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New to me McCormack DNA-225

The last thing it could be is DC on your mains. First power can cause transformers to hum, but generally that will be heard from the amp itself. Dirty power is a pain.

Another thing to check is your speaker cables and their routing. Running parallel to any power cables? Rolled up in loops turning them into antennas? Is there a ceiling fan or other motor nearby? How about fluorescent lighting?

The issue could be radio frequency related rather than power or ground.
 
Assume you mean tighten the bolts that hold down the transformer.

I'll be talking to the seller. I doubt they want me opening anything up. I think it has a noisy transistor and/or filter cap. That's coming from a layman as far as electronics go. I am not really qualified to do circuit board work. Wish I had kept my dad's old Heathkit to tinker around with. I imagine a transformer replacement would be pretty significant. Hopefully it's not that.
 
A23gt said:
The last thing it could be is DC on your mains. First power can cause transformers to hum, but generally that will be heard from the amp itself. Dirty power is a pain.

Another thing to check is your speaker cables and their routing. Running parallel to any power cables? Rolled up in loops turning them into antennas? Is there a ceiling fan or other motor nearby? How about fluorescent lighting?

The issue could be radio frequency related rather than power or ground.
Click to expand...

I have a furnace in the basement and also a small refrigerator. The furnace is in another room and about 15-18' from the system with a wall in between them. On a separate breaker of course. The fridge does have a fan and I did run a test with it unplugged but no change. No fluorescents on that floor. I could trip the furnace breaker and see if that does anything. Washer and Dryer are on another panel and far from the system.

I would not say the speaker cable is just straight. I'll try to reroute it some and see if that helps. They don't run parallel to any power cables. I checked that first thing.

I have thought about just running a dedicated circuit for the system. I will most likely get that done regardless of this situation.
 
So I spoke with Patrick at Smcaudio about the issue. This amp has Xlr balanced inputs and rca. At some point they realized that this configuration could introduce hum if the rca inputs were used. They later installed a switch to bypass the xlr inputs if using the rca’s. My amp was made prior to this change and that is where the problem lies. I wish I had called him earlier.

He also said the impedance on my pre is higher than they recommend for the DNA-225 resulting in the highs being somewhat rolled off. I did notice the highs were a little rolled off and made a few adjustments for this in my speakers and tone control. I can get all of this taken care of with some mods. A fairly significant investment. I may do it. Decision time.
 
vintageadict said:
He also said the impedance on my pre is higher than they recommend for the DNA-225
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800Ω output impedance of the CJ is too high?

That had me worried but the preamp I have driving the DNA-1 has an output impedance of 250 and the other preamp is 600Ω which might be adequate after I get around to refreshing that preamp.
 
FYI, I bit the bullet and sent the DNA 225 out to SMcAudio to fix the problem and make a few upgrades. I feel like it is worth it considering how it sounds in my system. I think it could be an end game amp for me. Can’t wait to get it back.
 
opnly bafld said:
Output impedance 800 and input impedance minimum 73,000 (stock version) = no worries.
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The previous mods done to the amp took input impedance down to 10k. Pat said Returning it to the original impedance (100k per Pat) would be a better match for my pre. I’ve never gotten this detailed on impedance matching so new territory for me. We’ll see what kind of difference it makes. The hum will be gone so that is worth the work by itself.
 
73,000 was the lowest measured value from the Stereophile test, specs had 100k.
I wouldn't be worried with 800 to 10k, most consider 10 times minimum, but more can be beneficial.
My tube preamp is 2k, so the builder recommends 20k min.
 
I have to say it’s really revealing. On another level from the Marantz 300 DC. Bass at lower volume levels is excellent. That was my main goal. These ADS really like some power.
 
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