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Old 21st July 2014, 06:56 AM   #1
tyger23 is offline tyger23  United States
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Default NAD 2100 DC Offset

All, Sorry for the *noob* question, but I've been working on this guy for quite a few months now and I'm not getting anywhere.

I've got a NAD 2100 Power Envelope amp, and it's working well and sounds great. I recapped EVERY electrolytic (Nichicon FG), changed out the preamp to an LM4562, pulled out some Sanken output drivers that a previous owner had once repaired the amp with (I replaced them with Toshiba 2SA1943/2SC5200), and set everything up as best I can.

My issue is with the DC offset. One channel is right at 110mV at turn on and around 90mV once it warms up. The second channel is around 40mV.

This is not technically outside of the service manual specs, but I'd still like to get it down and I don't know how. I read one thread that suggested changing the input resistance (R403), so I put in a pot there. It did make a MINOR difference, but only down to about 70mV and only when the resistance lowered to about 1K or less. That was too low for my taste, so it went back to 10K.

One thing to note: the DC offset measured exactly the same BEFORE I did any of the re-cap or transistor changes. So, I know my changes did not affect anything regarding this issue.

Here's a snapshot of one of the amp's channels:
Click the image to open in full size.

Here's are two different links to the full service manual. The second one is a better copy, but it requires you to register:

NAD 2100 Service Manual free download, schematics, eeprom, repair info for electronics

NAD 2100 Manual - Monitor Series Power Amplifier - HiFi Engine

Any help and guidance you guys can provide would be greatly appreciated. Thanks!
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Old 21st July 2014, 07:52 AM   #2
Mooly is offline Mooly  United Kingdom
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A 100mv offset in practice isn't a problem. If you calculate the power into 8 ohms and the current flow you'll see why... however

You could try removing one of the two pairs of the 47 ohm resistors and replacing them both with a single multiturn preset.
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Old 21st July 2014, 07:55 AM   #3
Mooly is offline Mooly  United Kingdom
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Old 21st July 2014, 03:04 PM   #4
tyger23 is offline tyger23  United States
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Quote:
Originally Posted by Mooly View Post
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I'll try and give that a shot. I assume that the center pole of the 100-ohm pot will feed the collector of "tapped" transistor?

For example, if I replace R413/R415 with the 100-ohm pot, then the collector of Q407 will be fed by the center pole?

Thanks for your suggestion. Any help is welcome. I was certainly surprised to not see a DC offset adjustment on this amp.
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Old 21st July 2014, 03:06 PM   #5
Mooly is offline Mooly  United Kingdom
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That's it
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Old 21st July 2014, 11:32 PM   #6
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On a side note, how did NAD's Power Envelope circuitry work?
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Old 22nd July 2014, 07:18 AM   #7
Mooly is offline Mooly  United Kingdom
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Wasn't that term applied to the switched rail designs where transient peaks could be delivered briefly.
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Old 22nd July 2014, 02:41 PM   #8
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Yeah, but I don't remember exactly. Wasn't it something with additional transformer
windings, giving a higher secondary voltages, to compete with PSU sag at load?
Sadly I wasn't able to find any specific answer.
However, if it worked this way, I don't think it has been a principle worth staying
awake at night. Hard to beat a real solid PSU.
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Old 23rd July 2014, 12:18 AM   #9
Eldam is online now Eldam  France
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Location: At home, sweet sound home...
Sorry, I'm totally off topic :

I found adding 1500 uf Panasonic FC with two copper wires (from a speaker Self) on the littlier main caps had a great improvement !

Put myself KZ muse 100V but the first stage (not the pre) with little Panasonics FM caps. For the pre section caps were changed but the two first Lelon which were great here ; put after mainly Silmic I (not the II !), I keeped all the green radial caps and putted 2 Silmic I also on the pots pcb !

Wow... very nice amps, I agree ! Tones are even better, musicality increase, and bass are better, not dull anymore.... drive nicely and with energy without harchness my Kef 104/2 ref ! (PS : I changed the 2 x 10K uF Elna caps for Jamicon...after ten years...)

I had to try the oap Mooly advised and that you puted : LM4562 ! (also BB2604 ! )

But change the 5K volume pots (one are very tired and shutdown alone sometimes !) : Could you an idea were can find such pot value please ?

Last edited by Eldam; 23rd July 2014 at 12:26 AM.
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Old 23rd July 2014, 07:17 AM   #10
Mooly is offline Mooly  United Kingdom
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Are they single or dual gang pots ? any tappings for loudness ?

Standard pots in 5k (or 4k7) should be easy to find I would have thought.
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