sound practices reading club /21

After a small hiatus, we kick off the line amps topic. Bruce Berman has the honour, in sound practices issue 13: My Type 76 Line Preamplifier. In it Bruce discusses in detail many aspects of constructing this line amp: tube/topology/schematic selection; part selection; grounding; power supply; chassis construction; tube rolling.

teaser quote: ‘The design of this line stage preamplifier began with the realization that the overwhelming majority of recorded music, be it LP or digital, is very mediocre in terms of technical quality, and while this is certainly no revelation to most of us, I needed a system which would play most of my records all of the way through. Having a harsh, unnatural-sounding system that will only play Cheskys, Audioquests, Living stereos, and other audiophile-grade recordings to our satisfaction, is foolish and deprives us of the emotions to be realized from the tremendous quantity of music that exists.’ Yes, those were really only two full sentences.

my take

After last week’s brief but deep one-pager, we now have a christmas turkey-sized article that goes deep in its own way.

One thing I like about this article is how Bruce is not trying to sell this line amp to us as a universal solutions to everybody’s problem. Instead—while pointing out that it sets high standards in objective terms—he stresses the system context in which this line amp performs well and for and in which it was optimised.

Meanwhile Bruce spends a lot of time sweating the details and yes, some of it is beautiful to behold. The overall result is certainly very classy, but in my world view there are bigger fish to fry:

• parts quality has ten times more potential than construction quality;
• component values have ten times more potential than parts quality;
• circuit architecture has ten times more potential than component values;
• caveat: a catastrophe, no matter how low the potential of the level, can sabotage everything.

Judging from the article, Bruce spent most of his time and effort on the low-potential stuff. Only occasionally are component values discussed, but that is where it stops.

Component values is exactly where I would start, if I were to work with this basic schematic. Because it is the summer 2010 instead of 1997, I would after strategic decisions about R1, R2 and R3 in the line amp go straight to simulation to see the effects of C1 and C2 then iterate them in tandem to the lowest values needed to get enough bass out, with low phase shift.

C1 in the line amp is of course also part of the power supply, so next would be a trip through PSUD. That cascade of same/similar inductance and capacitance values—OK, it doubles/halves through paralleling in the final stage inside the line amp—looks like a ring fest waiting to happen. So stepping the current draw in PSUD and checking for any voltage wobbles is what needs to be done.

But that is, if I were to work with this basic schematic. We get to that part next time.

Now go and read the article, see you next week.