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: