The β22 Stereo Amplifier

The circuit board

The β22 circuit board is made of high quality FR-4 glass epoxy, double-layer with plated-through holes as well as silkscreen and soldermask. The copper layers are 2 ounce heavy duty construction for low impedance. There is a ground plane on each side of the board for lowered ground impedance and improved shielding.

The following diagram illustrates some important board dimensions. It can be used as a reference when you work on the chassis enclosure.



Below are photo images of the top and bottom of circuit board. They are not in actual size.





The circuit board layout is shown below. The top layer is shown in red, the bottom layer is in blue, areas where there are traces in both the top and bottom layers are in lavender, and the top silkscreen is in light grey.



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Heatsink options

There are three MOSFET heatsinking options, one onboard and two offboard, as shown in the following illustration.

Onboard heatsinks

The default configuration is to use four onboard heatsinks (e.g., Aavid-Thermalloy 529802B00000). This is the easiest option and allows the β22 to drive any headphone load, or light-duty speaker applications. The enclosure housing the β22 should be well ventilated. If higher sustained output currents are required (e.g., when driving speakers at moderately loud volumes continuously, or when using inefficient or 4Ω speakers), then larger, offboard heatsinks should be used.

Offboard-heatsinks - with angle brackets

This option requires two angle brackets per board, one for each side to couple the MOSFETs to an offboard heatsink. The bracket should be no thicker than 15/64" (6mm) to assure that there is enough remaining solderable MOSFET pin length on the bottom side of the board. The MOSFETs' pins should be bent 90° back. See the following picture for bending dimensions:



The following illustration shows an example angle bracket made from 3/4" x 3/4" x 1/8" stock:



Aluminum angle stock can be found at your local hardware stores, or internet metal shops such as Online Metals who offers custom cutting service.

Offboard-heatsinks - bottom-mount

For this option, the MOSFETs are mounted below the board, and secured to either the case bottom or to a piece of large heatsink. The MOSFET mounting screws can be accessed through the holes on the board, using a miniature screwdriver. You may also enlarge the holes slightly if needed, as long as the ground plane or traces are not breached. The MOSFETs' pins should be bent 90°, and soldered to the board on the top side. The pin bending measurements are the same as in the angle-brackets option, except the pins are to be bent forward instead of backward. This method requires that the board standoffs to be no higher than 5/32" (4mm) in order to have enough remaining solderable MOSFET pin length on the top of the board.

Heatsink sizing considerations

The heatsink should have low enough thermal resistance to support the amount of heat to be dissipated. Bigger and more efficient heatsinks have lower thermal resistance specifications. To determine whether a heatsink is adequate, the power disspation must first be calculated.

Since there are four MOSFETs per board (the two closest to the supply rails are the cascode MOSFETs, and the two closest to the output terminal are the amplifier MOSFETs), the power dissipation per board is the sum of the dissipation of each of these devices. If you are using individual board-mount heatsinks for each MOSFET, then this calculation is done on a per-device basis. If all four MOSFETs are to be mounted on a common heatsink, then the calculation must be done based on the total board dissipation.

Use the following formula:

P = V * I

Where,
  • P = power dissipation in Watts. This is given off as heat.
  • V = voltage across the MOSFET. The amplifier MOSFETs' each have 8V across them, and the cascode MOSFETs each have (V+ - 8V) across them. For example, if the supply voltage is ±30V, then each cascode MOSFETs have 22V across it.
  • For headphone amp application, I = quiescent current of the output stage, typically 160mA. For speaker amp application, I = output current. To determine this current, we use the following formula (assuming a load impedance of 8Ω):

    I = √(Po / 8)

    Where Po is the output power of the amplifier. For ±30V rails, this would be 18W, and for ±24V rails, this would be 12W. Note that this gives the continuous current assuming that the amplifier is playing a sine wave at maximum output all the time. When playing real music the average current is a lot less, you can safely use 30% (which corresponds to about -10dB) of the calculated current, as long as the amplifier is only to be used in a home audio application.
Next, we determine if a heatsink (given its thermal resistance rating in °C/W) is sufficient. To do that, we calculate what the MOSFET internal junction temperature would be and check if it would approach the maximum rating of the device with some headroom.

The formula is:

Tj = Ta + (P * (Rjc + Rcs + Rsa))

Where,
  • Tj = junction termperature
  • Ta = ambient temperature
  • P = power to be dissipated
  • Rjc = thermal resistance of junction to device case
  • Rcs = thermal resistance of device case to heatsink
  • Rsa = thermal resistance of heatsink to ambient
For the IRFZ24N and IRF9Z34N MOSFETs, Rjc is 3.3°C/W. Ta is the highest ambient termperature you expect the circuit will operate under (note that if the circuit is going to be in an enclosure, then Ta will be higher than room temperature!). P is the power dissipation we calculated. Rcs is the sum of the thermal resistance of any mounting isolation pads, heatsink compound, etc., and you can look that up from their datasheets. And Rsa is the thermal resistance of the heatsink itself.

Calculating for Tj, and comparing it to the MOSFETs' maximum junction temperature of 175°C, you can determine if the heatsink is good enough. Even though these IRF MOSFETs are very rugged, you should avoid letting the MOSFET junction temperature exceed 100°C to prevent reduced device lifespan. Under extreme conditions, forced air cooling (e.g., fan) may become necessary.

Other heatsink notes

  • Heatsinks should always be mounted with the fins oriented vertically for effective convection.
  • TO-220 isolation pads must be used on the MOSFETs for all the offboard configurations.
  • For the offboard heatsink with angle bracket option, heatsink compound should be applied to the mating surface between the bracket and the heatsink to aid heat conduction.
  • The angle bracket mounting holes on the β22 circuit board has only a small clearance between the edge of hole and the bottom side ground plane. To avoid a short circuit (which could occur if the soldermask is breached), you should use an electrically nonconductive washer between the screw head or nut and the board surface.

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