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Old 15th May 2012, 12:43 PM   #1
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Default Tekko builds OnAudio's SYMEF Amplifier

The build has started, updates will come as i work my way ahead, but heres the progress so far on channel one:
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Old 15th May 2012, 01:58 PM   #2
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Alittle more progress, i was unable to find the exact value for some resistors so they are the nearest higher by about 10 ohms, i hope this will not cause any problems, but they will be replaced when i hate the currect values in stock.
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Old 15th May 2012, 03:57 PM   #3
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Default moving swiftly

Tekko, Tekko, Go Go Go . I see you have decided to test the formula. All the best, awaiting data. Hope she drips chocolate for you.
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Old 15th May 2012, 04:46 PM   #4
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Well no such luck.

I tested it on 2x35VDC but it does not work, goes up to +20V ou the outputs, with a full line level signal it starts jumping between 20V and the positive rail.

I dunno if its a board error or a misplaced resistor value. When touching the board in strategic locations i can get it to swap to the negative rail and/or make it oscillate.
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Old 15th May 2012, 04:50 PM   #5
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Post both bottom and top of board. Will try debug.
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Old 15th May 2012, 05:34 PM   #6
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Turned out to be the input transistors, forgot that BC557 and 2N5400 has reversed pinout.

I still have a problem though, i cannot bias the amp with the proposed 6.8k resistor, i have to add another 12k across it to get in range, but when i add any bias, the amp starts to oscillate :/

Did yoiu actually build this irl to make sure it works of did you rely on ltspice only ?
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Old 15th May 2012, 05:56 PM   #7
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A video: SYMEF by OnAudio - YouTube <-- without those two bodge resistors, it just goas straight up to around 1A bias no matter the pot position.

And as you can see, adding any kinda bias maes it oscillate and sound generally bad.
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Old 15th May 2012, 06:24 PM   #8
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Oscillations cured!!

The 39pF caps on the predrivers are miswired, they are supposed to go between B and C of the predrivers, not between B and GND.

I solved this by scraping off the traces that went to GND and instead replaced them with a short piece of wire going to the rails, that fixed it, no more oscillations!!.

The fix on schematic: http://i.imgur.com/KIaAM.png

Last edited by Tekko; 15th May 2012 at 06:30 PM.
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Old 15th May 2012, 08:36 PM   #9
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Hi Tekko,

Thank you for all you have put in making it work. Could you send me close ups of top and bottom.

kind regards,

Harrison.
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Old 15th May 2012, 08:52 PM   #10
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I hope these are good enough:

Top: http://i.imgur.com/A8tqb.jpg

Bottom: http://i.imgur.com/kQLlC.jpg
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