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adjusting DC offset for NAD 2100

gogofast

gogofast

Addicted Member
hello,
i just got my hands on NAD 2100 service manual and i think the amp has idle current adjustment feature but not DC offset adjustment. i don't know if they are related.....if i adjust idle current to spec, will DC offset be also adjusted to normal reading? if not, how can i adjust the DC offset?
 
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Idle current is your bias adjustment . As far as your question about offset I would need to see the service manual..May not be any adjustment for offset.Have you measured the offset?If its less than 15-20mv I wouldn't worry about it. Does the service manual mention anything about a centering adjustment ---this would be offset.
 
avionic said:
Idle current is your bias adjustment . As far as your question about offset I would need to see the service manual..May not be any adjustment for offset.Have you measured the offset?If its less than 15-20mv I wouldn't worry about it. Does the service manual mention anything about a centering adjustment ---this would be offset.
Click to expand...

thanks for your response.
the manual mentions 'center voltage check' and 'idle current adjustment'.
which i did both. there's no adjuster for DC offset - i don't think since it only talks about measuring the center voltage, but not adjusting it. (these measurements match measurements at the speaker terminal).

before the adjustment of the bias current, DC offset was 15.3 for left and 78 for the right. after the adjustment, 4.5 for left and 53 for right.

i guess there's more cause for DC offset than idle current being off. so where do i go from here? if i need to change the input differential transistors (according to echowars' sticky), how can i find these from the schematic? i can send you the service manual via email. thanks.
 
anyone know how i can identify input differential transistors?
would it be a good idea to check all the transistors anyways, and can i do it in circuit or do i have to take them out?

here's a complete DC offset reading for both amps:

amp1
left: 4.5
right: 53

amp2
left: 37
right: 89

this is after the bias current was adjusted to 15mv.

on amp 2, there's a thump when the amp powers on. also, when speaker button is engaged, i can hear a clicking sound from inside when amp 1 does not do this. i don't know which is normal. i hope by replacing the input differential transistors, these problems will go away....i hope

oh, one more thing: when the bias current is adjusted while impedence is set at 8 ohm, and then switched to 4 ohm while in bridged mode (this is what manual recommends), the bias current goes down about 2mv for both channels. should i set the current while the amp is set at 4 ohms? the manual says that i should always set it as 8 ohms when adjusting the bias current.
 
BalancedXLR said:
They tell you to recheck after 5 minutes but I'd give it longer to settle and readjust if necessary.
Click to expand...


i would also add,unless things are obviously WAAAY out of whack [IE if you've just done some work to the amp and need to baseline things],i wouldn't take any measurements as accurate or adjust anything if the amp is cold and just switched on..check it but then let it play for a while,listen to a cd or 2 at normal volume and check it again...what might seem bad when cold could improve markedly as the amp warms up to temp..or vice versa!
 
BalancedXLR said:
There is an allowance for the centre voltage between 0 +/-100mV. Q401,402,403,404 are the differential amp transistors. As far as adjusting the idle current do it with the amp switched for 8ohm. They tell you to recheck after 5 minutes but I'd give it longer to settle and readjust if necessary.
Click to expand...

thank you for that info!
401 & 402 are PNP and 403 & 404 are NPN - so i guess i can replace them with
2SC1775A or 2SC2547 for NPN and 2SA872 or 2SA1085 for PNP according the suggestion in the 'DC offset' sticky?

Rodzilla said:
i would also add,unless things are obviously WAAAY out of whack [IE if you've just done some work to the amp and need to baseline things],i wouldn't take any measurements as accurate or adjust anything if the amp is cold and just switched on..check it but then let it play for a while,listen to a cd or 2 at normal volume and check it again...what might seem bad when cold could improve markedly as the amp warms up to temp..or vice versa!
Click to expand...

yes, i noticed that. at power up, the DC offset readings are above 100mv, but it settles down to 70's after 10-15 minutes.
 
ok, after searching for those parts, i have to ask these few questions:

1. given the fact that these amps will be used in bridged & 'lab' mode, replacing those transistors with upgrades will not have any negetive effect?

2. i tried the web and not much luck there. a few sites have listed them, but require big quantity to be purchased. is there any place i can buy just a handful of each kind at most?

thank you once again for any help.
 
Great thread. Do you have a copy of the offset instructions? I haven't checked or set the offset on mine yet - I just recapped one of my NAD 2100's last week, and will do the other soon. If you end up doing a recap, I'd be happy to post the cap list here.
 
Ian C. said:
Great thread. Do you have a copy of the offset instructions? I haven't checked or set the offset on mine yet - I just recapped one of my NAD 2100's last week, and will do the other soon. If you end up doing a recap, I'd be happy to post the cap list here.
Click to expand...

there isn't an option of adjusting the DC offset as far as i know. you can measure the offset using ground and R471 & R472, but this is same as measuring at the speaker terminal. you can also measure idle current and actually adjust by connecting your meter ends to P402, 403 & P404, 405 and turning R437, 438 for each channel to reach 15mv + - 2.5mv. have you replaced input differential pairs? if you did, where did you get the parts? let me know what your DC offset is? thanks.

i dont know if i want to do the recap. if i decide to use these everyday, maybe i should. did you replace all the caps? power filter caps? or all electrolytics? that's gotta be a lot of work.....
 
BalancedXLR said:
There is an allowance for the centre voltage between 0 +/-100mV.
Click to expand...
wait, so is that mean if your reading is within 100mv from zero, the amps should run ok? i read that part, but didn't really think much of it since EchoWars suggested offset over 50mv is not good. if the allowance is really +-100, then i don't even need to worry about replacing those transistors.....yay!
 
Well, the real pain was looking up the caps - I didn't have a schematic so I actually ead them off all of the caps. It looked like a lot of work, but it took about 2-3 hours I think. I replaced every one of them. I haven't really say down and listened, as it's just driving my sub now, but the big difference I noticed, is that when I used the gain controls, it would get louder up to the 12 O'clock position, then as I turned the gain knobs from 12 O'Clock to all the way, it would get quieter again. I don't know why, but it works now, so I'm happy.

I'm not sure which transistors those are you're talking about without the schematic, but I didn't do any of that. I've considered the following mod and
would be interested to hear anyone elses opinion of this:

from
http://www.audiokarma.org/forums/archive/index.php/t-111434.html
---Begin Paste----
The part you are looking for is the NJM2043 which is a dual opamp. This was OK in its day but I would recommend that you replace it with the OP275 which is obtainable from Digikey and others. This is a very low noise, low distortion device and drops straight in. I replaced the 2043s in my NAD 2200 which was a great improvement...
 
Ian C. said:
the big difference I noticed, is that when I used the gain controls, it would get louder up to the 12 O'clock position, then as I turned the gain knobs from 12 O'Clock to all the way, it would get quieter again. I don't know why, but it works now, so I'm happy.
Click to expand...

i thought those gain control become active only when the amp is in 'normal' setting instead of 'lab' setting where you bypass the filter stage OR when you bypass the preamp and use the power amp only with the source.

i did see that post about upgrading the Q301 ic and thinking about doing it. if i can find the input differential parts at digikey, i might as well order one of those too. well, whoever gets to it first, will post about the results.
 
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