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Old 3rd March 2001, 06:20 PM   #1
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Hi everyone!

This is my first post in this board!

Anyway, I'm looking for a DIY tube preamp with balanced inputs and outputs. Know of any? Most of the projects and kits uses unbalanced RCA input & output.

The other thing I'm curious is does anyone one know of diy preamp design utilizing tubes normally used only in an output stage of a poweramp?! E.g A preamp that is driven by 300Bs! Check this link!

http://www.zenn.com.sg/Zenn%20prod.htm

Unfortunately this is only available as an assembled product. Any DIY tubed preamps like that eh? A 2A3 preamp? Or a 211 preamp? hehehe.
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Old 3rd March 2001, 07:50 PM   #2
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Off the cuff, I'd expect that it'd be a dicey proposition to put together a preamp with power tubes, as they generally have a pretty low mu (amplification factor). That's not to say it can't be done, particularly if you string enough of them together.
I'm short on time at the moment; will look at the link later.
How much gain do you need for your balanced preamp? If all you're doing is listening to CD or tape, then you can get by with, say 6 to 10dB gain (although most preamps have more). A simple differential front end, followed by cathode followers would do the trick. Use the tube of your choice: 12AX7, 6DJ8/6922, 6SN7, etc. If you're planning on phono, consider a little more gain, even though most of the work will be done in the phono stage.
There are zillions of DIY sites out there: hit the links and start fanning out looking for schematics for balanced stuff.

Grey
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Old 7th March 2001, 11:27 AM   #3
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Actually I did search for schematics and couldn't find them. Which is why I said I only found those on unbalanced and was hoping that someone here may know of some. Oh well...

I'll do a 211 preamp. Though it is more for novelty than anything else.
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Old 10th July 2002, 12:27 PM   #4
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Default WOT & mu stage

Hi all,
What is the advantage and disadvantage for preamp which use WOT and mu stage (ala Allan Kimbel)?
How is the sound ?
How is that configution compare with SRPP ?
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Old 10th July 2002, 01:45 PM   #5
tiroth is offline tiroth  United States
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Although it is by no means a complete preamp, you could take a look at the 5687 LTP on my site. This circuit would form the basis for a balanced preamp. Reduce the degeneration resistors until the gain is where you want it, add a pot or stepped attenuator, etc.

There are also a number of preamp schematics for the 5687; I know because I referenced some of them when working on my page. Try a google search--some people like the sound of this tube.
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Old 10th July 2002, 06:30 PM   #6
dice45 is offline dice45  Germany
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Grey,
Quote:
that it'd be a dicey proposition to put together a preamp with power tubes
you have no idea ho dicey (see moniker) this proposition is ROTTFLMAO have exactly that in mind .
MC-input trannie deeding the grids of a long-tailed pair of choke loaded EC8010 (could also be triode-wired E180F), directly coupled to a long-tailed pair of choke loaded 12A4 or 6900, directly coupled to a long-tailed pair of PP-output-transformer-loaded 12B4A.
That's for phono; split RIAA tucked between the stages. For line, a complex switch circuit replaces the direct coupling between stage 2 and 3 by an input transformer, Lundahl LL 7901.
The output stage of my preamp is nothing but a straight lil' PP power amp as linestage, with enuff headroom. OPT: Lundahl LL1660. outputZ tuned to 600Ohm.

The 12B4A is as juicy as a 2A3, just indirectly heated and gain a tennyweeny bit higher. In fact, it is 2A3's grandchild and also its replacement in similar industrial and military applications

No idea if this will work or how it will sound, but my tube guru friend Manfred was unable to find the hair in the soup of my circuit design albeit he feels uncomformtanble about it -- burned child avoides fire, but in his case direct coupling (mis-)happened in single-ended topologies. WE both feel my intended circuit has huge sonic potential and i wil find out if it is worth the hassle . But it will have hot-pluggable phono EQ. Yippie! just the right thing for my shellac collection.

tiroth,
second your recommendation for the 5687 as linestage. Makes also a worldclass SE line stage in combination with the Shilton pa102.

Vinylsavor has directly heated triodes for linestages since i know him. Always Xfrmr-coupled, mostly the 10/10Y/801 with gain 8. Raved elsewhere about his system. But he told me he was not happy with longtailed pairs of OPT-loaded 10s due to the stages virtual ground collecting hum and noise unbearable at this signal level.
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Old 11th July 2002, 06:06 AM   #7
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Here is one balanced gain stage;

http://laplaza.taos.nm.us/~fvuotto/mustage.html

If you have access to a library with current and back issues of the Japanese audio magazine MJ that would be good place to your search for extreme preamps. There is another Japanese magazine, its name escapes me at the moment, but that one is only good for drooling over the pictures.

ray.
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Old 11th July 2002, 02:07 PM   #8
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Peaceseeker and all,

preamps with power tubes, especially DHTs have big sonic
advantages. Power tubes are usually run at rather highish
bias voltages (compared to the usual preamp tubes). Even
if run at reduced power dissipation compared to power amp
applications. This gives the preamp huge headroom. In my
opinion you can't have too much headroom. A typical CD
source has 2V RMS output, some more than 3. 2V RMS is
about 5.6V pk-pk. That means the line tube needs a minimum
bias of 2.8V. But that would be absolutely zero headroom.
Even with a moderate 6dB headroom a bias of 5.6V is needed.
Better yet 12dB or more!

Phono is an entirely different story. As Bernhard reported, some
records can have peaks of 17dB above nominal 0dB output!

Typical power triodes would run at bias levels of 20-40V.
Plenty of headroom, no danger to even come near the
region of overload.

This also means that the tube will never swing anywhere
near the non-linear reagions. At line levels power triodes
will operate with extremely low distortion.

There is a prejudice that power triodes will have less resolution
ate line levels. Not true! In cintrary due to the above reasons
they will have heaps of detail.

And last but not least they will bw able to provide some power,
which is good to drive long cables, or as in my case low
impedance line level cross overs in active systems.

Of course all this does not come for free. As somebody already
mentioned, they typically have low mu. With a step down
transformer at the output, even this low mu will be eaten up.
That's why I prefer tubes like the 10 or 801 for line level
application. It's mu of 8 is not so extremely low.

Other factors to consider are microphonics and filament
supply. DHTs with their flexible filaments tend to be womewhat
microphonic at such levels and definitely need some means
of shock mounting. The filament supply needs to be DC, no way
to get away with AC here. Especially thoriated tungsten
filaments tend to show significant sonic differences depending
on the filament supply. Best solutions are current rsources,
or passive filters with end with a choke.

Peacekeeper, I'd love to hear about your progress with the
211 line stage. I'd expect more microphonics due to it's size
and the filament supply will be quit hefty. Which kind of
transformer do you have in mind ?

Ciao ... Thomas
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Old 21st July 2002, 04:20 PM   #9
Coulomb is offline Coulomb  England
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Default Re: Preamp Schematics

If you are still interested, AudioXpress published excellent DIY article in June & July for a Tube based Preamp.

Anthony
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Old 21st July 2002, 07:28 PM   #10
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Default Re: Re: Preamp Schematics

No discussion of balanced pres would be complete without pointing out Allen Wright's RTP3s.

A more thorough discussion of this circuit in his Preamp Cookbook -- a hoot to read even if you don't build anything out of it.

dave
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