Chassis Construction

Pi-Amp Aluminum Chassis

Milling

Chassis Parts Description No. Supplier Stock No. USD ea. Total
Aluminum Plate 24" x 36" x 0.125 1 OnlineMetals 5052-H32 47.70 47.70
Aluminum Angle 3/4" x 3/4" x 0.125" x 8' 1 OnlineMetals 6061-T6 6.88 6.88
Aluminum Channel 3" x 1" x 0.125" x 1' 1 OnlineMetals 6063-T52 4.24 4.24
Machine Screws/Nuts/Washers #6-32 Round and Counter Sink Head 50 NA NA 0.10 5.00
Total NA NA NA NA NA 63.82
  1. The aluminum used to build the chassis should be cut on a table saw with a non-ferrous blade. A non-ferrous blade is designed to cut metals that do not contain iron. The teeth are carbide set with zero degree rake. The kerf will be slightly narrower than the wood cutting blade (3/32" as opposed to 1/8") so you will need to keep this in mind when setting the fence if you use a fixed scale.
  2. Before cutting, lubricate the blade teeth using a candle stick. Touch the end of the candle stick to the teeth with the saw running. Always lubricate the blade between cuts. Cut with the blade set at minimum height to reduce vibration. The teeth should just break the surface as the cut is being made.

    Pi-Amp Table Saw Blade Cutting Aluminum
  3. Cuts must be made with a slow feed rate. The first cut may be a little un-nerving but cutting aluminum on a table saw is a smooth operation.
  4. Mill the following pieces from 1/8" thick aluminum plate -
    • Base, 1 piece, 17-7/8" x 15-5/8"
    • Rear Panel, 1 piece, 19-1/4" x 8-1/8"
    • Rectifier Post, 1 piece, 5" x 2"
    • Toroid Mount, 1 piece, 4-1/2" x 5 7/8"
    • Amplifier Fin, 2 pieces, 5-7/8" x 6-1/2"
    • Controls Panel, 1 piece, 5 3/4" x 4 3/4"
    • Drive Head Bridge, 1 piece, 6" x 5-3/4"
    • Drive Head Side, 2 pieces, 6" x 6-1/2"
    • Ground Clip, 1 piece, 2" x 1-1/2"
  5. Mill the following mount brackets from 3/4" x 3/4" x 1/8" angle -
    • Rear Panel, 1 piece, 17-7/8"
    • Amplifier Fin, 2 pieces, 5-7/8"
    • Controls Panel, 1 piece, 5-3/4"
    • Toroid Mount, 1 piece, 4-1/2"
    • Rectifier Post, 1 piece, 2"
    • Zobel Bridge, 1 piece, 2"
    • Drive Head Mount, 2 pieces, 3/4"
    • Reservoir Capacitor, 2 pieces, 3"
    • Drive Head Bridge, 2 pieces, 5-1/4"
  6. Make the Hard Disk Drive (HDD) mount brackets from 3" x 1" x 1/8" channel. Use the table saw to cut off one of the flanges to give one piece of angle 1" x 2-1/2" x 1/8". After trimming off one flange, cut the brackets to length as follows -
    • HDD mount brackets, 2 pieces, 4-1/2"
  7. Smooth all edges and corners of the pieces cut in steps 4 - 6 with a flat file or sanding block.
  8. Cut slots in the amplifier fins and toroid mount as shown below. Begin by drilling a hole at the base of the slot. Finish by making two straight cuts with the jig saw. Smooth the edges with sand paper or small flat file. Particular attention should be given to these slots because wires from from the amplifier main circuit boards to the power transistors will pass through these slots. Why slots instead of holes? Easy installation and removal of the power amps.

    Pi-Amp Toroid Transformer Bracket
  9. To facilitate drilling holes in the angle brackets, fashion a support jig using a 24" long scrap of wood as illustrated below. The slot is cut, 1/2" deep, using the the table saw. Place the angle in the jig, measure, mark and drill.

    Pi-Amp Aluminum Angle Drilling Support
  10. Drill 5/32" holes in all mounting brackets as shown below. The view is looking at the apex. Unless specified, the holes are 3/4" between adjacent holes and ends of the brackets. Use a scratch awl to make a starting mark each hole before drilling.
    • Take a 1/2" bit in hand and trim the burr on each hole left behind by the exit of the 5/32" bit.

