AKSA ASSEMBLY

 
OVERVIEW

These assembly instructions initially describe the recommended power supply for the AKSA 55 amplifier. Note: Exactly the same principles apply to the AKSA 100, but the voltages are slightly higher, and parentheses will indicate the values for this amplifier.

Following the Power Supply section is a SAMPLE of the assembly instructions supplied with A SUPERSEDED VERSION of the AKSA 55. You are strongly advised NOT to purchase power supply components in advance of ordering the kit, since the documentation, for commercial reasons, differs slightly from that given here for public display. These website instructions are provided as an illustration of the detailed instructions you will receive with your kitset and may not be precisely the same.

DISCLAIMER

All reasonable care has been taken in the design of this kitset amplifier to ensure that it neither causes electric shock nor damages any loudspeaker or other transducer to which it is connected.

In normal, routine operation with both 8R and 4R speaker loads, and providing all assembly instructions have been followed carefully, no faults should cause any damage either to the amplifier or any connected transducers.

However, since both the assembly and operation of the kitset are beyond the control of Aspen Amplifiers, neither Aspen Amplifiers, its agents nor its employees take any responsibility whatever for any damage resulting from the use or abuse of this amplifier by any person or persons to any product to which it is connected or otherwise attached.

Once the product is purchased and the packaging opened, goods are deemed non-returnable, and by your action you agree to be bound in law by this disclaimer.

WARNING:
Mains wiring must be performed by a qualified electrician - Do not attempt the power supply unless suitably qualified. Faulty mains wiring may result in death or serious injury. All local regulations pertaining to mains wiring must be rigidly adhered to.

POWER SUPPLY:

The amplifier is NOT supplied with a power supply. The 55W AKSA is designed for a 36V rail voltage; that is, a bipolar supply of +36 volts and –36 volts, giving a rail to rail voltage of 72V. The 100W AKSA is designed for a 49V rail voltage; that is, a bipolar supply of +49 and -49 volts, giving a rail to rail voltage of 98V. THESE VOLTAGES SHOULD NOT BE EXCEEDED TO INCREASE POWER OUTPUT.
 
The recommended power supply uses two separate mains transformers and two independent, full wave rectifiers, one for each amplifier, preferably using ultra-fast, soft recovery diodes such as GI's UG10DCT, or the SBYV28-200. This DUAL supply requires four [8] 4,700uF capacitors, two [four] for each full wave rectifier, and gives superior electrical separation, conferring strong immunity to crosstalk.  Use of two, independent supplies confers palpable mage depth, a quality often absent with solid-state amplifiers and a hallmark of tube (valve) amplifiers. Full schematics of these two power supplies appear below (NOTE: this is for a 55W AKSA).
 

 
Preferred Power Supply

Suitable transformers for both the 55W AKSA are made by the Avel Lindberg company of the US, Antrim of Ireland, and a variety of manufacturers right around the world. The 55W AKSA requires two 160VA transformers, and the 100W AKSA requires two 230-330VA transformers. Both can be toroids, an ideal choice for this application as they are relatively inexpensive, and slimline.

Both power supplies must use star earthing, with ALL power earths returned to this single point, shown as a triangular earth symbol in both schematics. Loudspeaker 'cold' terminals are also returned to this point, but the signal earth for the input is connected to the signal ground pin on each amp module.
The best rectifiers are ultra-fast slow recovery types, which greatly reduce switching noise. Surge rating on these devices is 55A [100A], which suggests it would be difficult to destroy them in normal amplifier service.

ASSEMBLY

Assembly is best performed in steps. Before commencing, read these instructions very carefully. This will ensure you follow a disciplined sequence, which almost guarantees you get it right first time. Re-works in electronics are nearly always costly, particularly if they result in destroying the output stage – very easy with a solid state amplifier. Make sure you are comfortable, all tools are to hand, and arrange adequate lighting. Directions given here are for ONE pcb; assembly for the other board is identical. Assemble one pcb at a time to avoid duplicating a mistake, and check each and every component carefully before and after installation. If it is incorrectly installed, wait one minute to allow it to cool, desolder and remove the component, then re-fit a second time.
 
STEP 1 - Check the Printed Circuit Board ('pcb')
Check the printed circuit board for damage in transit; cracks or abrasive damage which might cause assembly problems. Carefully examine each hole; from time to time solder can block component holes and must be cleared.  Correct this with a compass point; if a hole is missing altogether (check first!), carefully drill it out manually with a handheld 30 thou (0.8mm) drill.  Compare the board with the overlay diagram included; it should be 86mm deep and 75mm wide.
 
