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Thread: class A amp project for Mark

  1. #16
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    More chunks of aluminium...

    The diodes would not seat property on the little pcb heatsinks so I had to look for another solution.

    Given the surface area of the lower outer panels I used a 20x 25mm solid bar between the diodes which will be thermally bonded to the 80 x 260mm panel.

    This was non routine as the design had to be adapted to suit the circumstances. I am glad it turned out this way because the heatinking is far more substantial and the recifier will be located close to the filter bank which will minimise noise emmissions.

    The bar was tapped with M3 holes for the rectifier pcb and M4 to attach to the panel.

    Also an image of the soft start circuit and the Mitre saw.

    A soft start circuit is used to moderate the inital high surge current from the large toriodal transformer and intial charge of the filter capacitors.

    It consists of the time delay for a relay that closes after about 3 seconds. The current initally passes via a thermister that has cold resistance of about 10R (ohms).

    Next week we will look at installing the switches and audio connectors.

    Ian
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  2. #17
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    Worked on completing the router table today

    Spent the day milling out some slots and drill some big holes with the big drill.

  3. #18
    Senior Member Guido's Avatar
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    FET mounting

    Ian,
    do you use these Kapton isolators?
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  4. #19
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    Up until now I have only used mica .

  5. #20
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    The construction of the router table is nearing completion.

    Sorry I forgot the camera.

    I worked on the guide rail for the router today. The guide is a 1 metre aluminium extrusion with a 3/4 inch steel bar that is attached to the router base and runs the full length of the guide. I made a temporary base out of 6mm masonite for the initial trials.

    The table has an overall length of 1 metre and an effective travel of about 500mm.

    The idea is that the router will slide atop a substantial alumnium channel that forms a clamp to press down on the work (plate aluminium)

    At each end of the channel there are large spring loaded hex bolts that will draw the channel down on the work. A length of 3mm hard neoprene has been attached under the channel which will squeeze against the work under pressure of the bolts.

    There are also four guides to hold the channel in an extact lateral position E- W , N - S. Another set of guides in the form of a 500mm stainless steel ruler will be the work stops. The image below was taken in the early stages but you can see the channel construction

    I saw a table by tool company today about 1/2 the size of this diy table and it was about $1100. The BOM so far is about $250 (less router)
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  6. #21
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    Not much to report here due to a bad dose of the flu.

  7. #22
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    I am nearly ready to throw the big switch.

    Completing the final fit out of switches and conenctors today.
    Most of the infra structure is done.

    Need to buy a cutter for the 24mm connector holes. A 5 minute job with the right tool.

    Musing some ideas with Mark on the cosmetics.

    Ian

  8. #23
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    We looked some options including anodising but that was difficult to arrange in small quantities and with cut and machined work.

    So after a lot of consideration I have arranged to get the exterior plates brushed and nickle plated.

    Some of the work was dropped off at the plating works today so I should have something to show you next week.

    I think it will look look quite nice.

    Ian

  9. #24
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    I made more progress on the amps this weekend.

    The usb uplink cable of the Nikon broke so I will attempt to descibe what has happening behind he scenes.

    1. Yesterday I cut out the holes on the rear 26 x 8cm aluminuim panel for the Speakon and Canon 3 pin sockets and drilled the M3 mounting holes for the connecters. The panel will go to the plating works sometime this week.

    2. The amps were striped down today to nut and bolt level and the remaining extrusions are now ready to be sent for plating this week.

    3. The heatsinks were polished and cleaned ready for final mounting of the power fets and associated Pcbs. This is to ensure a smooth flat surface free of oxide build up. Everything will be carefully cleaned and degreased again before final assembly.

    4. I also made a nifty Jig for bending the power fet legs to precise measurements so they pop through the pcb holes in just the right position.

    5. Final wiring was soldered to the power Fet pcbs earlier in the week and are ready for installation.

    6. The final power filter for the CRCRCpcb was fabricated and I have managed to fit it neatly under the main pcb. The benefit of this is the whole assembly represents a bolt in module and the filter capacitors are within an inch or so of the main pcb and the output stages. There is 44,000 uf on the final filter bypassed by 2,200 uf on reach supply rail. The supply is rated at 12 amps continous current +-25 volts.

