Toroidals for LM3875

[Nordic]

I am planning to build 6 channels worth of LM3875, I can have toroidal transformers wound localy pretty much for the same price as importing, only much faster... but I need to know what I have to ask for.
[list=1][*]Would a toroidal with 6 secondaries be feasable, or 2 with 3 secondaries be better? [*]If I have multiple secondaries is it posible to only use one or some of them until all amps can be connected ?[*]Would much appreciate if someone could tell me what to ask for i.e. 230V primary to 3 x 24V DC at (x)VA and or (x)mA[/list=1]

I was told that overspecifing the values, would push up the price needlessly.... I live of a dissability pension so don't have money to waste.

The plan is to modularly build-up to 6 channels, 2 per month over next 3 months or so... then to build a powered subwoofer, then to get a creative DDT-100 decoder to connect it all up to the DVD player, Satelite dish, tuner and LP player.

[Wynand]

I would ask for a transformer with two 24v secondaries.

Remember, you can't ask for a 24v DC x-former it's AC.

The VA's you'll need is roughly VxA so (24Vx2) X Current rating.

Read the datasheet for that. If you have big enough smoothing caps then the peak current drawn throught the transformer should be lower.

I'd ask for a transformer with two 24VAC secondaries. Then decide the current rating for the secondaries. The manufacturer will use the right VA core anyway.

I personally don't see the need for seperate transformers for each channel, but I'm still new. If that holds true, then how will seperate secondaries help. KISS

My opinion is that me, with a limited budget can improve my sound more with R1500.00 speakers, than with R1500.00 transformers.

On your second question. There should be no reason that you can leave secondaries unused. Just terminate them safely.

DC

[Wynand]

If anyone can point me to a site or something that shows me that seperate secondaries/x-formers improves sound significantly i'll be glad for the read.

[Nordic]

Hey Wynand, glad to see another Capie on here, where do you get your transformers from? I have been dealing with Renam.

[Nordic]

I'm trying to understand the example given on the datasheet.
are they saying a 30.3V secondary will give 38.3VDC after regulation?

Equations (5), (6) give:
40W/8W Vopeak = 25.3V Iopeak = 3.16A
Therefore the supply required is: ±30.3V @3.16A
With 15% regulation and high line the final supply voltage is
±38.3V

I am also still trying to figure out how much miliamps the transformer must be rated for per amp assuming a 56w continuous average, with 100W peak output

Is there a way to convert the 120VA per amp recommendation to miliamp for my ordering purposes?

[OzBradW]

Hi

Here is a good article on building power supplies (you may have already read it)

http://www.tnt-audio.com/clinica/ssps1_e.html

Part two has calculations for working out power requirements etc to size transformers.

Hope it helps

Not sure from your last post what you didn't understand from the data sheet (don't have the data sheet), but remember that when you recitify AC Volts to DC volts you multiply AC volts by 1.41 to get the resultant DC volts minus a couple of volts for diode loss. The article explains this.

The authors web site is

http://www.zero-distortion.com/start.htm

More articles.

Brad

[Nordic]

Many thanks, wow did that first article open up my eyes....read it, and read it, and probably going to read it again now...

Who would have been able to think that even building for 8ohm/50W, you need to provide power for 2ohm/200W, realy starts makeing sense why good PC PSU's are so expensive, not to mention high end audio stuff... lol at least 'd be able to afford 1 capacitor per month or so.

[OzBradW]

Nordic

You are welcome. With this article you are able to decide to go the whole hog, or be some what conservative.

I unfortunately have only built one amp and only one power supply so far. Currently doing my second at the moment So I can not say how much expensive caps, individual transformers per channel etc improve the SOUND over cheaper offerings and one transformer for a dual mono amp. The thing with DIY is you can build the most over the top power supply for your amps and then find that the speakers are the weak link, So you do those over or make new ones, then you find that the amp does not really drive them that well, then you change the amps, then you find the power supply is the weak link.............................................. .................................................. .........................................

[Nordic]

Yeah it was quite a learning experience for me....
Never knew how the power realy gets to the speaker cones or how much power, yikes nearly 4 amps! ON a small amp!!!

So far in terms of getting good 10000ufd caps, it seems ebay is the way to go, you can get 10 highest quality caps for the price of 1 or 2... still mondo expensive, considering what you take home...

I have infact decided to not go the whole hog, although I will be dong the powarsupply with dual rectifiers... just not to the degree suggested...., doest seem like the LM3875 has the powerhandling abilities to make it worthwile...

Question...
I read that to power 50W into 8 ohms will take say 5amps of power, but that it would take 10 amps to push 100W into 4 ohms at the same voltage... (rounded off amps for example)....

Wont it kill the chip if you connect it to a 10 ampere powersupply?

[OzBradW]

Nordic

I am not sure on the LM3875 that you have choosen, but suspect that it is the same as for the LM3876 that i am using (you can check the data sheet from National Semiconductors) these chips have current limiting built in, which I guess will cause them to clip when driving difficult speaker loads (low impedence) at high power levels close to the amps output ceiling. With amp design like these, relatively low power outputs more consideration of the speakers you will be using is important for the SPL you wish to achieve (how loud you expect it to be). Bi-amping using these amps is a way of gaining more SPL. Speaker loads don't require such careful consideration if you throw a hi-power amp at them (200w plus) for hi SPL levels. Remember that amp clipping can damage tweeters.

I am not sure if you have checked this site soundwest.com.au.

Lots of articles by this author on a more conservative (read value for money / cost vs gain) approach to power supplies, amps etc.

I have purchased his PCB's for my chip amps for a bi-amped system.

Brad