NAD S300 integrated amplifier - DC Offset? - Help

Hi all! My name is Sergio and I'm from Portugal.

I'm the very proud owner of a NAD S300 integrated amplifier (it's such a great amp and I was very lucky to get it at a price I could actually afford).

I have a small glitch with my S300... when it is turned ON but without any music playing (all other gear turned OFF), when I turn the volume knob, I can see the speaker woofer drives move a lot, forth and back, reacting to the turning of the volume up and down.

This actually stops happening after I turn the volume up and down repeatedly 10 or 11 times, by then the speaker drives don't move... If I leave it alone for 2 or 3 minutes and then try the same thing, turning volume up and down, the speaker drives move again...

I was told that this can be caused by DC Offset not being ok on both channels, where a minor difference can cause this to happen.

Does any owner (or previous owner) of NAD S300 have any details about this? Do you have this speaker drive movement symptoms when turning the volume with no input feeding the S300?

Thank you very much for your help!

Best regards,

SergioRZ

 

Hi Sergio,

Does this only happen just after the amp is switched on?
It the problem disappears a few minutes after switch on I wouldn't worry at all. I've seen it many times in amps and it simply indicated that the capacitors inside the amp are settling/stabilising after power up.

Have you measured the DC offset at the speaker terminals?

If the S300 has a passive pre stage it could be that your source (phono stage/tuner/Cd/Dac) has DC on it's output and is feeding this into the S300. The coupling capacitors inside the S300 will take some time to charge/discharge in response to the DC and every time you move the volume pot you cause the cycle to repeat.

Measure the DC offset on the amp and all your sources.

 

Hi RobHolt, thanks for your help!

It's funny actually, with all the testing I've been doing regarding this issue, I found that it does happen all the time, not just at power on.

But, if I turn up the volume to the maximum position, and leave it there for 6 or 7 seconds, the speaker woofer goes forth slowly... then goes all the way back... then it centers neatly, like the amp was adjusting the output automatically or something.

After I do this, the woofer stops reacting when I turn the volume up or down, as if it was adjusted and the DC output is now stable (I guess).

Then, I listen to some music... 15 minutes later I try again (CD player OFF), turn the volume up and down, and the woofer is moving again...

Turn the volume all the way to the maximum position, there it goes adjusting itself, and the problem is gone again...


I'd like it very much to check the DC offset on the speaker output terminals... is there some "how to" article about how I can do this?

Thank you very much!

I also have the blinking power led at power ON... that could indicate a "protection mode" being activated, but then, all other S300 I know do this. Oddly, the manual says it should not blink at power ON...

The amp is working perfectly (for a year now), even with these minor symptoms that exist since day one... so, this could all be by design features or something?

Thanks a lot for your help!

 

Ok, I've bought a cheap multimeter... followed a web article showing exactly how to measure DC offset on an amplifier...

- Turned multimeter knob to V ... 200m
- S300 turned ON, volume at minimum
- Nothing connected to the S300

The multimeter shows values in 00.0 format.

My results for the speaker output terminals are about +05.0 but the values don't stop changing, anywhere between - 15.0 to +15.0, they keep changing up and down... Is this normal?

Can't see any difference between L and R channel... both show the same value variation between -15.0 to +15.0, and although they never stop changing, they seem to be more comfortable around the +05.0 spot...

I did the same test on my Yamaha AV Receiver, just to make sure... and the Yamaha has constant values around +23.0, that don't change almost nothing, they stay almost 100% fixed.

Is there a problem with my S300 being so "nervous" with it's DC output?

Thanks!

 

Hi Sergio,

+/- 15mv is nothing to worry about and is in fact quite low for a power amp.

Check the outputs of the source components in the same way by connecting the meter to the phono socket. You should see very low/no DC.

What speakers are you using?
Ported bass drivers of good efficiancy can look scary woth small amounts of DC or LF as produced by a turntable.

 

Hi RobHolt,

the S300 doesn't have a phono output that I can use to test.

I have a Linn Ikemi CD Player, NAD S300 integrated amplifier, and B&W CM1 speakers. Balanced XLR cables connecting CD to amp. Mission Solid Core Speaker cables.

You can see some pictures of my setup here:

http://www.forumhifi.com/galeria/thumbnails.php?album=74

I'm still trying to understand two things:

- Why the measurement show DC always changing between the -15.0 and +15.0 values... (shouldn't it be stopped at a specific value?)

- Why do the speaker woofers move so much when the volume is turned up and down and there is no music playing?

Sure, they move a lot with music (these little speakers are famous for their long course woofer drives that sometimes look like they're going to pop out of the cabinets hehehehe), but when there is no music it's odd...

I tested with another integrated amp to see if the same thing happened, but no, no movement from the woofers when turning the volume.

I wonder why the S300 is doing this if there is no major problem with DC Offset...

 

No, I mean measure the output of the Source, ie the Linn CD player. This could be passing DC into the amp.

 

I'll check the CD

But anyway, the symptoms occur even if nothing is connected to the NAD S300 amp (that is, all cables disconected except speakers). So, the moving woofers when volume is turned is something that must be generated by the amp alone.