Beskrivelse: Stereo forforstærker med indgangsvælger
 Varenr: SK1050
Byggesæt:
En Hi-Fi stereo forforstærker til den kræsne. Trykknapomskifter til valg af inputs fra: Tuner (200mV), båndoptager (200mV) og Pick-up med RIIA korrektion (3mV). Scratch filter, mono/stereo omskifter og kontroller for bas, diskant, volumen og balance. Frekvensområde 15Hz til 30kHz. Kan anvendes som indgagnstrin for enhver udgangsforstærker. Udgang til effektforstærker 1,5V. Mål: 72x275mm
How it Works
The circuit is very simple. It consists of four transistors for each channel which form three distinct stages. Please note that throughout the description of the circuit and the construction instructions the components that appear in both channels are marked with two numbers in their code, one of which is enclosed in parenthesis (i.e. TR1(2)). The number in parenthesis is that of the same component in the other channel and is marked thus to help you find it in case you have to make a repair, and also to help you identify the component when you are building the circuit. The first stage which is built around TR5(7) & TR6(8) is the P.U. (magnetic head) preamplifier and RIAA equaliser. As you probably know the information on the record is recorded with a lot of emphasis added to the high frequencies and it is necessary to bring the signal back to normal before listening to the record because otherwise it would be unnaturally distorted with very predominant high frequencies. After the P.U. input circuit there is another voltage amplifier stage using TR3(4) and the output of this circuit is passed on to the tone control which is a classic, BAXANDALL type, passive tone control circuit. The final stage which is also the output of the preamplifier uses the fourth transistor TR1(2) and in it there are the volume and balance controls of the circuit. There is also a power supply stage on the circuit which is common for both channels and which consists of a rectifier bridge, a smoothing capacitor and a zener diode to keep the supply voltage steady. The switches that can be seen in the circuit are of the push button type and are the input selectors for the TUNER, TAPE and PICK-UP (S1,2 and 3 respectively), MONO-STEREO (S4) selector and SCRATCH (S5) function switch. The recording output is taken just before the tone controls so that the signal which goes to the recorder does not get distorted in any way by the controls. The potentiometers are VR1,2 (BASS-TREBLE), VR3 (VOLUME) and VR4 (BALANCE) controls and they are all twin STEREO types which helps to keep crosstalk between channels as low as possible.
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