dvadura@rose.UUCP (07/02/87)
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I'm working on building a small audio pre-amp using a 12AX7 type
tube. I'd just like to experiment with some old-style technology. It's
been suggested that I could obtain the less than 330 volts needed by
directly rectifying the line current and putting it through a voltage
doubler, eliminating the power transformer. This could reduce the cost
of the thing quite a bit *iff it's a safe (for both me and my toys)
thing to do*. Are there any experts out there who'll expound on the
wisdom (or lack there of) in eliminating the power transformer? I'll
definitely put one in if there's any doubt.
Thanks.
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UUCP: {ihnp4|utcsri|utzoo|decvax|allegra}!watmath!rose!dvadurahenry@utzoo.UUCP (Henry Spencer) (07/03/87)
> ... I could obtain the less than 330 volts needed by > directly rectifying the line current and putting it through a voltage > doubler, eliminating the power transformer. This could reduce the cost > of the thing quite a bit *iff it's a safe (for both me and my toys) > thing to do*. Are there any experts out there who'll expound on the > wisdom (or lack there of) in eliminating the power transformer? ... The trouble is that the negative end of the power supply is not isolated from the AC line and hence can pretend to be "ground" only if the whole gadget is self-contained and well-insulated, and you avoid connecting things like oscilloscopes to it. I believe some TV sets do their power supplies this way, much to the irritation of TV service people who need to haul around an isolation transformer if they want to connect test gear. If the thing were self-contained, e.g. a TV set, I would rate this dubious but acceptable given precautions. For a pre-amp, you are presumably going to be connecting it to other equipment. Isolating those connections is going to be hard, probably too hard to be worthwhile. Accordingly, I would give this a big black NOT ACCEPTABLE. Use a transformer. -- Mars must wait -- we have un- Henry Spencer @ U of Toronto Zoology finished business on the Moon. {allegra,ihnp4,decvax,pyramid}!utzoo!henry
ken@rochester.arpa (Ken Yap) (07/04/87)
Don't do that! It ties your audio circuit to the power line in an undesirable way. It probably wouldn't work because 1. The neutral of the 110 V input is close to ground potential. 2. The negative rail of your will be at some 140 V below the neutral, assuming the full wave doubler, or at neutral, if you use the half wave doubler. 3. In the first case, this makes the ground point of your audio -140 V DC live wrt true ground, assuming it doesn't short first. In the second case, this creates a giant ground loop which will make your woofer vibrate with hum. Go buy a transformer. For your safety, listening pleasure and peace of mind. Ken
neal@weitek.UUCP (07/07/87)
In article <8240@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes: >> ... I could obtain the less than 330 volts needed by >> directly rectifying the line current and putting it through a voltage >> doubler, eliminating the power transformer. [...] > >The trouble is that the negative end of the power supply is not isolated >from the AC line and hence can pretend to be "ground" only if the whole >gadget is self-contained and well-insulated, and you avoid connecting >things like oscilloscopes to it. I believe some TV sets do their power >supplies this way, much to the irritation of TV service people who need to >haul around an isolation transformer if they want to connect test gear. They don't call 'em "hot chassis" for nuthin'! Yeow! > [...] For a pre-amp, you are presumably going to be connecting it to other >equipment. Isolating those connections is going to be hard, probably too hard >to be worthwhile. Accordingly, I would give this a big black NOT ACCEPTABLE. >Use a transformer. One way to reduce the bulkiness (but not neccessarily the cost) of the setup is to rectify the AC directly, and use that as the power supply of a DC-DC converter. Not only do you get the isolation (thx to the ET transformer) but you also get better regulation on the B+ (300V) supply. However, you still need filament voltage (a small/cheap transformer can do this.) Another approach is to use a low (filament) voltage transformer with enough excess capacity to drive a "flyback" DC-DC converter for the B+ supply. You get isolation, simple B+ regulation, and filament voltage too. However, if you're lazy (or dislike working with switchers), you can probably scrounge an all-in one B+/filament transformer pretty cheap from a surplus house. You'll have to filter the B+ supply with some big HV caps, tho'. Ick. One advantage: transformers tend to "swallow" line spikes. Henry's right, tho' - whatever you do, do not hook the circuit innards directly to the AC line. I wouldn't recommend the use of an "auto-transformer" either, since these connect you to at least one leg of AC (50-50 chance it'll be the "wrong" one.) Be careful. -Neal
