Transformers for LM3875 Monoblocks

[dfdye]

I hate to be a broken record when it comes to this, but Steve at ApexJr.com has a good selection of surplus transformers for much cheaper than you would get elsewhere. Plus, he supports the forum, so go support him!

On a side note, I used 80VA transformers--dual mono--to power my office amp and have never had an issue, but then again, I am not really trying to peel pain with them.

[Nuuk]

the complexities of separating the PSU from the amplifier are not easy for a newbie to comprehend.

I don't understand Andrew. Surely we are only talking about physical separation of the PSU (transformer/rectifier) into a separate 'box'. How can that be difficult to understand? As far as I know, it doesn't change anything electrically except that we need an umbilical connection to the 'box' with the amp section.

[AndrewT]

Originally posted by Nuuk


I don't understand Andrew. Surely we are only talking about physical separation of the PSU (transformer/rectifier) into a separate 'box'. How can that be difficult to understand? As far as I know, it doesn't change anything electrically except that we need an umbilical connection to the 'box' with the amp section.

questions that need to be asked
1.) where to put the rectifier?
2.) where to put the smoothing?
3.) where to put the power ground?
4.) where and what needs a Safety Earth?
5.) where to put the Audio ground?
6.) where and what size of Local Decoupling?
7.) do we keep the various ground separate through the umbilical?
8.) will making the amp a two channel version require further thought on the grounding?
9.) will the PSU be two channel or shared or dual output?
10.) probably many more.

Now, see if we can reach agreement on the answers to each of these questions.
Will a Newbie even understand why these questions need to be asked?
Will a Newbie be able to weigh up the pros & cons for each of the conflicting answers?

Do you see why I said ?

the complexities of separating the PSU from the amplifier are not easy for a newbie to comprehend.

I know that I don't have all the answers.

[Nuuk]

Many of those questions, if not all of them bar the rectifier location can apply to an all-in-one amp too.

My first ever GC was a monoblock design with the PSU in a separate case and worked fine. Of course, I am always assuming that a newbie will start off with one of the most basic designs (and an easy to drive speaker).

[dfdye]

Originally posted by AndrewT

questions that need to be asked

IMHO the quick and simple answers:

1.) where to put the rectifier?

Close to the transformer, away from the amp inputs.

2.) where to put the smoothing?

Do you mean rail capacitors? If so, right after the rectifier. Also, you should always connect your amp rails directly to the caps, rather than then rectifier pins.

3.) where to put the power ground?

If you are using a center tapped transformer with one rectifier, it should be connected to the ground of each rail cap and then back to the star ground. Don't connect the transformer's center tap directly to ground for best results.

For dual secondaries/ rectifiers, again, take the ground from each rail cap and not the rectifier or transformer.

4.) where and what needs a Safety Earth?

Any metal chassis should be connected to ground. If multiple parts of the chassis are insulated from another (like, say, by a plastic standoff for the lid or something goofy like that) any metal part not in good electrical contact with the main chassis section should be tied back to ground.

5.) where to put the Audio ground?

I always take it directly to the star ground. If there is any problem, you can put a low value resistor in there to "isolate" the signal ground.

6.) where and what size of Local Decoupling?

Decoupling caps (meaning the ~0.1-1uF caps) should be as close to the power input pins of your chip as practical.

7.) do we keep the various ground separate through the umbilical?

If you have the extra wire, sure! Why not? Let's put this caveat in, though: I'm assuming you have already grounded your power supply and are pulling these two ground wires from the star ground.

8.) will making the amp a two channel version require further thought on the grounding?

Not really. Everything still goes back to the "star". . . . I don't quite follow this questions.

9.) will the PSU be two channel or shared or dual output?

Aren't we talking about a dual mono design here? I am confused by this question.

10.) probably many more.

Now you are just being difficult! Despite the admonitions of many here, chipamps are really difficult to screw up. You will more than likely get a pretty good amp if you can follow the simple build guide that comes with pretty much every chipamp kit.

Now, see if we can reach agreement on the answers to each of these questions.

NEVARRR!!!!

Will a Newbie even understand why these questions need to be asked?
Will a Newbie be able to weigh up the pros & cons for each of the conflicting answers?[/B]

No, but if you aren't willing to learn and experiment, you should go to Best Buy. Not knowing is a starting point, but knowledge is a journey. (sorry, I couldn't resist)

[eketehe]

Hi There,
Sorry for jump in,

Dear All,
as I've got somewhere, that A= ( VA * PF / V ).

is that means that we will have more Ampere if we use the lower voltage?
I mean 6A for 18-0-18VAC and 4.5A for 24-0-24VAC from the same 220VA? ( assuming power factor almost 0 )

Brgds
Eka

[AndrewT]

Hi,
Thanks Dfdye for your opinions on the separated PSU from dual channel power amp.

I happen to agree with answers 1 through 3.
I disagree, or am undecided, with the answer or adequacy or guidance offered for all of the remainder of the questions 4 through 9.

Does anyone else want to contribute?

[dfdye]

Well, I like things nice and compact. As long as we are dealing with torroidal transformers, there isn't much leakage that you have to worry about, thus the compact recommendations.

[Chip-Audiophile]

Thanks all for your comments. Although this is my first Chip-Amp, I think myself to be fairly electronically minded and with all the help from forum members don't think there will be many problems...

I would have expected that separating transformers from amplifiers would only make for a better, lower noise amplifier in the end without any major complications to its design... I am planning to build the enclosures from 40x40mm western red cedar slats laminated together with an aluminium bottom and back that all the electronics are mounted to. So provided that the aluminium bits are earthed correctly to the mains earth there shouldnt be too much of an issue. Maybe I will have to run an additional earthing wire from the metal parts of the amplifier enclosure, through the umbilical, to the metal of the psu enclosure and then to safety earth...

[Chip-Audiophile]

I have made some revisions to my enclosure design and am now looking at going for only 2 enclosures. Basically just trying to keep costs down. One for each monoblock, each one housing the toroidal and amp etc.

Still undecided about what voltage toroidals to go for though...

Is a 3mm thick aluminium plate sufficient to sheild the amp from the toroidal?