Dual Mono Power Amplifers

The following slides are from the Avondale Adio DIY Audio CD, available on request.

Slides from the Avondale Audio DIY Audio CD, available on request.

December 2009

Slides from the Avondale Audio DIY audio CD, available on request.

These notes are informal but you might find them useful if you are building an NCC200 amplifier module

Kindly compiled by Malcolm C - (misterc6 on PFM)

These notes are from my own experience of building three pairs of boards and are very much a ‘work in progress’.

Some of the holes on the PCB need enlarging so you are going to need some small diameter drills:

I used 2.8 mm male spades for the test points and had to enlarge the holes to 1.2 mm. (Standard PCB pins may fit without enlarging)

Check the holes for the 6.3 mm male spades, depending on the spades you have they may also need enlarging – I used 1.4 mm.

I found it easier to fit the driver transistors (TR7 & TR8) if the holes were slightly enlarged.

If you are using the recommended Silver Mica for C8 the hole sizes should be increased to 1 mm.

Some of the PCBs I’ve received from Avondale have had minor errors in the silk screening and some even had a small piece of track missing but that was easily bridged using the component lead.

The 6.3 mm male spades are expensive (£8.90 + VAT) if you have to buy 100 from Farnell. I suggest asking Avondale if they can help out with these, and possibly the test point terminals as well.

There are various possibilities for the driver transistor heat sinks. Again, you could ask Avondale to help out with these. These do not need insulating kits but you will need M3 mounting hardware which will depend on whether the hole is tapped or not.

Somewhere on the Avondale web site there’s an excellent instruction leaflet on how to mount the power transistors.

You will need some 18SWG enamelled copper wire for the inductor. I hope to provide more advice on this shortly.

Before installing the trimmer use a meter to check that it is set to mid-range.

After completing the construction of the board, but before attaching the power transistors, you should apply power to the board and check that the voltage between TP3 and TP4 is between 1.6 and 1.7 V d.c. If you can’t achieve this by using the trimmer you need to seek further advice before risking the power transistors.

When doing any testing the power supply rails should be fully discharged before connecting up to the board. A 1k0, 5W resistor is a suitable value to use and you should connect two, one from each rail to the 0V terminal.

NCC200 COMPONENTS

The values of C7 and C14 are not all that critical and I use 220pf for both.

C3 – Avondale and others like the solid aluminium BC128 capacitors that I’ve listed in the spreadsheet. I prefer wet tantalums which I source from eBay. You could use a standard 10uf, 35V tantalum bead or a better quality axial such as a Kemet T110 type (1457559) from Farnell at £3.70 +VAT. Other users swear by 2.2 or 3.3uf film capacitors such as the WIMA MKS2 in this position.

C2, C4 & C12 – there has been some confusion over these with values of 100nf and 470nf being quoted. The silk screen calls for 100nf which works well in parallel with the 100uf and that’s what I’ve included in the spreadsheet.

C6 is the feedback capacitor and has the largest effect of all component choices on the sound of the amplifier. Avondale’s preferred Vishay does a good job here but the 100uf, 10V Kemet T110 type (1457560) comes very close and is almost half the price of the Vishay at £3.30 + VAT. I have also used wet tantalums in this position. Many users really like the 47uf MMK film capacitor which was the subject of a pfm group buy but be aware that this is too large to be mounted on the board and requires flying leads.

C11 – I’ve listed the BC128 but again I prefer a tantalum here. Farnell have a tantalum bead (9708235) at 76p + VAT (min. qty. 5) but I use a wet tantalum from eBay.

TR1 & TR2 – it is now well known that these have to be selected in order to keep the output D.C. offset at a reasonable level. The Hfe of TR1 should be greater than that of TR2 by about 10%. The spreadsheet shows the minimum order quantity of 10 but you would have to be extremely lucky to get two suitable pairs from such a small sample. The devices are cheap enough and I would suggest buying 100 to be reasonably sure of being able to select what is required.

TR9 & TR10 – The MJ15003 is perfect for this application and much better value than the BUV20 which the original spreadsheet listed. In my opinion the ON Semiconductors device I have listed at £3.96 + VAT is worth the extra cost compared to the Multicomp device at £2.99 + VAT.

Initial Testing Procedure:

After building the PCB to the point where the output devices have yet to be installed on the heatsink, there is a simple check “built in” to the NCC200 to ascertain if all is well up to that point.

It requires nothing more than a digital voltmeter set to a low DC Volts range and the two probes are then connected to the test points TP3 and TP4 on the part-built PCB. Applying DC power as if the board were being installed, the voltage across these two test point should have a reading of between 1.0 Volt and 2.5 Volts according to the setting of the trim pot. Adjust the setting to read 1.6 – 1.7 Volts and observe there is no significant drift in this figure.

After final assembly of the heatsink and output devices, this setting will ensure that there will only be a small bias current flowing though the devices at power up time.

Hoping all above is clear and you thoroughly enjoy the fruits of your labours.

January 2010 - Les