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Old 12th March 2012, 11:37 AM   #21
Zen Mod is offline Zen Mod  Serbia
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Old 13th March 2012, 09:48 AM   #22
Mfly is offline Mfly  Netherlands
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Zen Mod, thank you for the testing schematic. I build the filter/LED combo yesterday and connected it to the output. Played some test files, removed the 10K from the input, tested again (full volume). No burning LED's. So no definitive conclusion on what's going on.

Placed a bid on an 20Mhz oscilloscope. I'll post an update as soon as I got one.

Thank you all for now..
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Old 27th March 2012, 08:27 AM   #23
Mfly is offline Mfly  Netherlands
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Well no oscilloscope yet. But some good news.

The distortion I experienced with the 10k on the input, came from the source material. So still only distortion issues with no attenuation on the inputs.

I found out distortion only starts when I flip the last switch on one channel. So you can run one channel with no attenuation on the input without distortion even with half of the second channel, but not both channels in total. Because it's full dual mono and the two channels are only connected via ground I changed the grounding that they connect only at the chassis point. This reduced the faint 50Hz hum to an even almost inaudible hum , but didn't solve the distortion issue.

Then I read somewhere about possible issues with the protection zeners on the input, high capacitance cables, resulting in stability issues on the source side, where even a little resistance on the input solves the problem. So I just swapped the 10k resistors for 1k. Clear sound and no issues.. Good enough for me.

When I get an oscilloscope, I'll have another look at what's going on, but so far.

Quote:
Originally Posted by generg View Post
I just remember.....

I put the same 680pF on the lower side in the same place to the ZTX450 and the osci was gone....
But I do not know if this had any disadvantages, can anybody comment?
Also tried this, but this made things quite worse. Seemed like this resulted in some antenna behaviour where the distance of my hand to the caps was proportional to the amount of distortion.
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Old 27th March 2012, 03:20 PM   #24
generg is offline generg  Germany
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Quote:
Also tried this, but this made things quite worse. Seemed like this resulted in some antenna behaviour where the distance of my hand to the caps was proportional to the amount of distortion.
Hi Mfly

perfect that my advice helped you to make music with your hands......

isn´t it marvelous that an advice that helped me makes things more worse in your case......

happy you got your solution with the 1k pot...I will keep it in mind.....but I am sure when I do it the circuit will oscillate......whatever part of my body I use!

Nevertheless Aleph P was and is one of my three favorites of pre....

good hearing now!
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