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Old 2nd September 2005, 05:45 PM   #1
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Lightbulb Differential input stage

Hi all, would like to ask if anyone out there could help me with a schematic or ideas for a differential input/pre-amp stage.
I can easily achieve this with op-amps but am toying with the idea of using valves, well, because I like them so mutch.
Many thanks,
Carl.
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Old 2nd September 2005, 08:34 PM   #2
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The attached is my EL34 triode-strapped PP amp which starts with a differential (LTP) splitter, using a pentode as current sink for the tail. If you build it you may have to experiment a bit with the bias network to get a useable range. I run the El34s at about 45mA but my particular tubes are el-cheapo so-called "Svetlana" (not winged C) and have unusually low transconductance.
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Old 3rd September 2005, 04:35 AM   #3
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Here's another one. It's the front end / driver for a power amp, but would work just fine as a differential linestage. It's a little odd in that it uses a dc supply from the output tube bias to power the current source on input. You could use a 6.3Vdc heater supply for your application. Or similar.

The current source is key to optimal operation here. It provides better ac balance for the outputs (plates), plus it offers input common mode rejection (small range).

Click the image to open in full size.

jh
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Old 3rd September 2005, 04:48 AM   #4
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Default Re: Differential input stage

Quote:
Originally posted by Dynsdale
Hi all, would like to ask if anyone out there could help me with a schematic or ideas for a differential input/pre-amp stage.
I can easily achieve this with op-amps but am toying with the idea of using valves, well, because I like them so mutch.
Ideas? Here's an idea: Use a transformer. They make for one hell of a differential input stage in my opinion.

I like valves too, but I like trasnformers even more.

se
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Old 3rd September 2005, 08:49 AM   #5
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Thanks for the schema Ray, I'll give it a go, I've just built a test rig to play with circuits. I think I have those triodes in my box at home,
I like EL34'2 too, they are readily available here in ZA and more bang for your buck.
Cheers.
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Old 3rd September 2005, 11:04 AM   #6
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http://www.tubecad.com/2004/blog0008.htm
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Old 3rd September 2005, 11:07 AM   #7
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Thanks JH, looks like an interesting design on the whole, not sure if I can get the tubes, if not, I'll try the idea out with similar valves.
Best regards, cb
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Old 3rd September 2005, 11:07 AM   #8
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Thanks too to SE, I have been looking at Jensen's transformers and many others seem pleased with their results, worth a try if I can get my hands on one.
Best regards, cb
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Old 3rd September 2005, 08:29 PM   #9
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Quote:
Originally posted by Dynsdale
Thanks too to SE, I have been looking at Jensen's transformers and many others seem pleased with their results, worth a try if I can get my hands on one.
Yeah, the Jensens are very good quality transformers and I've used them for over 10 years. However I've more recently found CineMag's transformers to be just as good in terms of quality but at prices significantly less than that of the Jensens.

se
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