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In the Forum: Melquiades Amplifier
In the Thread: DHT driver & input
Post Subject: filament suppliesPosted by guy sergeant on: 2/1/2007
I made a simple SE amplifier using the 6B4G for a friend. Initially, it was ac heated and sounded fluid & lively with a very agreeable gentle quality particularly with strings and woodwind. But, however I arranged the ac supply & hum bucking methods I couldn't get it sufficiently quiet and did feel that there was some strange upperbass imprecision on almost all of the music I tried. I wasn't sure whether this aspect was due to the output transformers or some by product of the remaining hum.
I started by supplying DC via a simple bridge circuit using fast recovery diodes and capacitance which significantly reduced the noise but which also flattened the pallete of colours the amplifier seemed to have at its disposal. Where it had been eager and colourful it became much less engaging. I experimented with various regulators but still felt that this 'washed out' character persisted. It may or may not be because of the fact that the passive components used in heating the cathode are in the signal path.
I read about the Tentlabs boards and liked the idea of using a high impedance supply which the signal might not 'see' When I fitted them the life, colour and spirit I had enjoyed with the ac heating returned (but without the hum) The output transformers, in this application, were fine.
I'd been meaning to try an all DHT amplifier for a while but had put it off because I hadn't been sure how to make it quiet without compromising what it might do well. For me, these boards work. There are other people in the UK thinking along similar lines (but different approach) to Tentlabs, and I've just received some 7.5v boards to use in a 10Y linestage I've been working on. It will be interesting to see if their solution has the same effect.Rerurn to Romy the Cat's Site