| Octal Phono [message #29967] |
Tue, 26 February 2008 08:41  |
SteveBrown
Messages: 330
Registered: May 2009
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Grand Master |
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I wanted to share a recent success in completing a version of Hagerman's Octal Cornet. I know that over the years much has been published on this, but wanted to encourage folks to give it a try if you're looking for a fun project. I made a couple small changes that I think helped: 1) I used Russian 1579 (6SL7 equivelents) for the 1st two stages. These are amazing tubes. If you've ever worked with 6SL7's in high gain circuits you know they can be terribly microphonic. With the structurally superior 1579's I can rap right on the tube and not hear it in the speakers. 2) changed the 6SN7 to a 6BL7 for more drive on the output - sounds great! 3) the final PS decoupling cap was changed to 6.8uf Obligatos for what I feel is a smoother and richer tone. If I had the $$ I swap in more of these. I built the amp on a standard Hammond 10x17x2" chassis (aluminum). I placed the phono section at the far end, spacing the tubes 2" apart. This allows the component leads to make most, maybe all, of the connections and shortens signal path without having to resort to micro-surgery to get parts in there. The PS is on the far right, well away from the audio circuits. This leaves about 7" of space between audio and PS which in the future might just accomodate a line stage... ;-). Anyway, I've been really impressed with the sound quality of this phono, and I've built lots of phono stages! My first go around was with the RCA phono, then switched it to an octal version but could not get rid of the microphonics - even had feedack issues if the amp wasn't placed correctly! So the 1579's are a big improvement. I don't have a digital camera with me, so no pictures, sorry. But happy to describe the build further if anyone is interested. Grounding can be a challenge, so if you take this on and want to know what I did, let me know. By the way, TONS of gain in this guy! I can use my Grado 0.8mv Sonata and my 0.3mv Dennon 103 DIRECTLY into the phono stage w/o a step up. My line stage has good gain too, which makes this possible (it is a 7044 based stage). So.. happy soldering!
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