Hi-Fi: Leak Stereo 20 Modifications
This involves essentially changing the biasing of the phase splitter stage to allow direct coupling from the primary stage, reducing gain and fitting the more linear ECC82 valve for the original ECC83 type. I also added the resistors to isolate the transformer windings from the output valve screens.
Take R6 and R11 (the only 3.3M resistors in the circuit) and join them together in the air, isolating them from R8 & R9. Now remove R9 (22k) and replace with a link, remove R8 (1k) and replace with one of 100k. This last one is between the valve socket and circuit board. Remove C3 (0.1u) and replace with a link. Add the 100R resistors to the transformer connections (on my unit, the white and purple wires should be removed from the transformer solder tags and the 100R resistors inserted in line). Replace V2R and V2L with ECC82 valves.
The above will give you a significant improvement for very little expense. The source for this design improvement is Hi-Fi World magazine. Other improvements, depending on how much you want to spend: 1) Replace the two electrolytic reservoir caps with RATA 32+32uF Ansar polypropylene types. The old caps will be good for nothing if they are original. The RATA units are physically much larger. You must use cap clips and drill two holes in the base plate to mount each one. This gives a useful improvement in dynamics and transparency and much greater bass impact. This is (in my opinion) the single best mod you can make after the above circuit change. 2) Replace the GZ34 rectifier valve with a RATA fast recovery solid state module (this plugs into the valve socket). This improves the focus of the sound stage. 3) Replace all the 47uF (or 50uF) 25V electrolytic capacitors. Use 47uF 50V Black Gates for the output valve bypass caps and Sanyo Os-Con 47uF 25V types for the other two. This cleans up the transparency a lot. 4) Replace all the remaining 0.1u (or 0.22u) caps with Kimber Kaps (expensive). This cleans up the transparency a little. The combination of all these mods gives you a valve amp with incredible transparency and detail, and power which defies its 20W per channel rating. It really is a tribute to the original circuit design, and especially the output transformers, before Leak went the way of transistors like so many other great British manufacturers.
3 October 2000