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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: 18 Sep 2016, 19:53 
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Joined: 01 Sep 2016, 17:19
Posts: 7
Location: Hertford, NC 27944
Wow, you guys kick a$$!! Have never been on another forum where folks are so helpful. Thanks for no "search function" comments (even though I DID use it right after I asked about the RCA design!)..

Looks like a pretty simple circuit - if I'm not careful, I'm going to have more phono stages than tables, heh heh...

Mack


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PostPosted: 19 Sep 2016, 10:15 
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Joined: 07 Dec 2015, 14:21
Posts: 135
gofar99 wrote:
Hi, One thing I would change for sure if you are going to build this preamp is to not use a ceramic output cap. Back then it was probably a cost effective thing to do as large polys were costly. Not any longer.


The output cap will be moved to after the follower circuit, and I assume most people are using a quality film cap here.

gofar99 wrote:
BTW it does work OK but not up to current standards. S/N is not great and it is sensitive to loading. I would not want to use anything below about 50K on it.


They recommend not to load it with anything under 200k, a cathode follower will have a much higher impedance so nothing to worry about there, I recommend placing a follower after the circuit if people didn't hear me the first time. I will post a revised schematic with a follower to make things easier to see.

As for the S/N ratio it's not an issue so long as you have a good regulated power supply. What ripple is on the B+ will be -10db on the plate. With that Maida regulator I can't even measure ripple at the output but I am told by guys with better test equipment it's ~20uV at the output. And besides I don't think one can say the unbypassed "SRPP" phono preamp presented on this site has a better S/N ratio because it doesn't.

DC heaters keeps noise down too. I built this circuit and have 105db sensitive speakers and I don't hear any hum. The key for any phono amp is the power supply, I won't build one without a regulated supply with practically unmeasurable ripple.


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PostPosted: 19 Sep 2016, 10:42 
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Joined: 07 Dec 2015, 14:21
Posts: 135
In case you didn't know you want to not use any ceramic caps at all to be clear. Polystyrene is best IMHO.

Back to the S/N.

A 22uF cap has a reactance of 60 @ 120Hz, the first stage filter (15k/22uF) will knock the 20uV ripple down to .08uV. The next filter stage (39k/22uF) will put it down to .08nV. S/N is well below -100db.


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PostPosted: 19 Sep 2016, 16:57 
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Joined: 07 Dec 2015, 14:21
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famousmockingbird wrote:
In case you didn't know you want to not use any ceramic caps at all to be clear. Polystyrene is best IMHO.



Wow my Grammar and proof reading is terrible. I meant to write that you don't want to use any ceramic caps at all.

Sorry for the multiple posts but I was past the edit time limit.


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PostPosted: 07 Dec 2018, 05:53 
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Joined: 26 Nov 2016, 16:59
Posts: 1
I know this is an old thread, but i just built one of these. Sounds really good to my 50 year old ears :). Just wondering if any of you fellas could suggest any tweaks I might try?


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