This guide is to help you build the DOUK KSL-M77 Preamp clone kit.

[Kanders1]

The mains filter, I bought a filter/fuse and mains socket combined to replace the cheap supplied one and to keep thing neat. It is very slightly larger so a small modification with a flat file is required.
BULGIN IEC EMI FUSED INLET FILTER PS20/A0320 250V AC 3A RATING POWER ENTRY

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The additional rectifier/regulator Board. (To replace the original on board heater DC supply, this is the primary cause of the HUM)
Supplied by - Jim’s audio (E-bay) LT1963A very low noise 1.2V-20V 1.5A - 6A power supply PCB w/soft starting! This comes as just a board; you will have to build it. Its not hard and you only need one side of the board to supply the clean DC for the valve heaters. The board uses SMD components so a pointed soldering iron, some tweezers and a steady hand is needed. (YouTube training if you have never done it plus you can practice on the spare side of the board. As some of the components come in qty of 10 only).

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I used a Texas Instruments TPS7A4501KTTR IC, REG LDO ADJ 1.5A 5DDPAK regulator chip but the LT1963AEQPBF (PBF - lead free) is available at RS Components. The rest of the items I got from E-bay and Mouser (postage is expensive). Jim’s Audio supplies a parts list.
Note: - The 100uf 50v 1812 SMD cap (C25 C26) is not easy to get. The voltage does not exceed 12v at this point so a 100uf 16v can be used (mouser have AVX 100uf 16v 1812 size) I didn’t use the 2 pin terminal blocks, easier to solder directly onto the board. Also you will need a heat sink for this. I modified a computer processor one to fit but they are easy to find on the web.


Volume control, I used a China 24 step attenuator to start as I wanted the stepped volume control. As it happens it seems very good for the money! My intention was to upgrade it but can’t really fault it. The only issue is fitting it in! They are big fortunately the China copy does not have its resistors soldered the same way as the real thing so you can bend them out away from the valves located in that area. In hindsight the ‘ALPS RK27112 Audio Potentiometer 100k stereo’ would be best for packaging. (Beware of China copies! Info on the web how to tell the difference)

Audio connections and wire, I used silver Phono sockets and RS wire to connect the inputs/outputs. I started with screened wire but found it not so easy to work with around the multiple signal in area of the board and also I think it may also have given me some grounding issues. So twisted single core wire for all the in’s and out’s.

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Building the amp.
I put mine together as is from DOUK using the finished board. Most of the connections are oblivious. And labelled on the board. The AC connection to the board are 6.3v 250v-0-250v 12.6v I soldered them from below purely for neatness. They are labelled on the DOUK transformer but mine had yellow as 250v - 0v but actually was orange.

The new mains filter socket has the safety capacitor already fitted, it is slightly larger than the supplied socked hole but a file is all that’s need to make it fit.

I drilled an earth point for a nice solid ground point to the chassis next to this socket and connected the earth to it and then the earth wire from the transformer. And also an earth for a phono connection, I drilled mine in the middle but its better located near the divide as there is more room for the earth wire to the board and the banana plug socket. (The board earth is located in the centre)

The Transformer is mounted on 10mm acrylic spacers to lift it from the chassis (this was a recommendation from DOUK when I queried the hum from the finished stock unit, not sure if it really made any difference TBH). The transformer is still connected to the chassis via nuts bolts and spring washers. I had to drill new holes for the Transformer fixings.

The main board, its important to get this in the right place if you use the same box as I did as its a tight fit. There are pre drilled threaded holes but these are for the M7 board so no use. Also bear in mind where the chassis feet will go. There are pre drilled holes but even they interfere with some other threaded pre drilled holes.( I ended up moving the feet and using 3M double sided tape for a temporary fix whilst building).
Fit the volume control into the front panel along with the selector switch. The on/off switch was not a nice one supplied so I went for a good old heavy-duty toggle switch; this is a problem though… as the heat sink for the power transistor gets in the way of the switch. I fixed this by removing the end two legs from the heat sink and then slightly bending the next two outwards.