NAD 2150 Bridging problem - pulsing sound

[k9gardner]

I have just implemented bridged mode on my NAD 3150 and 2150 to drive my Magneplanar speakers. However, after the system is on for several minutes, a low-frequency pulsing sound starts up in the left speaker (driven by the 2150), pulsing at about 150 bpm, though it speeds up and slows down a bit unpredictably. It is not dependent on there being a signal from the 3150 preamp, and switching L/R at the input side doesn't change things; it's still just the speaker driven by the 2150 power amp. I would like to solve this problem!

Here's the back story.

I had been running the preamp outs of my 3150 to the 2150 power amp for over 10 years, in the belief that I had a bad left power amp channel in the NAD 3150. I had originally bought the 2150 to permit operating both amps in bridged mode with these Maggies, but then this problem developed where I was getting no output on the left channel, so I just left it as-is all this time.

Today I was preparing to finally take it to a local electronics shop to have it fixed, but I wanted to give it one more quick going-over to make sure it wasn't something bloody obvious. And as fate would have it, it was. The damned connection from the left main-in RCA jack to the sort of blade connector on the backside that connects it to the circuit board was apparently not making good contact due to years of corrosion/tarnish. A few shots of DeoxIT and jiggling and pushing and pulling the connector (I couldn't remove it completely), and all was fixed! Also fixed the scratchy volume and balance pots and everything was getting better.

I carefully followed the directions on bridging setup in the NAD manual, and the system does work, and there is definitely a power increase to the speakers, but this pulsing problem has also started. I just don't know whether I should go back to the way it was or... or what. I want to use it this way, I've been wanting to do this for ages!

One thing I'll add is that the 2150 is getting its AC power via the 3150 convenience outlet. I first had it plugged into the switched outlet, but then tried the unswitched to see if that made a difference. It didn't. However, when it was in unswitched, I once turned off the 3150 via the power button, and the pulsing sound got considerably louder.

If anyone has similar experience or can offer any diagnostic pathway for me, I'm all ears and eyes. Please let me know!

 

[E.Man]

Can't really offer anything except that I did Bridge NAD C372 & C272 with both units plugged into the wall power outlet. Had no problems and can confirm a big power increase.
I must have referred to the instructions a hundred times haha.

 

[Bob]

assuming you have eliminated the possibility its introduced earlier in the sound
chain (eq. not some low signal from the pre that, when amplified, shows up)

from your description, and based on my assumptions, it's a form of motorboating.

usually a cap. since its only one channel (under all test conditions?), I'd look at the
decoupling caps for the channel in question. these will be from the power supply
line to ground, and also for the negative PS side in case the amp circuit is dual
voltaged,

other than that, you'd need to use a scope and set it for the CPS/CPM and look
for where it's introduced.

 

[leesonic]

So it works in "normal" mode, driven by the 3150 as a pre-amp? Hmmmm... If I'm reading the schematic right, the bridging switch shorts the input of the left channel to ground, and instead feeds a signal taken from the speaker output of the right channel via C219. I would have a look at this cap, and maybe the caps C601 and C603. Of course, re-capping the whole thing wouldn't be a bad idea as well. Look for dry solder joints, some of those big resistors in the power supply and elsewhere in the power amp (R629 and 641) can get hot and sorta kinda "unsolder" themselves. Maybe some DeOxit on the bridging switch?

Lee.

 

[k9gardner]

I didn't know the term motorboating, but yes, that's exactly what it seems to be. It sounds like a capacitor charging and partially (?) discharging periodically at about 2.5 Hz. I believe I did eliminate the possibility of trouble being injected earlier in the chain, by swapping L/R at input, and also disconnecting pre-main connection, in both of which cases the problem remained, and remained on the same side.

My favorite idea here is DeoxIT on the bridging switch, which peculiarly I did not do, even though I had the can of stuff sitting right there. But are you (Lee) suggesting that this might help with the situation even if the problem is something with one of the caps? Or is this a separate idea you're also exploring here? I have no experience at all testing capacitors; I only have a meter, no scope. If they need to be tested I'll find a way to do it. But I'll also inspect the solder joints and see how they look. I don't really run the thing that hard, but I'm sure it's possible.

Thanks to both of you, Lee and Bob, for your input here. I will not be able to get back into it for a few days, maybe one or two weekends from now, and will let you know what I find!

p.s. I did want to ask whether it was a surprise that it would work under "normal" circumstances, as I had it (3150 preamp outs to 2150 inputs, non-bridged), but that this problem would only be introduced in bridge mode? Were you surprised by this, or just asking? Thanks again.

 

[k9gardner]

So it works in "normal" mode, driven by the 3150 as a pre-amp? Hmmmm... If I'm reading the schematic right, the bridging switch shorts the input of the left channel to ground, and instead feeds a signal taken from the speaker output of the right channel via C219. I would have a look at this cap, and maybe the caps C601 and C603. Of course, re-capping the whole thing wouldn't be a bad idea as well. Look for dry solder joints, some of those big resistors in the power supply and elsewhere in the power amp (R629 and 641) can get hot and sorta kinda "unsolder" themselves. Maybe some DeOxit on the bridging switch?

Lee.

When I read your response I wasn't aware that in order to test capacitors, you'd have to remove them from the circuit! Ugh. I'm not currently up for this. Too many other projects going on. It was really getting annoying so yesterday I ended up undoing the bridges, and am just using the 3150 integrated amp alone. Back to 50 watts. But they're nice watts; I've always liked those watts.
I'm using a pair of JBL L40s at the moment instead of my Magneplanars (SMGa), so there's certainly no shortage of amplitude available. Even with the Maggies, though... I mean, I live in an apartment, and have never had the volume up anywhere approaching the limit. I know, headroom, etc., would be improved, but still, in spite of rhetoric to the contrary, 50 watts is not insufficient for Magneplanar SMGa speakers.
Once I finish the regluing project, I'll get back to the amplification!