Sound distorted from amp configuration, and one transistor variety getting too hot

Question

The sound is distorted from my configuration. The transistor I'm using is an MPF4393. I need to add resistors and capacitors and how do I get more power (to make it louder) What should I add? Also when I use a 2N4401 instead, it gets too hot.

The configuration is: positive input at the base, emitter grounded, collector to positive speaker terminal, negative terminal hooked up to a variable voltage source (1.3v - 15v) & negative input source to the collector

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

Try something like this (see the 2nd schematic, for your MPF4393 FET usage).

This bipolar circuit, fine for the 2N4401, uses the 100 ohm resistor to limit current and provide biasing to approximately VDD/2. The 10Kohm provides base current. We use a ratio of 100:1 for base_resistor to collector_resistor, because many bipolar transistors have approximately Beta=100. If 100, then the circuit self-biases the Collector to approximately VDD/2, giving maximum linear operation and predictable power dissipation.

If input is 1 millivolt PeakPeak, with with transistor biased at 15mA and with amps_out/volts_in of 600 milliAmps per Volt, you will only have 0.6 milliAmps useful output (signal-related) current. Using Power = I^2 * R, or 0.6mA * 0.6mA * 8_Ohms, your signal power at the speaker (in the mid-band audio, say 1KHz) is 0.36 microWatts * 8 = 30 microWatts of power at Vbase of 1milliVoltPP. [error: accounting for converting from PeakPeak to RMS, the output power is about 4 microWatts]

For inputs much larger than 1mVPP, say about 10mVPP, you'll begin to hear lots of distortion in the tone/voice/music. An emitter resistor, say 1 ohm or 2.2 or 3.3 ohms, will help linearize the amplifier.

^{simulate this circuit – Schematic created using CircuitLab}

But you were first experimenting with MPF4393, an FET Nchannel, that with gate-source shorted will provide some current; rated at 5ma -- 30mA, with VDD of 15 volts. That wide range of current: 5ma to 30mA, makes the FET difficult to work with. That is why I offered the bipolar. I've seen NJFETs be very popular for very low noise preamplifiers for vinyl-record audio playback signal sources, BUT the experimenters take lots of time to PRESELECT the FETS.

At 15 volts, with gate at zero volts (grounded) and source also grounded, the MPF4393 may become (quickly) way too hot; the max heat is 0.35 watts in air of 25 degree Centigrade; if the FET (gate and source both at zero volts) has 30mA (0.03 amps) at 15Volts, the power is 0.45 watts which by exceeding the rated 0.35 watt will likely reduce the lifetime.

So you can preselect your FET (as people on the diyAudio.com "simplistic NJFET RIAA" have done), to operate at 10 or 15mA at 6 or 9 or 12 volts. OR you can insert a resistor in the source, value 100 ohms or 47 ohms, to greatly dial back the worst-case drain current.

Like this

^{simulate this circuit}

For your 2nd FET project, you might build one of the world's lowest noise RIAA_compensation 3-transistor preamplifiers. The experimenters did preselect for 6mA at 10 volts with gate-source both tied to ground. The circuit has low-noise first stage, followed by RIAA compensation (some people search the Internet for special Russian Teflon capacitors and Mica capacitors; result can be stunningly accurate left-right music imaging), then followed by more gain, followed by a buffer stage (Av = 1) to drive a power amplifier thru a few feet of coax cable. Like this

^{simulate this circuit}