Preamplifier for Turntable
The problem is that turntables are becoming increasingly harder to obtain and my dad has about a ton of those. I am talking Harry Belafonte and the Sea Shanties, Cowboy Songs and of course those Readers Digest Classicals. Until I was 15 I thought Readers Digest was some kind of record label. How was I to know?
The following circuit is that of an ordinary preamplifier that is usually used with guitar amplifiers. I will be using it with a turntable for vinyl records at the input and a power amplifier at the output.
As you may be more of an electronic boffin than I am, ( not that I bof at much) you are welcome to skip this bit. Just for the hell of it here is the way the cicuit works anyway.......
The circuit uses a TL082 dual operational amplifier to amplify the signal from the turntable.
The first op-amp provides a high-impedance input. This has the advantage that very little current is drawn from the source, in this case the turntable.
The output of the first stage is fed to the second stage via C5. Feedback is obtained through R1 back to the input. The purpose of feedback in these circuits is to limit the gain of the first stage and insure linearity in amplification. This results in a cleaner signal with a broader bandwidth, thus grabbing the lower and higher end of the frequency spectrum, which isn't very big because us humans only hear sounds in the 20 Hz to 20 kHz range.
The signal is fed to the input of the first stage via C2 and R4. The output of the first stage is then fed through a passive filter and this is where the tone control bit comes into play.
The filter is known as a BAX ANDALL filter and makes use of C4 and C7 to filter the bass out and controls the tone with R 14.
C5 and C6 passes the bass sounds and lets the treble through. The output of the second amplifier is then strong enough to drive a final amplifier circuit.
I do believe that is the basic idea of what goes on here, eventhough I haven't heard of the BAX ANDALL filter system before. If you have some more info let me know.
The circuit above is available in kit form from Smart Kit No. 1091 and the manual is in a couple of languages so if you ever need a tone control circuit feel free to contact them. The kit is of good quality and they even supply you with some solder wire and the component layout is printed on the PC board.
Well that is the tone controller unit. The rest will follow as I work on the thing. As you can see I like the Smart Kits. the quality is good and I haven't found a faulty one yet.
Stay tuned for developments on the audio-amp front.......
