Voltage is not an issue as we are working well below the limits of usual varnish insulated magnet wire. But the working voltage is no more than 50v, so have some margin on that. I think it’s going to be whatever the standard voltage is. Toroidy is good stuff but made in EU and shipping to US will really make it quite a luxury item. 
One more spec to give - needs air gapped core. Circa 0.25mm to 0.5mm.
One more spec to give - needs air gapped core. Circa 0.25mm to 0.5mm.
Someone should try the MD-903AMR MOT. It seems to be rated 0.1ohm DCR based on test by seller. Might be worth the $38 cost to get one and measure for your self. The 903’s seem to be replacing the 803’s as they are harder to find.
8II13 TRANSFORMER MD-903AMR-1, FROM MAGIC CHEF MCM1110B MICROWAVE OVEN, TESTS OK | eBay
8II13 TRANSFORMER MD-903AMR-1, FROM MAGIC CHEF MCM1110B MICROWAVE OVEN, TESTS OK | eBay
Without core saturation, the choke should be able to accept 10 VRMS at 20 Hz, 3A DC current.
Does not seem to have air gap as E and I parts are welded together; besides the laminations do not seem to be audio grade. M6 is what you'd need or better.
I wonder if this is the air gap, as the weld only the edge. There is on on the other side too. Or it may be internal - I have seen someone do a tear down of one of these and I recall there is a plastic thin liner wedged in there somewhere.
It’s smaller than a 5 mil feeler gauge though if it is a gap. Looks very parallel.
It’s smaller than a 5 mil feeler gauge though if it is a gap. Looks very parallel.
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I’m not going to complain about air gaps for $30 MOT.
Don't underrate the importance of that choke; it will make or break the performance. Exactly as important as the output transformer required for a quality tube amplifier
Gap does not go by Amps. That depends {EDIT: also} on number of turns.
It is typical for gap to be *about* 1/1000 of the total magnetic path(*). If that is 4 mil gap and 6 inch path in iron, it's reasonable.
(*)Iron conducts magnetism 1,000 to 10,000 times better than air. So it takes only about 1/1000 of air for the air to dominate the reluctance.
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Only the number of turns?? Don't think so...
Properties depend on core dimension (excitation), number of turns (inductance; DC current capability), DCR, winding capacitance, to name the most important.
In the case of this amp the chokes' DCR is the "first" parameter as it biases the Jfet as a function of the DC current.
Next comes the required inductance and the core geometry/gapping to withstand the DC current without core saturation, and this should perform cleanly with some 10 VRMS at 20 Hz.
Properties depend on core dimension (excitation), number of turns (inductance; DC current capability), DCR, winding capacitance, to name the most important.
In the case of this amp the chokes' DCR is the "first" parameter as it biases the Jfet as a function of the DC current.
Next comes the required inductance and the core geometry/gapping to withstand the DC current without core saturation, and this should perform cleanly with some 10 VRMS at 20 Hz.
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JPS64 has already started the placement of the key parts for the LuFo amp PCB. As you can see, long and skinny for mounting 4x TO247 size devices on a UMS compatible heatsink. We are also adding provision for use of M2X/Yarra format front end preamp boards. There will be an option to bypass the front end for driving with an external opamp.
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Testing it with some bigger 6.5in 2-way reflex speakers. I can hear the bass that this amp is capable of. It sounds punchy and tight with deep extension. Also playing with different input preamps. Today it is the one designed by Hugh Dean that is a modification of the Lender topology. It’s also SE Class and cap coupled output. It’s notable for being able to drive 40Vpp cleanly. That’s enough for a 25w amp anyhow.
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In anticipation for this upcoming build, I used my Peak LCR45 meter to test 5 md-803amr MOT’s in my stash:
0R57, 51.85mH
0R54, 47.70mH
0R57, 59.64mH
0R55, 54.10mH
0R90, 7.90mH *BAD*
Looks like the DCR is very consistent, but the inductance varies +/-6mh from the average. Doesn’t seem too bad, unless you have the 47mH and 59mH as a pair.
JP,
The layout progress is looking Sweet!!
0R57, 51.85mH
0R54, 47.70mH
0R57, 59.64mH
0R55, 54.10mH
0R90, 7.90mH *BAD*
Looks like the DCR is very consistent, but the inductance varies +/-6mh from the average. Doesn’t seem too bad, unless you have the 47mH and 59mH as a pair.
JP,
The layout progress is looking Sweet!!
I left the LuFo on all day yesterday - playing same Norah Jones song over and over again in the background while I was out. Returned to listen after 18 hrs of burn in. The sound is just better. Smoother. Fluidly dynamic. It’s only in mono and I can say it’s one of the best amps I have heard. Makes you want to listen more and stay longer. I was up until 430 am listening. You know it’s special when an amp engages you in the collection of same old songs you have heard a thousand times. Must be capacitor burn-in? Or the amp is now channeling Norah?
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