Finally completed my V0R0 DIY Front End preamp!
I planned on using this preamp in my living room, so I thought the case should look a bit better than an aluminum box. I had an old PS Audio Superlink DAC in the cupboard that I elected to repurposed for the job. The biggest challenge to using this chassis was to reconfigure the capacitive-touch buttons for control of power and input switching. Since I was not able to find a ready-made interface to these buttons, I decided to go old-school and use pushbutton switches. I was able to use the existing holes in the front panel for the switches, indicator lights and volume control. The old lettering on the front panel is a bit distracting, but I am going to live with it for now.
Previously, I had built a line-level preamp with removable circuit cards so I could compare different op amps (the AD845 wired for unity gain turned out to be my favorite at the time), so I decided to put the DIY FE on the same card so I could inter-change them for making comparisons. Since single ended and unity gain worked for me previously, I decided to stay with that topology. I decided to socket mount all the transistors and the gain setting resistors so experimenting with alternate arrangements would be easier. I elected to use a regulated +/- 20V for power, 2.2 µF input and output capacitors, and an Alps 50K input potentiometer.
I used relay input switching in my previous design, and since I still had some relays available, I decided to go that route again. Also common with the previous design is the external unregulated power supply.
Measurements on this unit look very good. This is, by far, the quietest preamp I have ever had in my system. The residual noise and distortion areabout as low as I can measure. The distortion level and profile are the same as the QA401. Some of the measurements are shown in the attachments. One interesting result is the crosstalk measurements, the right to left is not as good as the left to right. I might be able to fix that by re-routing some wires, but I wonder if it is worth it, considering that I am working with such low levels.
So, how does it sound? Before I tried to make that assessment, I measured the voltage of a 1KHz test tone at the speaker for each preamp. For the DIY FE preamp, with the volume set at maximum, it was 1.46Vrms. For the preamp I have been using the past few years, a tubed Assemblage L-1, with the volume set at the level I normally use, I measured 1.67Vrms. If I did my math correctly, that 0.2V amounted to about a 6dB SPL difference with my 4Ω speakers with an 86dB / 2.83V sensitivity. That’s a larger difference than I expected, but I went ahead and listened to a few tracks before I made any volume adjustments. On 8 out of 11 tracks I compared, the DIY FE preamp sounded just as loud as the Assemblage preamp. That 6 dB difference should make the DIY FE preamp sound about half as loud, but that is not what I found. Not sure what that means, if anything. Maybe someone can offer some ideas? But what I did notice was that music through the DIY FE preamp sounded crisper, with greater clarity – using the visual analogy – it is like a thin scrim has been removed. All told, I could not be more pleased with the look and sound of this preamp.
Thanks Mr. Pass.
Cheers,
ceulrich