    Pi-Amp Aluminum Angle Hole Layout
  11. Drill 7/32" holes in the sides of the two HDD mounting brackets as shown below. All 3.5" form factor HDD have a standard side mounting hole spacing and use #6-32 machine screws. The 7/32" holes allow mounting tolerance. Note that there is a right and left bracket.
    • The right hand bracket, as viewed looking at the front of the drive head, is the top bracket shown in the illustration below.
    • The HDD mounting brackets function as heat sinks for two hard drives.
    • The HDD's are shown in silhouette by the dashed green lines. The end connectors face toward the rear of the drive head.
    • Detailed 3.5" HDD layout dimensions can be found here (mm). Note: Limit screw penetration into hard drive case to 3/16".

    Pi-Amp Hard Drive Mounting Hole Layout Schematic Side Elevation
  12. Drill 5/32" mounting holes in the top of the HDD mounting brackets as shown below -

    Pi-Amp Hard Drive Mounting Hole Layout Schematic Top Elevation
  13. Drill 5/32" holes in the Amplifier Fins for the printed circuit board (PCB) stand offs (inner set of holes), and heat sinks (outer set of holes) as shown below. A template for holes can be found here. Measure the template to ensure that the scale has not been altered. The four holes for the PCB stand offs, are counter sunk to allow the heat sink to mount flush against the face of the Amplifier Fin. Note that the fins are left and right after counter sinking the holes. The holes for mounting the Amplifier fins to the Base will be matched drilled later.
    • The term "match drill" means align one piece to another piece and mark an outline of existing holes on to the other piece, center punch the outline and then drill the hole. This will assure good alignment.

    Pi-Amp Amp Fin Hole Layout Schematic
  14. Drill holes for the control hardware in the controls panel as shown below. Start each hole with a 1/8" pilot hole. This layout is suggested but not mandatory. If you are considering adding additional controls refer to the drive head assembly to for clearance restrictions between it and the controls panel. The hole sizes below are based on hardware called out in the Amplifier section. If you are using different hardware, check the mounting hole sizes. If unsure of exact hardware, drill only the pilot holes.

    Pi-Amp Front Panel Hole Layout Schemtic
  15. Drilling and cutting access holes on the rear panel is done in two stages. Stage one, holes for amplifier power, speaker binding posts, line-in RCA plugs and Zobel bridge, are fixed dimensions and should be done first. Stage two, the cut outs for the main board I/O and card bracket, and ATX power supply, are determined by the fitted elevation of the main board tray and should be cut later. After the chassis is assembled, the main board tray and power supply will be test fitted so that the exact elevations can be marked and final dimensions applied. The cut out dimensions for micro ATX main board and ATX power supply are standardized, however the elevation and separation between them will be determined by the shape of the salvaged main board tray.
  16. The diagram below shows the hole layout for eight speaker binding post, two RCA jacks, Zobel bridge bracket and IEC C14 socket. The RCA jacks are mounted to a rectangular Plexiglas bushing and the bushing is mounted to the inside of the rear panel using the two 5/32" jack bushing holes. The RCA jacks protrude through the 1/2" holes but do not touch the panel. This ensures that signal ground is isolated from chassis (earth) ground to prevent hum. You may elect to use RCA jacks that come mounted on an insulated bushing, however the hole spacing will be different and the jack quality may be poor.
    • The small square cut out, in the upper left, is for the IEC C14 socket that connects 110V to the amplifier power supply. The cut is made by marking the cut out, drilling two starter holes in opposite corners and cutting the remainder with a jig saw. Use a small flat file to clean up the cut out. Fit the C14 socket into the cut out and match drill two 5/32" mounting holes.
    • The blue outlines show the approximate location of the ATX power supply (on the left) and the main board I/O and card bracket. These will be cut later.

    Pi-Amp Rear Panel Hole Layout Schemtic
    Pi-Amp Rear Panel View

Assembly

Basic Chassis
  1. Assemble the chassis by attaching the Rear Panel, Amplifier Fins and Controls Panel to the base. The two illustrations below show the chassis in elevation and plan view. The mount brackets are shown in blue. All hole sizes are 5/32" and some holes through the Base are counter sunk on the under side to permit the chassis to slide into the case without interference. All screws are #6-32 x 1/2" machine screws.
  2. Begin by aligning the rear panel bracket to the under side of the chassis base, flush with the rear edge. Hold the bracket in place with a couple of C-clamps and match drill. Counter sink each hole from the top to accept counter sink heads. These screws are counter sunk to prevent interference with the main board tray.
  3. Attach the bracket to the chassis base with counter sink head machine screws.
  4. Align the rear panel to the rear panel bracket. The bottom of the rear panel must be flush with the bottom edge of the bracket and the edges spaced 5/8" from the edges of the chassis base. Clamp and match drill. No counter sink is required for these holes.
  5. Attach the rear panel to the bracket with pan head machine screws.
  6. Scribe three lines on the chassis base to position the amplifier fins and controls panel bracket. Position each bracket on the scribe lines. The fin brackets must be flush with the outside edges of the Base and the Controls Panel bracket must be dead center. This will ensure a 3/16" gap between the inside edges of the Amplifier Fins and the adjacent edges of the Controls Panel. Match drill the brackets to the chassis base and counter sink each hole from the bottom.
  7. Match drill the respective brackets to the Amplifier Fins and Controls Panel. Ensure that the four counter sinks on each amplifier fin face toward the front of the chassis. Attach the fins and panel to the chassis with pan and counter sink machine screws.

