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Old 27th July 2011, 05:58 AM   #11
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Thanks Mike, was never set on a tube pre amp, but thats all that popped up on ebay. I like the warm bass from a tube amp, but a tube amp i may need to take more care with. So pre-amp in the link is good suggestion in your mind? i'll go with it if there is nothing else worth considering.

rick
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Old 27th July 2011, 06:18 AM   #12
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Distortion ? From a laptop's output ? What kind of files ? Mp3 ? Oh ,well
3,5 mm minijack output from the laptop is intended to power an headphone ,so it may be compared to a good buffer ,being able to drive low loads .
In case I misundersood and you are using some line level output ,it may necessitate of some gain to make the amplifier reach its full power .
But I would stay away from NFB based chips configurations ...
At least you could try it , it is cheap and then you can make your considerations
of sound quality.
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Old 27th July 2011, 06:22 AM   #13
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One of the easiest things to do would be to connect another pair of 8 ohm speakers in parallel. Providing a 4 ohm load, which I understand your paralleled chip amp was designed for, and nearing twice the power output. Ideally, I'd want to possess signal strength and amp sensitivity/gain numbers for building a dedicated preamp. I think I'd consider now building a standalone preamp to use with this and other amp projects and headphones. Have you seen these Oatley preamps? They appear to give good results.
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Old 27th July 2011, 06:45 AM   #14
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One more thing, I strongly recommend you check for DC offset on the chip amp boards, and make adjustments if necessary before putting them to regular use. Failure to do that could cause problems. If you need help with that, I'll chime in tomorrow, right now it's way past my bed time, and judging from what I'm seeing in the mirror, I need beauty sleep, LOTS of beauty sleep.

Mike
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Old 27th July 2011, 09:31 AM   #15
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Thanks Mike, we'll do that, i'm not sure how to check the off set, but i'm sure i have the tools. chat later.

rick
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Old 27th July 2011, 10:27 AM   #16
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Quote:
Originally Posted by picowallspeaker View Post
Distortion ? From a laptop's output ? What kind of files ? Mp3 ? Oh ,well
.
NO MP3's for me mate, (laughs)
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Old 28th July 2011, 01:14 AM   #17
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Hi Richard, I'm Back...
DC offset is the presence of a DC voltage at the outputs of an amplifier that can cause the speaker voice coil to be displaced off center in the magnetic gap and excesive heating and power dissipation or even damage in extreme cases. Amplifiers that don't have a DC blocking cap or a transformer at thier outputs are prone to DC offset problems, this includes practically all modern solid state amplifiers.

Here's how to adjust your chip amp boards for minimum DC offset:
1) With nothing connected to the input or output, and no power to the board and power supply caps fully discharged, set the multi-turn trim pot wipers to center position, they are most likely 10 turn pots, but to be sure, use a small screw driver to move them from one end stop to the other and count the turns, then set for the half-way point.
2) With nothing connected to the speaker output, short the input to ground and apply power. Using a DVM, measure the DC voltage from pin three (third pin from the left looking at the front of the chip) to ground on each chip, and adjust the adjacent trim pot for minimum DC voltage, you should be able to get them to, or near, zero volts. WARNING: Be careful not to let the probe tip slip and create a short from pin to pin, that could fry the chip or worse. There may be some interaction between adjustments, so re-check each one and tweek if necessary.
3) Remove power and the short at the input, connect speakers to the output, and a signal source to the input, apply power and play music at a normal level for about 15 to 30 minutes to warm the chips to operating temperature.
4) Remove power and repeat step 2 to verify lowest DC offset and re-adjust if necessary.

That's it, you're done! Enjoy your new amp.

Mike
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Old 28th July 2011, 05:53 AM   #18
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Hi Mike, I did step one and i'm sure theres gonna be alot more turns than 10, i got to 15 turns one way then turned it back to original before going further.
Ok step two, will i leave the input shorted the whole time while i test the DC voltage across pin 3 and ground? I just did it like that and the heat sinks became warm during the test. Is this correct test method?
And a note i tested all the trim pots(rated 50K), i got between 6.5-7.5mV on pin 3 and 0mV on pin one, if thats tested correctly. I'm not gonna continue further untill i get some confirmation of my method.

Last edited by richard.C.; 28th July 2011 at 05:58 AM.
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Old 28th July 2011, 06:54 AM   #19
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OK, sounds like we might have a failure to communicate. For step one you need to center the wipers of the pots as a neutral starting point, so since they're 15 turn pots, you need to turn the screw all the way to the stop at either end, then go back the other way 7.5 turns, then the the wipers will be half way, so at that point you should measure about 25k from the wiper to either end. If the pot wipers are at one end of thier rotation or the other it could cause the chip heating you experienced. For step two you should be reading the voltage of pin three (output) on the LM3886 not the pot, and the first pin on the LM3886 should be at V+ voltage of the power supply, so if that's not right something is wrong there.
Here's the LM3886 data sheet: http://www.national.com/profile/snip.cgi/openDS=LM3886
Mike

Last edited by Michael Bean; 28th July 2011 at 06:57 AM.
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Old 28th July 2011, 07:28 AM   #20
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OK, well i get back to ya with this one shortly, in a day or so, i'll keep post on here.

Cheers Mike
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