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Old 7th November 2006, 04:01 PM   #11
CarlosT is offline CarlosT  United States
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Quote:
Originally posted by I_Forgot
...Since you are making one unit, and the price difference between a 50VA and a 100VA transformer is pretty small, why not just put in the bigger transformer?...

I_F

Thanks, IF. I'm actually making a few (lotsa kids in the family and they may be Xmas gifts ) and it's a question of packaging...it's all gotta fit under (inside) an appropriate speaker cabinet for just a single 5" woofer and leave reasonable space for the LM3875 and PSU.

Thanks to all you guys that responded.

Again...basic idea is just one LM3875 being driven to put out about 30-40W to a single 5" driver. I'm shooting for low voltage...maybe like 18-25V DC to the rails. What is the minimum VA that'll work OK? It seems that the answer is about 60VA or so. The Bicron units of this VA rating have a regulation % of 12-13%.
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Old 8th November 2006, 08:35 AM   #12
AndrewT is offline AndrewT  Scotland
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Hi,
All the following apply to ClassAB and ClassB amplifiers i.e. the vast majority.

The MINIMUM VA that will work is probably of the order of half the maximum output power. The performance is likely to be terrible.

A sensible minimum would be VA = maximum output power but the performance will be compromised. This will give a "cheap" amplifier.

The optimum VA ~=1.5 times maximum output power. This gives good performance and good value for money.

VA>1.6times maximum output power generally gives only small or very small gain in performance and value for money decreases rapidly unless a cheap or secondhand transformer becomes available.

Be carefull to check the full range of voltages that can come out of your PSU, particularly if you hang another voltage limited IC across the rails.
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Old 8th November 2006, 10:52 AM   #13
sreten is offline sreten  United Kingdom
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Hmmm.......

Like I said, lots of tosh expounded on this subject.
For your active speakers you need VA = power output.

/sreten.
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Old 8th November 2006, 11:42 AM   #14
AndrewT is offline AndrewT  Scotland
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Hi Sreten,
you are entitled to your opinion. It is just as valuable as any other.

I have explained where and how I have arrived at my conclusions.

A similar explanation from yourself would help your opinion to carry more weight. Or maybe you think KISS and let the enquirer decide?
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Old 8th November 2006, 03:16 PM   #15
CarlosT is offline CarlosT  United States
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Quote:
Originally posted by AndrewT
...The optimum VA ~=1.5 times maximum output power...

...Hi Sreten,
you are entitled to your opinion. It is just as valuable as any other...

Guys...guys...your inputs are greatly appreciated and actually I thought that you're both converging around the same answer...1.5 times max power output.

I found a nice 90VA 20V+20V with 7% regulation Avel Lindberg on the cheap and that seems to meet the 1.5 times criteria for a single LM3875 driven moderately. Do you agree?
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Old 8th November 2006, 03:29 PM   #16
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If the 3875 GC kit guide recommends a 160VA or higher transformer, it was based not on numbers but actual listening tests. PS is very critical to this amp design and the recommendation was given for a reason. Personally, I wouldn't go less than 200VA per channel.

Of course, the amp will work with 40VA transformer, but I thought you approaching "this GC LM3875 as art..."
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Old 8th November 2006, 03:38 PM   #17
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I just read in other thread about "a small single powered speaker for a kid to plug in his iPod" so my recommendation is not valid here. Go with small transformer as recommended by others here. 90VA should be fine.
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Old 8th November 2006, 04:32 PM   #18
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by AndrewT
Hi Sreten,

you are entitled to your opinion. It is just as valuable as any other.

I have explained where and how I have arrived at my conclusions.

A similar explanation from yourself would help your opinion to carry
more weight. Or maybe you think KISS and let the enquirer decide?


Hi AT,

I thought you'd read Self's Power Amplifier Design book ?

If you have/do, then you'd understand why I'm not attempting
to go through Selfs reasoning other than stating that for music
replay as opposed to steady state sine waves at full power,
VA = clipping power level is a good commercial choice.
(That is : instead of oversizing the power supply build a more
powerful amplifier with that supply simply because you can.)

For active speakers in particular you don't have to generalise
the load, and say always design for 4ohm power output. If
the drivers are 8 ohm then VA = 8 ohm power RMS clipping.

In the context of this thread where in the application space is
tight I'm just being insistent on what is engineeringly required.

The VA needed without major compromise is the designed power
output, in this case 30VA to 40VA, with a decent capacitance level.

On music signals the continuous average power output even
with excessive levels of clipping should not exceed 10W, which
is not a major strain on a 30VA supply.
30W continuous sine waves for hours are a very different matter.

/sreten.
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Old 8th November 2006, 05:14 PM   #19
CarlosT is offline CarlosT  United States
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Peter...it's art...pure art, man
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Old 8th November 2006, 05:17 PM   #20
CarlosT is offline CarlosT  United States
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BTW have you ever seen some of the rare stuff offered by Angela Instruments? Pretty cool resistors and other vintage stuff...I was thinking of "warming up" the sound with some of the more esoteric resistors out there. This is mainly a tube amp place but the stuff was interesting...artsy
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