Custom Passive Pre Amp

The Passive pre amp that I chose to build is a very simple 3 input design with 1 output and no tape loops. Each of the left and right channel inputs are connected to a very high quality 2 Way 6 position Elma selector switch (3 are redundant) with gold plated contacts. The selected input is then fed to the Dale Vishay stepped attenuator which is connected to the output phono sockets.

I have used a brushed aluminium case to house the pre-amp which measures 100 x 60 x 130 mm (WxHxD) which was bought from Maplin Electronics for about £3. The custom built pre-amp uses 8 teflon insluated gold plated phono sockets which cost £2.20 each. For the internal wiring I used sharkwire pure silver cable interconnect cable which is available from Maplins as well priced £26 metre which is very expensive but 1 metre should be enough for this design. The Elma switch and the Vale Dishay Stepped Attenuator were bought from Hi-Fi Collective priced £40 and £45 plus VAT respectively.

Silver solder was used to solder the wires which are quite awkward given their size. I could have used another type as Hi Fi collective can also supply some HGC pure silver wire which is a lot easier to work with but I used the Sharkwire because I had some spare and it made sense to use it as my interconnect cable was made from the same cable.

Once everything was soldered in place, a multi meter was then used to check the resistance between the input channel and output channel which for a 10K stepped attenuator like the Dale Vishay I used should measure about 2.6Kohms, and the resistance between the ground and the input should be zero. Once I was happy that the wiring was correct, the lid was put on the amp and the pre amp was connected to my system consisting of a Quad 67 CD player, Quad 11L speakers and Quad 405-2 power amp.

Even though the Vishay Dale resistors need about 100 hours burn to reach their sonic optimum it was evident straight away that this pre-amp had lifted a veil off the sound that the previous Alps "Blue" based pre amp had masked. The sound was so much smoother and the treble was much brighter and detailed without ever been harsh - it was an incredible and very rewarding experience to hear such detail from the Quad equipment and well worth the outlay for a superior and transparent pre-amp. With the new pre amp in place every note in the mix just seemed to last longer with the decay in the studio clearly audible and instruments taking on a more natural coherent soundstage with a top end that I almost thought impossible from a solid state amp.

I could write more but the only way to do a pre amp like this justice is to listen to or build one yourself. More so than ever now I am convinced a passive pre amp is better solution sonically than a well designed active one.

This entry was posted on Saturday January 6th, 2007 at 11:34 AM and is filed under Amplifiers. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response.

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