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hifiknutter 27th October 2013 10:17 PM

Transformer choice for LM3886 ChipAmp
 
I've got my new 'Dual mono kit from ChipAmp this week and have a few transformer options available to me and am looking for a little advice.
Option 1 2x 100VA 2x20v frame transformers. Good voltage but a bit low on power.
Option 2 1x 160VA 2x30v toroidal. Its suggested this voltage may be used, but is it big enough to run both channels.
Option 3 1x 330VA 2x35v toroidal. Plenty of power to run both channels, but is the voltage to high.
(I have all the transformer's on the shelf awaiting a home).
Option 4 Buy in some new ones, not my preferred option but something I'm prepared to do.

I will ultimately use the completed project with ART audio Illusion 1 speaker's which have a nominal impedance of 6ohms.

Open to all suggestions.
Mike

davym 29th October 2013 02:53 AM

I wouldn't go above 25v if I were you. Most UK transformers are rated at less than 240V which means that they will put out more than the stated voltage. I'm using a 2x22v 300vA for that kit (it's based on a rated input voltage of 230). My mains reads steady at 245v, I get 25v from that 22v toroidal and an 18v toroidal I have puts out almost 21v.

Remember that after rectification, the DC voltage will be higher. Have you read through the build instructions? there are recommendations for suitable transformers included.

If you are using two separate power supply boards for proper dual mono, it's best to use a separate transformer for each channel. A pair of 2x160vA 22v transformers would be excellent in that case.

It's a very decent amp if put together well. Good luck, have fun and post some pics on the chip amp section when your finished.

ByronInPortland 29th October 2013 04:26 AM

1 Attachment(s)
According to TI's data sheet (http://www.ti.com/lit/ds/symlink/lm3886.pdf) rail voltages up to 28V are OK for 4R speakers, and voltages up to 35V are OK for 8R speakers. As I recall, the ChipAmp.com instructions recommend a single 300VA to 350VA transformer. Keep in mind that the LM3886 has some protection circuitry to protect against overvoltage.
I kind-of like the idea of buying either a second 160VA, 30V transformer, or a second 100VA 20V transformer, and going dual mono. You'll have to choose between slightly underregulated transformers or slightly too high rail voltages.

hifiknutter 29th October 2013 11:33 AM

Thank you for your info, the tech spec useful has more info on it than the one I had.. I am leaning towards the full dual mono approach and do have 2 of the 100VA 20v transformers, in fact have a box full.

hifiknutter 29th October 2013 11:38 AM

Thank you for your suggestions, it is dual mono route I wish to go with. Have read instructions but wish to try and use what I've got hanging around if possible. Would take the plunge with new transformer's if I like what it does. Fortunately the speaker's are an easy drive and my listening room small, so don't need huge power.


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