I saw the "Jobs" movie yesterday and it revived some ancient memories of my dealings with Jobs and Holt in the "old days"! When I returned home, I researched Rod Holt on the Internet and ran across your Power Supply Blog, which I found most interesting. Perhaps you can add my ensuing comments to your blog as you see fit.
In 1973 I started a company in my garage in Cupertino to design and manufacture custom Magnetic Products. It was called Mini-Magnetics Co. Inc. After a few months I was forced out of the garage into a small office complex on Sunnyvale-Saratoga Road, and had about 5/10 employees.
I believe it was around 1975/76 or so, I had a visit from a insulation and wire salesman named Mike Felix. He informed me that I may soon be getting a call from a new /start-up company called Apple Computer located just a few blocks away in Cupertino. He gave them my name when he was asked to recommend a Magnetics manufacturing house.
I promptly forgot about it, as I was already quite busy and I never had to solicit business or even advertise. A week or two went by, and I received a call from a female at Apple who set up a appointment for the next day with a guy named Steve Jobs. She gave me the address and it turned out to be located in a office complex located just behind the "Good Earth" restaurant.
The next day I went over to the location and knocked on the door, and it was opened by Jobs, with Wozniack in the background and a young hippie looking girl at a desk in the corner talking on the phone while eating. That was Apple Computer. They had just moved out of their garage into this new location. It appeared to be a large room with "stuff" scattered hap-hazardly all over, on benches and on the floor. From Jobs' appearance, I was a bit afraid to even shake hands with him, especially after getting a whiff of his body odor!
He immediately took me over to a bench that had a few cardboard boxes on it and showed me some transformer cores, bobbins and spools of wire, and unfolded a hand written diagram of the various magnetic components that he wanted me to wind and assemble for Apple.I took a quick look, and while it was all quite sketchy, looked do-able. He said that he needed them within 10 days and I said ok, since he was furnishing the materials.
I told him that I would call him with a quote after I got back to my office and he said ok and as we were parting he mentioned that if I had any technical questions to get a hold of a guy named Rod Holt and wrote down a phone number where he could be reached.
As I recall, there were about 5-6 magnetic components from simple toroids to a complex switching main power transformer. I believe that the price came to about $10.00 per set,and they wanted 35 sets, so the entire matter would be about $350.00. I called it into Apple the next day and they gave me a Purchase Order number over the phone. When I asked if they would be mailing me a hard copy confirming the order, they had no idea of what I was talking about!
I figured, what the hell, worst case, I would be out $350 bucks if they didn't pay the bill. No big deal.
After I got into examining the sketches I discovered something quite interesting about the power transformer. In all previous designs that I had seen, there was a primary, a base feedback winding and several output windings. What Holt had contrived was a interesting method of assuring excellent coupling of the base winding by using a single strand of wire from a multi-filar bundle that was custom ordered from the wire factory. For example, I think that there was a bundle of 30 strands twisted together, which were all coated in red insulation and one strand of green insulation also twisted together in the bundle, which gave a precise turns ratio together with excellent coupling between the windings.
I am uncertain if that contributed much to improving the efficiency of the switcher, but it seemed clever at the time I discovered it. That transformer, is the one that is shown with the copper foil external shield pictured in your blog. I did speak with Holt once or twice but never met him in person.
The 35 sets of parts were delivered on time and much to my surprise, we were paid within 10 days. I attributed that to the arrival of Mike Markkula onto the scene who had provided some money and organization to Apple.
At the time, after seeing the Apple operation, I wouldn't have given a nickle for a share of their stock if it had been offered! Ha!
I had been involved with power supplies for many years prior to this Apple issue, and can say that switchers were known for a long time, but only became practical with the advent of low loss ferrite core materials and faster transistors as your blog implies.
So, thats the Apple Power Supply story ! Be happy to answer any questions that you may come up with. Regards, wpf