FAQ
TL;DR: A 1 MHz input‑triggered oscillation destroyed SC200 Q14; fixing the insulated RCA ground stopped it. “An open input-ground connection would explain the oscillation.” [Elektroda, Anonymous, post #21679355]
Why it matters: If your SC200 howls the moment you plug in a PC sound card, check the input ground before swapping parts.
For: SC200 builders troubleshooting ultrasonic oscillation, blown outputs, or unstable behavior with source equipment.
Quick Facts
- Reported oscillation: about 1 MHz at ~0.4 Vpp, halved to ~0.2 Vpp by paralleling another 100 nF output cap. [Elektroda, Anonymous, post #21679340]
- Symptom trigger: plugging in a PC sound card with insulated RCA ground; open ground caused oscillation. [Elektroda, Anonymous, post #21679354]
- Stable conditions: input shorted or left open showed no oscillation on the faulty channel. [Elektroda, Anonymous, post #21679339]
- Output network: L2 ≈ 2.2 µH in series with output, paralleled by 6.8 Ω damper to suppress RF issues. [Elektroda, Anonymous, post #21679338]
- Transistor choice: BC546A vs BC546B gain did not materially change loop behavior in this design. [Elektroda, Anonymous, post #21679348]
How do I fix SC200 oscillation when I connect a PC sound card?
Verify the RCA shell actually bonds to amplifier ground. The reported fault was an insulated input jack that left the source ground floating. Once the shell was grounded, the 1 MHz oscillation disappeared and normal behavior returned. [Elektroda, Anonymous, post #21679354]
What’s the quickest test to confirm a missing input ground?
With the input otherwise open, clip a jumper between the PC sound card’s sleeve (ground) and the amp input shell. If the oscillation appears or changes when you move the jumper, the ground path is suspect and must be fixed. [Elektroda, Anonymous, post #21679353]
Can a bad load make things worse during debugging?
Yes. A temporary 5 Ω load pulled the positive rail into current limit and extinguished the rail LED. That indicates heavy current and risk to outputs. Use a proper dummy load and current‑limited supply during tests. [Elektroda, Anonymous, post #21679340]
Does using BC546B/BC556B instead of A‑grade devices cause instability?
No. The designer noted the stage gains and impedances make hFE differences negligible here. Quote: “The SC200 is an excellent design.” Keep B‑grade parts; fix grounding and compensation instead. [Elektroda, Anonymous, post #21679348]
What simple RC tweak can tame ultrasonic oscillation during diagnosis?
Try a 1 nF capacitor in series with 100 Ω across the collectors of Q1 and Q2. This compensation network has stopped HF oscillations in similar topologies. Remove after root cause is fixed if not needed. [Elektroda, Anonymous, post #21679341]
Will adding more output capacitance help?
Paralleling an extra 100 nF from output to ground halved the measured oscillation amplitude from ~0.4 Vpp to ~0.2 Vpp. It doesn’t fix a wiring fault, but it can stabilize testing. [Elektroda, Anonymous, post #21679340]
Should I install the input ferrite bead or an inductor?
Yes. If the bead is missing, fit it or try a small 2.2 µH inductor at the input. The goal is to attenuate RF ingress that can excite the loop. [Elektroda, Anonymous, post #21679343]
How do I check whether the oscillation is entering through the input ground path?
Scope both ends of the 10 Ω input‑to‑ground resistor with the sound card connected. If no RF appears across it, inspect the RCA shell isolation and ground wiring first. [Elektroda, Anonymous, post #21679343]
What is the ‘Miller capacitor’ in this context?
It’s the small capacitor across the VAS transistor that sets the dominant pole and overall loop stability. Replacing it alone did not cure the reported fault because the true issue was the open input ground. [Elektroda, Anonymous, post #21679342]
Could magnetic coupling from the output inductor L2 trigger oscillation?
It can contribute. The tall air‑core coil radiates field. Try moving steel or aluminum near it and observe changes, but treat this as secondary to correcting input grounding. [Elektroda, Anonymous, post #21679353]
What’s the safe three‑step process to diagnose SC200 oscillation?
- Power via current‑limited rails; verify DC offset near 0 mV at no load.
- Confirm RCA shell continuity to signal ground; fix any insulated jack issue.
- If needed, add 1 nF//100 Ω across Q1–Q2 collectors and an extra 100 nF at output for testing. [Elektroda, Anonymous, post #21679339]
Does the problem appear with input shorted or left open?
No. The faulty channel was stable with input shorted or open, and only oscillated when connected to the PC source with the ground floating. [Elektroda, Anonymous, post #21679339]
What statistic characterizes the failure mode?
The instability presented as roughly 1 MHz oscillation at the output. After adding 100 nF in parallel at the output, amplitude dropped by about 50%. [Elektroda, Anonymous, post #21679340]
Will replacing Q7/Q8 or small caps fix a ground fault?
No. Replacing Q7, Q8, the BAV21, and compensation caps did not stop the oscillation until the RCA ground issue was corrected. Prioritize wiring integrity. [Elektroda, Anonymous, post #21679347]
Do commercial amps react similarly if the source ground is left floating?
Yes. The user observed comparable misbehavior on CYRUS and QUAD units when the input shield was not grounded, confirming the importance of that connection. [Elektroda, Anonymous, post #21679354]