STEP 2 - Fit the four (4) spade connector tags
Insert the four spade tags, in the positive, earth, negative and output positions.  The power supply tags are easily inserted from the component side into the two drilled holes.  They are pressed into place with a pair of long nosed pliers straddling the pcb and tag and squeezing firmly into the board.
 
STEP 3 - Fit the four (4) fuse clips
Insert the four fuse clips on the left and right component side of the board marked FUSE 5A.  First push each clip hard and flat against the board.  Heat the fuse holder prongs with a hot iron, then solder to the board ensuring that solder covers the full distance between the prongs.
 
STEP 4 - Fit the 15 resistors and one preset (bias) pot
Fit all resistors except R13, R14 and R15. These are R1 through R12 on the component (non-copper) side. R13, R14 and R15 are fitted on the COPPER side at completion of step 6 and not before. Note that these three resistors require no lead holes. Note that R6 (82K), R7 (2K2), and R8 (120K), are mounted vertically on the component side;  the top lead is bent around with the finger from the lead end so that it is 5mms separated from, and parallel to, the bottom lead.
Colour coding with 1% resistors works this way; the first three bands give the first three digits, the fourth band gives the number of zeroes following the three digits, while the last band is much thicker than the rest and gives the tolerance, in this case brown – 1%. Thus a 12K 1% would be brown, red, black, red, BROWN. When decoding, always orientate the thickest band on the right.
It is a good idea to actually measure each resistor with a multimeter prior to fitting. This guarantees you install the correct components. Here is the standard resistor colour code:

 

Black	Brown	Red	Orange	Yellow	Green	Blue	Violet	White	Grey
0	1	2	3	4	5	6	7	9	0

A gold band before the last band means that the resistor value lies between 1 and 99 ohms.  Use the overlay diagram enclosed to identify the various component positions.  The overlay diagram is viewed from the component side.
Dress all leads so that they are bent neatly to line up with the holes in the board.  This means they can then be fitted without stressing the leads, improving reliability.  After soldering the resistor leads to the circuit board, trim them flush with the soldered joint using suitable side cutters.

Note that R13, R14 and R15 are mounted on the copper side. R15 (10R) should be mounted betweeen the pads marked 'X' on the copper side only when Step 6 is complete.

Now fit the 25 turn trimmer pot which controls bias, P1 (100R). This is a three terminal, vertical device, with a small brass adjusting screw on top.  This screw adjustment, located on the left towards the centre of the circuit board, must be wound counter-clockwise at least 20 full turns before assembly.

DANGER: Failure to wind back the bias pot fully may seriously damage the output stage upon initial switch-on due to severe overbias.  Take care!
 

------------------------------Some steps removed from this sample for brevity------------------------------------------------

 
SETTING UP AND TESTING

Now both channels are assembled, move the completed power supply close to the stereo module for setting up the bias and testing.
 
STEP 11 - Attach the power supply leads to each channel in turn
There should be three duplicated output leads from a SINGLE power supply; two positive (red), two earth (green), and two negative (black).  Each complete set connects to the pcbs.  If you are using a DUAL power supply, ie one (or two) transformer(s), with two rectifier blocks and two pairs of reservoir capacitors, then there will also be a pair of each; ie positive left channel and right channel, earth left channel and right channel, and negative left channel and right channel.  In both cases the earth terminal(s) from the power supply should be connected to a sturdy star earth, with the earth connection to each channel also taken to star earth. This approach minimises hum.

The three leads of the power supply connect directly to the amplifier power connectors. Looking at the amplifier module so that all overlay writing is legible and the output devices appear at the top, the positive (red) lead connects to the left connector, the earth (green) to the centre connector, and the negative (black) to the right connector.  The leads may be attached using a standard 6.3mm female spade connector, or (preferably) by soldering the lead directly to the spade at the board.
 
STEP 12 - Set up protection and wind back bias
In the two fuse holders, wire two 100R 0W6 resistors by winding the leads around each holder clip.  There is no requirement to solder these resistors; they are used only for protection in the event the amplifier is incorrectly biased during initial setup.

To ensure there is almost no bias drawn upon switch-on, use a jewellers screwdriver to wind the bias pot adjustment fully counter-clockwise until a clicking sound is heard each time it is turned.  In any event, the adjustment must be turned counter-clockwise at least 20 full revolutions.
 
