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White Noise Audio - Headphone Amplifier Kit MKll

Following on from the original headphone amp WNA have introduced the MKll Headphone amp. There are now two new versions to suit different applications: (Please note: WNA supply higher quality parts with their amp kits, I populated the board pictured above with metal film resistors whereas WNA supply Welwyn RC55Y resistors and Polypropylene Capacitors with their full kit)


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The first version ( type 1 pcb ) is similar to the previous one ( and can also be used as a buffer amplifier ) except the pcb is a little larger; there are now two pairs of output devices per channel to deliver more grunt into low impedance headphones; the rail splitter is preceded by a capacitance multiplier to make performance less dependent on the quality of the external power supply; The component count on MKll has increased also, 12 x transistors (instead of 6)  6 constant current diodes (instead of 2)  9 x 100uF decoupling caps (instead of 2) and a ferrite bead replaces the R4 jumper. The output caps now mount on the pcb; and each channel is now completely independent i.e. the amp is dual mono so that you can have one power supply per channel. The latest version of the cascode power supply accomodates this because it consists of two independent regulated outputs ( which you connect in series if you need a split rail power supply ). The price for the kit is now £160 and the assembled and tested version is £210. The case is the same as previously but the pcb mounting holes will be in different positions.

The second version ( type 2 pcb ) is as for the first except that the pcb is larger again because the volume control and headphone jack now mount on the pcb which is really handy and saves wiring the pot and head-socket to the board.

Part of the circuit


PCB type 2


The Rail splitter Section



Dimensions of PCB's:

Original: 74 x 105mm
PCB1: 90 x 105mm
PCB2: 90 x 129mm



_______Putting it together________

It took about an hour and a half to populate the PCB 2 and it's now a lot easier with the potentiometer mounting directly onto the PCB. Certainly a lot less wire floating about on the PCB which makes for a very tidy job indeed. The output of the rail splitter power supply is not connected to the power inputs of each amplifier channel (in case you want to bypass the rail splitter and use one PSU per channel) so it's necessary to connect the power from the rail splitter to each channel using hookup wire.... even then it's only a small run of wire and can be done either on top of the PCB or underneath if you want to conceal it. You also have to connect the left and right signal outputs on the PCB to the pads next to the headphone socket but, again, this can be done under the board to keep things tidy.

All in all the MKll is extremely easy to put together thanks to a very easy to follow construction manual, you wouldn't believe the MKll has 30 more components than the MKl when you look at them side by side.. the MKll is just a lot tidier without all the wire kicking about:

                                          MKl                                                                               MKll



Completed MKll


Another view


Superfast LM6171 at the heart of the WNA


Dual Class A




Close up of the LM6171, Non polar output cap and incandescent lamp current limiter



Provision for a LED power source and resistor on the rail splitter.

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