the octal phono preamplifier

Posted on Feb 6, 2014

Octal Preamplifier can also be built as a stand alone unit without the linestage part. This is not feasable since it has a split RIAA, which means the RIAA EQ circuit is split into two parts, one after the first stage and another one after the second. This means the second RIAA part`s accuracy depends on the loading resistor that follows after it. The frequency response of a stand alone phono stage should not be influenced

the octal phono preamplifier
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by the load it drives. Another problem would be the high output impedance of that phono section. After I introduced a stripped down version of the 6CB5A amplifier. I thought it would be a good idea to also develop a matching simplified version of the Octal preamp. This time the concept will be such that the phono section can either be integrated into a full function preamp or used as a stand alone unit. As you can see in the schematic, this version uses different tubes as the original Octal Preamp circuit. Instead of the 6SL7 and 6N7 this phono section uses 6SF5 and 6J5 which I presented in the two recent tube of the month articles. Let me explain this tube choice. Since this phono circuit should be usable as stand alone unit, it needs a bit lower output impedance. But I wanted to keep it as a two stage circuit. So a output tube is needed which still provides some gain at a reasonably low output impedance. I chose the 6J5 as output tube since it is extremely linear and availability of the tube is good. The 6J5 has a lower amplification factor as the 6N7, hence the need for a different input tube which has a higher mu as the 6SL7 to compensate for that. I chose the 6SF5 again for it`s superb linearity. I also wanted to keep the Octal theme. Another benefit of these two tubes is their availability as metal tubes which have shielding built in. Being a stripped down version this circuit is supposed to be used with a power...

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