Need a piece of advice about DIY tube preamp

Hi everyone !

As I've got a little bit of time by now, I'm planning to build my own tube preamp for my B*rhinger B2 (not the Pro one).

Here are the things I found interesting to use:
- no input and output tranformer to avoid bandwith problem.
- 48V phantom power
- based on one 12AX7 (one should be enough in my opinion)

A few things should be cleared anyway:
- how can I make a balanced stage with tubes ? WITHOUT tranformer, don't forget, it's one my goal.
- I need high voltage gain to get nearly +4dBu output. But I also need low output impedance on my preamp. I plan to use a common cathode stage (for high gain), followed by a cathode follower to lower the output impedance...
- Between those 2 previous stages, I could implement a dephaser to recover balanced output. But it would add an other tube.

I'm interested in all your ideas !

Thanks to all of you in advance !

Thanks for your fast reply buttachunk.

Well, in fact the main problem with tranformers is that good ones are expensives ones

[quote author="buttachunk"]I don't know of any way to make a balanced mic pre with 1 12ax7-- as your description suggests...[/quote]
Well, if you got ideas with more than one 12AX7 I'd like to hear about it

Fred Forssell has made a few designs using tubes and no transformers. I understand though that this approach trades off noise performance (with certain mics) for bandwidth and distortion. In Fred's latest designs he's using a FET for a first stage and then using tubes. Supposedly this gets around the noise issue.

http://www.forsselltech.com/fetcode.htm

Cheers,

kris

DrFrankencopter, thanks for pointing me out to Fred Forssell, because I found this :
http://www.forsselltech.com/schematics/VT%20Opamp.PDF

Seems to be a good way to start my project

Ahhh, if I had three grand I would buy a FETCODE. Fred makes kick-ass gear. :thumb:

- no input and output tranformer to avoid bandwith problem.
- based on one 12AX7 (one should be enough in my opinion)

A few things should be cleared anyway:
- how can I make a balanced stage with tubes ? WITHOUT tranformer, don't forget, it's one my goal.
- I need high voltage gain to get nearly +4dBu output. But I also need low output impedance on my preamp.

Does not compute, not for an all-purpose mike amp.

One CC 12AX7 plus one CF 12AX7 gives unbalanced input, gain of 75, output impedance about 1.5K, output current about 2mA max.

There is a reason all the good old tube mike preamps used two transformers and a lower-Mu tube than 12AX7.

A long-tail differential pair will give you a balanced input. Two triodes, and higher noise voltage than a single tube. (And note that even one tube has higher noise voltage than a microphone.) 12AX7 used that way will give gain about 30 (diff in, single out). You need another gain of 30, plus an output buffer that can drive your load (600 ohm or 10K ohm depending on your studio).