Transformless Supply Amplifier


2001-04-07 10:03 pm
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..
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.
Be careful.

Suicide Mission?

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.



2002-01-09 11:22 pm
Re: Linn

arnach said:
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.

Dont know the specs for Linn Klimax, check one of the many tutorials on the net on switching power supplies.

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

The one and only
Paid Member
2001-03-29 12:38 am
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. ;)


2001-09-05 6:37 pm
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
I'm Not Alone!

This discussion sounds very familiar! I want to (yes, I still do!) make an amp with a power supply directly rectified from American 240V, so there would already be a center tap. With power supply droop, it should come to about +/-150-160V. Everyone on the forum should know this project well by now, unless they're new to the forum. Nobody thought the idea was wise, because of the danger of not having isolation, but there would be isolation transformers at the inputs and low level outputs for added safety in failure scenerios, however, the main speaker hookups could pose a danger in this amp if not treated with proper safety practices. It is also very hard to design an amp that has +/-160V rails because of slew rate and stability among other things, but I plan to do it anyway. I personally think it's a good idea, but only in certain very rare and special amplifier projects, and if you plan on selling such a thing, make sure it's as safe as possible and that the buyer understands the special safety issues. Also, the power supply capacitors alone in a supply like this will be far more expensive than the power transformer used in even some of the biggest commercial and DIY amps.

Good Luck!
Yeah i agree.. I've even gotta get my linear power supply amps checked by an electrician and tagged or the insurance goes out the window and they have step down transformers in them. Some direct line rectified amps have been approved by the various agencies involved but to my knowledge, they have all been subwoofer amps that have sat inside the sub with only the power cord and low level signal input sticking out the back... and in all cases, the signal input has an isolation transformer. I also think that most if not all of these amps were designed and built with very close co-operation with the various agencies involved.

As far as ever selling that amp killowatt, forget about it...
That's interesting. Has anyone thought about just why that is? You won't find an isolation transformer in your toaster, oven, dishwasher, power tools, or anything of the sort. These things have no trouble being approved by the UL or CSA and are not considered unreasonably dangerous. What is it with amps? Like those subwoofer amps, my amp will have isolation transformers at everything but the speaker terminals, and the speaker cables would pose no more danger than an extension cord in my opinion. I find it strange that amps, which have nothing dangerous exposed, except perhaps around the speaker hookup area, in the case of my amp, are considered to be so much more of a fire hazard than oil lamps, wood stoves, and gas ranges. That's so stupid I can't even find the words to explain it. The same goes for my amp being considered more dangerous than a big Tesla coil, or a large Jacob's Ladder like mine, or an electric motor that has exposed power terminals. Believe me, there are things more risky to have in your house than my 7200W amp, or any other amp with a line supply, but those things are all UL and CSA listed. I believe that with some consideration, my amp and other line-powered amps could be made very safe indeed.

I'm not trying to start an argument, just prove an interesting point. I'm very curious to see what people will say about it.
As in my example and as in most of your examples, the device has no exposed high voltage / high current outlets.... sure the motor is an exception but having said that, even using bridged amps of a few hundred watts is potentially deadly if you get between the terminals because they can often supply over 100Vac RMS @ many amps.... so while i agree that some of these laws don't make sense because of the lack of consistency, the underlying principle is sound.... from memory your 7200W amp will be capable of @ least something like 200V RMS across the terminals when it is working hard... @ this kind of voltage, 100mA can quite easily kill in a split second.
Perhaps the reason for it all is that with other things that have big, dangerous HV terminals, the only people who will be working with them have good working knowledge of electrical safety, but most people do not know that audio hookups can be dangerous. That would be a good point, but I still do not understand why you have to have you amps inspected. As far as commercial marketing goes, there is something that many of us are familiar with called a "warning label." Perhaps these could be effective in some situations, like the output of big line powered amps. Anyway, I guess laws are laws.
Also, perhaps if such amps were to be marketed, the speaker hookups would have some sort of special plug or something, like an electrical receptical, to increase safety.
Kilowatt... the law here states that no one other than an electrician can even wire a power supply that in anyway connects to mains power or another HV source of over 100V (i think) so i can not even 'legally make the power supply for my amps in fact the law here says i'm not even allowed to plug anything into such a power supply even if the electrician made it or i bought it from the hardware store etc. (i gotta get the electrician to do it ... um yeah rite!!!) ... legally i would have to get an electrician to make the power supply (which would be all enclosed etc) then just stick it in or next to the amp chassis and get the electrician to wire the power supply to the amp... luckily in practice many electricians are willing to 'test and tag' the power supply as if they did it themselves.....

It is also effectively impossible here for an EE or computer technician to gain even a limited electrical licence which effectively means that they are all working illegally..... now that really makes sense dont it :)

I wonder what Grey, Geoff and others would say about this :)


2001-09-05 6:37 pm
PaulB, you are not correct in saying that UL etc would not approve such a thing. I have had such designs approved by Ul, CSA and SEMCO. The proviso is that the amps are built into a subwoofer so that the output terminals are not accessible, and the input terminals are transformer or opto isolated. In fact in Europe, even conventional high power output amps have to have their speaker terminals shrouded because of the potentially lethal signal voltages present at full power.
The cost of the power supply caps actually works out lower than conventional amps, because of the popularity of switched mode PSU's, which directly rectify the line.
Just wanted to put my foot in this discussion, "probably in my mouth". ;^)

In the USA, residental power is supplied from a center tapped (earth grounded) secondary winding of the utility transformer. For 115 VAC loads you connect across either of the two hot wire and ground. For 230 loads you connected across the two hot wires.

What Kilowatt has proposed doing is using the 230 VAC circuit as though it was coming from a normal center tapped power transformer and using earth ground for his signal ground reference.

This idea will work. The big reservation I have about doing this in your home is that there are other loads (like lights, TV, stove, heat pump, etc) connected to the same utility transformer. This opens you up to problems with interference and unbalanced loads.

To fix the first problem you will need some serious noise filtering in the line input to the amp, both to prevent outside noise from getting into the audio and even more importantly to prevent diode switching noise in the power supply from being transmitted back out through your household wiring (should make a great antenna. This filtering will probably end up taking the form of a pair of inductors and some caps. I expect that inductors that can handle the voltage and current will cost as much if not more than a normal power transformer of the same power rating costs.

The second problem (unbalanced AC loads) will result in the supply rail voltages becoming unbalanced with each other as household appliances switch on and off. This is not readily solvable in the amp (imagine using regulated power supplies for this beast), except by dedicating a utility power transformer to powering just the amp (this will probably be required anyways since Kilowatt is looking to get upwards of 7500 W out of his amp).

I can see that this is doable without too much risk to life and limb, but I don't see that you will save any money. I also expect you will not be able to get insurance coverage for this brute and I would suggest hiding it if any electrical inspectors ever come around, I expect they would go ballistic at the mere sugguestion of this idea.

Speaking of rectified line supplies, about how many uF do you think I should have in a +/-170V supply for a 7200W RMS amp? I'm thinking they would not have to have as much capacitance as one might first think, because of the higher rail voltage, so any ripple voltage present would be a smaller percent compared to the rest of the rail voltage.

Also, what would be a good value for some inductors in the power supply to provide fairly good filtering from line noise?