How to choose a transformer for project.

[CorySCline]

Hello all! I have decided to proceed with a chipamp based on the LM3886TF in a mono I/O setup. I wish to use a 4OHM 70Watt speaker. This is not purchased yet, and can be changed as needed . I have only laid that as a point to base the rest of the project off.

The datasheet calls for +-28v at a 4ohm resistance. I am under the impression that a 20V transformer will yield me the required 28V after being rectified. However, I do not quite feel comfortable picking out a transformer from digikey as I do not have much more knowledge than that. What resources are available that are reliable and intuitive about choosing a transformer? The only ones I can find detail choosing them for houses...

Thanks!

[AndrewT]

see my other post.

The recommended transformer rating minimum is VA equal to the maximum output power upto a maximum of VA equal to twice the maximum output power.

The amplifier will work with a transformer outside this range.

For 60W of maximum output you should pick a 60VA to 120VA transformer for each channel of output.

BUT !!
Small transformers do not allow good performance from power amplifiers.
I recommend using the above rule and using greater than or equal to (>=) 160VA

160VA can power amplifiers with a total maximum output from 160W to 80W. And will also power amplifiers outside this range.

[!]

20V transformer will give you slightly under 27V after rectified. The question is do you want to build with the specs (and limitations) of your speaker in mind or build in versatility in having more power into 8 ohm speakers too?

Digikey charges a lot for theirs, here's a 22x22V, 200VA for $43 delivered Antek - AS-2222

A few more voltage and VA choices, should you someday want to have more VA for dual mono or more voltage for 8 ohm speakers.
Antek - Transformers - Grid View[]=0

Every now and then you can find a better price on ebay but in voltage ranges most suited for chipamps they get snatched up pretty quick and I didn't see any suitable on ebay when I looked a moment ago (but I might've missed them). Well, there is a 20x20V center tapped, 150VA used one for $40 delivered eBay - New & used electronics, cars, apparel, collectibles, sporting goods & more at low prices

There's a 160VA 24x24V on the following page for lower cost but not much in the way of detail about it and the page is so old I don't know if they are still available nor what the shipping cost would be, miscellaneous.html

[bcmbob]

I used four of the Apex Jr inexpensive units successfully with both the BrianGT and the Ref-C amps when I started. If you call him with your specs he will try to match a product from his inventory.

I'm now buying direct from Antek (mostly for the added magnetic shield) as needed.

[lnh]

Don't mean to post jack, but how do you like the Antek transformers bcmbob? I was looking at getting one for my project cause they're inexpensive, have the added shield as you noted, and I can get quad secondaries to run my +-15v and +-30v boards separately.

[bcmbob]

Hi lnh,
About all I can say is the construction is top notch and the outputs read as they are rated. Honestly, I have not experienced any hum or other interference problems with the less expensive ApexJr units but intend to use multiple PGBs in some builds. Having the extra shield is preemptive.

[!]

Cory, ignore the ebay link I posted previously (figured you would anyway since it isn't as good a deal) as it is only spec'd for 230V primary.

[CorySCline]

OK. Thank you for all your information. I have narrowed down my desired goal (through reducing my incompetence ) to wanting to use push 35V through a single transformer / rectifier combo into a mono application using a single 8OHM speaker with 50W RMS expected. With this in mind, I have calculated that a Toroidal transformer with a 115 primary and 24 or 25V secondary with a VA of at least 150. The LM3886 shows a peak output of 135, but I dont believe, for this project, that volume knob will get that high . At least if it does, my neighbors will have the cops at my door lol. Do these specifications sound reasonable?

[!]

yes, though if it's only 8ohm now and with your mention of volume limiting neighbors, a 100VA part starts to look reasonable too, Antek - AS-1224

[CorySCline]

Yea, If you recall my initial goal is an LME49811 . With that in mind, I have no need to design this one with large volumes in mind. This is only an in between project to further my knowledge and experience before I undertake a large project like that. However, 100VA sounds kind of low. How much will that limit it in practice?