FAQ
TL;DR: Up to 42 °C heatsink temperature rise after 5 min at 50 % output, “BD911/912 failures are now 3× likelier due to fakes” [Elektroda, Baskhaal, #19205810; Elektroda, Ibuprom, #19217425].
Why it matters: Selecting robust parts and layout tweaks prevents costly transistor or speaker damage in Rotel-style DIY amps.
Quick Facts
• Original Rotel RA-820 output pair: 2SD896/2SB776, 80 V/10 A/60 W each [Rotel Service Manual].
• Common replacements: 2SC5200/2SA1943 (~11 PLN per pair) rated 230 V/15 A/150 W [Elektroda, Ibuprom, post #19222481]
• Soft-start relays become useful only above ≈300 VA; 120 VA trafos draw ≤1 A inrush [Rosini, 2019].
• XS3868 Bluetooth modules show −90 dBm RSSI without antenna, −55 dBm with external antenna [Elektroda, Owen27, post #19205091]
• Typical quiescent current for this copy: 30–40 mA per channel at ±35 V rails [Elektroda, Owen27, post #19205091]
What caused the repeated burnout of BD911/BD912 output transistors?
Forum builders link the failures to counterfeit BD911/912 parts that lack die area and safe-operating margin [Elektroda, Owen27, #19205091; Elektroda, Ibuprom, #19217555]. Substituting genuine audio pairs such as 2SC5200/2SA1943 cured identical amplifiers [Elektroda, Ibuprom, post #19217425]
Is a soft-start circuit necessary with a 2×27 V / 120 VA toroidal transformer?
No. Inrush current for a 120 VA toroid is well below relay contact limits; builders call the soft-start “unnecessary for such low power” [Elektroda, rosomak19, post #19207302] Omit it unless you later upgrade above roughly 300 VA.
How hot do the enclosed heatsinks get?
With fins partially blocked they reach about 42 °C above ambient after five minutes at 50 % rated power and approach 60 °C at full power [Elektroda, Baskhaal, post #19205810] Ensure top and bottom vents remain open.
What grounding mistake can trigger oscillation?
Connecting the small-signal ground (R604/C604/C606) directly to the power ground invites parasitic loops. Run a separate thick trace or wire to the main filter-cap ground, or insert a 10 Ω isolating resistor [Elektroda, MUGOL 2, #19223316].
Why does the XS3868 Bluetooth board hum during silence?
The PCB omits output filtering and proper decoupling. Users report tape-like noise and idle hum [Elektroda, tytka, post #19217392] Adding a 100 µF + 0.1 µF pair across VCC/GND and a 1 nF shunt on the audio outputs reduces the artefacts, but swapping to a VHM-314 module eliminates them entirely [Elektroda, tytka, post #19224637]
Can I mix the XS3868 audio ground with system ground?
Yes in this build: Owen27 ties AGND and GND together without audible interference [Elektroda, Owen27, post #19211925] For portable gear, keep them isolated to avoid digital noise [Elektroda, Owen27, post #19211925]
What is the easiest transistor upgrade path on a TO-220 board?
If the PCB limits pin spacing, install TIP35C/TIP36C (25 A, 125 W) which share the TO-220 footprint [ST Datasheet]. For optimal audio performance, redesign pads for TO-3P/TO-247 and fit 2SC5200/2SA1943 pairs.
How do I stop toroidal transformer hum coupling into the preamp?
- Rotate the toroid on its bolt while listening; the field has a null [Elektroda, Ibuprom, post #19212938]
- Add a steel or copper flux band around the core.
- Route AC wiring at least 50 mm from signal traces and twist the leads [AD UM-1225]. Edge case: some 1.5 kVA mains-grade toroids radiate so strongly that only an audio-grade shielded core fixes the issue [Elektroda, rosomak19, post #19210401]
How were the PLA control buttons mounted without visible screws?
The printed blue plastics are cyanoacrylate-bonded (“Super-glue”) to the aluminium faceplate; the PCB then screws into the plastic posts from behind [Elektroda, padus ↔ Owen27, #19237119].
What current can the speaker-output Zobel coil handle?
Commercial 0.6 µH air coils rated 5 A were used, adequate for the amp’s ≈3 A peak into 8 Ω loads [Elektroda, Owen27, #19211925; Coilcraft Data].
Edge case: What happens if you hot-plug a source with volume up?
A hot connection can inject a large HF spike, forcing the bias servo to latch high and short-ing the output pair—exactly how both BD911/912 sets were destroyed during setup [Elektroda, Owen27, post #19205091] Use muted relays or power down before swapping inputs.
How can I add MCU-relay isolation to prevent firmware freezes?
Insert an optocoupler or at least an RC snubber (100 Ω + 100 nF) on each relay coil line. Long unshielded control leads have caused STM32 hangs during switching events [Elektroda, katakrowa, post #19206048] Expert quote: “High-voltage pulses directed directly to CPU pins are problematic.” [Elektroda, katakrowa, post #19206048]
3-step guide: Replacing fake BD transistors with genuine audio pairs
- Desolder BD911/912, clean pads, and verify driver bias (≈0.7 V across Vbe multiplier).
- Fit 2SA1943/2SC5200, attach to insulated TO-3P pads, and set quiescent current to 35 mA.
- Load test at 1 kHz, 8 Ω, monitoring idle dissipation; readjust after 30 min thermal soak.
