phono and line preamp

Posted on Feb 6, 2014

I have been hacking the Dyna PAS chassis with various cascoded and cascaded phono circuits all employing negative feedback RIAA EQ in search for a circuit that will satisfy my musical requirements. My research ended when I discovered thisunglamorous2-stage cascaded passive RIAA EQ phono preamp circuit at the back page of an RCA t

phono and line preamp
Click here to download the full size of the above Circuit.

ube manual. The most important lesson I learned in this excercise isnegative feedback chokes the sound!I did not alter any component values except for using Ni-mH batteries for cathode bias which I thought improved detail and definition. I strongly advice against the use of a cathode follower and make sure the circuit drives aminimum load of 220Kas originally specified otherwise you will lose bandwidth at both frequency extremes. I use an Alps 250K but have also tried the cheap Alpha 250K dual log taper carbon pot sold by AES. It does not have the tracking precision of Alps but at normal volume settings the channels are balanced enough. I have been using this phono circuit since the early 90s starting with aPAS chassiswith Telefunken ECC83/12AX7. Through the years I developed fondness for octals and nowadays I almost exclusively use 6SL7s/5691s which further improved the bandwidth and depth due to its slightly greater transconductance, vis-a-vis, the 12AX7. There are people who swear by using high transconductance tubes like WE417A or 6DJ8. To hear what this was all about, I bread boarded a famous circuit using these tubes but did not find the sound as appealing, must have been just a matter of taste. I find the superior linearity of true audio tubes like a 12AX7 or even better yet 6SL7 octals play a lot more LPs. For MC cartridge use a transformer step up device like this Tamura TKS-83 or mic transformer to boost the...

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