Let me detail a project I have been thinking about, mainly while bored in math class..
A transformless, directly rectified, class A/B power amplifier. A bridge-recitifer directly off the outlet power supply..!
So we will have around 120V rails. Maybe buy bulk 50 or so film caps and put them before the rectifier. A big recitifer, a 50 AMP bridge, or 60-amp X 4 individuals, etc. will do the job.
We now have the capability for an absolutely huge power supply. Bias this sucker for a little bit and hang maybe 50 to 100 devices in a push-pull operation in it.
Anybody ever thought of doing this? I read somewhere that someone saw one pushing 1000W+ into 8 ohms or so.
I think I might try it, of course with the proper precuations so I don't burn my house down..;
As for casing I was thinking of milling 1 inch alum panels and putting the transistors inside two of them, sandwiched, I have seen a similar design..
I believe Nelson Pass did this already many years ago. Direct voltage doubling off the input supply (transformerless). There is no doubt in my mind that it will work.
There is a safetey issue with running transformerless. Have you considered SMPS?
Your rail will end up more on the order of 170V. You'll need a ground in there somewhere (unless you're planning on going single-ended), which will need to be floated, either with a center-tapped inductor, or a pair of resistors. The resultant rails will be about +-85V. You will always need to remember that ground on this thing isn't ground in the real world. You'll have one rail at ground potential (AC neutral), one rail at 170V, and "ground" at 85V.
One thing about not having a transformer is that high frequency nasties on the AC line come straight in. There's no isolation. Same for spurious stuff on your rails--right back into your household AC. A cap across the incoming AC line would help.
What about the kind of power and amplifier of this design can produce?
Grollins, what would be the values of those resistors?
In my opinion it is very close to a suicide mission what you propose.
While i agree that it would be very nice to have a powerful amp without substantial investments in power supply magnetics, i think it is a step too far to go without a safety transformer of some sort.
SMPS is one way, as Petter mentioned, but needs expertise in design and specialized transformers which are hard to get. That said, i also don't have the expertise for it.
Better than rectifying the line directly would be to use a big isolation transformer (the industrial stuff, find one used for cheap and have several kVA on tap) with two secondary winding (common to adapt US gear to the rest of the world). Still you will have the problem of very high supply voltage and comparatively low current.
A big step-down transformer is a better solution for audio amps IMHO, like +-60V.
There is another way, too, if you have three phase power in your house. That's the route i chose, took three 500VA transformers and built a 1.5kVA three-phase supply with a ripple frequency of 150Hz and high DC content in the rectifier output. 1.5kVA is enough for me :-)
and the supply does a fine job of powering my Aleph 3 clone with 0.25F with choke input (20mH).
For your own health's sake, don't break the rules of electrical installation safety. Don't rectify the mains directly.
[Side note: some industrial gear does this, but it does not connect to other gear or uses alot of precautions with optocouplers etc.]
BTW, i think it was Bob Carver and not NP who built a mains directly rectified amp.
The Linn Klimax uses a switching power supply, yes? Anyone know any technical details on this? I'm looking for alot of power in a small size.
But in a Sunfire subwoofer Bob Carver just rectifies the 120V mains to +-160V DC. After the rectifier he uses high voltage caps. Then he lets one or more power mosfets (that can deliver high currents) switch (or chopp) the 160V to a pulse width modulated signal, then he reconstructs the voltage from PWM to DC with a low-pass filter, and then this voltage is dropped across the power transistors in the output stage.
This design however could be very dangerous if not handled correctly...
Nelson Pass did indeed do this many years ago.
I think this was covered previously on one of
Kilowatt's threads. The amp had +-170 volt rails
and was bridged, so that it could swing a peak
of about 300 volts. Into 8 ohms, I believe that
would be about 11 kilowatts.
I worked fine, but was quickly retired as it was too
scary to run day-to-day. Electrical safety per se was
not the issue, rather you could imagine the results of
a bad input transient. ;)
I would not advise this approach for the DIY builder, as the amplifier and all parts conneted to it are live. I did design such an amplifier for use in powered subwoofers for both Infinity and Pioneer, but this was done in cooperation with UL safety labs to guarantee a safe product. It utilised a small wide bandwidth signal input isolating transformer to safely isolate the input. The driver and box were specially designed to prevent access to any of the live parts. Although the technique allows cost/performance advantages by eliminating the need for a costly power transformer, there are lots of resultant design pitfalls to overcome, so I would again not recommend it as a DIY project
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