confimation needed (gainclone)

[kmj]

Hi, sorry for posting yet another gainclonetread but after reading a bunch of threads at different forums I got many questions with different answers. So I just need confimation on a few things
It's Brians Basic stereokit lm3875 btw.

1: Do you get 56W out of this kit? I will be usings this amp to drive a pair of shelfspeakers with efficiency in the lower 80db's and they seem to like some power to sound right.

2: What transformer is optimal for 8ohm speakers? 2x22V and 100-150VA/channel? I've found these:
2x18@220VA
2x20@220VA
2x22@220VA
2x18@300VA
2x20@300VA

3: Is it enough to mount the 2 chips at a bottomplate of aluminium roughly 20x15cm with 1-2cm airgap underneath?

4: Regarding a Pot to the volumecontrol, (no stepped attenuator), it seems like most people recommends a Linear pot with a value between 10-50 Kohms. Is this right?

that's it

[edjosh23]

I have Brian's kit also. I'm going to use a 330va 30v + 30v, with 8 ohm bookshelfs.

Brian recomended me the Noble pot from Percy Audio

They was a thread I recently read just about pots and here it is

and the heat sink idea sounds like it might be alright. But the more power, the hotter your chip gets. You may want to think of alternatives.

[Vikash]

these questions have been thrashed out a hundread times at least. regurgitating them here will not offer any more definitive answers, but nevertheless...

1) single lm3875 is capable of continous 56w at 8 ohms.
2) neither of those transformers is optimal. the chip ceiling is around 28v before rectification. I would suggest a 2x25 to allow for some mains variation.
3) how thick is the aluminium? If anything >= 6mm, then I would say no problem.
4) 10-50k will be fine. you will want to law-fake if using linear. with cheap pots, i feel the bigger issue is channel matching not linear vs log or R value.

[kmj]

these questions have been thrashed out a hundread times at least

Yaeh , I know. I've read and reread and the answers differs depending on the thread and forum. Some recommend 2x25v as you while others say that is to high and 2x18V is enough but anyway, the dealer have 2x24v in both 220 and 300VA so that's close enough i hope.

But is 220VA enough for two channels?

The aluminum is only 1-2mm so i guess i'll have to salvage something more robust.

The pot i'm going to use is quite ok, it was just the value i was wondering about since i've read a couple of threads stating special values but perhaps that was for one of the inverted versions.

[Vikash]

the lower rail transformers are better if you intend to use lower ohm speakers, i.e 6 or 4. Considering these are only nominal values, I can see why some people will recommend lower rails for 8 ohms+ too.
However, some people say the chip sounds best with higher voltage. Since your goal is to get max output in 8 ohms, higher voltage is what you're looking for.

220VA is enough. I find the difference in cost between 2xx and 3xx little, in which case I prefer to go for the larger.

You will need a lot more than 1-2mm heatsink, especially if going for highest ouput = more voltage.

[mcs]

I would suggest the 2*22V transformer. With that transformer you can expect 37W (see my measurements here. Using a too small heatsink is no problem - the chip will protect itself

Just try it - if the chip switches off or starts distorting (or the box get's too hot) use a larger heatsink.

Best regards,

Mikkel C. Simonsen

[kmj]

mcs& Vikash:
thanks, since there's no difference in price i will go with 24V@220VA and use some heatsinks from an old computer.

[kmj]

oops, it should say 24V and 300VA.