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Old 18th January 2010, 12:51 AM   #1
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Join Date: Nov 2009
Default Unity coupled power stage


I am a new member ,located in Germany 49 years old and since the age of 14 I have been fascinated by some glowing parts in a radio.
After some years of being occupied with other things, I would like to build another tube amp.
I have been attracted by a design called "unity coupled" formerly used by McIntosh.It seems that it might be possible to design a tube amp with all the advantages of such a unit combined with a higher damping factor than in conventional tube amp designs.
I own some Telefunken V69's and like them much but would prefer a better speaker control the low frequency range.
Has anyone made some experiences with a unity coupled design,perhaps with the Van der Veen toroidal output transformers?

Greetings from Germany Ralph
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Old 18th January 2010, 02:10 AM   #2
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Join Date: Apr 2004
Location: Monroe Township, NJ
If you are going to get involved with unity coupled design, study Frank McIntosh's designs, like the MC240.

Completely different in topology is the Harman/Kardon Citation II, but it too controls woofer voice coils very well. Plitron offers toroidal O/P "iron" with the bandwidth needed to "replicate" Stu Hegeman's design.
Eli D.
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Old 18th January 2010, 02:21 AM   #3
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Location: Hickory, NC
Jan V. did a Mac project:
BigMac, Unity coupled output amplifier

Likely some threads here on diyAudio too. Seach for McIntosh or CFB or unity gain.
If it isn't broken, take it apart and mod it.
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Old 18th January 2010, 10:21 AM   #4
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Join Date: Jul 2003
Location: Ann Arbor, MI
search also for 'LW6-180' an design using TV sweeps.

The unity coupled design requires large drive voltage swing; it is moderately troublesome. There are a few ways to solve it. Examine the 12BH7 part of the McIntosh circuits( on the 75/275 for example ). There is positive FB there to deliver the headroom required. One can deliver the headroom without positive feedback by substituting a CCS for the 12kOhm resistors( the 12BH7 plate loads ).
the Tnuctipun will return

Last edited by Bandersnatch; 18th January 2010 at 10:24 AM.
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Old 18th January 2010, 11:27 AM   #5
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Join Date: Nov 2009

thank you for all the answers.I knew that the unity coupled design is a bit tricky because of the recommended high voltage swing for driving the output tubes.I considered a common cathode phase splitter as first stage followed by a mu stage driver in combination with a white cathode follower.(comments welcome...)When I find the time needed I will mount these stages for doing some measurements before buying the "iron".

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