Mechanical Layout and Design Considerations
The mechanical design adopted depends very much on the intended market and production and tooling resources, but I offer a few purely technical points that need to be taken into account.
All power ampliﬁers will have a heat sink that needs cooling, usually by free convection, and the mechanical design is often arranged around this requirement. There are three main approaches to the problem.
a. The heat sink is entirely internal and relies on convected air entering the bottom of the enclosure and leaving near the top (passive cooling).
The heat sink may be connected to any voltage, which may eliminate the need for thermal washers between power device and sink. However, some sort of conformal material is still needed between transistor and heat sink. A thermal washer is much easier to handle than the traditional white oxide-ﬁlled silicone compound, so you will be using them anyway. There are no safety issues as to heat sink temperatures.
Because of the limited ﬁn area possible inside a normal-sized box and the relatively restricted convection path, this system is not suitable for large dissipations.
b. The heat sink is partly internal and partly external, as it forms one or more sides of the enclosure. Advantages and disadvantages are much as just described; if any part of the heat sink can be touched, then the restrictions on temperature and voltage apply. Greater heat dissipation is possible.
c. The heat sink is primarily internal, but is fan cooled (active cooling). Fans always create some noise, which increases with the amount of air they are asked to move. Fan noise is most unwelcome in a domestic hi-ﬁ environment, but is of little importance in PA applications. This allows maximal heat dissipation, but requires an inlet ﬁlter to prevent the build-up of dust and ﬂuff internally. Persuading people to clean such ﬁlters regularly is near impossible.
Efﬁcient passive heat removal requires extensive heat sinking with a free convective air ﬂow, and this indicates putting the sinks on the side of the ampliﬁer; the front will carry at least the mains switch and power indicator light, while the back carries the in/out and mains connectors so that only the sides are completely free.
The internal space in the enclosure will require some ventilation to prevent heat build-up; slots or small holes are desirable to keep foreign bodies out. Avoid openings on the top surface as these will allow the entry of spilled liquids and increase dust entry. BS415 is a good starting point for this sort of safety consideration, and this speciﬁes that slots should be no more than 3 mm wide.
Reservoir electrolytics, unlike most capacitors, suffer signiﬁcant internal heating due to ripple current. Because the electrolytic capacitor life is very sensitive to temperature, mount them in the coolest position available and, if possible, leave room for air to circulate between them to minimize the temperature rise.
It is important to realize that the buoyancy forces that drive natural convection are very small and that even small obstructions to ﬂow can seriously reduce the rate of ﬂow, and hence the cooling. If ventilation is by slots in the top and bottom of an ampliﬁer case, then the air must be drawn under the unit and then execute a sharp right-angle turn to go up through the bottom slots. This change of direction is a major impediment to air ﬂow, and if you are planning to lose a lot of heat then it feeds into the design of something so humble as the feet the unit stands on; the higher the better for air ﬂow. In one instance the ampliﬁer feet were made 13 mm taller and all the internal ampliﬁer temperatures dropped by 5°C. Standing such a unit on a thick-pile carpet can be a really bad idea, but someone is bound to do it (and then drop their coat on top of it); hence the need for overtemperature cutouts if ampliﬁers are to be fully protected.
A toroidal transformer is useful because of its low external ﬁeld. It must be mounted so that it can be rotated to minimize the effect of what stray ﬁelds it does emit. Most suitable toroids have single-strand secondary leadouts, which are too stiff to allow rotation; these can be cut short and connected to suitably large ﬂexible wire such as 32/02, with carefully sleeved and insulated joints. One prototype ampliﬁer I have built had a sizeable toroid mounted immediately adjacent to the TO3 end of the ampliﬁer PCB; however, complete cancellation of magnetic hum (hum and ripple output level below -90 dBu) was possible on rotation of the transformer.
A more difﬁcult problem is magnetic radiation caused by reservoir charging pulses (as opposed to the ordinary magnetization of the core, which would be essentially the same if the load current was sinusoidal), which can be picked up by either the output connections or cabling to the power transistors if these are mounted off board. For this reason, the transformer should be kept physically as far away as possible from even the high-current section of the ampliﬁer PCB.
