I think my interest in passive preamps started here
DIY passive preamp
I planned on building this preamp for months. During this time I read countless of reviews and heard a lot of opinions on passive preamps. Some people told me that passive preamp is the only one to consider. And then there were the other people who warned me about lack of dynamics and restricted frequency range. Finally I decided to build the preamp but I have to admit I was a bit scared was little bit scared since I had never heard a passive preamp and I wasn't sure that my setup was suitable for using a passive preamp. Fortunately the outcome was even better than I dared to expect. If I had to describe the sound with one word, it would be flawless. I had gotten tired with a dirty hifi sound but now the sound was so clean, as if somebody had cleaned the window and you were finally able to see what's really out there.
Pics Klick to see a bigger image
The component and material choices were eventually quite easy.
A passive preamp usually consists of a potentiometer or a stepped attenuator. For example the famous Creek passive preamp is based on Alps Blue potentiometer. Many manufacturers use Alps Blue in their active preamps and integrated amps. It costs only about 20 euros and it also comes with a motor which enables the use of remote control. The potentiometers are ok but quite lousy if you compare them with a good quality stepped attenuator.
There is a countless number of stepped attenuators out there. Despite this the choice was clear because the DACT CT2 was supposed to be the ultimate choice. The design of CT2 is different from all other stepped attenuators and apparently fo a reason.
Luckily DACT has a retailer here in Finland so I didn't have to order the attenuator from abroad Kruunuradio
Here is a few examples of stepped attenuators you can find in the internet:
DACT CT2 - Stepped Attenuator
Elmas switches are great. The quality is excellent and they are easy to find allmost anywhere.
I chose 2x6 switches that have 6 pairs of inputs. The switches can easily be modified to 2x2,3,4,5 switch.
They cost 25 euros each.
Choosing the sockets was also easy. A local store (Biltema) sells sockets that seem to be mechanically quite decent but they cost only 2.9 euros per pair so there has to be something wrong with them. The insulation is most likely something else than high quality teflon and gold plating must be quite thin. Who knows what metal the conductor is. The amount of sockets I needed was so high that I had to settle with these cheap sockets. Otherwise the preamp is non-compromising. If I had had a lot of money I would have bought WBT sockets that cost about 50 euros per pair.
Internal wiring design was a hard nut to crack. I thought about several different solutions, but luckily I got help from a Finnish tube amp guru Markus Hihnala. He runs a web store RPM Developments and is the man behind awesome GLIM tube amps.
The wiring is made of low capacitance coaxial cable with stranded silvered copper wires. The cable is only 2.54 mm thick.
The earthing is should be connected to one common point. As Markus Hihnala advised I used a silvered copper rod and connected it to the enclosure from one point. You can see a little bit similar earthing system for example in Classe amps.
The third wire in the picture below is a wire that I was supposed to use but it was too stiff and thick (in the middle).
The enclosure was quite difficult to build. First of all finding suitable aluminium is suprisingly difficult and secondly it is hard to cut without proper tools. The anidized aluminium that I used was quite expensive. I recommend buying normal aluminium and making the finishing afterwards.
As they tell in DACT internet site, the material of the enclosure should be made of un-magnetic material. Magnetic materials affect on components magnetic fields and therefore change them.
I cut the aluminium with a normal metal saw. The cuts were not straight so I spent a lot of time to file them straight.
The enclosure turned out great. The secret of making perfect conjunctions and accurate holes is that I always glue the two pieces together before I drill the holes. I use such glue that the pieces are easy to separate after drilling.
One of the most important things to consider was the seamless and non-compromising integration of multi-channel home theater setup and two channel setup. I had been thinking about this for about two years so the idea was chrystal clear. I simply had to add one extra switch, that makes the choice between preamps. I made two inputs for external preamps (AV receiver pre out ) and the switch switches between these two preamp inputs and stepped attenuator. The output of the switch is of cource connected to power amp. I believe the drawing below tells a lot more than my "explanation".
This is probably the most purist way to deal with multi-channel and two channel integration. In some integrated amplifiers and active preamps you can find a so called processor loop switch which allows you to adjust the volume level constant for one input. However the idea of leading the signal through two active preamp doesn't seem wise.