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If an amplifier precedes the mixer, its gain makes intermodulation at the mixer worse. We'd like to keep signal levels down as low as possible going into the mixer, yielding high dynamic range. … It is a general "rule-of-thumb" that amplifier noise figure is superior to mixer noise figures. …

OP's circuit does generate voltage spikes at MOSfet drain that can considerably exceed the DC supply voltage. Switching current off quickly to an inductor is the cause. The circuit fragment of the few …

R4 keeps the amplifier stable, and protects it from short-circuit loads. R6 is a "pop" protector. You may notice that on power-up it will still "pop" as C5 discharges. … It is standard to remove DC offsets from amplifier outputs. That's basically what C5, R6 does. …

It is not a class-A amplifier - loading is asymmetric and seems to cause amplitude and frequency instability. If you intend to use a buffer stage, it should operate linearly (Class-A). …

TLDR: Not always an indication of instability - a peak might be intended. However, an internally-compensated op amp is very often used to amplify linearly, where two resistors are chosen to set closed …

Why doesn't the output then swing to the positive or negative voltage rail if the voltage at the negative input terminal fluctuates sinusoidally? Voltage at opamp "-ve" input doesn't fluctuate from …

[...] the author discusses the function of RC1, mentioning that RC1 helps set Ic1 equal to Ic2. Why is it important to equalize Ic1 with Ic2? This is a matter of DC bias stability. With no input sig …

Those high-frequency components could be filtered with an LC filter, but the low-frequency 120 Hz component requires a different approach. One must be careful that the 120 Hz component doesn't ride in …

That's a lot of open-loop gain, and breadboard construction doesn't help. Oscillations could be via Vcc or ground-loops. Add a monster electrolytic from TIP41C collector to TIP42C collector (Vcc-to-GN …

Your simulation drives the amplifier input with a signal source having zero source resistance - very unfair. … That 4400 ohm shunt feedback is the cause of such low amplifier input Z. Q2 will run fairly warm. Its 83 mA collector current is quite high. …

Mixing in high-level AUX audio along with low-level electret microphone. Can be done a few different ways: Add two sources in series Apply one source to LM386 "+" input, other source to LM386 "-" inp …

At the bias point of \$V_{CE}=1V, I_C=100mA\$, the transistor is in its linear range; for example when used as an amplifier. …

Visual heat darkening is of concern but can also lead you astray. Black charring is a definite red-flag. You might measure Q801 emitter "E" with a DC voltmeter - it should be near +15V above GND. It …

A simulation... One cycle of a 1kHz wave, with a ridiculously large DC supply voltage (so as to minimize the transistor's base-emitter cross-over loss of about 0.6V). The transistors are LTspice defau …

OP's first reference circuit is a regenerative amplifier. … But grounding may be important for the high-gain amplifier that follows. Should I shield my circuitry? …