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Wednesday, June 1, 2011

The Octal Preamplifier Mk2: Assembly

Hi!

As promised in the last post with the circuit, here some photos of the assembly. Let's start with a picture of the finished preamp:


Construction starts with the predrilled and engraved plate which will carry all parts:


Mounting of sockets and connectors:


Capacitors attached to the plate as well as input selector. Some initial wiring, heaters and input signals to the selector switch:


More wiring, adding of ground bus and some resistors:


Transformer volume controls added and wired. Also the first plate choke. Since I didn't have the intended Lundahl chokes at hand, I used 3 small chokes in series which yields the same inductance. MC step up transformers are also mounted already:


Everything completed and tested. Due to the use of 3 small chokes for each plate load, things got quite crowded. RIAA circuit is underneth the chokes:


Preamp inserted into the wooden frame, it just fits in:


Finished preamp, PSU is in an external chassis, not shown:




After some burn in, the preamp opened up a lot. In fact it is remarkably better compared to the Mk1 version. Especially the phono section, but also with line level inputs it is improved. The 6SC7 proved to be perfect for the phono input. It is very quiet. Also the 6AH4 turned out to be an excellent performer. I like it much more than the 6SN7. The preamp sounds very detailed, yet musical. I'm especially critical about the articulation of s-sounds in the phono section. It performs very well there, approaching the level of a LCR phono. Of course a well executed LCR phono will be better yet, but the result is truely excellent for a RC RIAA. It turned out to be better than I expected.

Best regards

Thomas

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