Simple DIY Rectifier?

cogitech · 2013-03-26 4:13 pm

Hello all,

I am currently building (cobbling together) a hybrid integrated amp.

The tube pre-amp board requires ~12v AC

The amp board (Tripath 2020) requires ~12v DC

I currently have:

- 110/120vac-to-12vac transformer rated at 3.33 amps

- 110/120vac-to-12vdc transformer rated at 2 amps

It all works quite well during testing (see photo), however I would like remove the DC supply and implement a simple rectifier between the 12vac supply and the amp board.

An externally hosted image should be here but it was not working when we last tested it.

I have read and understand the theory of a diode bridge and output smoothing. Diode bridge - Wikipedia, the free encyclopedia

I have a box full of spare parts with all kinds of the square metal bridge rectifiers and capacitors.

Is it as simple as hooking the 12vac leads to the AC terminals of the bridge rectifier and then connect the load to the DC terminals, with a large capacitor in parallel?

cogitech · 2013-03-26 4:17 pm

By the way, the pre-amp board only draws a max of .833 amps from the 12vac supply, which would leave 2.5 amps for the amp board, which is still more current than the dedicated 12vdc supply.

Another question; with that huge Mallory in the picture, is a paralelled "output smoothing" cap still necessary?

Mihkus · 2013-03-26 4:38 pm

That longass wires to the cap is like 1000uf cap on pcb.
Try 10k uF on pcb it might sound a bit better.

And 12V supply, well thats way too low

cogitech · 2013-03-26 4:40 pm

Actually, stock configuration had 3300uF on PCB.

It sounds much better this way, and this is a test setup. Things will be optimized/relocated during final build.

But this is off-topic anyway. Do you have any comments about simple 12vac to 12vdc rectification?

DF96 · 2013-03-26 4:52 pm

If you rectify 12V AC RMS you get about 16V DC, not 12V DC.

cogitech · 2013-03-26 4:54 pm

DF96 said:
If you rectify 12V AC RMS you get about 16V DC, not 12V DC.

Interesting. This seems important!

So the next question is, how to I bring that back down to 12v? Resistance?

Looks like I found my answer... http://www.raltron.com/cust/tools/voltage_divider.asp

But if you have any other pointers, I'd be happy to hear them.

cogitech · 2013-03-26 5:14 pm

According to the TA2020 white paper, it looks like I can safely run it at 13.5-14 volts without issue, so I will aim for that when designing my voltage divider circuit.

FoMoCo · 2013-03-26 5:21 pm

Mihkus said:
That longass wires to the cap is like 1000uf cap on pcb.

No. More like a 10,000 uF cap in series with a small amount of inductance and resistance. If there's decoupling on the PCB, those wires aren't that big of a deal. It's not pretty, it's not good engineering practice, but it will work perfectly fine. In no case does the capacitance change!

DF96 said:
If you rectify 12V AC RMS you get about 16V DC, not 12V DC.

+1
Since the OP doesn't seem to understand this, the formula would be 1.414 x 12, minus two diode drops.

cogitech said:
So the next question is, how to I bring that back down to 12v? Resistance?

Voltage regulator. But, most cheap and dirty three terminal regulators won't do the job because of the 4 volt drop.

cogitech said:
Looks like I found my answer... Voltage Divider Calculator

Not hardly. This isn't viable for more than milli-amps.

sofaspud · 2013-03-26 5:26 pm

Why don't you use a voltage regulator?

cogitech · 2013-03-26 5:27 pm

FoMoCo said:
No. More like a 10,000 uF cap in series with a small amount of inductance and resistance. If there's decoupling on the PCB, those wires aren't that big of a deal. It's not pretty, it's not good engineering practice, but it will work perfectly fine. In no case does the capacitance change!

This is what my gut was telling me, but I don't know much about electronics so I decided to keep my mouth shut.

Voltage regulator. But, most cheap and dirty three terminal regulators won't do the job because of the 4 volt drop.

Not hardly. This isn't viable for more than milli-amps.

Hmmm. OK. Well, I really only need to drop down to 14 volts. Not sure how much that changes things.

cogitech · 2013-03-26 5:29 pm

sofaspud said:
Why don't you use a voltage regulator?

Because I have no idea what I am doing, basically, and I am frugal.

If I can cobble something together from my box of components, then I'd rather do it that way.

Edit: I just looked on ebay and it appears as though a voltage regulator is as big or bigger than the existing DC supply I already have in there.

The point is to get rid of the DC supply and get ~13.5v DC from the 12v AC supply with the fewest, cheapest components, many of which I probably have in my box o' goodies.

FoMoCo · 2013-03-26 5:34 pm

cogitech said:
Hmmm. OK. Well, I really only need to drop down to 14 volts. Not sure how much that changes things.

Cheap and dirty solution: A few diodes in series. They drop about .65V a piece for plain old silicon. I'd use 6 in case of high line. The voltage rating (PIV) doesn't matter. Switching performance doesn't matter. Standard cheap silicon diodes are fine. Current rating does matter. I'd use a 6 amp diode. (Anywhere from 5 to 10 should be okay.)

AndrewT · 2013-03-26 5:36 pm

It is sometimes considered poor practice for digital signals to share PSUs with analogue signals.

cogitech · 2013-03-26 5:37 pm

FoMoCo said:
Cheap and dirty solution: A few diodes in series. They drop about .65V a piece for plain old silicon. I'd use 6 in case of high line. The voltage rating (PIV) doesn't matter. Switching performance doesn't matter. Standard cheap silicon diodes are fine. Current rating does matter. I'd use a 6 amp diode. (Anywhere from 5 to 10 should be okay.)

Nice! I have some diodes laying around. Not sure on the current ratings, but I'll see what I have and do some tinkering.

Thanks!

AndrewT · 2013-03-26 5:38 pm

1n400x are 1A,
1n540x are 3A.
The pulse rating for these is very much higher.

FoMoCo · 2013-03-26 5:41 pm

On second thought... Your preamp may require an isolated supply. Without seeing the schematic, I can't be sure. You may wish to just leave things as it is.

cogitech · 2013-03-26 5:44 pm

FoMoCo said:
On second thought... Your preamp may require an isolated supply. Without seeing the schematic, I can't be sure. You may wish to just leave things as it is.

Will I damage it by trying, or will it simply not work? It's a $22 board, so I'm not super paranoid about it. I can always revert back to dual supplies...

A part of me just wants to see if I can do it.

Know the feeling?

alexmoose · 2013-03-26 5:46 pm

Why do you want to get rid of the second supply? If I may ask

cogitech · 2013-03-26 5:50 pm

alexmoose said:
Why do you want to get rid of the second supply? If I may ask

I dunno. It just seems so... superfluous.

Also, the larger supply has more reserve juice available for the amp, which is never a bad thing for these TA2020 chips (AFAIK).

But mostly... because I want to see if I can build a rectifier and make it work.

I'm one of those dangerous people with no formal electronics training and way too much enthusiasm and spare parts. :shhh:

FoMoCo · 2013-03-26 5:56 pm

cogitech said:
Will I damage it by trying, or will it simply not work? It's a $22 board, so I'm not super paranoid about it. I can always revert back to dual supplies...

Without seeing the schematic, I can't say. I'd assume that it'll trash both the amp and the pre-amp to be safe.

Simple DIY Rectifier?

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