A simple 2-transistor pre-amplifier
Sometimes we feel the urge of level-shifting an audio signal, example, guitar pickup, low-level device such as mp3 player (I hate mp3 apart from in-vehicle/plane music playing) ...). Sometimes we face a very simple application, requiring a single power supply with a very low power consumption, somewhere around 25mW so it may be battery-operated, 9 or 12V.
. Building a pre-amplifier out of opamps is possible at the cost of added complexity and comsumption, but I preferred something very simple to build, where all parts could be bought at your neighboring store.
2 BC550B or C grade transistors, 5 1/4W resistors and 3 small electrolytics will do the job. Flat frequency response, 20dB (10X gain), moderately low noise and distorsion are required.
The circuit is built around a degenerated common-emitter Q1 running at around 200uA of current for low noise which gain is approximately
the ratio of R3/R4. Q2 works as a common-collector buffer in order to lower to 100Ohm the output impedance which would otherwise be high (1.65KOhm). Input impedance may be increased by increasing R1 and R2 keeping its ratio. Increasing R1 to 470KOhm and R2 to 47KOhm will increase input
impedance to about 35KOhm while maintaining the other specs reasonably intact. Increasing beyond this is not recommended.
Measured specs @12VDC (Audio Precision Portable One Plus):
| Gain | 18.3dB |
| THD | 0.3% @0dBu,1kHz |
| Saturation | +13dBu (THD 2.5%) |
| Input-referred noise | -88dBu, 22Hz-22kHz unweighted |
| Frequency response | 20Hz (-1dB) to 32kHz -3dB, 20kHz -1dB |
| IMD | 50Hz/8kHz, 0.65% -3dBu, 0.2% -18dBu, 1.4% +3dBu |
| Input impedance | 4kOhm |
| Output impedance | 100 Ohm |
Try it, you may probably like its performance versus simplicity ratio... Usable at 9VDC as well, headroom will lower in consequence.
