[tomb]

  Fine tuning!!
 
As I mentioned previously, headphones show different impedance under varies frequencies and/or voltages. Unless you have a very low impedance circuit, your amplifier may experience varies gains under different frequencies.
In order to solve this problem, I added some reverse feedback that can further lower the distortion, increase the bandwidth, and lower our output impedance. The R18 and R19 form the reverse feedback network and the overall gain is controlled about 10dB. Actual lab measurement shows that it can drive loading over 30 Ohms and none of the performance is lost.
 
Why did we choose to use just 10dB? This portion is where you can get the best fine tune over the volume. It will not run above the destination gain when tuning up and will not cause too little gain while tuning the volume down.
 
The trouble of using the dual triode tube, however, is that each half of the tube may not perfectly match the other. In other words, it may generate different gains from each half of the tube output. Question is, can you tolerate the 1dB volume difference between your left and right ear?
Our enhancement of our feedback network has successfully reduced the gain gap down to less than 0.1dB! This is important since the use of headphones placed right above the ears. Any minor gain differences can result in poor performance. This is why the designing a speaker amplifier versus building a headphone amplifier is different. 

 
I won't argue the idea of limiting feedback, but when you claim flat frequency response to 200kHz, maybe you should look at the other end.  With C14 at 470uf, the 30 ohm load you mention will experience a -3dB cutoff at 11.29 Hz.  That's fine as far as it sounds, but what it really means is that with a 30 ohm load, the 20Hz frequency response is already down by -1.2dB.  That would be plenty OK for a headphone, but probably not for an amp.  Still, it's probably OK in a majority of headphone use, but compared to your claim of 200 kHz on the other end, it seems misplaced.
 
That's sort of beside the point, though.  I won't try to offer comments anymore after this.  However, looking at your low Head-Fi post count and the totality of your posts, I can't help suspecting that this is some sort of marketing ploy.  That might still be OK if you eventually presented a kit (as in DIY) or got around to explaining how the circuit gets built and put together (as in DIY).  I'm beginning to believe that's not the point.  This seems more like a blog about something you've already built or even more - something you are going to sell.
 
If that's wrong, I apologize.  Anyway - good luck.

 

[OJNeg]

audioboozer: That's a nifty little circuit! I've built something like that myself actually. Single-ended gain stage (not a tube though

) into a Class-A follower. You mention that the anode is directly coupled to the output stage which is nice, but I would have to agree with tomb that AC coupling the load is less than desirable. What I would do is a put a coupling cap in between the anode and base. That way you can use a much smaller value which I'm sure everyone will agree will result in higher performance than a large valued electrolytic on the output

. Then use a classic DC servo to hold the base at 5V so that the output offset can be nulled. While you're at it, get rid of that input coupling cap so long as you're sure that you don't have any DC output coming in from the previous stage.
 
What sort of plate current are you running through the 12AU7 there?

 

[audioboozer]

Thank you for the suggestion on output transformer. I am still digesting for your advice. It is going to be a DIY tube amp for sure. It is my pleasure, if someone wants to buy the design. I am willing to share my final circuit and part list on here.