LM1875 PCB, Which To Use

[AndrewT]

look up the datasheet and see what load impedance will suit 20Vac (~28Vdc).
240VA is not a problem, it just uses up valuable resources that may be better elsewhere.
240VA could power 80W + 80W

[zymurgn]

Here is my 1875 amp. I finished it a few months ago.

The two assembled PCBs....

Power Supply powered and ready to go.

The Chassis

[ollebolle]

Looks great Good work!

[danielwritesbac]

Originally posted by AndrewT
required VA~=1times total output power to 3times total output power.
But no less than 80VA.
eg. output = 20W+20W then VA can be between 40 and 120. But lower limit is 80VA, so use 80VA or 100VA or 120VA.

Why is there a lower limit of 80VA?

[AndrewT]

I see two reasons. There may be others.
1.) The transformer regulation is too high for small transformers.
2.) The very small transformers don't sound as good.

I suspect these two reasons are interlinked and come from massive variations in the PSU voltage as the current sent to the load varies.

[danielwritesbac]

Thanks man!
I was recently involved in (got roped into) making an extremely small amplifier with a 28VA transformer. This deal sounded rather good, but didn't have much bass.

Comparing a very similar design using a 160VA transformer, well that amp on the big transfo doesn't lack for bass.

Personally, I never found a substitute for a big linear supply; but, Surely there's some more elegant options than using a great big transformer every time?

[pacificblue]

Originally posted by danielwritesbac
Check out the PDF files at national semiconductor. The lm1875.pdf and lm1876.pdf are quite informative.

Yes, they are.

Originally posted by danielwritesbac
The spreadsheet works from the variables in lm1876.pdf. It is 2x, LM1875's internally

No, it is not 2 x LM1875 internally. The specs and equivalent schematics in the above mentioned datasheets are quite different.

[danielwritesbac]

Originally posted by pacificblue
. . . No, it is not 2 x LM1875 internally. The specs and equivalent schematics in the above mentioned datasheets are quite different.

Thanks for correcting this.

[mosfets]

Originally posted by Nordic

Can the input components be modified so only low freq Hz will pass ie 150Hz and below? You know, to drive a little subwoofer! This looks very neat BTW!

[mosfets]

Furthermore, can this preamp be used to drive three of these LM1875 PCBs?

Left/right full tone PCBs and a third LM1875 PCB to provide the low? Kinda like a sub-satelite computer speaker set with more balls?

Also, I'm more interested in DIY at home my own PCBs so if you know of a preamp with basic tone control and volume that I can fabricate with free pcb layouts etc I would be very happy.

DIY to me means I should be able to make my own PCBs and NOT have to purchase them, though I don't think purchasing PCBs is a bad thing for most if not all people.