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analag · May 30, 2007

http://www.twin-x.com/groupdiy/albums/userpics/hybreed.JPG

How about an active loaded tube pre. I find the 12AX7 to sound a million times better when it's plate is held at steady current. P1 and P2 are 2K trimmers that are used to set U1a and U1b at half the B+ voltage respectively.
I have yet to determine the value of the resistors around the switch. You will notice that U2 is not configured in the usual WCF topology, this is a personal preference of mine. It helps to nullify some of PSU induced effect upon the sound of the pre.

adamasd · May 30, 2007

You will notice that U2 is not configured in the usual WCF topology, this is a personal preference of mine. It helps to nullify some of PSU induced effect upon the sound of the pre.

I didn't notice it while staring at the optical comp schematic but now that you mention it, it jumps out. You are mixing in out of phase power supply noise to cancel it out. Why are you doing this at the final stage as apposed to earlier on before it gets amplified by the previous stages?

I will have to play with actively loading 12AX7s when I get to working on my tube pres in not to long. I really do not like the 12AX7 much at all, it is by far my least favorite of the High-Mu triodes. It will be interesting to see how an active load affect the sound of a 12AX7.

skipwave · May 30, 2007

Aikido-style:

http://www.tubecad.com/2004/blog0011.htm

analag · May 30, 2007

Actively loaded 12AX7s become a new beast, you can also force it (with CCS) to behave more electrically consistent among the various brands.

Aikido style it is.... I tried it a few years ago and I've been using it since.

skipwave · May 31, 2007

I'm curious if the active plate load would also help with the consistency of the amplifier sound at different feedback settings. I've noticed that in tube preamp designs with variable feedback gain controls they seem to sound best in a particular range of feedback, but often the volume of the source and output level of the microphone don't work with that gain range (clipping!). You end up padding the input and/or output of the amplifier enough to put you back in the sweet spot of the preamp. That is fine most times, sometimes the noise penalty hurts, but it requires a lot of controls to tease the best possible sound out of the preamp.

Maybe this would be more consistent across the feedback range. There is already feedback going on with the unbypassed cathode trimmer/resistors, so maybe the amount of overall feedback would be less affecting.