A one transistor amplifier connected to iPad both for reproduction and production

At your own risk.


Apn amolifier with one electronic amplifying device is perhaps the first (1907? by Lee de Forest) and miraculous amplifier. It operates in class A (electrons flowing all the time).


A classic example of such an amplifier with just one electronic amplifying device is the amplifier inside the Neumann U47 microphone. It uses the VF14 electron tube with underheated cathode.


Now back to iPad or other devices.


A one transistor class A operating amplifier can be connected to the headphone output and drive a loudspeaker. For example the Pleiadss 2N3053 driving the high impedance Philips AD5046N loudspeaker. (Construction details are found on YouTube on one transistor amplifiers). The quality is so high that it is dictated by what is in the input perhaps with no limit. The better the quality at the input the better the quality at the output. The speaker is the collector load itself instead for example of a collector resistor. This Pleiades setup is usually used connected to a Sony portable CD Walkman at its bridged to mono line output. A series resistor to the transistor base terminal controls volume. A coupling capacitor is not used but it may be needed for an iPad etc, at your own risk. Bias to the transistor is by a resistor from collector to base. A heatsink should normally be used for safety and protection including thermal runaway.


Now how about connecting such an amplifier at the input so as to amplify an external microphone and then connect to mic in of iPad or other mic in device.


So far a JFET K117 mic booster amplifier has been described on previous posts. The Pleiades K117. The drain resistor is provided by the iPad itself while supplying power too from its mic headphone socket. Various world class microphones have been connected such as the Sennheiser MD21 HN with fantastic sound quality and ultra low noise. (Many devices have an internal low cut filter).


A very low anode voltage electron tube mic head amplifier with the iPad supplying the anode voltage from its load resistor is briefly described on a recent previous post. Pleiades bias is by a high resitor from anode to grid. This schematic is essentially a Pleiades V6 schematic with no output transformer. Electron tubes are miraculous devices for production.


But transistors too. So why not connecting a bipolar one transistor mic amplifier to the iPad's input in order to boost a microphone. Again the collector resistor will be the internal device load resistor or voltage supplying resistor. The role of a load resistor is to convert the amplified output current to output voltage, by Ohm's law. (Power is exactly the same as on the usual electret microphone powering of the internal mic JFET.)


Bias can be from collector to base by a suitable resistor. Adjusting the resistor can lead to very interesting distortion for electric bass synths etc. The mic, electric guitar, etc is connected to the transistor base through a coupling capacitor. And an inout transformer may be used too. A guitar or synth may give a too hot signal. At your own risk.


The common cathode, emitter of source amplifier. The simplest and perhaps the most fabulous sounding amplifier on the planet.


Inspiration for using bipolar transistors on top quality studio amplifiers can be found on the classic Neve preamplifier circuits. Again close to the first amplifier made on our planet. The class A one stage circuit.





Bibliography:


Tubes vs Transistors (vs op amps), is there an audible difference? - Russel O. Hamm - JAES