Use an AC output wall wart. 12.6V AC or some such. In the device itself you can run a smaller transformer "backwards" to get the high voltage to make B+. 12V to 120V transformer. It can be small as it won't be handling heater power. The wall wart does that. No powerline hazards to deal with.
I agree with the moderator for not allowing the schematic. Even with a red "X" and a Death's Head stamped over, some fool is going to build it just to prove the the moderator wrong. The odds approach 100% that said fool, spouse, friend, relative or at least the cat will soon be toast.
Rod Elliot's latest amp is a 1500W monster. It comes with Rod's express warning not to build it. To me this was an article in the spirit of Swift's "Modest Proposal". Sure enough I've heard of someone setting off to build it.
Rod Elliot's latest amp is a 1500W monster. It comes with Rod's express warning not to build it. To me this was an article in the spirit of Swift's "Modest Proposal". Sure enough I've heard of someone setting off to build it.
Sorry, but I totally disagree with your logic. An effective way of teaching is to show the pro's and con's of doing things a specific way. I sincerely doubt that the only people capable of understanding would be engineering students and persons with engineering degrees. The amateur will still try to build the power supply thinking that he or she is smart enough to overcome the engineering pitfalls. Listing the reasons why a specific design will or will not work will make people wake up and think. Unfortunately hiding a design under a basket will only make the uninformed more willing to try it.
solid state power supplys like computer PSU aren't a choice ?
i allways try to put in and transformer... but i'm wondering on a simple thing... an 211 SE amp 1200 V with one of those switching power supplys...
i think it safer don't you ?
and there are out there some switching PSU small enough.. i guess *-)
i allways try to put in and transformer... but i'm wondering on a simple thing... an 211 SE amp 1200 V with one of those switching power supplys...
i think it safer don't you ?
and there are out there some switching PSU small enough.. i guess *-)
Back in the day then tubes ruled the world there was some thing called all current (direct Swedish translation) radios.
Those radios could be supplied with both Ac and DC power. How? They operated without transformer. A rectifier direct after
power input so both DC and AC find path trough. We had with other words a live ground. This worked fine as the only electrical connection to outer world exposed to user was the power cord and everything else was insulated to user. If external antenna connections existed they were though an antenna transformer.
The same thing applies to earlier TV sets. Internally there was indeed a transformer but for high voltage to CRT, ground still live.
Many accidents did happened when less knowledgeable tried to for example connect external amplifiers direct from inside of transformer less equipment.
That could dependable on mains polarity result in mains voltage in external equipment chassis or short circuit in case of grounded equipment.
So it is possible to design without transformer on mains but you really need to know what you are doing and the efforts needed to succeed far outweighs tradeoffs introduced by a transformer, especially today with small switched units.
I agree with the sharp warnings.
Those radios could be supplied with both Ac and DC power. How? They operated without transformer. A rectifier direct after
power input so both DC and AC find path trough. We had with other words a live ground. This worked fine as the only electrical connection to outer world exposed to user was the power cord and everything else was insulated to user. If external antenna connections existed they were though an antenna transformer.
The same thing applies to earlier TV sets. Internally there was indeed a transformer but for high voltage to CRT, ground still live.
Many accidents did happened when less knowledgeable tried to for example connect external amplifiers direct from inside of transformer less equipment.
That could dependable on mains polarity result in mains voltage in external equipment chassis or short circuit in case of grounded equipment.
So it is possible to design without transformer on mains but you really need to know what you are doing and the efforts needed to succeed far outweighs tradeoffs introduced by a transformer, especially today with small switched units.
I agree with the sharp warnings.
EC8010 said:
Well, as I said, even if they use a transformer they can mess it up completely and connect the primary and secondary sides or something equally hazardous, if they don't have the knowledge. And then there is ground and and earth and all the safety issues there.