    7. I managed to source some TO3P Mica washers during the week from a repair centre. I am certain they wondered why I counted out 32 washers..they are now out of stock. Mica is now quite hard to obtain. They are by far the best thermal washers.

    8. In order to expedite the final assembly the cutting of the top and bottom panels will be out sourced to a shop. Completion of the router table will be pursued at a later stage when I have more time.

    So its all coming together at last.

    In the attached image taken at an earlier stage looking down into the works you can see the primary capacitor bank.(44,000 uf, 0.235R 30 watts, 44,000uf, 0.235R 30 watts). The final stage of capacitors mentioned in item (6) will be mounted inverted under the bar stock bracket holding the main pcb at the top of the enclosure. So there will be a central core if you like that will be the power supply. Since that pic was taken the brackets have been adjusted and the top bracket has been raised about 15mm and the side vertical edges trimmed in the Makita Mitre saw so they are flush with the side vents which extend the full perimeter of the chassis.

    Fitting everthing in has been fun but a painstaking and tedious task at times and as a result quite time consuming.

    While the capacitors are rated at 105c (for a given ripple current) there should be sufficent temperature/pressure differental between the bottom and top vent channels to ensure adeuqate convection cooling. The amp is designed to run at 46-50C (in an ambient temperature of 25-30c) without forced air cooling.

    This is the idle temperature and the optimum operating point for best measured performance. The amps will take about an hour to reach full temperature. The gain of the fets and some their characteristics change with increased temperature so it is important that the bias and other tweaks be done at full operating temperature.

    I would expect the subjective performance to be at its peak in well under an hour. Forced air cooling a frowned upon in class A amplifers because the ambient background noise masks important low level resolution. Even the quietest fans are audible so we cool these amps with sheer heatink real estate. There is collectively 800mm of 150mm high heatsink in each amp.

    In the early stages I talked to Mark about his situation and as a result designed the amps for the Perth WA summer. So on a really hot day (inside the house) with an ambient temperature of 30c the amps would be running at 51c. That is hot to touch but not hot enough to burn someone (Mark's kids). 55c is on the threshold of pain and 58c is too hot to keep your hand on the heatsinks.

    Typically temperatures in Perth can soar to the high 40's in the summer and allowing for air conditioning I would not expect anyone to be in a listening room at 30c ambient temperature running a pair of class A amps pumping 500 watts into the room but we design this into the amps. A thermal breaker opens at 75C.

    The biasing and absolute Dc offset can take some time to settle with this design, typically an hour. Absolute dc offset is the potential difference between the positive and negative loudspeaker outputs and ground. Because the positive and negative outputs float above ground (the amp outputs are balanced SE) in normal operation we provide a load of between 50-100 ohms to help control this offset at turn on. Some common mode feedback is alse used to help control this offset.

    The loudspeaker does not see this offset and typically its 5 volts at initial turn on and then slowly settles to within 100mv after an hour. The actual DC offset is only small, typically 10-20 mv at all times.

    So you can see the operation of the amplifier is closely allied to the physical parameters and we measure key indicators when certain conditions are me.

    The final adjustments will be done over several days.

    Ian
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  10. #25
    Senior Member Krunchy's Avatar
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    Wow, I stand humbled.

  11. #26
    Senior Member Audiobeer's Avatar
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    Great read Ian. can't wait to hear more!

  12. #27
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    Thanks,

    I can't wait to here it go.

    Need to buy a new usb cable for the camera!

    Ian

  13. #28
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    One thing's confusing me sat here in England Ian.

    Where's the tubes?

  14. #29
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    Here are some recent images of assembly and fabrication.

    My favourite tool..the big drill. Image of 24mm hole's being drilled for the connectors. Out of frame is a large clamp holding the work.

    Next image of the rear panel (un plated and finished at this stage.

    The pcb assembly modules showing the final filter capacitor storage stage under the main pcb.

    .

    Ian
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  15. #30
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    Here we are again,

    Another angle of the pcb modules and the heatsinks stacked up following polishing and cleaning.

    Ian
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