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cogitech 26th March 2013 04:13 PM

Simple DIY Rectifier?
 
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.

http://www.cogitech.ca/photos/speake...d_Amp_Test.jpg

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?

http://upload.wikimedia.org/wikipedi...othing.svg.png

cogitech 26th March 2013 04:17 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 26th March 2013 04:38 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 26th March 2013 04:40 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 26th March 2013 04:52 PM

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

cogitech 26th March 2013 04:54 PM

Quote:

Originally Posted by DF96 (Post 3428395)
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 26th March 2013 05:14 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 26th March 2013 05:21 PM

Quote:

Originally Posted by Mihkus (Post 3428365)
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!

Quote:

Originally Posted by DF96 (Post 3428395)
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.

Quote:

Originally Posted by cogitech (Post 3428400)
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.

Quote:

Originally Posted by cogitech (Post 3428400)
Looks like I found my answer... Voltage Divider Calculator

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

sofaspud 26th March 2013 05:26 PM

Why don't you use a voltage regulator?

cogitech 26th March 2013 05:27 PM

Quote:

Originally Posted by FoMoCo (Post 3428438)
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.


Quote:

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.


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