ACA Mini

The ACA mini by Nelson Pass.

A fantastic amplifier, intended to be used as this boxless PCB, which is a fun industrial look, and saves the major expense of an enclosure.

Power is 5W into 8ohm, 8W into 4ohm.

This amplifier sounds fantastic, if you have any interest at all, you really should build one. :)

Please read the notes and click through all the photos.

Also, download and read Nelson's ACA Mini article

Print the schematic and have in front of you when stuffing the PCB

Ensure you are using the correct Bill of Materials. The current BOM used in the completion kit is ACA Mini BOM V1.1.

PCB front. All components mount on this side.

This is a beautifully simple kit.

PCB

24V 90W PSU brick. Double insulated, Universal input.

Photo 1 - Connectors and hardware

Photo 2 - Rear 15,000uF capacitors. Front 1,000uF (Black cans), 3.3F (blue cans), 1uF film (blue box)

Photo 3 - resistors, potentiometers. Please note the pots in the current batch of completion kits require their leads bent before installation, here is how to bend the leads.

Neat fact - 3.3F is 3,300,000uF

Rear - IRF520 (N-channel Q4) and IRF9520 (P-channel Q3)

Small signal Jfets can be LS parts or Toshiba.

LSK170, 2SK370, 2SK170 can all be supplied for the N-channel devices intended for position Q1

LSJ74, 2SJ74, 2SJ108 can all be supplied for the P-channel devices for use in position Q2

Photo 3 - Heatsinks

Power resistors are mounted first.

Leave some room under them for airflow. The nut is a good gauge.

You can use the heatsinks as an impromptu stand for holding the PCB.

Neatness counts - Bend the leads so the value is readable, and so it reads left to right. The value markings are R33, R75, 1R0.

Low-ohmic resistors use "R" as a decimal point. "R33" means 0.33ohm. "1R0" is 1.0ohm.

It's difficult to measure low-ohmic resistors because the resistance of your multimeter leads (normally 0.2-0.4Ω) will be added. You can measure your leads and subtract the value, just trust the values which are printed on them, or build a low resistance value test rig.

Install and solder the small resistors next.

Measure every one before installation.

Face them so the heavier brown stripe is on the right. (Or bottom, for the three resistors that point up.)

Photo 2 is provided as a reference to help stuffing, but it's much better to measure every one and refer to the schematic before installation.

In all cases, make sure the connectors are flat and properly aligned with the PCB before soldering.

It is very helpful to tape the jumper pins to the PCB before soldering. This will hold them in place and keep the vulgar language down to an absolute minimum.

Don't try to hold them on with a finger whilst soldering, that is a fantastic way to burn yourself.

At this point you should have the edge connector items stuffed, and the potentiometers.

Power switch orientation does not matter.

LED needs to be stuffed with the long lead in the + hole.

If you want to swap the LED for a different color, feel free, it's used only to indicate power on/off.

Stuff the small transistors. Watch the orientation, the flat of the device lines up with the flat drawn on the silk.

Leave the leads long. I place the PCB on the tabletop, solder one leg from the top to keep it in place, then flip the board and finish the rest of the legs.

Small capacitors next. Long lead in the + hole.

The - of the capacitor can is marked, but the convention shows the + marked on the silkscreen. So be careful.

Install the large capacitors. Orient the - marks on the cans all to the inside.

A little heatsink goop is helpful if you have some.

1) Attach the Mosfet to the heatsink but leave slightly loose.

2) Insert Heatsink/Mosfet assembly into PCB.

3) Solder heatsink to the PCB first. There's no trick to this, it just takes some time with the soldering iron, as it is a heatsink after all...

4) Snug down (modest torque, please don't crank it) the Mosfet attach hardware.

5) Finally solder the Mosfet.

The Q3 positions use the IRF9520 devices. The IRF520 devices are in positions Q4.

Please observe the positions for the power mosfets.

IRF520 on the front of the board.(power switch end)

IRF9520 towards the back of the PCB. (wiring end)

The provided metal standoffs and their associated screws are used as feet.

You may use something else if you choose to, this is DIY after all... :)

Member Charles Port drew this wonderful diagram. Thank you!

Refer to article until this guide is completed

If you want to DIY a base of some sort, here are the PCB dimensions.