3 Tube Stereo Amplifier Without Transformer

4 watts per channel max. output

This is a simple amplifier with tone and volume controls for the do-it-yourselfer. It features a line level input, a loudness type of volume control, and separate bass and treble controls for each channel. These controls may be ganged together or be a separate control to control the left and right channels individually. Also a feature of this amp is a separate center channel output for low frequencies. This amplifier is so simple and easy to build that several could be built cheaply for surround applications.

Schematic:

As you can see from the above schematic, a half wave voltage doubler is used in the power supply. Also note the use of a three-prong plug. 

Notes:

• Volume control is 2 meg with a tap at 1 meg
• Output transformers T1 and T2 have a primary impedence of 1700 ohms. The secondary should match the speakers you intend on using. T3, which is the center speaker is 4400 ohms, center tapped and should also match your speakers.
• Unless noted, all resistors are 1/2 watt, carbon composition
• Unless noted, capacitors are in micro Farads (mF). Micromicro Farad caps (mmf) should be silvered mica, 500v. Electrolytics should be 350v rated, and all other capacitors should be metalized polyester, 630v rated.
• Since the 12AX7 tube is getting expensive because of audiophiles, two 6AV6 tubes with their filaments wired in series may be used, or two 12AV6 tubes with their filaments wired in parallel may be used in place of the single 12AX7. Each triode unit of the 12AX7 is identical to the triode of the 6/12AV6. Tie the plates of the 6/12AV6 diodes to the cathode. Consult a tube manual for proper basing.
• Speakers having an efficiency greater than 85db/watt should be used.

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