OK, Paul. Let’s take this imaginary 2 channel amp for example. It has this specification.

It needs a 13.8V power supply.
It outputs 500W total into 4-ohm load. (250W per channel)
It consumes a maximum current of 90 amperes.

How can I determine the type of transformer I need based on this 3 specification. (How many VAC for primary winding and how many VAC for secondary winding)

Also what about the VA? 500VA, 800VA, 1000VA, so many kinds. How to determine the value I need based on the above 3 spec.

When parallel or series those windings, should I solder them together? Or I can just tie those cable together?

The diode bridge also called rectifier, is that correct? Or are the two different things.

How much money do raw electrolytic caps usually cost? I found ready-made car caps are not cheap.

Also, what will happen if the caps UF value is really high? Like, 900,000 UF. Is that value higher the better?

Thanks, Paul. I am the beginner and need some professional guidance. Hope you have patience.

"OK, Paul. Let’s take this imaginary 2 channel amp for example. It has this specification."

If you have an imaginary 2 channel amp, then you can build a killer imaginary power supply for it for nothing.

"It needs a 13.8V power supply.
It outputs 500W total into 4-ohm load. (250W per channel)
It consumes a maximum current of 90 amperes."

The 13.8V rating is a nominal value. There should be a maximum voltage specified. Maybe the manufacturer has this spec. on their web site.

13.8V @ 90A max means 1242Watts of power (max) consumed. This will give you some idea of the power requirement of the transformer. Wttts and VA are somewhat similar (but not identical). Watts is a special case of VA (Volt-Amps) which involves trigonometry.

"How can I determine the type of transformer I need based on this 3 specification. (How many VAC for primary winding and how many VAC for secondary winding)"

If you are plugging in the transrormer into the wall outlet, you are giving it 120VAC (about).

What you need OUT of the transformer is about 11.3VAC, as I stated in my first post to this thread, with further elaboration in my second post. As the amp puts out more power, the voltage out of the transformer will decrease. A transformer rated at higher VA will do this less than one rated at lower VA. When the amp is putting out nothing, the transformer will be putting out a higher voltage. You must make certain that, when this transfomer voltage is rectified and converted to DC, that this DC voltage is NOT too high for your amp. Or else fried amp. You should find out what MAX voltage you amp is rated at.

Too much voltage: fried amp.

Too little: you don't get the full rated power out of the amp or it doesn't funcion properly (in an extreme case).

"Also what about the VA? 500VA, 800VA, 1000VA, so many kinds. How to determine the value I need based on the above 3 spec."

1242Watts of power (max) should give you a clue.

"When parallel or series those windings, should I solder them together? Or I can just tie those cable together?"

Solder is best.

It depends on the particular transformer and your needs.

As in the case of the transformer that had two primary windings, each rated at 120VAC and two secondary windings each rated at 25VAC each, the 25VAC is way too much. Beings there are two primariy windings, the user has some added options. Connect the two primary windings in parallel (the proper way, noting polarity of the windings) to get 25VAC out of the secondaries. Connect the windings in series to get 12.5VAC out of the secondaries. The secondaries would then be connected to each other in parallel so that you would get 12.5VAC with about twice the current of each secondary by itself.

"The diode bridge also called rectifier, is that correct? Or are the two different things."

A bridge rectifier (for single phase use in this application) has four rectifier diodes in it. One of these type of diodes, by itself in it's own package is sometimes called a rectifier.

It would be best (and more expensive) to get a bridge rectifier rated at 90A or more. These things have a "surge" or "peak" current rating that is a lot higher than the "steady state" current rating, so a bridge rated at 50A may be OK. Four rectifier diodes can be hooked up to funcion as a rectifier bridge. Note that the rectifier circuit (bridge or 4 diodes) will be dissipating some power (90A max times about 1.6V gives about 144 Watts max). The average power dissipated will be much less. So, a heat sink is a good idea.

"How much money do raw electrolytic caps usually cost? I found ready-made car caps are not cheap."

Yes, they are not cheap, especially if you buy them one at a time. Depanding on the capacitance and voltage rating, anywhere from $5 to over $100.

I'd look at surplus electronics places and see what I could find on sale. If I had to parallel a bunch of 20V 10,000UF capacitors, I'd condider that if the price per capacitor was low enough.

You do not need a cap rated at more than 20V, so don't waste your money on higher voltage caps (unless you get a really good deal). Also, a bunch of 10,000UF to 50,000UF caps in parallel may be less expensive than one big cap. There are other benefits of using a bunch of smaller caps as well.

"Also, what will happen if the caps UF value is really high? Like, 900,000 UF. Is that value higher the better?"

The higher the better, up to a point. The more capacitance, the lower the ripple voltage on the DC voltage and the more steady the DC voltage when the amp is putting out a large burst signal.

But, the higher the capacitance, the higher the peak current through the rectifier diodes and transformer. So, very large capacitance values can give problems, too.

In addition, when first turning on the power supply, there is a huge surge current as the capacitors first charge up. More capacitance makes this worse. Some amps have a resistor in series somewhere in the power supply ciruit to limit this startup current. When the caps get charged up enough, a relay kicks in to bypass the resistor.

If you have to pay list price for every item (switch, fuseholder, fuse, transformer, indicator lamp(s), rectifier bridge or diodes, maybe a heatsink for the rectifier, capacitors, chassis, and other misc. parts) you may find that an amp for home use may be a better option. Buying electronics parts one piece at a time is expensive.

Paul

Also, is this really a project that should be undertaken by someone who has to be taught basically the entire theory?

This would be a project suitable for someone who HAS had a basic electronics/electricity course (assuming he/she actually learned the material).

For someone who does not understand the basics, one should have a mentor. Otherwise, building this sort of power supply by trial and error could be very expensive and dangerous.

Paul

lol, i agree.

It would be easier to just build a 120 volt amp from scratch!

i would agree with hullguy!
do they still make kits like the old HEATHKIT tube amps of yesteryear?

The Heathkit company is still around. The Heathkit Virtual Museum has some good information. The kits are no longer made.

so whats the story with this project?