Yet another voltage doubler question

[edvard]

Ok, so I know about the [ur=http://en.wikipedia.org/wiki/Voltage_doublerl]standard voltage doubling[/url] techniques, but there's one I've run across lately that I want to use, but it doesn't work in LTSpice or Falstad.
I have an old consumer-grade stereo with a dead CD player that I took the power amp out of (TA8612H TA8216H - 13-watt stereo chipamp) which is rated for 40V.  The power supply is a dual-secondary 12v transformer that runs into two voltage doubler circuits the likes of which I've never seen before.  
It's basically a standard bridge rectifier, but with each diode paralleled by a 20nF capacitor.  It spits out a nice ~20V out of each end, both positive with respect to ground, so between both ends the voltage is zero.  I suspect one end was to power the amp chip with 20V and the other end powers the CD player, radio, pre-amp, etc.
Long story short - how does this work, why does it not show up in the standard literature on voltage multiplying, is it better or worse than any other voltage doubling technique, and why does it not work in simulation?



(BTW - 12V in, 20V out)

[rring]

This is not intended as a voltage doubler - those are noise filtering caps to snuff out noise across the diodes from AC harmonics. I have used the technique in numerous circuits. Remember that 12V AC RMS will charge to peak @ 17 volts when rectified and charged across a cap. 14V AC will charge to 20V.

[edvard]

Aha! Thanks for the explanation. I knew about the RMS/Peak relationship, it just didn't strike me at the moment.  The original transformer puts out a solid 13.3 VAC.  So, I can safely double it with a standard bridge doubler and get just shy of 40V to safely drive the chipamp?

[Minion]

I don"t see why you need a Voltage doubler ?? You say your transformer is a 2 x 12v AC which will give you 2 x 17vDC or 34vDC .....
A 13w amp would not need a 2 x 40v supply .....

The pic you posted just shows a regular diode bridge and would only work as a voltage doubler with a single 12v AC supply and no transformer ground connection  .....

I think you are barking up the wrong tree .....

Cheers

[defaced]

A 13w amp would not need a 2 x 40v supply .....

Alot of chip amps require bipolar supplies, so in this case you'd end up with +/- 20v rails.  This detail is usually in the chip amp data sheet, but I'm not having any luck finding one for the TA8612H chip in the OP.  Reverse engineering the board to produce a schematic would be your best bet at understanding what's going on. 

[edvard]

I think you are barking up the wrong tree .....

I've been known to do that on occasion....  

I thought it was a voltage doubler because it was 12 in 20 out, so I assumed it was doubling with diode drop accounting for the 4 missing volts, but as rring pointed out, it's not a doubler at all.  I was also trying to avoid using the original transformer, but it just may be the way to go.

Alot of chip amps require bipolar supplies, so in this case you'd end up with +/- 20v rails.

Because the original circuit produces +20V at either end with respect to ground, I also assumed it was not bipolar
The datasheet schematics say Vcc and Ground, so it's safe to say it's not, and the front specs say 24 volts max with a 4 ohm load, so prolly not going to go with the spec max of 40V either.
P.S. I made a typo in the chip part number... It's actually a TA8216H:
www.datasheetcatalog.org/datasheet/toshiba/1152.pdf

Either way, thanks for the replies and apologies for the barking, I'll be in the corner warming up my solder pot...

[PRR]

> TA8216H - ...which is rated for 40V

It blows-up at 41V.

Most of the specs are given for _28V_, at which point it gives two times 13 watts into 8 ohms.

Unusually, the 8216 chip is not designed for bipolar supply.

And voltage-doublers are very unusual in power-amps.

Therefore I am sure they ran the chip on 13V*1.4= about 20V, for about 6 or 7 Watts per channel.

I am also sure the power transformer is just-big-enuff for such power level. (The CD/tuner didn't draw anywhere near as much as a 2*7W power-amp.)

Using a larger PT and aiming at 40V:

If you touch 41V it may die in months or seconds. (Probably not, but it can.)

At 37V you can call it 24 Watts (8 ohms) per channel BUT the chip dissipation peaks near 20 Watts. The chip will do 25 Watts on an Infinite Heat Sink, but there's no such thing. On a quite large sink the chip can shed over 15 Watts. Perhaps the heatsink from an old/fast Pentium would do, as long as the fan ran.

With this PT: build simple 13VAC to FWB to large cap for +20V DC. Use original-size heatsink on 8216. Accept 6 Watts a channel.

Trying to up-force the voltage, you will need a PT with more iron, and if you do then you will need more heatsink. Like: I can force air into my car engine for more pressure and power, but I need enough block to hold that pressure, and if I do then I need more radiator than I got now to handle the inefficiency losses.

[edvard]

Understood; thank you sir, your input is always respected in my world.

These wild goose chases I get myself into usually start with a mistaken assumption, this one being that the voltage was doubling in the first place, when it was not.
And yes, 20V would be just fine. I did trace out the circuit, and indeed one side fed the power amp, the other side a 12V regulator and siphoned off to run the radio/CD player/turntable.  The original heatsink was just a big hunk of aluminum that also served as a support frame for the circuit board the amp was on.  I'm a little sketchy about using a CPU heatsink as those are designed for being fan-blown rather than passive, so I'll prolly go with re-using the original or a sink designed for passive radiating.  As far as power, since the transformer is dual-output, I can parallel it for better power handling if need be, not to push the amplifier, but maybe go a little easier on the PT.