Hello Experts, I have a question regarding the replacement of transistors in my AudioAnalyse A9 amplifier (Pure Class A, 50WPC). This amp delivers excellent sound quality, which I really enjoy (sweet, calm, with a wide soundstage). Unfortunately, two MJ15015 transistors have shorted, and after reviewing the schematic, my technician friend raised a few concerns and suggestions. I would appreciate your expert opinion on the following:
(Schematic attached)
- Why are the same transistors (MJ15015/MJ15016) used both in the output and driver stages?
- Can we replace the MJ15015/MJ15016 transistors with MJ21193/MJ21194 across the board?
- Considering the different HFE values (20 vs. 75) and Ft ratings (1MHz vs. 4MHz), would this be an upgrade or potentially cause issues?
- If MJ21193/MJ21194 can be used in the output stage, should the driver stage transistors also be replaced?
- Or should we stay with the original value transistors?
(Schematic attached)
Attachments
1. Because it is being biased into class A. One can get away with low speed driver devices if the outputs and drivers NEVER cut off. It’s the cutting on and off that requires speed. Normal “driver” types are often fragile at high power dissipation, and a more rugged device is advised here. Since the rugged power device doesn’t cause any other problems, it is used.
2. Yes. The originals were chosen because they were cheap. At the time, they were. Now they still are cheap*er* but the gap to other, much better, types is closing.
3. Potentially an upgrade, although one is not likely to really hear it. Just less likely to end up with shorted transistors again. That circuit topology is used in regular ordinary class AB amplifiers all the time, with higher gain faster driver transistors (and often outputs too).
4 I would, but not a requirement. As a general rule I don’t like MJ15015/6. Gain can be pathetically low on most of them (20 at 4 A, which is “enough” here but falls to single digits as low as 7 A), and very poor uniformity and matching among like type. When used class AB where current swing is high they basically suck.
2. Yes. The originals were chosen because they were cheap. At the time, they were. Now they still are cheap*er* but the gap to other, much better, types is closing.
3. Potentially an upgrade, although one is not likely to really hear it. Just less likely to end up with shorted transistors again. That circuit topology is used in regular ordinary class AB amplifiers all the time, with higher gain faster driver transistors (and often outputs too).
4 I would, but not a requirement. As a general rule I don’t like MJ15015/6. Gain can be pathetically low on most of them (20 at 4 A, which is “enough” here but falls to single digits as low as 7 A), and very poor uniformity and matching among like type. When used class AB where current swing is high they basically suck.