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The non passive pre becomes interesting

Posted 3rd August 2013 at 09:03 AM by hbc

Lots going on, my making things mojo has been busy. I have been reading 6 moons, and was interested by the idea of the directly heated triode preamp which sounds quite well apparently, and also repelled by the thought of using a huge output tube, slightly. So after some thought, I realised that the old battery radios used the directly heated ones, so a few weeks later some 3A5s were winging them selves to me.

Oh, (sigh) I had spent some time tweaking it first (the not passive pre) the most exciting feature was that the copied guru circuit pulled the output cathode to the HT rail when the heater is turned off! (I have big reservoir caps and small bleed). To be fair guru circuit is direct coupled so things are all over the place, but Anyway.

I decided to graft in the 3A5 in to this circuit, which was simple in principle, but ended up with a complete redesign of the power supply several times, as the addition of that one little tube hugely increased the heat generated in the little transformers for some reason I don't understand. So I now use a 30VA toroid, with a small boost converter for HT, and a 5 volt switcher for the DHT heater circuit, and AC heaters.

The reason I have persisted with these rebuilds is that it is making very nice noises. That also maybe because I am halfway through upgrading the anonymous IB 2ways mentioned earlier with some ring radiators, but first I must build my integrated not-Walin jig.

The Walin jig is marvelous in principle, but still a chaotic pile of wires and boxes, i.e. Walin jig, mic preamp usb sound card, amplifier multiple wires, wall wart. So I am going to integrate all in to one box with a diagnostic meter and also enough IO to test amplifiers.

Then I will be able to test to see what I am doing more succinctly, oh to dream.

I have had the good fortune to acquire a pair of original Maxims (very nice actually), and some Wharfdale linton XP2, need to measure them to find out why they are not as good as I had imagined. I think the bass resonance is to high, but then my amp may have to high an output Z??? So many things to think about.

Guru pre circuit:

Allen Wright - The Tube Preamp Cookbook [English]
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