But OK, a few schematics then of the basic correct ways to use a transformer, and the rectifiers and text explanations of what is important not to change and what will happen and why it is dangerous if violating this. Then at least there is no schematic of something lethal, so they have to figure it out for themselves if they absolutely want to ignore what we say.
I did some stupid things as a young teenager, not understanding it was dangerous. After getting a "kiss" I realized that something was wrong and figured out what (and that was not skipping the transformer). If somebody had only shown me a schematic of how I should do it, I am not sure I would have followed it, since i thought my solution was just as OK. If somebody had showed me a schematic of what I did and explained why it was dangerous, I would have understood and done it differently, even if costing me a bit more. Fortunately, I've never got anything worse than a current path between fingers on the same hand, which usually isn't lethal.
Edit:
Of course, one should by all means avoid even such current paths between fingers, since they can cause serious injuries, which is why I wish I had seen a schematic of my stupid solution with explanations of why not to do it. I would then have solved my problem differently and never had that nasty experience. Other cases was pure carelessness. I was repairing a friends light organ, which actually was a transformerless circuit, since it was intended to drive 220 V bulbs. I was holding the PCB with my hands when troubleshooting it, of course shorting two live traces with a finger. That is, never hold any PCB with your hands when powered, even if connected via a transformer. Even low voltages can be dangerous under undfortunate circumstances.
You don't want to
"I've never got anything worse than a current path between fingers on the same hand, which usually isn't lethal."
ive got that either *-) but when repairing an amp. one cap blown away one wire disapeared and o got the current thru the digital multimeter.
one of the probes was not good... lack on isolation... and lack on my care :S
so i got new probes
after that not very good experience...
ive got that either *-) but when repairing an amp. one cap blown away one wire disapeared and o got the current thru the digital multimeter.
one of the probes was not good... lack on isolation... and lack on my care :S
so i got new probes
after that not very good experience...
Ryssen said:I am not going to build anything whitout a transformer,but would anyway like to know why I shouldn´t.
Since the neutral wire of an outlet is directly connected to ground, grounded objects near the device under test (desk, lamp, concrete floor, oscilloscope ground lead, etc.) may be at a hazardous potential difference with respect to that device. By using an isolation transformer, the bonding is eliminated, and the shock hazard is entirely contained within the device.
Thanks to wikipedia for that concise description.
Ryssen said:I am not going to build anything whitout a transformer,but would anyway like to know why I shouldn´t.
Read about "Stone The Crows." Please.
Ryssen said:I am not going to argue with you.
Yes. Your widow will still be able to use your stereo.
SY said:
Hah! I have been smacking my head trying to think of exactly that, and here you keep posting it. Stone The Crows was the band where the guy died from touching the mike. Yes, that is it!
Also, using a hair dryer in the bathtub would be another analog. Wasn't there a B movie about that?
And design limits of ~130V of 1/2 wave rectified noisy B+ and series connecting heaters up to line voltage are just yuck.
Mains transformers are your friend. Find room in your wallet and stereo.
And with respect to the fuse, fuses are often misunderstood. The fuse is not there to protect people, the fuses protects the house wiring and upstream fuse/circuit breaker. The fuse will blow as a result of you absorbing a lethal shock.
This thread was started about a guitar preamp. In a guitar amplification situation. The guitar, and the strings on the guitar are always in contact with an often sweaty user. Now if that guitar was connected to one side of the power line because there was no isolation transformer, and the microphone (or anything else in the performers reach) is grounded (or connected to the other side of the line by a similar situation), the performer will be fried when he touches both of them (a likely situation).
In this circuit there was a bridge rectifier connected directly across the line, and the negative side of the rectifier was connected to circuit ground, as was the guitar. This connects either side of the line directly to the operators hands on alternate half cycles of the power line frequency. Isolation transformers are cheap, and life is priceless, use a transformer in any equipment that you build.
