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Powering LM 3886 from Headphone Output, MAC. problem
Powering LM 3886 from Headphone Output, MAC. problem
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Old 5th April 2008, 03:28 AM   #1
dxtr is offline dxtr  United States
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Angry Powering LM 3886 from Headphone Output, Mac. problem

Hi I have built 4 LM 3886 amps so far and have no problems with any of them when powered thru my Dynaco PAT 4 or as the output stage for my Dynaco SCA 50 [last project]. But when I hooked one up, which had over a month's use, to my iMac Headphone via an FM 5 thru it's AUX input the amp just quit. It ran well at all power levels then quit after about an hour.
Any Ideas, do I have to put a "real" pre-amp in the circuit? Or is there an easier way to do it, don't really want to use the PAT 4 that I am upgrading to sell or build another pre-amp.

I have included some pix of the SCA 50 [date on pix is wrong, too lazy to reset date on camera]
Used orig Dynyo trans and used the center tape to split the volatage to +- 32v.
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Old 5th April 2008, 12:58 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
which amplifier topology did you adopt?
inverting or non-inverting?
AC or DC coupled?
Volume control?
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Old 5th April 2008, 08:07 PM   #3
dxtr is offline dxtr  United States
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non inverting, used the pc boards from chip amp site, twin 21.5 v toriod, snubbed power supply but added a 10k uf/63 to each side [total 20kuf on each side]. Used 100uf on board and added 1000uf in parallel to each side. Used the Dynaco stile output cap on each side [3300uf into 6ft of 20 ga wraped arround the cap and parralells with 10 ohm 3 watt resister, o.1uf mylar in series with 0.47 ohn 2 watt resisters.
No volume control [am using the test amp I built now with 20k pot]
AC or DC Coupled, don't really know, on this amp the ac is not grounded.
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Old 6th April 2008, 10:03 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
you need to identify if your amp has a an output offset.
or,
Is the source sending an offset that the amp is amplifying.
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Old 6th April 2008, 07:17 PM   #5
dxtr is offline dxtr  United States
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Hi had time to do some testing and found that two of the neg side diodes on the power supply blew [over heated?????, added another fan just in case].

Would love to be able to show the schematic but cannot seem to attach it where can I store my images so I can add them??????????????


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Old 6th April 2008, 08:00 PM   #6
dxtr is offline dxtr  United States
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I am going to try to upload schematic again found a free file host.[IMG]
An externally hosted image should be here but it no longer works. Please upload images instead of linking to them to prevent this.
[/IMG]


And here is a pic of the SCA-50 Project that I just finished remember that the date is wrong on the pic, to lazy to change date on camera

[IMG]Click the image to open in full size.[/IMG]
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Old 6th April 2008, 09:05 PM   #7
AndrewT is offline AndrewT  Scotland
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have another look at your schematic.
It does not look right.
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Old 7th April 2008, 01:20 AM   #8
dfdye is offline dfdye  United States
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How hot do your "heatsinks" get when it is/was running? The fan doesn't look like it will do much of anything since it is blowing between the two "heatsinks" and not directly on either one. Be sure if you are adding fans to have the fan pointed at the aluminum plate to more effectively cool it.

Also, what is the power rating of your bridge? If it blew, then something is not right (obviously!). If only half of the bridge blew, that lends a little to the DC offset argument (but I am not quite convinced yet )The chips themselves are pretty well protected, so I am guessing they didn't blow.

I'll second that your schematic looks a little off. Double check your feedback loop. I am guessing this is a drawing issue and not an issue with your circuit.

Finally, if you make small, compressed images, you can attach them directly to your posts without having a 3rd party hosting.
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Old 7th April 2008, 01:57 AM   #9
jaycee is offline jaycee  United Kingdom
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Your bridge probably blew because of the extra capacitance you have used. If i read you right, you have 20,000uF of supply capacitance!

Personally I would use a 35A bridge bolted to the metal case rather than the discrete schottky diodes, but that's me.
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Old 7th April 2008, 03:13 AM   #10
dxtr is offline dxtr  United States
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Default Ooops

Had a close look as the schmatic and you are right, attached is the right one, the amp was built with this one.


[IMG]Click the image to open in full size.[/IMG]
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