    Pi-Amp Aluminum Chassis Side and Top Elevation Schematic
  8. Test fit the chassis to the case. It should slide into the case from the rear with no interference. The front edge of the chassis base should show a 1/8" reveal along the case bottom rail. The chassis rear panel should fit flush with the back of the case with a 1/8" reveal along the case sides and top.
Drive Head
  1. The function of the drive head is to mount two HDD's and one optical drive, serve as a heat sink for the drives and provide easy access to the drives. The optical drive defines the width of the drive head so care must be taken during assembly to ensure that the optical drive fits snugly in the slot. The width of the slot is fixed by the width of the drive head bridge. All 5.25" optical drives have an enclosure width of 5-3/4", the same width as the drive head bridge. Sufficient adjustment can be made through the drive head brackets to ensure that optical drive fits properly.

    Pi-Amp Drive Head Multiple Views
  2. The drive head is shown below in front and side elevation. The Drive Head Bridge and HDD Mount Brackets are shown in blue, the drives are shown in green, and the controls panel is shown in red. The clearance between the the Controls Panel and the HDD's is 3/4". The controls in the middle and top portion of the controls panel should not extend (wiring included) beyond about 1/2". The Drive Head is fixed to the Controls Panel by the two 3/4" x 3/4" Drive Head Mount brackets (to the left and right of the HDD) and two self taping sheet metal screws. The drive head is removed by removing the two sheet metal screws and lifting straight up. The bottom rear corners of the drive head sides are angled to allow passage of wiring. Note that mounting holes are shown on the Drive Head Mount brackets in the front elevation view. This is for illustration only. These holes will be match drilled to the controls panel after the drive head is assembled and fitted in place.

    Pi-Amp Drive Head Front Elevation Schematic
    Pi-Amp Drive Head Side Exterior Elevation Schematic
  3. Prepare the drive head sides by making an angle cut across one corner of each using a miter gauge and table saw. Clean up the cuts with a flat file.
  4. Drill the four large HDD mounting screw access holes. Position the Drive Head Bridge brackets 1 3/4" from the top and 3/4" from the front and match drill to the sides. Position the Drive Head Mount brackets 3/4" from the front and match drill to the sides. Attach the Drive Head Mount brackets only to the sides.
    • Check to ensure that the angle cuts are on the correct corners.
    • Build one as left hand and one as right hand.

    Pi-Amp Drive Head Side Interior Elevation Schematic
  5. Prepare the Drive Head Bridge by match drilling the Drive Head Bridge and HDD Mount brackets as shown in the illustration below. Notice that the width of the drive bridge (5-3/4") is equal to twice the thickness of the HDD brackets (1/4") plus twice the width of the bridge brackets (1-1/2") plus the width of the HDD (4"). With this fact in mind, begin by match drilling the first bridge bracket followed by the adjacent HDD bracket. Make a 4" wide spacer from a scrap of wood and use it to position and match drill the next HDD bracket. Position and match drill the other bridge bracket. Counter sink the eight bracket holes.
  6. Attach the four brackets to the Drive Head bridge using #6-32 x 1/2" counter sink machine screws.

    Pi-Amp Drive Head Side Bottom Elevation Schematic
  7. Join the Drive Head Bridge to the sides using #6-32 x 1/2" machine screws. Check the fit the optical and HDD'(s).
  8. Fit the drive head to the chassis. The controls panel should fit snugly in its recess and the bottom edge of the drive head sides should fit flush with the chassis base. Transfer an outline of the drive head mount holes on the controls panel to the Drive Head Mount brackets. Drill these holes with a 1/8" bit to accept a #6 machine screw. Final fit the drive head to the chassis.
PSDU and Toroid Mount
  1. The rectifier post, post bracket, capacitor brackets and ground clip join to form the power supply distribution unit (PSDU). The PSDU serves as a mount point for the bridge rectifier, reservoir capacitors, fuse buses and central ground. The capacitor brackets are attached to the rectifier posts with any type of epoxy based glue because screws would cause interference. The fuse buses are attached to the rectifier post with #6-32 machine screws. Washers are used to provide stand off between the fuse bus and the rectifier post to allow passage of plastic wire ties. These wire ties secure the capacitors against the brackets. The rectifier is attached to the post and the post serves as a heat sink. The ground clip attaches to post bracket. Assemble per illustration below.

