Replacement of blown output transistors
I realize this post is 11 years old, none the less; there is a tremendous amount of information this was not addressed. Knowing these items will help you find and replacement transistors in any amplifier, or any circuit application. These are some of the steps an engineer would follow in the design of an electronic circuit:
First: Determine and remove all defective silicon device. With no load on the output, power the amplifier and measure the power supply voltage. If possible disconnect the power supply and take your measurement, the prior method is acceptable, unless you are a novice!
I have never seen inside a Bryston amplifier, I do have a complete library of there amplifier circuit schematics.
1. Knowing the power supply voltage will give you the ability to calculate the basic output power, not taking into consideration of minor losses, like voltage drop across the transistors during the full-on condition and the drop across Emitter resistors, or in this case Colleertor resistors. The Bryston 4B has a power supply of +-80-VDC or 160-VDC total, across a totem pole design. A Totem pole design has 4 sets of transistors in series, this will be important later in your analysis. With 160-VDC power supply, what would be the maximum AC Voltage before clipping at the output? To convert a DC value to the equivalent AC value is E/2.828, or 160/2.828 = 56.58 Vrms = E2/R = 56.58Sqared/8 = 400.1 watts, no doubt this is a 400-watt amplifier into an 8-ohm load. When selecting a suitable replacement transistor in this case, to get 400 watts RMS you need to have 4 times the amount of transistor power ability. In the 4B, 80 Vpeak Sq/8 x 2 = 1600-Watts p-p. This is due to the peak-to-peak power of the sin wave. We need a transistor that can handle a minimum of 200-Watts x 8 transistors = 1600-Watts.
2. A transistor secondary Brake Down Voltage, this is the voltage when the transistor will go to a none-destructive short circuit, Primary Breakdown is when the device has catastrophically failed. The device needs to have a breakdown voltage of at least 20% above the maximum voltage the device will operate under. Now back to the totem pole, the reason for this is, in 1988 there where few devices that could handle a 160 volt supply rail and when you did find one, it was not cost-effective, also the majority of high voltage devices at that time were suitable for flyback power supplies, not audio! The answer was, instead of having a two-transistor push-pull configuration, engineers went to a 4 transistor totem to lower the required voltage of the output transistors. Instead of a 200-volt device, you can use 8 200-Watt,100-volt devices. The bottom two transistors of the design are current sinks whos current output ane determent by the voltage applied to the emitters of the current sources, by the above complimentary voltage followers.
3. No matter what you do, besides using 200-V devices, on turn on it is possible for the amplifier to become unstable and for a brief moment, one transistor can experience the full 160 volts across it and it will take out all eight devices. Clearly Bryston figured this out, as they no longer use this design.
4. In 1988 we only had the 2n3773, an excellent device but it had no compliment. 150-watts @ 140-V.
5. The final thing you MUST do is bata match your devices, hears why the minimum Hfe (current Gain) is 15 of a 2N3773 but it can be 60 or more. Let us assume you are using 4 transistors, 2 have a bata of 20, 1-15, and the other 60. The two devices of 20 will operate together, the device that has a bata of 15 will not keep up and cause the two devices of 20 to work harder, making up for the bata of 15, now the last transistor with a bata of 60 is going to try to handle the full load, because it will give more current gain than the total of the other three devices. the amount of current than the other devices, under full power this amplifier will fail. If the devices have good consistency you can get by with buying 50% more than you need, if the devices have a poor yield you will need to buy at least twice as many devices than you need, so you might get the number of matched devices you required to repair the amp correctly, to factory spec
I hope this helps people trying to figure out power. My background is 47 years in the professional music business as Electronics Engineer. If you have a question you think I can answer, please do not hesitate to ask. It is my turn to give back!
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