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Old 31st August 2013, 04:15 AM   #11
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Join Date: Aug 2013
Originally Posted by JonSnell Electronic View Post
If the junction of R6 and R7 is not at 1/2 rail. (=12v) pin 1 will be low causing pin 4 to go low and as pin 4 controls the inverting input pin 2 and sets a low voltage on there, pin 2 will try to return pin 4 to a higher level. The overall control is pin 1, which is at the wrong potential.
Check the voltage across C6, it should be 1/2 rail.
Hope that helps.
Jon Snell Electronic
Hello again, well I've been dicking around with the resistors trying to get these voltages right. I've got it all reading pretty much ok but still no luck with any form of output. Here is the schematic with the voltages I'm now reading.
Click the image to open in full size.
I'd like to get it going but I don't know how long I'll persist with this. I'm feeling pretty defeated by the whole thing.
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Old 2nd September 2013, 05:26 AM   #12
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I replaced the LM1875 out of desperation, and as soon as I turned it on I heard the thump from the capacitor charging up. Hooray... this is progress. I am yet to get any actual real sound out of it but I am feeling more encouraged. I haven't taken any voltage readings yet but I think I'll need to undo all the resistor mods I did trying to get the voltages right. This may have been a bad chip all along... Fingers crossed.
I won't have a chance to do any more for a day or so but I now have some hope.
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Old 5th September 2013, 06:01 AM   #13
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Hooray!!! It works!!
but I'm not out of the woods yet. There is way too much hum at all times there is plenty of signal getting through.
Time to consult the forums on how to reduce this annoying buzz / Hum.
Thank Jon for all your help. I'd have been totally lost without you.
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Old 6th September 2013, 02:29 AM   #14
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Hi all. Still having issues.
I have had some email correspondence from Jon Snell and I'll paste it here for anyone whom it may benefit.
From Jon...

Pleased to be of assistance.

Is the sound a buzz or a hum?
If a hum check your power supply for ripple. Place a large value capacitor across the supply and disconnect the mains. Does the hum/buzz go away? If it does check your earthing/ground point.
Check the input C6 capacitor is decouplig the supply. Try another one from the ground point to pin 1 if the chip. If the hum is still there it is nothing to do with the valve stage.

All the best!
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Old 6th September 2013, 02:38 AM   #15
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I put a large capacitor across the mains and when powered off, the signal was clean while the capacitor drained.
I am a bit confused about checking the earthing/ground point. As far as I can tell the only earth is the - DC supply and that all seems fine.
I think there is some very basic electronics knowledge that I'm missing. I also have a 12v TDA 2003 guitar amp that I built inside an old radio and that has a similar hum issue. My solution with that was to power it off a car jump starter pack and that works fine. If I can fix this hum with the K281, maybe I can implement the same fix for my radio guitar amp.
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Old 8th September 2013, 07:17 AM   #16
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Overall volume is up and power supply hum levels are acceptable. It would be nice to get it quieter but it's really not bad.
I really just went overboard with shielding and making cables as short as possible.
So this brings my K281 adventures to a welcome end.

Thanks Jon
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Old 10th September 2013, 03:33 PM   #17
robrob is offline robrob  United States
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Location: WV
I really wanted to build a balanced headphone amp with two of these K281 kits but the shipping charge to the USA is over $50! Oh well.
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Old 10th September 2013, 08:09 PM   #18
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Originally Posted by robrob View Post
I really wanted to build a balanced headphone amp with two of these K281 kits but the shipping charge to the USA is over $50! Oh well.
I know your pain. I am in New Zealand and as such, am plagued with the curse of shipping expense. In the case of the Oatley kits I am fortunately close to Australia which makes the cost of these kits appealing.
I often have to contend with shipping costs from the US and it hurts my bank balance. Unfortunately we have no good supplier of electronic components here and everything must be sourced overseas. I can't tell you the number of times I have read "you can get it at radio shack" on these forums and felt extremely hard done by.
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