This amplifier has become a very popular one for DIY constructors, and it isn't hard to see why:
- It offers very high performance at low cost
- The simple design appeals to many people
- Inexpensive PCBs are available
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On page two I describe my P3A amp and on page three there are some measurements. This guide focuses almost entirely upon component selection, as this is one of the few areas where Mr. Elliot declines to go into great detail--probably because individual preferences and design needs will dictate choices.
Power
The first choice that needs to be made is the amount of power needed, and into what load. With the proper output transistors the P3A will operate from 0.5W-120W+. It is recommended that the range be limited to 10-90W unless you have experience modifying designs. Do not assume that more power is necessarily better; often it can be easier (and less expensive) to construct a more moderate amplifier without sacrificing dynamic range.
Use the following form to estimate your rail voltage requirements. R2R is the distance from the rail that the output can swing; 2V is realistic and 3V is more conservative.
Once you know your rail voltage and power requirements, you can determine the proper transformer to use. Both El (standard) and toroidal transformers may be used successfully, but the latter is preferred. The transformer must have a center tap.
A true "monoblock" design would utilize a separate transformer for each channel, but this is rather wasteful of resources. Two or three amplifiers can easily share a single transformer provided they have separate bridge rectifiers and supply capacitors.
The minimum VA listed is just that; many designers will recommend doubling the above figure. Personally, I feel this is unnecessary, but having more than the minimum is definitely desirable. Another caution is that the VCT voltage listed above MUST be the voltage under load, not the no-load voltage. It is not always clear if manufacturers are quoting the load or no-load operating point, so you should verify this prior to choosing your transformer.
Whole books can (and have) been written on supply design. Click here for some more information on supplies for this amplifier.
Transistors
The only crucial decision here is the choice of output transistors. The listed transistors may all be used interchangeably, provided you use both units from the complementary pair. (i.e. no mixing MJ2955 with 2SA1302) Popular opinion says that the various Japanese transistors are more linear and better matched than the 2955/3055 pair, and hence will have less distortion. They also have a larger safe operating area; 2955/3055 cannot be used with loads smaller than 8 ohms or supply rails above 35V. Pairs marked with an asterisk (*) below are recommended units.
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| MJE3055/MJE2955 | TO-220 plastic package; 10A - 60V - 75W | ||
| TIP3055/TIP2955 | TO-220 plastic package; 15A - 60V - 90W | ||
| 2N3055/MJ2955 | TO-3 version of above; 15A - 60V - 115W | ||
| MJ15015/MJ15016 | TO-3 same family, higher voltage/power; 15A - 120V - 185W | ||
| MJ15003/MJ15004* | TO-3 package; newer version; 20A - 140V - 250W | ||
| 2SC3281/2SA1302* | TOP-3 plastic package; Japanese BJT | ||
| MJL3281A/MJL1302A | alternate to 2S pair? | ||
| NTE2328/NTE2329 | alternate to 2S pair? | ||
| MJ21193/MJ21194* | TO-3 package; 16A - 250V - 250W | ||
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If you are a first time constructor I do not recommend using the more expensive transistors, for the simple reason that the output stage is very easy to blow up, and it is much less frustrating to destroy MJE2955T's at US$0.50 than 2SC3281 at nearly US$4, or MJ15003 which can go for $8. Either way, order some extras as backups. Make sure also to order thermal grease, mica washers and nonconductive bushings, as necessary. (The larger TOP-3 plastic packages do not require bushings)
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| BD139/BD140 | 1.5A - 80V - 10W | ||
| MJE340/MJE350 | 0.5A - 300V - 20W | ||
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Driver choices are less critical; whatever is easiest to acquire is probably sufficient. The BD parts may have a slight edge. Again, do not mix units from different pairs.
Resistors
Bulk 1% metal foil is sufficient for all resistors other than the 5W outputs. If one wanted to be excessive an expensive high power metal film (Caddock et al) could be used for these resistors, but there is unlikely to be much of a performance difference when compared to a basic 5W wirewound.
Capacitors
The most critical capacitors are the input coupling cap and the miller (100pf) capacitors. The former determines the low frequency rolloff and should be as large as is feasible (in the 1-10uF range) with a polypropylene or polystyrene dielectric. Polyester may also be used, but is less advisable. Metallized is generally considered inferior to foil, but this decision is less important than the dielectric. The miller caps should be polystyrene or polypropylene, and a close tolerance is advisable.
The choice of the supply bypass (C+ and C-), zobel (C7), and shunt (C2) capacitors is not critical. It is recommended to use film caps for all of these. For the higher valued capacitors electrolytics are unavoidable, and it is important only to choose capacitors with appropriate ratings.
Heatsink
Estimating the appropriate heatsink size is very difficult. It is advisable to get the largest sink that is economically and dimensionally feasible. The official construction guide (available on the private ESP site only) offers several different mounting options, and should be considered prior to any decisions on heatsinking. ESP also has an article on heatsinking. This page offers a way to estimate heatsink ratings.
If you will be running your rails >35V, sinking the drivers is also recommended. As 42V and above is approached, it is required.
Wiring
If you used the form above, the box below will list the minimum gauges for the mains wiring and the internal supply wiring. The mains wiring is worst case, and assumes that the amp will be producing 100% of its rated power constantly. This is never the case for most amps, so you may wish to consult this guide and choose for yourself. As for the supply wiring, larger gauges will minimize ohmic losses.
Ground wiring should use a star return to a central bus, preferably 1mm solid copper plate. This includes transformer center taps, supply caps, amp grounds, and speaker returns. Signal wiring should be shielded (ground only one end of the shield).
Layout
Proper layout is extremely important in a power amp design. Fully addressing this issue is out of scope here, but there are some basics that should be considered:
- Star grounding
- Minimize supply cable impedance
- Keep transformers physically separated from the amp boards
- Separate power/power-ground and signal wiring; cross at right angles where unavoidable
- Use a metal chassis for RFI shielding; a ferrous chassis will provide some EMI shielding
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