Hot on the heels of last week’s article (just turn over the page) is WOT Preamplifier by Diego Nardi, also from sound practices issue 10. In it, Diego first discusses the (non)sense of using a line amp, then requirements, topology and tube selection. Then he remains totally vague about the output transformers (boo!), followed by the circuit description. Next is a substantial section on the power supply and heater supply. After some construction tips there is a impression of the resulting pre amp. Diego finishes with throwing in a free phono amp schematic (don’t knock it, free is not worthless).
teaser quote: ‘When someone asks me to build something, especially if it’s apparently simple […], and the budget is not too strict I think there is really no point in giving him a product that may be similar to anything commercially available. A custom product must be real special and, from my standpoint, this is how this preamp had to be—special.’
my take
In its own convoluted way, Diego’s argument for when to use a line amp and when not is very sane. What I would like to add is that when you look at the requirements for a line amp then you see that they are also true for any source (CD, tuner, tape deck): they are also driving a cable into a next amplifier. For the ultra-fi constructor it is no problem to ensure that each source is up to scratch in this regard: rip out the weak old stuff and build a line amp with just the right amount of gain into each.
Once that is done you are free to either follow Diego’s rule about line amp versus integrated amp, or go for the ‘modern’ approach that came back into vogue after 1995: use a passive magnetic volume control which, by virtue of being an (auto)transformer, forms an efficient power coupling between your (now) husky sources and your amp(s).
I must admit that when I read this article for the first time 15 years ago the ‘put out power to grip the input of our power amps’ argument went right past me. As you could read last week and today, I have learned a thing or two in the meantime. It is good to see also here the AC efficiency of transformer coupling highlighted and ‘there’s really no comparison.’
Then there is the power supply. Diego tries to sell us his ‘acting as batteries’ solution. If you look at these as reservoirs that simply must maintain their B+ voltage, then these monster 1500µF caps are probably quite successful. But as I pointed out last week: every bit of audio that comes out of this line amp has made a full trip through these caps. And in that regard these vats of electrolytic chemicals are not exactly the coupling caps of your dreams.
And Diego confirms that he can hear these caps because bypassing them lead to ‘horrible high-frequency colorations.’ If I would work with the ‘acting as batteries’ approach, then I would keep the audio signal off them by following them by another LC group, where the C is sized to work out with the tube and inductance of the OPT and the L is sized to form a 5Hz corner with the C.
The heater supply still puzzles me. Diego complains about having one positive and one negative heater to kathode voltage. But with the same ‘filter time constant and regulator operation considerations’, even the same power transformer winding, he could have take 2 diodes for a full wave rectifier with double the current capability, build two positive versions of these regulators, grounded the centre tap of the heater and run it.
pop quiz
Diego says: ‘no 2A3 preamps,’ among others because of ‘excessive input capacitance.’ I thought I’d look up how bad that would be, the 2A3 compared to 5687. First we must take the Miller effect into account, then calculate:
- for the 5687, with an amplification of 18 in this line amp, Miler capacitance is (18 + 1) * 4pF = 76pF, plus 4pF grid–to–kathode is 80pF input capacitance in total.
- for the 2A3, with an amplification of 4.2, Miler capacitance is (4.2 + 1) * 16.5pF = 85.8pF, plus 7.5pF grid–to–kathode is 93.3pF input capacitance in total.
That is 17 percent extra for the 2A3. Now in audio everything works logarithmically and double the input capacitance would have impressed me, but 17% is just peanuts. Maybe the guys were right last week, who tried 2A3s and other power DHTs ‘with fine results’?
Surprisingly, this is it—as far as I am concerned—when it comes to line amps in sound practices. We will cover phono amps another time. From next week we will look at a a more holistic topic: complete audio systems.
Now go and read the article, see you next week.
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