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Old 8th May 2011, 12:11 AM   #191
Salas is offline Salas  Greece
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Quote:
Originally Posted by woodturner-fran View Post

So, looking at the front of the MJE350, plastic facing you, metal piece to the back, legs are numbered 1-3 L to R and you have E, C, B.... correct?


Fran
Correct.
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Old 8th May 2011, 12:37 AM   #192
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Thanks.....
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Old 8th May 2011, 08:11 PM   #193
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So I got the shunt rebuilt today and have it working OK again. FWIW, I rebuilt it using your the schematic linked back in this thread (i.e. the Salas one, not the Quanghao one).

But....I still have hum at an unacceptable level. I'm pretty much convinced that this is PS related because when I turn off the PS the hum dies instantly even though the phonostage stays playing for a while (lifting the earth makes no difference). I've checked the diodes on the HT - all check out ok. Looking back at the PS, I dropped the filtering on the first CRC to 100R (from 470). I wonder if thats coming back to bite me now.

Anyway I need to think a but more about this now and decide where to go next.

Fran

Fran
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Old 8th May 2011, 08:16 PM   #194
Salas is offline Salas  Greece
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You need a scope. Hum from the regulated rail is a bit unlikely. That would be buzz if there was a PSU problem. You know, 100Hz and multiples ''zzzz''. If it is deep like ''vooomm'' its some ground loop between stages and PSU. You use bus bar grounding right? Is it all well with that? PSU ground should attach at the end of the bar towards stronger stages.

*There is some good advice on star or bar grounding in that article http://www.aikenamps.com/StarGround.html
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Old 8th May 2011, 08:29 PM   #195
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Tiny bit of buzzzzzzz when its turned right up. Huuuummm though is the main issue.


Oh look!!!! I'm not under any illusions - I'm quite sure its my build!!

I will look again tomorrow night and see what I can do. Ground is a piece of 2.5mm copper running down along under the 4 valves - various grounding points connect to the bar as appropriate. PSU ground connects at the end nearest the upper tier valves, this is also where input and output grounds connect (separate wire to each transformer/rca jack).

Now that I think about it though, while the upper tier heaters are referenced to 1/2B+, the lower ones are not referenced to ground at all. That needs fixing I think.


Need to go back over grounding in particular now.



Fran
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Old 8th May 2011, 08:35 PM   #196
Salas is offline Salas  Greece
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Read that link I added above. Talks bar if you will scroll down enough. ''A properly implemented buss ground can be every bit as quiet as a star ground, and is usually neater looking. By the same token, an improperly implemented buss ground can be a humming, oscillating nightmare''. On a bar we ground at the input side to chassis only, and we go from lighter to heavier currents in a row on it ending at PSU. Something must be mixing there most likely.
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Old 8th May 2011, 08:38 PM   #197
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Thanks Salas, much appreciated.
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Old 9th May 2011, 06:55 PM   #198
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Salas - a big thank you for the ideas and the link above. I rearranged all the grounds and the hum is much much less now. It is now inaudible at normal listening levels. However, I think I can get it quieter (now that I know that I'm on the right track), maybe I might try a star ground.

the fix was as you suggested - simply moving the ground attachment points along the bus so that the input end was at one end of the bus and the output stuff, PS at the other.

I think complete separation into a couple of stars might be even better. However.... now I get to listen for a few nights first!!

Fran
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Old 9th May 2011, 07:32 PM   #199
Salas is offline Salas  Greece
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Now that the build got teething issues free enough you will be able to tune up much easier after familiar. Is it to your general liking territory initially?
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Old 9th May 2011, 07:42 PM   #200
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Actually its better than that - it's quieter than I said -I found i left the tt ground wire disconnected. Now that it's on, the itch is very quiet. Happy days!

Sound is very sweet, little more clarity than the pass ono clone so that's very good indeed. I still have only the panasonic pp caps for coupling so no doubt adding the ky42 should bring another jump.

For now though I want to monitor heatsink temps and voltages, plus see if I can add back in the wiring for mm input.

As I said back up..... happy days!



Fran
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