Stereo equipment is not immune, your CD player and turntable are connected to the amps ground system. If the amp is hot, so is ALL of your equipment. You are leaning against something grounded (even a concrete wall) or standing barefoot on a concrete floor, and you touch the metal case of the CD player, or your metal tone arm, and ZAP, you are toast!
There were plenty of transformerless (called AC-DC in the USA) radios, TV's and yes, guitar amps built in the 50's and 60's. They relied on plastic insulation to keep the user from touching the electronics. Many of these old guitar amps connected one side of the power line to the guitar input through a paper in wax capacitor. These amps are also DEADLY. If you find one, mount a 50 VA isolation transformer in the back of the amp before using it. Put a 3 wire power cord on it and connect the ground wire directly to the ground side of the guitar input.
Many vintage guitar amps including the popular ones from Fender, Marshall and others, have a two wire power cord, and a "ground" switch. This switch puts a cheap paper cap from one side of the power line to the chassis (and guitar). When these caps get old and leaky a shocking situation develops. I did not know about the "Stone the Crows" story, but I have seen the same thing happen a few times over the years. Fortunately none have been fatal. When I used to set up equipment for a friends band, I would run around the stage with a neon test light looking for hot equipment. This is real important when playing outside gigs where power is provided by old extension cords!
Take a neon test light, connect one lead to a known good ground. Touch the other lead to every exposed piece of metal on the stage. If the light glows, fix the problem before proceeding.
When building any new equipment measure the resistance from the ground prong on the power cord to all exposed metal on the equipment. You should have zero ohms.
I put an Electrical Safety page on my web site. It is based on many things that I have learned in 40+ years of playing with electricity.
http://www.tubelab.com/Safety.htm
In this circuit there was a bridge rectifier connected directly across the line, and the negative side of the rectifier was connected to circuit ground, as was the guitar. This connects either side of the line directly to the operators hands on alternate half cycles of the power line frequency. Isolation transformers are cheap, and life is priceless, use a transformer in any equipment that you build.
Stereo equipment is not immune, your CD player and turntable are connected to the amps ground system. If the amp is hot, so is ALL of your equipment. You are leaning against something grounded (even a concrete wall) or standing barefoot on a concrete floor, and you touch the metal case of the CD player, or your metal tone arm, and ZAP, you are toast!
There were plenty of transformerless (called AC-DC in the USA) radios, TV's and yes, guitar amps built in the 50's and 60's. They relied on plastic insulation to keep the user from touching the electronics. Many of these old guitar amps connected one side of the power line to the guitar input through a paper in wax capacitor. These amps are also DEADLY. If you find one, mount a 50 VA isolation transformer in the back of the amp before using it. Put a 3 wire power cord on it and connect the ground wire directly to the ground side of the guitar input.
Many vintage guitar amps including the popular ones from Fender, Marshall and others, have a two wire power cord, and a "ground" switch. This switch puts a cheap paper cap from one side of the power line to the chassis (and guitar). When these caps get old and leaky a shocking situation develops. I did not know about the "Stone the Crows" story, but I have seen the same thing happen a few times over the years. Fortunately none have been fatal. When I used to set up equipment for a friends band, I would run around the stage with a neon test light looking for hot equipment. This is real important when playing outside gigs where power is provided by old extension cords!
Take a neon test light, connect one lead to a known good ground. Touch the other lead to every exposed piece of metal on the stage. If the light glows, fix the problem before proceeding.
When building any new equipment measure the resistance from the ground prong on the power cord to all exposed metal on the equipment. You should have zero ohms.
I put an Electrical Safety page on my web site. It is based on many things that I have learned in 40+ years of playing with electricity.
http://www.tubelab.com/Safety.htm
The moderators have acted decisively here, and for that they can't be faulted.EC8010 said:
There remains the possibility that the missing schematic will pique the interest of some who will trawl through the w3 to do it on their own, for the reasons mentioned above.
Showing the schematic (with clear warnings) would allow the curiosity of some to be quenched, allowing them to put the matter to rest and walk away.
2c
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