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Old 9th October 2008, 04:23 PM   #71
AndrewT is offline AndrewT  Scotland
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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
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Old 9th October 2008, 10:55 PM   #72
zymurgn is offline zymurgn  United States
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Here is my 1875 amp. I finished it a few months ago.

The two assembled PCBs....
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Power Supply powered and ready to go.
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The Chassis
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Old 10th October 2008, 11:28 AM   #73
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Looks great Good work!
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Old 16th October 2008, 06:49 AM   #74
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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?
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Old 16th October 2008, 07:51 AM   #75
AndrewT is offline AndrewT  Scotland
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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.
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Old 16th October 2008, 08:01 AM   #76
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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?
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Old 17th October 2008, 05:11 AM   #77
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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.
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Old 17th October 2008, 06:45 AM   #78
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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.
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Old 31st October 2008, 03:19 AM   #79
mosfets is offline mosfets  Canada
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Quote:
Originally posted by Nordic
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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!
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Old 31st October 2008, 03:45 AM   #80
mosfets is offline mosfets  Canada
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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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