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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 13 Aug 2017, 19:04 
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Joined: 06 Jun 2008, 18:23
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Location: Australia
 After building and selling a number of Mimic MM  and MM_MC phono preamps, all based on Oddwatt's great design, I have here a Carbon film resistor version. All other builds use metal film 1/2W resistors. Carbon film 1W resistors have been used in a few of my tube power amps and the sound is rich and mellow. Here 1/4W fine carbon film resistors are used.

To make this build even more different with MKT caps I have also used Polypropylene and Russian Mil. PIO caps. The large red caps in the image above are 4uf PIO snubbed with 220nF polies. The usual OPA2134 chip and mil. spec. socket are used. Silver plated fine wire wrap wire is used for the short hookups. PS and amp exist on a 5cm square of vero-board. WBT silver solder holds it all together.

I had hoped this preamp would run from a single 9V battery but in the end two 9V batteries were required. This should provide 100hours of play between battery changes. A battery test point is at the front and an earth lug on the side. Initial test show good promise with a rich and warm sound from (at this point) limited listening. Interestingly on a short A/B test it sounded similar to my LM833 based two stage MM  preamp. Both have great warmth.
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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "Mimic Carbon" - carbon resistors and PIO caps. MM phono preamp
Website: retro-thermionic


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PostPosted: 14 Aug 2017, 21:34 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi Mark, I tried carbon films in one as well but liked the metal film ones better. The carbon film ones seemed to take some of the sizzle out of my system. Nice to listen to but not my style. This design seems to perform far better than it should. Simple as are most of mine, but carefully tweaked for best performance. I have been getting requests to build them. It seems that one person hears one and shows another and on it goes. Mine all use AC power though like in the original project. I'm going to have to order a bunch of parts to keep up. As for comparisons with some of my other preamps...I like the sound better than the Simaudio Moon LP3 and it is nearly as quiet (that one is amazinly quiet), they are not as nice as the several versions of the Groove, but then the tube ones are much more sophisticated, the ones built into things here (three Marantz, and a Hafler) are not in the same league as either the Mimic or Groove.

Good listening
Bruce

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PostPosted: 14 Aug 2017, 22:38 
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Location: Australia
Bruce, even with metal film resistors have you tried the LM833 vintage chip. It seems to have more bite and drive. If you can drop one in a Mimic and let me know your thoughts.

The other changes, as you would have seen, is PIOs on the output. It all adds to the sound.

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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "Mimic Carbon" - carbon resistors and PIO caps. MM phono preamp
Website: retro-thermionic


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PostPosted: 15 Aug 2017, 03:26 
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Joined: 28 Nov 2014, 15:21
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Location: Queensland, Australia.
G'day mate, now I'm quite confused! I seem to recall that you've had poor results and oscillation? with the LM833 in the past. Do I have that wrong? I've used LM833 many times in the past with no issues at all, although I prefer to use 'ON SEMI' types if possible as they appear to be free of possible issues from my reading on the 'net. Regards, Felix.


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PostPosted: 15 Aug 2017, 08:10 
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Felix wrote:
G'day mate, now I'm quite confused! I seem to recall that you've had poor results and oscillation? with the LM833 in the past. Do I have that wrong? I've used LM833 many times in the past with no issues at all, although I prefer to use 'ON SEMI' types if possible as they appear to be free of possible issues from my reading on the 'net. Regards, Felix.


The only time I can remember oscillation is with my first Mimic. I put a nF cap on the input and she sang. Cut the cap away and all was good. Only did it once and it was my very first Mimic.

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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "Mimic Carbon" - carbon resistors and PIO caps. MM phono preamp
Website: retro-thermionic


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