@dpaton.net        |