• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Differential input stage


2004-01-27 8:55 am
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.


  • rwm amp el34 triode ab1.jpg
    rwm amp el34 triode ab1.jpg
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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).


Dynsdale said:
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. :)

Dynsdale said:
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.