2x LM3886 In Parallel - How To Add DC Offset Adjustment

[ParkingLotLust]

Starting to order parts. To those who know their parts, are these good-quality capacitors and good diodes?

0.1uF
100uF
10,000uF PSU Filter Caps
Rectifier Diodes

Power Supply:
**broken link removed**

Transformer is a 20-0-20v 220VA or higher torroid (hopefully) chosen to produce ~25v after diode drops.

Also, the image above shows two separate grounds, is it fine to connect these two together for the normal ground like in is:?

**broken link removed**
**broken link removed**

EDIT My newly-minted eagle schematics, if someone could please check 'em over Starting on the board now.

 

[ParkingLotLust]

Both boards done Time for sleep!
**broken link removed**
**broken link removed**

All the terminals on the board are this style:
https://www.sparkfun.com/commerce/product_info.php?products_id=8235

Except the pot, which will have the wires soldered in.

 

[Nigel Goodwin]

Is there a particular reason for using two bridges instead of the usual one?.

 

[ParkingLotLust]

No, the only reason is that thats what the original design calls for (looks like they used a transformer with two separate secondaries).

Would a transformer with a single center-tapped 40v secondary (20-0-20), or one with 2 20v secondaries be less expensive? Cost pretty much dictates how this will be done

EDIT - here's the discussion on the power supply design. Perhaps there is a reason for the two bridges?

EDIT 2 - After lots of reading and SPiCE sims, its been recommended that the 0.1uF snubber cap be changed to an 0.33uF, as the "magnitude of the impedance peak to a little over 4 Ohm at 250Khz"

EDIT 3 - Or perhaps the style of transformer they used is less prone to resonate? Theres a discusson on that here

 

[Speakerguy]

Don't read that forum. It's full of cranks and crazies that can hear the difference in different brands of floobydust and unobtainium.

Trust me. I was a PWM amp engineer at Cirrus Logic in a past life. I know a smidgen about audio.

 

[ParkingLotLust]

Gotcha. Well, Im glad to have guys like you and Nigel and audioguru and bill around, & thats why Im here and not there

So what would you recommend - a 20-0-20 and a single bridge? Any other recommendations?

 

[Speakerguy]

FWIW I actually have that amp kit and have built it, just out of curiosity. The whole snubberized supply and all. Bought it from chipamp.com. Assembled it but never got around to testing it. It's never seen power, just soldered it up and then it got shelved for other stuff. If you want it it's up for sale.

If you are designing/building your own, just use a single standard bridge rectifier and skip the dual rectifiers with discrete superfast diodes. No need for them. Just use a standard dual secondary transformer with secondaries in series, ground the middle to earth and hook the other two to the AC inputs of the bridge. Skip the snubber.

 

[Speakerguy]

Constructive criticism - go ahead and use copper pours on your boards for power supply routing, especially on the PSU board. Everything on the right side of the diodes should just polygon pour with isolation of 10mils or more.

 

[Mr RB]

Floobydust?? Like using fast recovery rectifiers in a basic mains freq PSU before a 10,000uF cap?

You can just go with standard rect bridges. I also like to separate the main caps into a pi filter of 2 caps with a resistor inbetween. You will see that in a LOT more professional amps than you will ever see fast recovery rectifiers and other silly fads!! There's people on that forum saying you can replace the regular diodes in your amp PSU with fast recovery rects and the amp instantly "sounds so much better!"...

I built a nice gainclone style amp, see it here;
Roman Black's gainclone amp

and I cover some of the PSU issues there too. There are a few muppets on that DIYaudio forum that's for sure. Shame really, there are also some nice people/projects there.

 

[ParkingLotLust]

How much are you asking? Im looking to build the entire thing myself (for experience and fun) but depending on the cost, I may take yours, if it ends up being cheaper than what I can get parts for here.

So just use a standard dual-polarity supply? Is it fine to use an unregulated one? If I have to use regulators I will need to find something capable of higher current than a standard LM317

EDIT Then again I could just use big beefy bypass transistors, right?

 

[Speakerguy]

I dunno, $35 shipped for the fully assembled power supply board and two fully assembled amp boards sound OK? It is $70 plus shipping on chipamp.com. It was a stereo kit. You will still need heatsink + transformer. I totally respect if you want to make your own, it's just I have this very kit laying around and I'm not using it. I no longer have access to the Audio Precision that I used when I was at Cirrus, so I can't do any real testing on it like I wanted to.

ETA: there is no need for a regulated supply with these chips, most power amplifiers use unregulated supplies and just get by with very high PSRR.

 

[ParkingLotLust]

For the price, thats cheaper than I can buy the chips and capacitors for, nevermind having the boards made. Ill take it! PM'd.

 

[Speakerguy]

Responded. It is a good sounding kit, but it is also a good kit to study and see what they did that was either overkill or questionable/not necessary. So next time when you go to build something else or you are looking at someone else's schematic you can recognize them Glad someone will be getting some use out of it.

 

[Nigel Goodwin]

Would a transformer with a single center-tapped 40v secondary (20-0-20), or one with 2 20v secondaries be less expensive? Cost pretty much dictates how this will be done

Cost would be the same either way, and you can simply join two the two 20V windings together to make 20-0-20 (which is how most transformers are).

 

[ParkingLotLust]

I havent been downtown yet to the surplus places, but I did a bit of looking and found this from mouser (the ones on digikey were way more expensive):
160va, 2x18v Secondaries - 4.44A each

However, at nearly $70, that aint cheap Do I need a 160VA transformer or will a 100VA suffice?

EDIT I also found **broken link removed**, but its rated in watts, and doesnt really give very much for specs about it, and its 24v and not 18v

EDIT 2: Speakerguy directed me here where he has a toroid with dual 21.6v secondaries @ 2.7A each, for $20 + shipping. Im going to head downtown today and if I cant find something around that price, that seems like the best deal Ive seen so far.

 

[Speakerguy]

Hi PLL,

I PM'd you a link to a place that sells toroids. For everyone else's viewing pleasure, here is the site:

miscellaneous.html

Some good deals on toroids there.

 

[ParkingLotLust]

I picked up a torroid downtown with a few secondaries, one of which is a 21.6v-0-21.6v, which is right in the ballpark that I want it in, for $7.50. Worst case scenario, if its too low amperage (it didnt have accurate ratings, so Im finding the original datasheet), Im sure I can use it for something else.

Now, how do I go about hooking it up? In my earlier schematics, can I connect pin 1 from JP4 to JP5 to the center tap, and hook up the two outside wires to pin 2 on JP4 and JP5?

I also found this method of hooking it up, from the site speakerguy79 listed, but it only uses four diodes instead of eight. Whats the advantage/disadvantage of this? I havent drawn schematics or anything, but Im guessing it turns it into one rectifier, and creates a ground with the center tap?

**broken link removed**
**broken link removed**

 

[Nigel Goodwin]

I also found this method of hooking it up, from the site speakerguy79 listed, but it only uses four diodes instead of eight. Whats the advantage/disadvantage of this? I havent drawn schematics or anything, but Im guessing it turns it into one rectifier, and creates a ground with the center tap?

Which is what I've been saying - there's no point whatsoever in using two bridge rectifiers - just a single bridge is all that's required, and gives you full wave rectification on both supplies, with less voltage loss as well.

 

[ParkingLotLust]

Sounds good to me. Thanks!

 

[ParkingLotLust]

Is there any way to measure the approximate maximum current output of the secondaries on the toroid I bought? I cant find the transformer listed anywhere on the manufacturer's site.