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-   -   Fully balanced tubed LCR RIAA Head Amp (Phono Stage) under Development (http://www.diyaudio.com/forums/analogue-source/226981-fully-balanced-tubed-lcr-riaa-head-amp-phono-stage-under-development.html)

tiefbassuebertr 3rd January 2013 09:15 PM

Fully balanced tubed LCR RIAA Head Amp (Phono Stage) under Development
 
1 Attachment(s)
A German forum discuss a not transistorized (resp. not solid state) phono preamp include MC Step-Up stage - go to
Symmetrische RIAA-Vorstufe für SPU - Ideensammlung - Phono - Restaurierung und Selbstbau - Analog-Forum
perhaps of interest for some of the members here.

In this case also this thread could be of interest:
http://www.diyaudio.com/forums/tubes...aa-module.html

analog_sa 3rd January 2013 09:29 PM

Nothing can convince me that after passing through a 1635 and a 1660, the signal is worth listening to. Irrespective of how exactly was the riaa achieved. Likely a good concept but implementation is way more important.

kevinkr 5th January 2013 12:02 AM

Not to mention all of the D3A miller capacitance multiplied by the input transformer...

boli46 5th January 2013 07:28 PM

The schematic shows that the cartridge loading resistance consists of two resistors in series with the midpoint connected to the CT of the input transformer. This in turn is connected to ground through a voltage divider. This means that the Miller capacitance of the two tubes is in series. Therefore the total Miller capacitance is equal to half the capacitance of one tube. I would be more worried about the 22 uF cap in parallell with the ZZ1040 voltage stabilizers in the first stage.

Best regards,
Bo Lindstrom

kevinkr 6th January 2013 03:39 PM

Can you explain your analysis of miller capacitance, my belief and general experience is that they appear in parallel as far as the capacitance reflected to the primary is concerned, note that the center tap is at AC ground potential. (10uF bypass cap to ground)

Based on a cmiller of ~250pF per tube plus strays I calculated a reflected input capacitance on the primary of this transformer of around 0.13uF.. Consistent with my experiences with a single ended D3A driven by an LL1941 which is the more expensive amorphous core equivalent of the LL1943. I drive this transformer with an SPU as well. (I now use a cascode design with about 1/9th the input capacitance of the old D3A based design)

Am I missing something here?

Note that most of this cmiller can be canceled (say 90%) by cross-connecting ~3.0pF capacitors between the plates and grids of the opposing tubes. I would probably use hand made "gimmic" capacitors made with teflon insulated wire for this purpose.. Difficult to accomplish accurately and going too far results in an oscillator as you now have positive feedback.

The high miller capacitance of the D3A triode connected makes it a relatively poor choice for phono stage use imho, and I learned this the hard way by designing and building one. And yes as you would expect it is extremely quiet which was the appeal in the first place.

I can also see a potential problem with relaxation oscillations due to that 22uF cap across those gas regulator tubes. In general I have not found them to be a good idea in circuits where noise performance is critical. I would not count on PP PSRR to reject the broadband noise to a sufficient level for quiet operation.

boli46 6th January 2013 08:42 PM

Hmm. I suppose you are right about the Miller capacitance. I was thinking differential, but this circuit is push pull.
I like voltage stabilizers and I have never had any noise problems with them, but then I have never used them in phono input stages. I will try that in the near future though. I do use voltage stabilizers (STV/Telefunken 100/60 II) in my present linestage and it is as quiet as the grave. In fact, with my ear up against the loudspeaker I can´t tell whether the system is on or off. The loudspeakers are circa 100 dB sensitive Maison de L'Audiophile style Altec VOTT:s.

Bo

Cal Weldon 6th January 2013 08:55 PM

Quote:

Originally Posted by boli46 (Post 3313774)
L'Audiophile style Altec VOTT:s.

Pictures please. :)

boli46 6th January 2013 09:57 PM

Well, they don't look much different than the originals. They are fitted with "wings" to extend the low frequency response and they have the early style vertical bass reflex openings, otherwise they are pretty standard looking. They are however built from 25 mm baltic birch plywood.The curved part of the midbass horn is even thicker, about 40 mm. All inside surfaces are covered with a special quality 10 mm felt. There is also a felt curtain to stop higher frequencies frpm the back of the woofer to leak through to the bass reflex openings. My units are fitted with late model Altec 416-8A woofers (same chassis as 416B and C) and 511B horns with 902-8A drivers.
I bought the cabs as a kit in 1988 from the now long defunct Swedish distributor of the French Maison de L'Audiophile products.

Bo

kevinkr 6th January 2013 10:30 PM

Hi boli46,
Just wanted to mention that all of your posts are doubling up, perhaps there is some issue with your browser configuration or you are inadvertently reposting? I've done this countless times.

I use voltage regulation in all of my phono stages, but have recently moved away from gas references because I have found I am able to get better noise performance out of my regulators with well bypassed zeners.

I'm thinking down the road of using a variant on Morgan Jones' statistical regulator which is basically a whole bunch of low voltage zeners in series as it has a very beneficial effect on the noise levels of the reference. I use them in the cathode circuit of my cascode error amplifiers so both dynamic impedance and noise affect the overall performance of the regulator since it amplifies reference noise.

kevinkr 6th January 2013 10:32 PM

Quote:

Originally Posted by Cal Weldon (Post 3313788)
Pictures please. :)

Not here please, grievously OT.. :D


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