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One tube to amplify stereo?... smells funny
Old 13th September 2007
Lives for gear
Tibbon's Avatar
One tube to amplify stereo?... smells funny

I got Bellari 'tube' preamp for my turntable a while back. There's two problems I have with it- first there's no 'off' switch. How quick is this 12ax7 going to go dead leaving it on 24/7? Then again, it's like $12 bucks for a 12ax7... not a big deal.

More importantly... and maybe I'm missing something here. How is one tube amplifying two separate channels? Is a 12ax7 like a multiple op-amp like an LM-324 (which is basically 4 opamps in one?) and can do stereo?

I'm guessing it's using a common cathode gain stage, but how's that keep things separate? Could someone explain this a little better to me? Why have I never seen any other pro audio gear use a single preamp tube in this way?

I really need to crack this thing open and look in it. It sounds ok, but I'm afraid that it's a single tube sticking out the top, and its just got the heater hooked up to make the plates glow, and using a few cheap opamps for the actual preamp stage.

I can't figure out how to make this work. Does anyone have schematics of how something like this would be realistically made? Does this actually impart any 'tube' characteristics to the circuit, or is it all just a glowing light warming the room?

EDIT: Now that i think of this, I have seen similar before... where synths like a Triton had a single 'tube warming stage' or whatever bull**** to make it sound 'less digital'
Old 13th September 2007
Gear Maniac
corworld's Avatar

The 12AX7 is a twin triode meaning there are in effect two stages to the tube. These can be used separately. Many preamp tubes use this design. I have to admit I would have to see a schematic to know how it was used. The description said the output stage. Being a phono preamp it kind of seems like a gimmick.
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Old 13th September 2007
Lives for gear

As pointed out the 12AX7 is a 'dual' triode so it can do 2 channels. There is just about enough gain to act as a disc preamp but probably has other gear like transistors or an op amp to help it along.
One reason for not using it as setereo is that the 2 halves will 'crosstalk' to each other although in the context of a disc preamp even 'bad' crosstalk from an amp is minimal compared to the disc / cartridge combination.
With a regulated heater supply it should be good for about 10,000 hours, a good couple of years.
Matt S
Old 13th September 2007
Lives for gear

Just looked at the advert.
You can't make one half of a 12AX7 drive +10dB into an 8 ohm load and give sufficient gain (50 dB or so at low frequencies).
If it has a 'jam packed circuit board', what with since a RIAA equalisation stage uses around 10 components?
Matt S
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