Woodworking can test my tolerance

I spent 3+ hours obsessing yesterday over mere hundredths of an inch.  My dovetailing jig is cool, but when few hundredth's of an inch make a significant difference in the tightness of a joint, I think Rockler could have done a better job of making their jig more precise.

The general idea is that once you have the jig set up for a given width and thickness of blank, you should be able to clamp a new set of blanks in, route away and Bingo... perfect joint.  One problem is that the fence that sets the depth of the cut is attached to the upper clamp, but the clamp has a substantial amount of play.  So the fence can shift up to about 0.05" between clampings.  It took me a while to figure out that even though two passes had identical adjustments, things were shifting around every time I reclamped.  Ok, so I'll remeasure and readjust the fence depth every time.

The next problem is alignment/squaring of the blanks.  The main magic of a dovetailing jig like this is that you route both blanks at the same time.  There's a little alignment stop which is supposed to keep the two blanks aligned relative to each other and the jig.  My first beef is that it is free to float way out of square to the jig.  Ok, I'll square it each time I adjust it (3 screws).  But wait, it's made of fricken no-so-hard plastic and it flexes!  The upper part of the stop has 2 screws so squaring it pretty much keeps the upper blank square.  The lower portion has only one screw.  Trying to square it is like trying to benchpress with one arm.  Without a second tie point the stop rotates around the single screw and flexes based on the position of the upper.  That would be just fine if it was metal which doesn't flex, but as is, the lower blank is almost never square if you simply rely on the stop to keep it square.  In the end what should take about 1 minute of set up per joint takes about 10 and still may result in a bad (loose, tight or misaligned) joint.

To make up for looseness of tightness, the height of the router bit can be adjusted.  I found that the difference between loose and tight could be as little as 0.015".  You girls take note.  Yeah right, like any girls would read this blog.

Anyway, enough bitchin'.  After about 16 test joints and wasting about 10+ inches of length of my good blanks, I made the final cuts and I'm pretty satisfied with results.  It holds together without glue and only one of the 4 joints needs some filing to get the pins and tails to align.

The next step is to saw a kerf (slot) just below the top of the frame for sliding in the aluminum panel.

Enclosure layout design and mockups

Enclosure layout design, despised by many builders, is actually something I've always enjoyed.  Probably because I started making enclosures for my projects with my Dad when I was about 6 years old.  From cardboard with holes cut with scissors to drilled aluminum.  Those projects usually had just a few switches and lights, buzzers and bells, but the basic concepts are the same.  The main work is drilling the holes, but planning where to put the holes is definitely an art.  It involves ergonomic evaluation to make the device usable, but part placement must consider the functional constraints of the innards.

In an amp, that generally means making sure that components which connect to each other are close to each other and things that could induce noise to each other are far from each other.  Since most components connect to multiple other components, various placement permutations result in the need to consider oodles (technical term) of tradeoffs simultaneously.  Unlike semiconductor block/module placement and routing (which I did professionally for 10 years), the amp layout problem is solvable in polynomial time.  In fact, the problem space is so relatively small that even the meager human brain can figure out a set of component placements which are nearly ideal.

For me, noise is the number one consideration.  Electromagnetic induction is used to our advantage in many components of an amp.  Power transformers, output transformers and chokes all work because of electromagnetic induction.  However, that phenomena is also the leading cause for noise in an amp.  Any conductor (wire, resistor, capacitor, transformer, etc.) is susceptible to induced current from another conductor. To be induced, that current must be oscillating and thus has a frequency which can be heard if it or it's harmonics are within the audible range.  Most hum heard in an amp powered in the US will be either 60Hz like the incoming AC line voltage, or 120Hz (2 x 60Hz).  Either is certainly audible and annoying.

It's near impossible to eliminate the source of electromagnetic induction, but we can shield sensitive components from it, or keep sensitive components far from problematic sources.  Fortunately, induction has an inverse square relationship to distance.  So a small bit of distance between components can have a dramatic effect in the amount of induced current.  Thanks Physics!

Since transformers are the #1 source of EMF in an amp, I feel it's most important to place those first starting with the power tranny.  In this amp, I decided centralize all the power components in the center rear of the amp (see below).  Then the audio portions of the system can exist "far away" on either side. I did allow one twisted pair carrying AC to run along the side, to the front to the power switch and lamp.  This was the single case where I let ergonomics and aesthetics trump performance.

My other layout goals were:

• Arrange all inductors (transformers and choke) for minimal electromagnetic interaction (eg. Orthogonally oriented cores, spaced as far as possible or on opposite sides of a grounded shield)
• Keep AC lines > 2cm from DC rails or signal lines/components (AC lines include power entry/fuse/switch/lamp and tube heater lines)
• Keep AC lines as short as possible (power switch and lamp exempted)
• Arrange signal path components so the path is roughly linear without recrossing (ie. start from front center, out to the power tube, back to the output tranny, then back to the speaker terminals).
• Employ a "Star" grounding scheme with the lug very close to the amp center and close to the power channel
• Optimize heat dissipation.  Keep heat generating components (transformers, power resistors and tubes) away from each other and away from heat sensitive capacitors.

I decided to use Google sketchup for doing my layout.  It's a trip constructing in 3-D and took me a few nights to become productive.  It's an awesome tool though and one of the coolest things is that there's oodles of models for most everything you can think of online.  I found switches, jacks, tubes, etc.  The things I was left to construct myself were the James output transformers, power supply caps, power transformer cover, and circuit boards.  Here's what I came up with for the top view:

The output tubes don't quite look like 6B4G's, but good enough for government work...

... and after lots of jockeying trade-offs, the underside view:

... now "eyeballing" with real (top and bottom) parts: