An easy to assemble stereo amplifier for students and beginners in electronics. All the components and PWB are easily available in the electronic spare part market; and cost effective too.
This brand new unit from the world's best known amplifier manufacturer is simply bristling with exciting new features.To start with there's the power output; a genuine 35 watts per channel.
Furthermore, there's the distortion (or the lack of it); a mere 0.1% at 1000 Hz for all power outputs up to 25 watts per channel.
And, what's more, in addition to the features the discerning audiophile expects on a Leak amplifier we have made provision for the following:
The use of two turntables and pickups, selection between the two being made by the input switch on the front panel.
The use of a pair of loudspeakers in a location other than the main listening room.
The use of stereo headphones for personal listening as an alternative to loudspeakers.
The use of a portable tape recorder which can be plugged into the DIN socket on the front panel.
All this, backed by the famous Leak craftsmanship and reliability, makes the Stereo 70 the most technically advanced amplifier available, and it sounds it.
Circuit Diagram [Click on the schematic to magnify]
Power output: 35 watts per
channel into 8Ω (stereo)
Frequency response: 20Hz to 20kHz
Total harmonic distortion: 0.1%
Damping factor: 20
Input sensitivity: 2mV (mic), 2mV (MC), 10mV (MM), 30mV (ceramic), 250mV (line)
Signal to noise ratio: 56dB (mic), 56dB (MM), 66dB (line)
Speaker load impedance: 4Ω to 15Ω
Current adjustments (This is a critical adjustment, so do it with extreme care)
The quiescent [no drive] current in the input stages of the stereo-70 amp: has been set at the factory at 30mA and it should not be necessary to reset this current unless a driver transistor ot an output transistor has been replaced.
To re-adjust this current, two red flexible current links connect the collectors of T10L and T10R to 75VDC line, and these wire links should be disconnected, and an ammeter inserted [positive terminal to 75VDC line] to measure the quiescent current of the output transistors. This current can then be re-adjusted to 30mA by re-setting P5L and P5R.
Do not switch ON the amplifier without current links or ammeters in place as this will damage the driver transistors.
Do not short together the lead-out wires of the transistors as this will cause the failure of the transistor, particularly under drive conditions.
Frequency response: 20Hz to 20kHz
Total harmonic distortion: 0.1%
Damping factor: 20
Input sensitivity: 2mV (mic), 2mV (MC), 10mV (MM), 30mV (ceramic), 250mV (line)
Signal to noise ratio: 56dB (mic), 56dB (MM), 66dB (line)
Speaker load impedance: 4Ω to 15Ω
Current adjustments (This is a critical adjustment, so do it with extreme care)
The quiescent [no drive] current in the input stages of the stereo-70 amp: has been set at the factory at 30mA and it should not be necessary to reset this current unless a driver transistor ot an output transistor has been replaced.
To re-adjust this current, two red flexible current links connect the collectors of T10L and T10R to 75VDC line, and these wire links should be disconnected, and an ammeter inserted [positive terminal to 75VDC line] to measure the quiescent current of the output transistors. This current can then be re-adjusted to 30mA by re-setting P5L and P5R.
Do not switch ON the amplifier without current links or ammeters in place as this will damage the driver transistors.
Do not short together the lead-out wires of the transistors as this will cause the failure of the transistor, particularly under drive conditions.