wtm@neoucom.UUCP (07/09/87)
Hi, While we're on the topic of tube preamps... I learned a lot about the nearly lost art of tube construction methods by looking at old Tektronix 500 series oscilloscopes and their letter series plug-ins. The Tek scopes were about as far as the state of the art of tube technology went. In their hight gain amplifers, they often used DC on the tube heaters to help reduce 60 Hz modulation of the signal being amplified. The high gain preamps were also usually isolated from the chassis by rubber or springy metal strips. The idea was probably to combat microphonics from picking up vibrations of the fan. (Quite a big fan was necessary to cool down a 551 with its complement of 111 tubes! -- and it had a separate power supply with about a dozen more tubes for regulating all voltages). Shock mounting the amp would probably be useful in combating feedback from the speakers. One thing that has puzzled me is that some Tektronix plug-ins have flat rubber coated resistors. I've wondered what the purpose of doing so was. I've never seen any references as to why. One thing to watch out for in tubes is the potential difference between the heater and the cathode. 12AX7s permit about 100 vdc between the heater and cathode. The main impact is on the life of the heater, and possible dielectric breakdown of the barrier between the heater and cathode. You should be aware of this if you are designing a circuit that runs one or more of the stages with the cathode significantly above 0v. If so, you can float the filament xformer. The got-cha is if you feed DC to the heaters, and the DC supply is ground referenced. 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. Be sure to use good construction, to prevent RF from escaping. Also, you need to use good quality capacitors with a high Q rating. Regular aluminum electrolytics aren't very good. A small mylar capacitor in parallel can be used if you can't locate high grade electrolyic capacitors. (Save your $$$, and don't bother buying esoteric polyproylene caps for your power supply). Who knows, the polypropylene capacitors may make a difference in interstage signal coupling-- you be the judge of that. To me, mylar caps sound equivalent for audio coupling. Hope this was of some use. Bill (wtm@neoucom.UUCP)
rbl@nitrex.UUCP (07/13/87)
In article <622@neoucom.UUCP> wtm@neoucom.UUCP (Bill Mayhew) writes: > > ..... > > >The high gain preamps were also usually isolated from the chassis >by rubber or springy metal strips. The idea was probably to combat >microphonics from picking up vibrations of the fan. (Quite a big >fan was necessary to cool down a 551 with its complement of 111 >tubes! -- and it had a separate power supply with about a dozen >more tubes for regulating all voltages). Shock mounting the amp >would probably be useful in combating feedback from the speakers. > I've been told that the policy at Tektronix was that a 'scope should survive a fall from a 3 foot high table onto a concrete floor! The springies etc. helped the equipment fulfill this goal. >One thing that has puzzled me is that some Tektronix plug-ins have >flat rubber coated resistors. I've wondered what the purpose of >doing so was. I've never seen any references as to why. > From the fuzzy dark reaches of my mind, I recall this resistor configuration had something to do with reducing the inductance of the resistor. Thick-film technology, as I recall. > > >Hope this was of some use. >Bill >(wtm@neoucom.UUCP) There is an ART and there is a SCIENCE. VERY occasionally, a for-profit organization can afford to bring the state-of-the-art along with an excellent appreciation of the science and actually produce a OPRODUCT with the best of both! Rob Lake (rbl@nitrex.UUCP) Disclaimer: This in no way reflects any opinion of my employer nor any knowledge gained while in my current employment.