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Old 13th August 2008, 03:19 AM   #21
Sheldon is offline Sheldon  United States
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Regarding noise/gain relationships; with three stages of 20dB each, is it better to combine two stages at the beginning (stage-stage-filter-stage-filter) and split the RIAA with the third stage, rather than stage-filter-stage-filter- stage?

Sheldon
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Old 13th August 2008, 07:23 AM   #22
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Quote:
Originally posted by Sheldon
Regarding noise/gain relationships; with three stages of 20dB each, is it better to combine two stages at the beginning (stage-stage-filter-stage-filter) and split the RIAA with the third stage, rather than stage-filter-stage-filter- stage?

Sheldon

It would be if S/N was the only consideration. Unfortunately a very high gain prior to the HF pole creates the danger of clipping. Gain structure is always a compromise between S/N and headroom.
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Old 14th August 2008, 02:26 AM   #23
Sheldon is offline Sheldon  United States
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Thanks, makes sense. In this case, we'd be talking about 40dB gain in the first stage, followed by the HF pole.

If I assume a 3mVRMS cartridge, then music peaks would add about 20dB, for 45mV peak. Add 20dB for the HF RIAA boost to get to 450mV input. With gain from the first two stages of 40, that requires an output to the first filter of 45V peak. Might need a bit more headroom too. Not unreasonable for a small signal tube. So I guess the trade off would be noise performance for distortion.

Sheldon
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Old 16th August 2008, 10:21 PM   #24
rman is offline rman  Canada
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Hi guys.

I totaly see where Oldeurope is coming from. My rjm 2*6dj8
does put out some white noise. I had a gut feeling before
building it that as you say, the attenuation of the low frequncies
exceeding the gain of the first stage is not a good thing. I used
trimpots for the filter to set them as close to calculated valuse
as possible. maybe they contribute to noise more than a fixed
metal film resistor.

Having said that, I must add that I can't hear this when listening
to music and enjoy the sound of this stage very well.

I also agree with Mike on starting with something simple and
seeing what improvements more sofisticated designs offer later.

when I was planning to build this project, I contacted Richard
Murdey to let him know I would be trying it out, and for clarification on a small point on the design. He pointed out
that it was one of his early creations. The page with the
schematic said he was trying to make an easy to build and easy
to listen to circuit. I think he sucseeded very well. He did not claim
it was the ultimate in HI-FI phono stages.

Mike, I take it you have not built it yet, are you going to or are
you going to work out a split passive design instead?

Cheers.
Rolf.
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Old 16th August 2008, 11:24 PM   #25
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Rolf;

Finances are such these days that I am doing more planning than doing. :{ I am inclined to look at a split eq versions as it is not really all that more complex but with simple CC gain stages rather than the fancy active loads I see so often.

Right now my "doing" is limited to the solid state active crossover for my PA keyboard system as I have virtually all of the parts for that one. After that I will get to work seriously on the 6EM7 amp with crossover. After that the phone stage would probably come next. Somewhere in there I have to build the tapped horn subwoofer for the PA.
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Old 17th August 2008, 04:38 AM   #26
rman is offline rman  Canada
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Mike.

I sympathise with the finance issue, my car is sucking all my
money these days. However, I am pretty happy with my stereo
right now anyway.

Way back you said Gary Kaufman's operating points for the
6EM7 had no or very little third harmonics. Once I found
the formula to calculate this I came to the same conclusion.
This makes me think this tube at the same OP would be a
natural for a PP amp. Fred Nachbaur did a EM7 push pull
but it was biased like his miniblock SET. You have enough on
your plate but maybe someone else will pick up on this and
run with it.

Cheers.
Rolf.
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Old 18th August 2008, 07:28 AM   #27
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Default ... forgotten aspect ...

Hello,
thanks for your replies,
since this aspect is "forgotten" the moderators should make this thread sticky.
I added a link to this thread in my RIAA 2007 blog.

(After careful consideration I decided to build a passive RIAA in two stages. ...)

Kind regards,
Darius
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Old 17th September 2008, 12:42 AM   #28
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Hey oldeurope how about this approach. Using the multisection EQ approach with a nice low noise, low microphonics and medium to high mu twin triode and one of the small 9 pin vertical sweep tubes. What I have in mind is putting the bass boost section between the input triode and the small signal section of the sweep tube and putting the treble cut between the small signal section of the sweep tube and the "power" section.

This way we have a quite fairly non-microphonic tube for the sensitive input. The treble cut being after this section reduces the perceived noise even further while the very low rp and high current capability of the final high power triode give the preamp the ability to drive difficult loads.

What do you think?
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