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Comments on 71a PP Pre-amp
Comments on 71a PP Pre-amp
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Old 10th April 2003, 08:48 PM   #31
Joel is offline Joel  United States
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Default Re: power supply

Originally posted by 316a
...The very nature of push pull means most of the PS crud is cancelled in the output stage . Simple as that.
Yes, I thought someone would say that. I think you'd be better off thinking of any hum rejection as icing on the cake, rather than relying on it occuring. And since I would never design a push-pull amp without good PSU regulation, and plenty of capacitance - it really is not any simpler that I can see.
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Old 10th April 2003, 09:10 PM   #32
fdegrove is offline fdegrove  Europe
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I think you'd be better off thinking of any hum rejection as icing on the cake, rather than relying on it occuring.
Wow, two miracles in one day.

You're absolutely correct, Joel.

Moreover the PP needs to be well balances for this to happen so if people think they can get away with poorly designed PSUs because they're feeding a PP stage they'd be walking on thin ice.

Same goes for the distortion cancelation in PP too, no perfect balance no pay.

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Old 11th April 2003, 12:35 AM   #33
Brett is offline Brett
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Default Re: parafeed?

Originally posted by RichardJones
Or can get 2 100H chokes.... a really good OPT... and do parafeed PP output stage....

won't my PSRR be even better then?
PSRR will be better, but it will be better again with a CCS.
I like the PP parafeed idea, but why not try it with some 6C45's? One stage, low O/P Z, cheap, grunty and quite nice sounding, with some reporting it as good as a 45. For giggles I'm playing with some of these as a small (2w) poweramp, initially using another OPT for chokes, but if I like it, some v4 PMCCS. It would be running now, but I forgot to get some gridstoppers yesterday and will have to mailorder now.

But would the parafeed cap go from the midpoint of the output tranny to B+ or the cathodes of the 71a's?
Cathodes to B+ for the WE/ultrapath cap. For the blocking cap, either side of the primary will do, but if you can seperate the primary windings on the OPT, I'd put it at the centre, just because I like symmetry.
Lynn Olson has made some interesting posts about the WE/UP cap on Tube DIY at AA in the last few days.
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Old 11th April 2003, 08:22 PM   #34
RichardJones is offline RichardJones
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Default hmmm....


I am undecided now....

Still want to use the 71a's.... Push pull Class A. input tranny and output tranny.

If I use a step up phase splitter input tranny, don't I have to be careful about excessive miller capacitance reflected back from the 71a's?

OK... anyone out there, point me to 2 transformers that I can use with the 71a that will give me a total volateg gain of around 7.

Balancing the 71a is not an issue and will be delt with.

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