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In Reply to: RE: differential input, single ended output posted by rage on December 08, 2015 at 17:17:15
Piece of cake! Use differential gain block, AKA LTP. You even get the option to experiment with "absolute polarity". ;>)
Each channel gets a twin triode. Don't forget the tail CCS.
The line stage schematic I've uploaded shows the kind of circuitry needed. You would drive, instead of grounding, the non-inverting triode's grid.
Eli D.
Follow Ups:
If the purpose is to amplify a differential input that isn't referenced to ground, he needs a true differential amplifier. Why not use a version of the usual circuit (below) and take single ended output from only one of the two tubes?
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Buy Chinese. Bury freedom.
TK,
If I do that will still have the benefits of being balanced? (common mode rejection?
Read carefully. I said he had to drive, not ground, the non-inverting grid.
BTW, regardless of the amplifying device used, a tail CCS is in order.
Eli D.
Yep, sorry, I missed that. I will go on record though as being particularly uncomfortable (I'm being diplomatic here) with all those SS devices you toss into vacuum tube circuits. You probably already know that. :)
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Buy Chinese. Bury freedom.
I have a lot of experience using CCSs with LTPs. Cascode MOSFETs are way better than single MOSFET based CCS. IXYS 8N50D2/1N100D better sounding than DN2540s. But for currents less than 10mA a cascode BJT (BC549C for example) out performs any of the MOSFETs.
I guess if you HAD to use tubes a pentode based CCS would work well. That would be quite the pain in the ass though to implement. However, looking around this forum I do see many members of the "Hair Shirt" DIY audio bent. ;~)
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
You've never seen me use a SS voltage amplifier. As of now, the only place I'd be comfortable with SS voltage amplification is in Allen Wright's hybrid cascode LOMC phono setup.
BTW, I do own a Klyne model 1 LOMC "head" amp, which is 100% SS, but that unit has a well proven track record. Some folks can make SS "dance", Stan Klyne and Nelson Pass to name 2.
FETs, being high impedance, voltage controlled, devices, mate well with tubes. FETs make excellent current amplifiers (voltage followers). While it is an over simplification, think of a FET as a heaterless pentode. :> D
I'm a pragmatist. If a lump of galena yielded good sound (it definitely does not), I'd use it.
Eli D.
I read that as "heartless pentode" and thought, damn, that's harsh.
Doug
Somebody's "sand allergy" got the better of them. FETs make excellent "spear carriers" for tubes. Pick your spots carefully.
Eli D.
I'm over my sand allergy!
I am loving Cree schottky diodes and constant current sink cascodes with mosfets.....
Try the unbalancer circuit from John Broskie
http://www.tubecad.com/2011/03/blog0203.htm
Come on TK, Try it...your ears will like it ;> )
jkt - what transformer are you using?
eli - that looks really good. what happens if I remove the coupling caps and grid resistor? Right now I'm using 12ax7 direct coupled to 2a3..so B+ is 500 and I've got around 250v on the cathode of the 2a3.
You certainly can DC couple the 2A3's grid to a LTP plate. Notice in the preamp that the MOSFET source follower is DC coupled to the non-inverting plate.
That 500 V. of B+ makes for healthy sized LTP plate load resistors, which is good for keeping gain up and distortion down. 200 to 210 V. on the 'T7 plates and 3 mA. of plate current per plate sounds pretty darned good. A 6 mA. tail CCS is what you need.
Eli D.
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