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Sunday, May 29, 2011

The Octal Preamplifier Mk2: Circuit


In the first post about my Octal Preamp I already mentioned that I have an improved circuit in mind. In the meantime the circuit has been worked out and is already tested. The main reason for a change was the gain in the line section which is a bit high in the Mk1 for the ususal line sources. So a different tube was chosen instead of the 6SN7. The Mk2 uses a 6AH4 there. The phono section is completely reworked, going to LC coupling rather than RC coupling.

Here is the schematic of the Octal Preamplifier Mk2:

Quite a radical change, with lot's of iron! Since the line tube was changed to a less common tube, why not also change the phono input tube to something less often seen in audio? So I picked the 6SC7 instead of the 6SL7. Still all octal tubes, so the name of the preamp can remain.

Let's start with the phono section. Why go to LC-coupling? The 6AH4 line section would require a lower B+ compared to the 6SN7. This would offer too little headroom for the Phono section to operate with good overhead and reasonable operating points, so LC is the way to go. Lundahl has some nice plate chokes, the LL1667 which offers very high inductance. When chosen with a 5mA gap, it offers about 800  Hy of inductance. Enough for the rather high rp of the phono tubes. Even enough to skip the cathode bypass capacitors. Still the caps are drawn in in grey as an option. My first version of the preamp left them out. Works nicely. To keep the influence of the missing bypass caps low, rather lowish cathode resistors are chosen. The RIAA circuit is the same as in the MK1, split RIAA, but the component values needed some adaption. The second phono tube is still a 6N7. The triode section of both 6SC7 and 6N7 are paralleled.

The separate chokes in the B+ lines for the two phonosection are a bit overkill. I just threw them in for the fun of it and since I had some small suitable chokes available. They can be replaced with some resistors as well, about 5k to the 6N7 and 10k to the 6SC7 will do as well. The first choke should be kept for a low ripple B+ and decoupling per channel.

The preamp is playing right now and sounds very beautiful. Good resolution, musical flow, colors in tone. All is there. Not at the level as the LCR  phono which I showed in an earlier post but that was expected. I equipped it with a built in MC step up transformer, the Lundahl LL1933 wired 1:16.

The line section uses the Lundahl LL1692A which fits better to the 6AH4. Volume control is a AVC from Dave Slagle as in the Mk1 version.

The preamp is quite a bit more elaborated as the Mk1, so it won't replace it but is rather a higher quality option of the Octal Preamplifier series.

I will post another article with some photos of the building process in an upcoming post. Stay tuned!

Best regards


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