Optimal Transformer choice for a LM3875?

[zBuff]

Hi everyone, I was planning to build my first gainclone, first amp actually, and I need some advice on what transformer I should use.

I'm planning on getting one of Brian's NIGC kits, and was going to get a custom wound antrim toroidal from Harbuch. Now I've been reading up quite a bit on these boards over the last week and tried using the search engine. There's ALOT of information to absorb my head is swimming right now..

What voltage should I get ? there seem to be varying opinions, such as matching it with your intended speakers, that higher voltages sound better given with acceptable safety boundaries. I will be running some old acoustic research 6 ohm 87db speakers for a start, but play on building speakers as well at some later date.

Also should I get dual secondaries or a centre tap toroidal or something else? are there pros and cons?

Lastly, shielding should I worry about it or not. I have the choices of EN60742 spec, SAFETY SCREEN, ELECTROSTATIC SCREEN and AS3200.1/IEC601.1 spec.

[Peter Daniel]

I would suggest 300VA with 2 x 24V secondaries. Higher voltage sound better in my experience. I'm currently using 3 x 12V batteries per rail and it is improvement over 2 x 12V . My speakers are 6 ohm.

Go with dual secondaries as the kit is designed for such setup.

I have a pair of monoblocks with transformers having screen between primary and secondary, but I didn't really observed any big gains with that, so it's rather optional but not really a must.

[KT]

Peter,

The batteries sound great. What is your method of recharging?

Are you using a commercial chager, or did you build your own?

Thanks,
KT

[Peter Daniel]

I still didn't build my own (although I plan on that). I'm using 2 Velleman chargers which I switch between batteries.

[enzedone]

I want a 25v-0-25v transformer for mine but seem to be having problems finding one. J-car sell a 18v-o-18v one but i think this is too low. Remember that with 5% regulation, which is guite good, you will lose more voltage across the secondaries. If you find a good one in N.Z post a note up here so i can see it. I'm off to China for a holliday, so will look there as well. Good luck

[enzedone]

Yes i have also looked at Harbuch, but i found that around 300VA they all had 6-8% regulation. Thats a lot to lose. If you paid for the more expensive 600-800VA, then you'll get down to 5% reg. But this may be all garbage to you. If your not too fussed about secondary voltages.

[zBuff]

Thanks for the quick reply Peter, I think that answers all my questions

So your GC would be running at ~36 volts would it? I assume that would be about the same as 24v VAC after it's been thru the rectifier.

I did forget to mention that I'm building the dual mono kit and powering off one transformer, but I think you assumed that, so that's cool


Enzedone

I was getting my from Harbuch which is in Australia, custom wound so better tolerances and quality than the ones u get at Jaycar and the like and shouldn't work out much different in price
Where abouts in the country are u? I'm in Chch. You could order with me and save on postage I guess, if you're around the area.
There are quite a few other places u should be able to find toroidals though. Altronics (global PC), and more professional places for example Radcliffe electronicals. I know a guy that does custom winds in Chch but they are pretty expensive. 2 x 25 V should be easy to find since it's one of the standard values. I hope u speak chinese mate, finding stuff over in China is hard enough as it is sometimes, thought I'm sure if u can find it it will be very cheap

[zBuff]

Are we looking at the same chart?

http://users.bigpond.com/harbuch/harbuch/toroids2.HTM

At 300 VA the regulation can be 6.5, 6 or 5 %
goto 625 VA and it drops to 4%
800 and 1000 to 3%

I thought I would use them cuz quite a few australian posters here seem to recommend and use them, and custom wound cores should have more attention to detail and tolerances are likely to be tested and be at specifications. Unlike there mass produced counterparts.

Buying from Australia is just the same as buying within NZ nowdays anyway.

[Peter Daniel]

Actually three 12V cells in series produce approx. 39V It still works fine with 6 ohm speakers and I prefer it over two cells.

[enzedone]

Have you got a price for the transformer from Harbuch's?
I'm in Auckland, so probably no use in ordering together unless we saved heaps, and one could send the other on....
I will have to have another look at the site again and check the ratings.