Which toroidal is ok for the LM3886 and speakers 4 ohm? - Page 3 - diyAudio
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Old 29th December 2006, 06:33 PM   #21
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Quote:
Originally posted by CarlosT

Right now each V+ and V- bank has a 10 uF electrolytic cap at the rectifier board and a 1,000 uF electrolytic cap at the amp board.

I just did not understand the function of a cap across V- and V+.
Carlos is this statement correct, the resevoir cap, the one near the recifiers should be the largest.

If the series transistor turns on it will swing high very rapidly and if the largerof the two capacitors is at the output of the regulator this would look like an instantaneous short circuit that could move the series transistor out of its Safe Operating Area.

It may be wise to put a 1000 uF at the recitfier end as well.

If I misunderstood you then I apologise.

Kind regards

Nico
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Old 29th December 2006, 07:03 PM   #22
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Quote:
Originally posted by Nico Ras

... the resevoir cap, the one near the recifiers should be the largest.

GC's need about 1000uF as close as possible to the chip. In the PSU the caps can be minimal, unless using a regulated or snubbered supply. Then go as high as you want!
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Old 29th December 2006, 07:06 PM   #23
MikeW is offline MikeW  United States
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Quote:
Originally posted by Nico Ras
Hi Mike,

I suggested starting with four ohm and if all is well go lower, if it is really necessary. Think of it 68W on the speakers is the equivalent to 132W on the devices. Stereo operation is 264 watts of heat. Think of it - 13 x 20 Watt soldering irons worth.

Something confuses me a little, that is their SOA does not really explain how they achieve such high power without destroying the device.

Can anyone shed light on this.

The devices will never dissipate that much heat, especially with a music.
If is is that much of a problem double up the 3886's.
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Old 29th December 2006, 07:31 PM   #24
CarlosT is offline CarlosT  United States
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Thanks, Nico. Most GC kits seem to have large caps near the chip...the TechDIY one is not an exception although they are a little farther from the chip than let's say the AudioSector ir Chipamp kit. These two latter kits differ a little on how the caps lay out on the rectifier board.

AudioSector Kit - Two small 10 uF electrolytic caps at the unregulated rectifier board, two large 1,500 uF at each V+ V- bank...no other caps anywhere at the default configuration.

Chipamp Kit - I can't really comment on this one but I think that it's a snubberized power supply design with very large caps at the rectiifer board.

TechDIY - For some strange reason, the designer does not bundle a rectifier board kit. There are two large 1,000 uF caps on the amp board.

The designer of the TechDIY kit (jackinnj) set up various layers of cap protection on his board. I skipped some due to interference with the heat sink and some just out of a minimalist/lazy impulse. The amps without all these seem to work fine but I am curious if I'm giving in on some easy enhancement. The distance between the rectifier board and the amp board is almost none.

NicoRas...thank you for your persistence on my inquiry...it is refreshing on this forum

Attached is a pic of the "test mule" amp showing the relative position of the components.
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Old 29th December 2006, 07:37 PM   #25
CarlosT is offline CarlosT  United States
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This is the layout of the various caps on the TechDIY board. I skipped all except the large 1,000 uF caps and the little 10 uF cap on the recitifier board, of course.

I skipped C2, C5, C6 and C7. I am very curious about the one on position C7 since that would be an easy add-on right now and the motivation for my original question.



Click the image to open in full size.
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Old 30th December 2006, 10:15 AM   #26
AndrewT is offline AndrewT  Scotland
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Hi,
what is the black link in the middle.
That electrolytic standing up from the board, what is it?

Why did you omit all those caps from your installation?

R6 still does not match R2. What is your output offset? have you tried to optimise this?

20k pot into 10k Zin is too high. Have you tried to reduce it to 5k or 2k?
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Old 30th December 2006, 11:05 AM   #27
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Quote:
Originally posted by CarlosT
This is the layout of the various caps on the TechDIY board. I skipped all except the large 1,000 uF caps and the little 10 uF cap on the recitifier board, of course.

I skipped C2, C5, C6 and C7. I am very curious about the one on position C7 since that would be an easy add-on right now and the motivation for my original question.



Click the image to open in full size.

With regard to your statement of lowest off-set being attained making R6 equivalent to the feeedback resistors in parallel is not entirely accurate.

Off-set is a DC component. The statement would only hold true if you did not have an input capacitor and the volume has set to minimum.

From what I suspect is that you got the lowest off-set with the 1K by coincidence and it may be different for another chip.
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Old 2nd January 2007, 10:16 PM   #28
CarlosT is offline CarlosT  United States
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BTW those were the default values from the TechDIY page. My adaptation is a lot simpler and that's why I'm asking about what to add back in.

This is how mine is laid out.
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Old 2nd January 2007, 10:20 PM   #29
CarlosT is offline CarlosT  United States
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Quote:
Originally posted by AndrewT
...Why did you omit all those caps from your installation?...
Well...for one thing, the layout of the AudioSector GC is very simple and has none of these items. I felt that I could put in a DC input block cap and go from there.

The other issue was packaging...the heatsink interfered with the solder holes for the two little caps (C5 and C6).

Again the result hasn't been bad...
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Old 2nd January 2007, 11:18 PM   #30
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Quote:
Originally posted by CarlosT
BTW those were the default values from the TechDIY page. My adaptation is a lot simpler and that's why I'm asking about what to add back in.

This is how mine is laid out.

Why are you going mono?
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