Line Stage Prototype
Many modern components are designed for PC mounting and they make a simpler project to build compared to chassis mounting and point-to-point wiring everything. However I have found that it is a good idea to build a prototype of a circuit on a perforated board before etching the printed circuit board. You can easily mount PC components and instead of land patterns, you have the flexibility of point-to-point wiring that is easy to change if needed. I think of them as a small chassis. I have built a lot of prototype digital circuits over the years so I am comfortable using 30-gauge wire to interconnect the components, and some folks assert that the smaller the gauge wire, the better from a quality of sound viewpoint. I guess that I could have put everything on a single board but for flexibility of packaging I chose instead to make a separate board for each channel. They are identical except for which half of the tube is used on each. This provides the ability to get a new tube when the tubes are swapped between boards. Both the LM317 and the DN2540 are TO220 packages that need to be mounted on a heat sink. I am conservative when it comes to heat sinks and the large ones that I used get only slightly warm to the touch. These heat sinks and the transformer are both are about 1.7 inches long, which determined the width of the board. The length of the board will depend somewhat on the capacitor you pick for the parafeed. Some high quality film and foil capacitors are three or four inches long, although most are no wider than 1.7 inches. The length of the board will need to be about 5 inches in addition to what is needed for the capacitor. I had some old Wonder InfiniCaps that were the right size in my stash of goodies that I used for the parafeed capacitor. My board came to be 1.7 inches wide by 6.7 inches long. Photo 1 and Photo 2 show the assembly. I am pleased with the resultant package, a compact module that can be easily mounted in a variety of mechanical enclosures.
As luck would have it, the next meeting of the Piedmont
Audio Society was held the Sunday after I had finished packaging the completed
prototype boards. I wanted an
independent assessment from a fresh set of ears so I took the line stage along.
The meeting was intended to audition new power amplifiers that some members had
built but there were couple of new passive line stages there that we listened to
first. Then came time to listen to my line stage. Well, how can I say it? The
guys were kind in their lukewarm reception of the line stage. I was disappointed
but they were right in pointing out a couple of deficiencies that I had
introduced in building the latest version. I had not done sufficient listening
of my own and let them slip by. First was a brittleness or hardness in piano
crescendos which was definitely there, something I had not heard before I
packaged the boards. Second was a buzz, or harsh hum under some conditions. It
showed up only when the variable power supply was set at its maximum value of
250V and the volume control was set at its minimum value. With the volume set to
a normal listening level there was no audible buzz. While reflecting on what I
had changed in these last few steps, I realized that I had done all my earlier
listening with a transformer ratio of 4:1 and then arbitrarily wired it up 8:1
for the new board because I did not need the gain. So I experimented with
listening to the sound of the different ratios and found that to me the 2:1
ratio sounded best. The lower ratio cured the hardness in the piano crescendos.
The cause of the buzz was harder to find. At first I thought that I had a ground
problem, because even though I had been careful about a single-point star-ground
system, one never knows when it comes to grounds. I tried all the tricks in my
book and nothing worked. In talking with Kevin about the problem, he mentioned that he
had seen hums and buzzes caused by oscillations. I hadnít thought about
oscillations because I had a gate stopper resistor on the DN2540 and a similar
grid stopper resistor on the 5687 as well as ferrite beads and a capacitor on
its filament. I thought I had it covered but I was grasping at straws and ready
to consider anything. I changed my tactics and started to get results. I found
that when I hung my scope probe on the drain of the MOSFET the buzz stopped.
However when I connected the scope to the output of the power supply in the
separate power supply chassis, there was no effect. The two points were
electrically the same; the only thing separating them was a few feet of wire.
Okay, this would be easy Ė simply substitute a capacitor for the scope probe.
Nope. It wasnít giving up that easily. Next I tried a ferrite bead on the
high-voltage wire without success. But I did notice something interesting. I had
to move the high-voltage wire to install the bead and now the nature of the buzz
had changed. I found that I could control the loudness of the buzz by moving the
wire. I kicked myself Ė I had seen this before. In contrast to the present
good-looking assembly, the original test beds were a ratís nest of wires. Now
everything was neat, with wires laid down and dressed. I hadnít paid attention
to what I was doing and had inputs neatly running together with both outputs and
power supply wires for several inches. It was the distributed capacitance
between the wires that got me. I re-wired the signals and high voltage wires
with shielded wires, taking care to keep everything separated. This not only
fixed the buzz, but also made a further improvement in the overall character of
the sound. High-level transients in the music had driven the circuit into
momentary oscillation. This was the hardness in the sound that we heard. While
changing the transformer ratio softened the problem, it wasnít the fix. I went
back and listened to the different ratios and found that although they sounded
different, there was not a problem with any of them. Difference now was a matter
of taste. The 2:1 ratio was fuller, more voluminous, and a little softer. The
8:1 ratio had firmer and more bass and the sound was generally more tightly
controlled. The 4:1 ratio was in between.
Happy with the design, I proceeded to layout and etch the printed circuit board. I reduced the size of the heat sinks to be more in line with what is needed and added some holes for mounting the board as well as tie points for cable shields if needed. I wrestled with what to do about the parafeed capacitor. It took up a large portion of the board, and I wanted to make provisions for a variety of different capacitors. My solution was to provide maximum flexibility by moving the capacitor off-board, allowing for connecting it to tie points on the edge of the board. The final board is shown in Photo 3.