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In Reply to: RE: Question on Lundahl CMR connection posted by Maxamillion on April 06, 2011 at 07:46:05
That's a good question. I don't have any experience trying this, but I would guess that it might be good for noise rejection to put both chokes in CM connection.
Kevin Carter
K&K Audio
www.kandkaudio.com
Follow Ups:
I think I will go with choke #1 in standard hookup and choke #2 in CM hookup. This way I use the full series inductance of the first choke to block the ripple from the first (small) cap and get the CM benefits from the second where there will be much less ripple to block. I know Lundahl says the CM mode also uses the full inductance, but it relies on the coupling of the two coils in the core, which is never 100% perfect, so there is probably a small loss.
I'll post my results - still soldering.
And it sounds excellent! More body through the midrange, more bass slam (a LOT more), and smooth everywhere it should be. It even sounds a little smoother on top to boot. I have attached a picture of the completed power supply; note that choke 1 is in normal series mode and choke 2 is in the special CM noise reducing mode. Both are Lundahl LL1638-10H.Wiring is all Cardas 15.5 gauge teflon insulated copper litz - I had some left over from wiring up my speakers and loved what it did there, so I used it in the PS. First cap is Mundorf Supreme, second is Solen, third is Solen bypassed with Mundorf Supreme and Mundorf Silver/Oil.
I went from 33uF/10H/100uF to 6.8uF/10H/47uF/10H/100uF and ended up with a B+ of 299.1 V for the new PS versus 300.0 for the old - got to love PSUD-II for modeling, saves SO much trial and error. The lower first cap completely eliminated the slight mechanical hum from the power transformer that the unit has has since I bought it. Has to be easier on the rectifier as well.
Overall I'm VERY happy with how this turned out.
Edits: 04/16/11
It's worth never forgetting what difference power supply changes can make, even in circuits that in theory appear to be "isolated" from the power supply topologically.
Good work!
Kevin Carter
K&K Audio
www.kandkaudio.com
For sure. Like I said, I have a cascoded CCS on the plates of the linestage tubes, and a shunt regulator using 350B tubes between the PS and the preamp, which should theoretically make the preamp circuit immune to changes in the PS.
NOT! I think we need better theories. The difference in sound is very, very obvious. Thanks for your help and excellent service, Kevin.
After a while with the second choke in the CM configuration, I rewired so that both chokes are in standard configuration and all three PS caps are star grounded to the same point as the PT center tap.
I think the sound has improved. It definitely sounds more powerful with more impact in the bass, and the rest of the range sounds about the same as before. I figure that with the caps all star grounded, there is no common mode noise to reject, as there is no ground bus running from cap to cap, only the + side of the circuit.
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