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Tube guy question: Long-tailed pair for gain stage in balanced hybrid amp?

post #1 of 3
Thread Starter 

Have any of you tube guys toyed around with a long-tailed pair as a headphone amp gain stage?  It seems like an elegant way to get a differential input/output amp.

 

I have a quartet of JISBOS buffers I'm thinking of using as the output stage, and a bunch of preamp tubes (12au/x/t7's) sitting in my parts drawer from other instrument projects/tube rolling.  Thought I might try to hack together a balanced hybrid. :cool: 

post #2 of 3

I have. 

Other people have. Look at a bunch of the Cavalli designs for examples. I'm not convinced he makes the best use of the LTP, with the way he likes to do asymmetrical loading, but they are great starting points for interesting stuff. 

 

Anyways, the LTP is pretty awesome. 

Lower gain than common cathode = lower miller capacitance = improved bandwidth. 

Better PSRR than a common cathode.

Ooh, yea, balanced/differential outputs from a single ended (or balanced... ) input are pretty sweet too. 

 

Really the only reasons NOT to use a LTP are that you cant afford (due to money, physical chassis space, or power supply power output) any extra tubes to make up for the reduced gain. In a hybrid headphone amp with the tubes you mentioned, the problem is almost never "I dont have enough gain" and since the amp is being designed properly from the start it is very easy to accommodate all the chassis space, heater current, and B+ current one could want. 

 

A negative power supply to bring the tail back to (even if SS tail) is VERY nice, and adds to the price in an amp with a tube output stage but in a hybrid the power supply for the output stage can easily function as the negative rail for the input stage's CCS.


Edited by nikongod - 12/6/13 at 7:02pm
post #3 of 3
Thread Starter 
Sweet! I've been pouring over Merlin Blencowe's excellent book on instrument preamp design for the past year. I know it's not a resource specifically for Hifi but I've learned a heckuva lot about what makes a tube tick. Still have lots to learn, I'm sure!

If I have to spend any more money it'll likely be for the power supply--I have a basic unregulated 200V CR filtered supply worked up for prototyping but I fear that's not going to leave me a lot of headroom for what should be a "clean" amp design--at least with the 12a/t/x-series tubes. wink.gif

Being able to run either balanced or unbalanced inputs is definitely a neat plus. I expect there's a significant difference in necessary gain between the two, though. For now I'll design towards balanced, since that's what my source will usually be. I figure I could always add a gain switch or internal jumpers if I thought I might switch to an unblanced source.

I have a Twisted Pear Audio LCBPS to feed the JISBOS output boards, so it sounds like that could feed my CCS. What have you used for a CCS?
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