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Old 23rd July 2014, 09:35 AM   #11
Eldam is offline Eldam  France
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Two single pot...super to acurate balance setup.
No loudness legs....
Nad sold it with an external pre....the amp has no sources selection, just two inputs: one is with the two pots and the second no attenuation...
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Old 23rd July 2014, 11:03 AM   #12
Mooly is online now Mooly  United Kingdom
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Pots should be readily available then as long as they physically fit. Conductive plastic types are pretty decent and cost effective.

PC16ECO4K7A - OMEG - POTENTIOMETER, LIN 4K7 | CPC From This Range

PC16ECO4K7B - OMEG - POTENTIOMETER, LOG 4K7 | CPC From This Range
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Old 23rd July 2014, 02:16 PM   #13
Eldam is offline Eldam  France
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Thanks Molly,

the pots are the old way : with a cut in the midle of the "harm" : 4 cm long... exactly like this one (but this one is a 5K linear, i surmise the Nad to use log pots !) : potentiomtre mono vrb-100m5 5k
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Old 23rd July 2014, 02:33 PM   #14
Mooly is online now Mooly  United Kingdom
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You could always add a resistor to fake a log law if you wanted but tbh, its not so vital a thing really.

See changing the law of a pot,
Potentiometers (Beginners' Guide to Pots)
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Old 23rd July 2014, 02:48 PM   #15
Eldam is offline Eldam  France
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thanks Molly

Sorry to the OP... little Highjack on the summertime holyday highroad !
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Old 24th July 2014, 03:12 AM   #16
tyger23 is offline tyger23  United States
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Mooly - I just wanted to follow up and say THANKS! Your idea worked great. I replaced the R409/R411 pair with the pot and was able to adjust the offset. When I got it down to 40mV offset, I stopped and pulled the pot out. The measured values stand at R409 = 60ohm and R411 = 38ohm. Given the rather large imbalance (typically 47-ohm each), does this indicate anything more troublesome, or can I keep those values as is and be happy?

Thanks again!
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Eldam, I have two suggestions for you:

First, I would try and use a contact cleaner like Caig's Deoxit D5 or F5. This will help clean the pots and typically gets my scratchy or noisy pots working well again.
Second, if you have a volume control in your preamp, you can simply use the "LAB" input on the amplifier instead of the "NORMAL" input. The "LAB" input will bypass the entire preamp board with the volume controls and op-amp. Enjoy!

Last edited by tyger23; 24th July 2014 at 03:19 AM.
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Old 24th July 2014, 07:30 AM   #17
Mooly is online now Mooly  United Kingdom
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Your welcome. Pleased to hear it worked OK and you can keep those values and be happy.

(you could possibly get it more balanced still by seeing how sensitive the design was to such things as minor changes in those two zener voltages or more specifically the current in the two 820 ohm resistors R417 and R419. Are they equal ? Just measure voltage across the resistors to check. Tweaking either of those would also trim the offset. This shows a similar front end. Look at the offset voltage and the effect of altering the 220 ohm. Its very similar to yours. )
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Old 24th July 2014, 12:20 PM   #18
Eldam is offline Eldam  France
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Thanks Tyger,

If if can't find cheap pots or will not go with Mooly option (which seems a better solution with a 2 euros pot), I will try to clean... unluckily here all those cleaner products are more expensive than a cheap pot !

About the "lab in" : yes I know that. But here it's a second system and I found the pre option just good for it and love the fine two mono pots for fine balance setup ! The fine re caped ps stage adds also a great improvement and I just ask myself about the opportunity to rep^lace the bad coax wires for signals between back and front panels with a good cheap Mogami e.g. ! Not too expensive in relation to the global sound quality of this little nice amps.... I'm again the tweakings which are more expensive than the device : here all stay reasonable : just oap, fews caps needed after 20 years of loyal service !

Try to add the two 1500uF FC caps and tell me : you won't believe it ! A good option can be also KZ muse 100V nears the power transitors... not expensive, but first the two FC added to the 10K uf is a huge improvement !
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Old 25th July 2014, 06:16 PM   #19
tyger23 is offline tyger23  United States
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Mooly, the current across the two 820's is very close (4.96V versus 4.95V) now, but I've already used the pot to adjust the offset. I see how adjusting this could affect the DC offset as well. Thanks again for this idea!

My only issue now is the fact that one of my two main power supply caps keeps bulging (slightly on the top of the cap, not to the point of rupture). I've replaced it twice now, and the caps seem to keep bulging. When I pull out the cap, it measures within tolerance, but I had some extras so I replaced it. I can't keep replacing $10 caps though. It's always the same capacitor (C507, which is the main 80V power supply cap for the +Vh rail). I've measured 71.2V across it during operation, and I've never seen it vary. C508 exhibits no such bulging, and there is no real heat in the area. Any ideas on that?

Eldam, are you saying to place two 1500uF FC caps across C509. C510?

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Old 25th July 2014, 06:28 PM   #20
Eldam is offline Eldam  France
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Hi,

IIRC C509/10: the two littlier 10K uF caps : Brandnews out of the boxes when I buyed it off-shelves they were Elna 50V... the two others are with golded letters and have higher case !
But I believe this tip can affect positivly the two inputs mode... choose the couple of 10K uF caps you use ! This DIY is not very clean : long wired plain copper leads add a lot of inductance... but it just sounds greater at home by a large margin, not a poor 220 uf but 1500 uf : FC panasonic 50V ! I didn't try the FM serie because no Pi filtering here so a lower ESR was prefered. Notice I didn't check with no more serious devices than my ears (no scope at home ! )
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