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Old 15th September 2008, 07:45 PM   #11
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Michael Koster's Avatar
 
Join Date: Jan 2008
Location: Eureka, CA
Lars:

I have a lot of sketches like your schematic, but it comes
down to ? DC stability of using a CCS-loaded driver tube
to provide fixed bias. I have not tried building it yet...

Replace the CCS with a choke and use combination bias
for better DC stability and that's pretty much the amp
design I've been building. Also the choke eliminates the
3rd power supply.

I don't expect a mains toroid to perform well at line
levels though, and I've been spending up to $150
each for a good line input transformer (repeat coil)
UTC A-20s are good at about $100/pair on ebay.
Again I have not tried the toroid as a line repeat coil
but maybe a 5VA unit with a tiny core...

I would like to figure out a build like your picture some
day with CCS DC coupling, just haven't come up with
the one yet.

Ken:

Can I do a Hall-effect servo with 3 parts or less? Hmm...

Michael
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Old 15th September 2008, 09:29 PM   #12
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Location: Eire
Interwinding capacitance is the killer in this application. I had to push 30mA of current through my driver tube to get a decent response. Even then its somewhat rolled off on the top end.
If you can design it with a 2:1 voltage step down I would expect it to perform much better. I have used one as an input transformer with a step down of 9:1 and it works great.

Shoog
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Old 16th September 2008, 06:48 AM   #13
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Michael,

I had a dedicated 1:1 line transformer in mind when I did the schematic. DC-coupling looks nice on paper but maybe is a little hazardous.


Brgds
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