STEP 13 - Switch on and adjust initial bias
Plug in the power supply, and switch on the mains. Taking the multimeter, immediately measure the voltage across the positive rail 100R 'fuse' resistor. It should be just under 2 volts, and should not exceed 5 volts.  Check the 100R resistor on the negative rail; the voltage drop should be almost the same; just under 2 volts.  More than 5V may indicate the output stage is biased on;  wind back P1.

Now measure the voltage across the rails.  From the positive rail to earth should be 42 +/-1 volts;  the figure should also be 42 +/-1 volts (negative) between negative rail and earth.  In both cases place the common terminal at the earth point, so that true polarity is indicated on the meter.
With the multimeter set to the 20V scale, measure the voltage between the speaker output and earth.  It should be less than 50 millivolts.  If this is so, set to the 2V scale and confirm.  If it exceeds 100mV, R2 (68K) may need to be increased or reduced empirically.  Gross voltages approaching the rails indicate a circuit fault;  check all semiconductors have betweeen 0.4V and 0.6V across base and emitter.  Then switch off, check all soldered joints and output stage transistor junctions.

Measure the voltage between R4/R5 (each of 2K7) and earth. It should be approximately 20 volts.  If this voltage is less than 12, replace capacitor C4 before proceeding.
Now adjust the bias with a jewellers screwdriver.  With the multimeter probes across the positive rail 100R resistor, slowly turn the adjusting screw on the bias pot clockwise until the voltage drop across the 100R resistor is 3 volts.  Be very careful as the rise in quiescent current through the output stage is abrupt.  Under no circumstances should this voltage exceed 7 volts.

Confirm the voltage between output and earth remains less than 100mV.  Switch off the mains.
The initial bias has now been set to 20mA. The amplifier is viable, and will function as designed.
 
STEP 14 - Fit fuses and check audio output
Remove the 100R resistors from the fuse holders of each channel and insert the two 5A fuses supplied.
Connect an RCA socket via flying leads to the input terminals,ensuring the pcb terminal closest to the star earth point (signal earth) is connected to the outer earth terminal of the RCA.
Connect the 'hot' lead of a loudspeaker to the output terminal spade located close to the heatsink ledge;  the other terminal should be connected to earth.  Take great care when attaching this lead, as any short to the body of the heatsink or supply rails may destroy the output stage devices.

With a standard RCA audio connector, connect a CD player with variable output to the RCA input of the amplifier module. Arrange the heatsink for airflow so that the amplifiers cannot overheat.
Switch on the mains power. There should be a barely discernible switch-on pulse at the speaker.

Ensure room temperature is a pleasant 20C, or 68F.  Start the CD, adjust volume to an acceptable level, and play music for 15 minutes at moderate volume so that the amplifier heatsink warms appreciably.
Now stop the CD player, carefully place multimeter probes between the two right hand terminals (emitters) of each output transistor on the 200mV range, and check that the measured voltage is less than 90mV.  If it is, do nothing.  If it is not below 90mV, reduce the bias to read 90mV.  This procedure ensures the 'warm' bias current does not exceed 95mA.
 
STEP 15 - Audio test and set final bias
Now you know the amplifier functions correctly, you are now ready to test the amplifier on music.
Again ensure room temperature is 20C or 68F.  This is important because it strongly influences heatsink cooling, which affects bias setting.  Switch on the mains power.  Start the CD, adjusting volume to a high level, and play music for twenty minutes.  There should be no distortion, with excellent dynamics and a strong sound stage.

After twenty minutes of loud playing, stop the CD player, again place multimeter probes between the two right hand terminals (emitters) of each output transistor on the 200mV range, and if necessary, carefully reset the bias with a jeweller's screwdriver so that the reading does not exceed 120mV.  This procedure guarantees that under continuous, heavy loading the bias will not rise above 130mA.  Watch the meter for a few seconds; now the amplifier is idling, the output stage quiescent current should begin to drop, settling after 10 minutes or so at around 80mV or less.  This is normal.

The stereo amplifier module is now complete and tested. You can now set about mounting it in a well ventilated cabinet with a power supply, and finishing the complete unit. Congratulations! You will derive many hours pleasure listening to this superb amplifier…

© Copyright Aspen Asmplifiers P/L
 2000-2005 All rights reserved
 
Hugh R. Dean
Aspen Amplifiers P/L
All rights reserved
Reproduction in any form is prohibited