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July 2, 2011

The AMB Mini³ (Mini Cubed) Portable Headphone Amplifier

The AMB Mini³ is a cute little portable headphone amplifier designed by Ti Kan of AMB Laboratories in 2007. The size of the amplifier when completed is approximately the size of a pack of cigarettes, or perhaps a stack of credit cards ;) 

The AMB Mini3 Schematic

Available from the AMB Laboratories website is the PCB board, a few selected parts, and pre-drilled panels that fit the Hammond 1455C801 or1455C802 enclosure if you'd rather not make them yourself. I'm not a big fan of the unanodized rim around the FPE (Front Panel Express) panels so I'll be drilling them out myself, but if one doesn't have access to a drill press, the pre-made panels would save a lot of effort. 

Because of the size of the board, one has to mount some very miniscule parts, including 1/8 watt resistors and some SMD opamps. SMD stands for Surface Mount Device and means that the device or component is not through-hole, so it is soldered on the side of the PCB that it is mounted on. Most modern components are made by machines using SMDs, but when soldered by hand, care is required. As explained on the Curious Inventor website, solder flux is first placed on the component legs, then solder is added to the tip of the iron, then the solder is carefully added, please view the video here

AMB Mini³ PCB, the size of a credit card
Because of sizing constraints, one really can't drop in boutique resistors, capacitors or other such items, but one can manage a little bit of customization. I chose to replace the 1% metal film resistors with 5% carbon film as carbon films can tend to sound a little more natural. I purchased about 10x the amount of carbon films that I needed and measured them with the multimeter, keeping only the ones that fell within 1% and matching them on both the left and right sides. This is much more obsessive than one needs to be, but I like drawing out projects a little bit longer to get more enjoyment out of them ;) 

My populated AMB Mini³ Board

The Mini³ has a built in battery recharge circuit that runs off a wall adapter. The battery is a very tough fit in the Hammond case. I had to file down the batter contact pins, remove the battery wrapper, sand down the plastic around the battery AND file the top of the inside of the case and it all just barely fits. I chose the 9V CTA 325mAH NiMH rechargeable battery that was recommended, perhaps the latest models are a little larger than before. As you can see below, the fit is quite tight. I also re-snipped some of the soldered leads at the bottom of the PCB to ensure they wouldn't short to the case. 

I wanted to make my Mini³ unique, as there are many out there that have already been built by headphone enthusiasts on HeadWize and HeadFi, so I chose to dremel out a vent area (purely for aesthetics) and have the case powdercoated in a textured antique copper. AMB has a nice set of printouts that you can use to drill the holes in the front and back panels, I simply lined them up, taped them on and used the drill press to make the holes. The vent area was quite a bit more work, requiring four drill holes to be placed in the corners, a cutting disc to cut the area between them, and a metal file to de-burr and even up the cutout. 

Chassis Prep
Once this was complete, I trimmed a small piece of perforated metal and used a permanent adhesive to attach it in place. The grill area is colored with a bright copper powdercoat to provide an accent to the antique copper chassis.

Unit Front without panel on, input, output, LED and potentiometer

Unit back without panel on. LED, power inlet and battery

Completed Mini3 unit with brass hardware and aluminum knob

Antique copper textured powdercoat
One might wonder why I chose to build a solid state amplifier since nearly everything on the Zynsonix blog is tube-based. There are a couple of tube-based portable amplifiers out there. Todd the Vinyl Junkie developed one a few years ago and there's the DIY Oatley Electronics K272A for a mere $27 that people seem to like. I haven't tried either of the two units, but as tubes tend to be fragile and microphonic, I'd be a little concerned about them banging around in my pocket for extended periods of time, even with damper rings. The AMB Mini³ offers a nice warm sound without the tubes, so it is highly recommended in my book. 

The Fine Print:
Please remember that building circuits and performing circuit modifications can be dangerous to you and/or your surroundings and should only be performed by a certified technician. The owner of this blog and all associated parties can not / will not be held responsible if you attempt a build or modification posted above and cause physical harm to yourself or your surroundings. Many electronics contain high voltages that can kill, and mods, if performed improperly, can be a fire hazard. Please keep this in mind. 

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