As usual with toroids, ensure that the bolt through the middle cannot form a shorted turn by contacting the chassis in two places.
There are several important points about the wiring for any power ampliﬁer:
● Keep the + and – HT supply wires to the ampliﬁers close together. This minimizes the generation of distorted magnetic ﬁelds that may otherwise couple into the signal wiring and degrade linearity. Sometimes it seems more effective to include the 0-V line in this cable run; if so, it should be tightly braided to keep the wires in close proximity. For the same reason, if the power transistors are mounted off the PCB, the cabling to each device should be conﬁgured to minimize loop formation.
● The rectiﬁer connections should go directly to the reservoir capacitor terminals and then away again to the ampliﬁers. Common impedance in these connections superimposes charging pulses on the rail ripple waveform, which may degrade ampliﬁer PSRR.
● Do not use the actual connection between the two reservoir capacitors as any form of starpoint. It carries heavy capacitor-charging pulses that generate a signiﬁcant voltage drop even if thick wire is used. As Figure 13.1 shows, the starpoint is teed off from this connection. This is a starpoint only insofar as the ampliﬁer ground connections split off from here, so do not connect the input grounds to it, as distortion performance will suffer.
● Driver transistor installation. These are usually mounted onto separate heat sinks that are light enough to be soldered into the PCB without further ﬁxing. Silicone thermal washers ensure good thermal contact, and spring clips are used to hold the package ﬁrmly against the sink. Electrical isolation between device and heat
sink is not normally essential, as the PCB need not make any connection to the heat sink ﬁxing pads.
● TO3P power transistor installation. These large ﬂat plastic devices are usually mounted on to the main heat sink with spring clips, which are not only rapid to install, but also generate less mechanical stress in the package than bolting the device down by its mounting hole. They also give a more uniform pressure onto the thermal washer material.
● TO3 power transistor installation. The TO3 package is extremely efﬁcient at heat transfer, but notably more awkward to mount.
My preference is for TO3s to be mounted on an aluminium thermal coupler bolted against the component side of the PCB. The TO3 pins may then be soldered directly on the PCB solder side. The thermal coupler is drilled with suitable holes to allow M3.5 ﬁxing bolts to pass through the TO3 ﬂange holes, through the ﬂange, and then be secured on the other
side of the PCB by nuts and crinkle washers, which will ensure good contact with the PCB mounting pads. For reliability, the crinkle washers must cut through the solder tinning into the underlying copper; a solder contact alone will creep under pressure and the contact force will decay over time.
Insulating sleeves are essential around the ﬁxing bolts where they pass through the thermal coupler; nylon is a good material for these as it has a good high-temperature capability. Depending on the size of the holes drilled in the thermal coupler for the two TO3 package pins (and this should be as small as practicable to maximize the area for heat transfer), these are also likely to require insulation; silicone rubber sleeving carefully cut to length is very suitable.
An insulating thermal washer must be used between TO3 and ﬂange; these tend to be delicate and the bolts must not be overtightened. If you have a torque wrench, then 10 Nw/m is an approximate upper limit for M3.5 ﬁxing bolts. Do not solder the two transistor pins to the PCB until the TO3 is mounted ﬁrmly and correctly, fully bolted down, and checked for electrical isolation from the heat sink. Soldering these pins and then tightening the ﬁxing bolts is likely to force the pads from the PCB. If this should happen, then it is quite in order to repair the relevant track or pad with a small length of stranded wire to the pin; 7/02 size is suitable for a very short run.
Alternatively, TO3s can be mounted off PCB (e.g., if you already have a large heat sink with TO3 drillings) with wires taken from the TO3 pads on the PCB to the remote devices. These wires should be fastened together (two bunches of three is ﬁne) to prevent loop formation; see earlier discussion. I cannot give a maximum safe length for such cabling, but certainly 8 inches causes no HF stability problems. The emitter and collector wires should be substantial, for example, 32/02, but the base connections can be as thin as 7/02.