LM1875 PCB, Which To Use - Page 8

AndrewT · 9th October 2008, 04:23 PM

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 · 9th October 2008, 10:55 PM

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 · 10th October 2008, 11:28 AM

Looks great

Good work!

danielwritesbac · 16th October 2008, 06:49 AM

Quote:

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 · 16th October 2008, 07:51 AM

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 · 16th October 2008, 08:01 AM

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 · 17th October 2008, 05:11 AM

Quote:

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

Yes, they are.

Quote:

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 · 17th October 2008, 06:45 AM

Quote:

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 · 31st October 2008, 03:19 AM

Quote:

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 · 31st October 2008, 03:45 AM

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.

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9th October 2008, 04:23 PM	#71
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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

9th October 2008, 10:55 PM	#72
zymurgn diyAudio Member Join Date: Apr 2008	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

10th October 2008, 11:28 AM	#73
ollebolle diyAudio Member Join Date: Feb 2008 Location: Linkoping	Looks great Good work!

16th October 2008, 07:51 AM	#75
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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.

16th October 2008, 08:01 AM	#76
danielwritesbac diyAudio Member Join Date: Nov 2007	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?

31st October 2008, 03:45 AM	#80
mosfets diyAudio Member Join Date: May 2008	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.