    Pi-Amp Power Supply Distribution Unit Bracket
  2. The PSDU and toroid mount are located on the chassis base as shown in the illustration below. Match drill, counter sink and fasten with #6-32 x 1/2" machine screws.

    Pi-Amp Toroid Bracket and Power Supply Distribution Plan Elevation Schematic
Main Board Tray
  1. Obtain a micro ATX or ATX form factor PC case. These two form factors are the the most common in the industry. Do not use the less common low profile case.
  2. Identify the main board tray with expansion card and I/O bracket. Depending on the case construction, they may be integral to the PC chassis, attached with rivets or removable. In most cases the tray and bracket will be riveted to the PC chassis. The rivets are easily drilled out with a 1/8" bit. The two pictures below show two common tray arrangements. The dashed lines show the portion of the tray needed for this project. The ATX form factor is shown to illustrate additional required cutting. A micro ATX main board will fit in an ATX tray. The micro ATX form factor has four expansion card slots and the ATX has six. A micro ATX main board is 9.6" x 9.6". An ATX main board is 12.6" x 9.6". The 3" difference accounts for two additional expansion slots in the ATX board.

    Pi-Amp ATX Main Board Extraction From PC Case
  3. Carefully remove the main board tray and bracket. Trim off the "ATX portion" using snips or jig saw and retain the "micro ATX portion". Notice the main board tray in the picture above on the left. The PC power supply is set precisely where it is needed so the portion of the tray that extends below the supply should be retained. The tray in the picture on the left does not support the power supply. That will not be a problem because the supply will sit next to the tray at a slightly lower elevation. Be sure to smooth the cut edges with a flat file.
  4. Determine the position of the expansion card and I/O cut out by placing the main board tray inside of the chassis snugly against the rear panel. The tray should sit 1/8" back from the left edge (as viewed from the front) of the chassis base. Check to ensure that there is enough space between the tray and the toroid bracket to fit the ATX power supply. If the tray was cut properly in the previous step this should not be a problem. The illustration below is the plan elevation of the main board, power supply and toroid transformer.

    Pi-Amp Aluminum Chasis Component Layout Plan Elevation
  5. Mark the location of the main board expansion card and I/O bracket on the chassis rear panel. Fit the main board tray to the chassis and hold it in place temporarily with tape. Transfer the position of the six corners of the bracket to the inside of chassis rear panel using a fine tip marker.
  6. Fit the ATX power supply to the chassis and make an outline the top and side edges on the chassis rear panel.
  7. Remove the main board tray and power supply from the chassis and remove the rear panel from from the chassis base. Use the measurements in the illustration below to finish making the outline of the bracket for cut out (blue lines). The dashed red circles mark the location of saw blade starter holes.

    Pi-Amp Rear Panel ATX I/O Shield Cut Out Schematic
  8. Use the template shown below to mark the ATX power supply mount and vent holes. A template for holes can be found here. Measure the template to ensure that the scale has not been altered. Cut out the template on the 3-1/2" x 6" square that defines the outline of the power supply. Place the template on the reference lines and tape in place. Use a sharp center punch to mark the corners of the cut out and the mount holes. Remove the template and connect the corners of the cut out using a marker or scribe line.

    Pi-Amp Rear Panel ATX Power Supply Cut Out Schematic
  9. Cut out the access holes for the expansion card and I/O bracket and ATX power supply. Begin by drilling saw blade starter holes at the positions shown by the dashed red circles. Carefully cut between starter holes, along the outline, using a jig saw. Remove the remaining metal with a flat file. Ensure that the cut out is smooth and free of any burrs.
  10. Drill the four ATX power supply holes with a 5/32" bit for #6-32 x 3/8" machine screws. If the mount holes do not line up exactly with the power supply they can be enlarged to allow additional play.
  11. Attach the main board tray to the chassis. Fit the main board tray in place and ensure that the expansion-I/O bracket is centered over the rear panel access hole. Fasten the expansion-I/O bracket flange to rear panel by match drilling 1/8" holes through existing flange holes and using #4-40 x 3/8" machine screws. Fasten the main board tray to the chassis by drilling two, evenly spaced, 5/32" holes near the front edge of the tray (opposite of the expansion-I/O bracket). Match drill these holes through the chassis base and counter sink on the bottom side. Place spacer washers between the tray and chassis to prevent deforming the tray when the screws are tightened.
Pi-Amp Aluminum Chasis Rear View