Gainclone Transformer

[lazaroff]

I'm thinking about making a dual-mono lm3886 setup using kits from chipamp.com.

Does anyone know of a cheap toroidal transformer for this application? 1 per channel would be okay, but dual secondaries on a single toroid would be perfect. Any ideas?

[bmwpowere36m3]

Antek, SumR, and Avel Lindberg. For my GC I used a 22+22V 300VA trafo from Antek, Avel also makes quality transformers (similar to Plitron).

[AndrewT]

Originally Posted by lazaroff

I'm thinking about making a dual-mono lm3886 setup using kits from chipamp.com.

Does anyone know of a cheap toroidal transformer for this application? 1 per channel would be okay, but dual secondaries on a single toroid would be perfect. Any ideas?

A monoblock needs a dual secondary transformer to produce a dual polarity supply.
You can achieve very similar with a centre tapped secondary.

A dual mono would need 4 secondaries to achieve dual polarity supplies for each channel. But, it would not be a dual mono any longer because the two channels share the primary and the core.

[gootee]

Originally Posted by lazaroff

I'm thinking about making a dual-mono lm3886 setup using kits from chipamp.com.

Does anyone know of a cheap toroidal transformer for this application? 1 per channel would be okay, but dual secondaries on a single toroid would be perfect. Any ideas?

Mouser Electronics - Electronic Component Distributor

And this place sometimes has them:

Antique Electronic Supply

Try Google!

[danielwritesbac]

TWO of these: Antek - AN-2225 pair of 25+25 vac, 200va transformer
will make dual mono, and that is the answer to your question because they are what fits the chipamp.com dual mono kit.

For reference, ONE of these: Antek - AN-3225 25+25 vac, 300va transformer
would fit the chipamp.com stereo kit.

I would also like to mention:
1). Arctic brand CPU paste (has filler properties) to use with LM3886TF chips (which are covered in coarse plastic).
2). A nice big heatsink, the bigger the better.
3). Air inlet at the bottom of the enclosure (preferably underneath the heatsink), plus, air outlet at the top (or highest point of the rear panel) of the enclosure. Let the cool air in at the bottom and let the hot air out at the top so that the amplifier enclosure isn't an oven.
4). Place the input filter cap between the potentiometer and the RCA Jack (not between the potentiometer and the amplifier). If you don't plan to use a potentiometer (because you have a preamp), then add/substitute a 22k resistor (as a load) between Input + and Input - in order to maintain the integrity of the design. aka, the chipamp.com kit specs about a 10k input load, half of which is the onboard 22k resistor and the other half is the expected 20k potentiometer. Frequent reported problems with DC offset are because of input loads with higher figures than 10k (or 12k).
5). The capacitor for speaker output zobel (output RC) included with the chipamp.com kit is the wrong type. An inexpensive little polyester/mylar dip cap is typically used for zobel, not high-efficiency polypropylene, so if you get poor quality upper treble, then replace that little cap.

P.S.
Terminology:
The chipamp.com kit isn't a gainclone because its very much unlike the 47 labs original.
The audiosector.com kit IS a gainclone because its quite like the 47 labs original.

[lazaroff]

okay, I understood how many secondary lines I would need, even if I don't quite have the terminology yet.

Would using a transformer with 4 secondaries reduce the output or each channel?

It's interesting how versatile the LM chips are, National's basic schematic is a great design, I'm not sure about the Gaincard, never seen or heard one, but it has really added to the DIY community with all of the different Gainclones out there.

Thanks guys, I'll look them up.

At this point I just don't want to be paying more for the transformers than the kits, but maybe that's just how it has to be.

[Stormrider]

Most people use between 120va to 160va transformers for stereo LM3886 builds. A 200VA Antek or two 100VA Anteks (for dual mono) is plenty.

Also, if you're in N. America, since the Antek's are rated at 50Hz, you'll get a 10%-20% increase in available current output when running on 60Hz. So that 200VA is now a 220VA-240VA transformer.

[danielwritesbac]

Originally Posted by lazaroff

. . .
At this point I just don't want to be paying more for the transformers than the kits, but maybe that's just how it has to be.

Usually, the amplifier board and power supply board is the least expensive part of the amplifier.

Better than anyone else's recommend, you could just find out the exact parts (transformer model#) that the kit manufacturers are using in their own homes. Chip amplifiers are extremely sensitive to differences. So, the "real" most economical thing to do is to start out with a "known good" configuration as specific as possible.

Generally speaking, it can be unseemly to use Smaller than 120va transformer with chip amplifiers, because smaller transformers have thinner wire and a tendency to ring (aka boomy/weak bass, weird mids, etc. . .) along with other noticeable frequency response artifacts. Larger transformers (within reason) are less trouble.

[lazaroff]

I went to Antek's site first and found the AN-2225 just from what I new that I would need, didn't even notice the recommendation. 2 transformers for under $100? Perfect!!! Bookmahk!!!

Thanks for the pointers too, especially about the filter caps and replacement caps.

It'll probably be a couple of weeks before I'm done with everything, I will make a post in the photo thread when I am done.

Thanks guys!