NAD 218 THX channel imbalance

[manfred77]

Bought a seemingly decent NAD 218THX power amp, very clean amp. But one channel has more output than the other. Took it to a local shop, all output transistors test okay and he couldn't find any out of spec components on the driver board. This amp is way out of my comfort zone as far as DIY, but would speaker relays be the cause of this problem? I will probably take it to the only other shop in the area, but would like an idea as to what they should be looking at.
Thanks,
Thomas

 

[rayma]

Bought a seemingly decent NAD 218THX power amp, very clean amp.
But one channel has more output than the other.

Clean and check both the BAL/UNBAL switch, and also the NORMAL/BRIDGE switch.

 

[manfred77]

Thanks, will get a chance to do that tomorrow. Hope it solves the problem.

Thomas

 

[rayma]

Hope it solves the problem.

I'd say the odds are 50-50. Do clean the switches with a proper contact cleaner, and
then exercise the switches a dozen or more times, to clear the crud from the contacts.

 

[manfred77]

Still the same, cleaned the switches and also all the small plug in connectors that I could find. Fins on the left side run warmer(but not hot) than fins on the right side. When run in bridged mode the output is the same for both channels. Will run it all day and see if things change.

I'm using a tube preamp that has left and right gain controls. Left side has to be about 25% higher than the right to be equal. Sound quality is okay though. At least it's usable. I do prefer the neutral sound of my rebuilt Hafler DH500 with Quaco driver boards.

Unfortunately, having audio equipment repaired in the Portland, Or. metro area is a crap shoot.

Thanks again,

Thomas

 

[bullittstang]

Have you checked your DC offset and the bias? Should be +/-0.02mV and 18mV, respectively. At least that's what my rebuild one has and it heats evenly on both heat sinks.
Try a signal straight from a iPhone, they are all digital and typically have accurate balance on the signal. Will tell you if the pre-amp has something off.

 

[Sqeek]

Yes as bullittstang mentioned trying a phone or if you have a cd player with volume output control you can use that straight into the amp to make sure it isn't your preamp do you have the service manual? if not it is easily obtainable on the web here is a page from the service manual on how to set/check the idle current and common mode rejection i would also change speakers from left to right to see if that changes anything and or the source cables and sources from left to right ect cd player/TT/tuner ect..