R2R Buffered Pre-Amp.

R2R Buffered Pre-Amp.

clip_image004Adam Walker has produced an incredible looking pre-amplifier, In this article he talks though his build. Although casework was manufactured for him by a couple of companies the design work for the front panel is his own. This project really shows off the resources that are available to the hobby builder from specialist firms. Adams design I think really shows that home built equipment can rival and surpass many commercially available audiophile equipment. In Adams words…

I started this project as I needed a source selector and a volume control for my valve power amp.

Design brief: Remote controlled source and volume control, As transparent sounding as possible, can drive long interconnects, Looks good.

Case: Modushop.biz offer superb quality aluminum cases, and as I am in Europe the shipping is very reasonable.

For the front panel I used Shaeffer, or front panel express as its known in the states.

Remote source and volume control:

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After much researching I decided to go for an R2R volume control, made by BBP of the DIYaudio forums. In one neat package I have a remote control source and volume control and an LCD display

Version 1 – testing

So I do a mock up of the circuit, just connecting it loose on bench, plug it into my amp and away we go. The remote kit works well, the R2r resistor network sounds clear….. but everything sounds a bit lifeless. Many people comment on passive preamps sounding a little dull and dry. I assume that is what I was hearing.

Buffer

Back to the research. I only need unity gain as my power amp is quite sensitive, any more gain and the volume control would all be between 6 and 9 on the volume knob.

I consider a cathode follower valve buffer, the nelson pass B1 and the Pedja Rogic.

I decided to give the Pedja Rogic buffer a go, its based on the tosh 2sk170, a well known ‘transparent audio jfet’.

I contacted Blair from https://www.justblair.co.uk. He had a few spare buffer PCBs left over from his own build, so I took a couple from him. They were easy to build, and the circuit boards good quality.

Version 2 – Buffered R2R

Placing the buffers after the volume control lowers the output impedance, which my valve amp is much happier with. Testing the circuit I notice a good improvement. Its subtle but the music has a bit more life and sparkle, just very slightly less veiled.

Buffer power supplies….

Blair designed his buffer PCBs with onboard voltage regulators to create a +/- 15v bipolar supply. It uses the common 7815 and 7915 regulators. I am sure these are ample for the buffer, but I had a spare shunt regulator board. Just to go over the top I used the very low noise shunt regulator.

The result

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The picture above shows

Top left: 15v toroidal transformer
Bottom left: 9v toroid for 5v regulator
Left: 5v regulator on perforated prototype board to feed volume control
Bottom: LCD and volume control (on front panel)
Middle – blue circuit: Shunt regulator (+/- 15v)
Top middle: relay board for input selection and R2R volume control
Right middle: Pedja Rogic buffers through Sonicap capacitors to output.

Overall system:

Source: Noodle Dac (Spdif / toslink in, Digital / Analog converters: CS4397 24bit 192khz DAC Digital receiver : Crystal CS8416 – DIY)
Amp: Baby Huey, Push Pull EL84 valve amplifier (DIY)
Speakers: KEF Q5

 

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