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Old 31st August 2014, 09:59 PM   #1
diyAudio Member
Join Date: Nov 2010
Default Octal Aikido with gain/buffer switch?


Long time lurker but overall beginner when it comes to audio circuits and tubes.
I am in need of a preamp and decided a tube preamp might be an improvement for my SS amp and Magnepans. Recently caught the DIY bug but limiting myself to projects where a PCB was available, I finally settled on Broskie's aikido PCBs. Problem is that with my SS amp and a mixture of sources, some sources require gain and others only need a buffer. Since the second half of the aikido line stage amp is almost the same thing as his aikido cathode follower I wanted to switch between using the whole amp and only using the second half.

Some searching found these two threads with similar ideas:
Aikido selectable gain
Aikido buffer/gain selector?

Downside is that neither thread seems to have posted complete schematics or results so I decided to try myself at creating a proper schematic. Please take a look at it and let me know if it will work. This is my first schematic and my first time using Eagle so sorry if the schematic looks a little funny. Eagle has quite the learning curve!

Some questions that reveal my poor level of understanding!:
Is switching R9 into there needed to correctly bias the top right triode? Did I do it right?

Would the normal values for R11, R12, and C1 work in both switch positions?

Did I put C3 in the correct position or should it go before the switch?

Should there be a capacitor in parallel with R5? Seeing this in some but not all schematics out there.

If the amp was turned on and I flipped the gain/buffer switch, would I get DC or a bad pop at the speakers? I'm thinking C2 might prevent that but I'm not sure.

Would there be any large changes in input or output impedance between the two switch positions?

In eagle, where in the library are the normal input and put symbols?

Noob question: Would the volume attenuator go between the input selector switch and the gain/buffer switch or should it go after the aikido's output?
Edit: Realized that putting it after the aikido would result in varying and very high output impedance which wouldn't be good.

Thanks in advance for all the help! Tubes and audio circuits have a challenging learning curve as soon as you get beyond the paint by numbers level of filling in PCBs.
Attached Images
File Type: png OctalAikidoWithSwitch.png (10.4 KB, 46 views)

Last edited by grogthegreat; 1st September 2014 at 01:50 AM. Reason: Answered one of my own questions.
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