levy@ttrdc.UUCP (Daniel R. Levy) (07/12/87)
In article <622@neoucom.UUCP>, wtm@neoucom.UUCP (Bill Mayhew) writes: > I like the idea of using a switcher to supply the B+. By using a > switcher, you eliminate bulk and the higher frequency means that > smaller capacitors can be used to provide equal ripple removal. Switcher or no switcher, you still have to take care of power line ripple. It doesn't magically go away with a switching power supply. Think about it. If the switcher is powered from unfiltered full-wave rectifier output, the high-frequency output will also be thusly "modulated." Sufficient filtering (and possibly some regulation, which will also have the desirable side effect of reducing ripple) on the rectified output must be provided. -- |------------dan levy------------| path: ..!{akgua,homxb,ihnp4,ltuxa,mvuxa, | an engihacker @ | vax135}!ttrdc!ttrda!levy | at&t data systems division | disclaimer: i am not a yvel nad |--------skokie, illinois--------|
dje@datacube.UUCP (07/14/87)
The cleanest, lowest noise power supplies are linear regulated. Switchers are *mostly* for high power applications since their main asset is efficiency. Their other assets are light weight and the ability to convert DC to DC. If you want DC to DC conversion that's clean, one possibility is a switcher followed by a linear supply. I missed the original posting, but I heard a few references to tube type preamps. That means relatively low power and I assume throwing efficiency and weight to the wind. If I was building one, I would use a standard 60 Hz power transformer followed by either tube or silicon diodes and a reasonable amount of filtering. Then a tube for a series pass regulator (cathode follower configuration) or maybe even a high voltage amplifier transistor. Tubes as diodes have more inherent resistance than silicon thus causing lower charging currents and lower 120Hz harmonics on the power supply. The linear stage then has an easier job. Tubes as high voltage series pass elements take abuse better than transistors. If you're really into nostalgia, use an 0A2 (or the like) as the power supply reference. They have a great glow. Dave Erickson ---------------------------------------------- ------ Datacube Inc. / /| 4 Dearborn Rd. ------ | Peabody, Ma 01960 || \| | ihnp4!datacube!dje || /|/ Human:(617)535-6644 ------ Fax: (617)535-5643
wtm@neoucom.UUCP (Bill Mayhew) (07/19/87)
OK, I'll conceede that if you want to perform linear regulation at tube type voltages, a tube for the pass element is a good idea. If you accidentaly short the output cathode to ground, you've a few seconds to rectify (*a pun *) your mistake, rather than the microseconds you'd get before you send a transistor off to stick in the ceiling. A 6L6 is an easy to get tube that makes a good pass element. A pentode is would be a good idea to isolate your 0A2, 0D2, or whatever reference you use. There are some VR circuits in the old RCA receiving tube catalogs and probably old ARRL ham radio books. As far as switchers go, they aren't too bad if you design carefully so that it can operate in the MHz region, rather than KHz. Any noise wouldn't be audible. With a switcher, you can use something like a .1 uF fancy "audio grade" capacitor to get all the filtering you need. It's getting downright difficult to get decent n hundred uF capacitors* that are required for 60 (120) Hz operation. 60 Hz magnetics weigh a lot. Of course, building RF switchers calls for very careful design, and care at bypassing leads, etc. I think I'd put a linear VR circuit after the switcher for extra insurance. * at least, not at tube voltages! We have RF switchers in medical equipment that amplifies uV signals and don't have any noise trouble. Using an RF power supply is required so that ~5 cm. air gap can separate the primary and secondary of the *air core* power transformer. This baloney is necesary for "patient safety".-- although our patients are lab rats. Bill Mayhew Division of Basic Medical Sciences Northeastern Ohio Universities College of Medicine Rootstown (what a name), OH 44272-9989 USA phone: 216-325-2511 (wtm@neoucom.UUCP ...!cbatt!neoucom!wtm)
ken@rochester.arpa (Ken Yap) (07/21/87)
Isn't this all getting a little of overkill, regulators and all that? As I recall the original poster only wanted to feed a couple of voltage amplifying triodes in his audio preamp. A centre tap 250-0-250 transformer @ 20 mA, a pair of 1N4007 diodes, a pi section R-C filter, or maybe two, should do the job just nicely. And a fuse on the primary of course. Oh well, back to the nostalgia discussion... :-) Ken
wtm@neoucom.UUCP (Bill Mayhew) (07/25/87)
The high voltage power supply over kill issue was brought about by the use of the word, "audio". From reading net.rec.aduio, I've learnt that circuits for audio applications must be over designed by a factor of 5 to 10 in order to convince audiophiles that some horrible bastardization of the sound is not taking place. For circuits that require a few mA of current, a switcher supply isn't too terrible. Recently, I wanted ~200v at about 25 mA. I was able to use a ferrite ring and a couple of power transistors from my junk box to build a "ringing choke". The output of this went into about a 1 uF mylar capacitor after being recitified. I used two 0B2 tubes to shunt-regulate the output. The whole mess, except for the tubes, fit inside of one of those little Radio Shark aluminum boxes and only weighed about 100 grams. I resorted to that since I didn't have any old 60 Hz transformers and big filter caps laying around. I followed the design (not really copied) from one of those big "we publish every basic circuit you can imagine" type reference books. Bill