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In Reply to: RE: I *finally* fixed 90% of my hum, late 90s NAD owners beware! posted by Yashu on March 08, 2009 at 14:15:08
Interesting. I use a C350 which I have completely revamped. I removed the preamp circuitry and installed a passive attenuator, Alps Blue. The preamp design is compromised, assuming to meet a price point. Noisy and distortive. But, as you say the power amp is excellent. The brown goop is glue, used mostly on the large PS caps, which I replaced with Nichicons. I have not had the need to earth ground the chassis, but I know for sure that the inputs ground is common with the whole amp circuit. I replaced the 470pf couplers with Relcap RTE's, the output filters with Multicap RTX, and the small value decouplers with Black Gate NX. The results are spectacular, sounds easily as good or better than any $1K amp I've heard. The power amp is sweet.
Follow Ups:
Right now I have my C350 dormant in the other room, I was going to begin using it again had I found out that my preamp was going bad (caps leaking and so on), but I am trying to find as much out as I can and with your answer along with more internet searching, yes I began to suspect it was glue. It is only on the tallest, largest caps, and I have been told it was to keep them in place while the unit is tested, as well as shipped of course.
If I could get this sound with "just" the C350, I would use it. Your mods are impressive. I doubt I would be able to figure it all out. You are the first person I have seen describe extensive modifications to the C350. I have seen one other thing, which was a guy that put a IEC Powercord connector on the back. Only one thing to ground instead of having to hack grounding for two things right now. It all really makes me see the C350 under a new light of potential. Perhaps you could write something up for someone with a C350 that is a little new to modding electronics (as in, I don't know much about what individual components are the best to replace, or bypass, or whatever. I know nothing of what brand of cap sounds warm, or what resistor is the one to die for, and what stuff to just leave alone. That kind of thing)
The issue now is this hum. I have grounded the preamp and also the amp-of-the-week to common ground. The same ground that is used by the DAC (I have been told the DAC should never be un-grounded, this was from it's designer), the DIP, a tubed output buffer for another source (It is off unless I am using it, and it doesn't seem to have an effect on amount of hum), and the PC/Monitor. The monitor is one of those big 21" Pro CRTs. It is not the source, I tested this. The monitor itself will react to noise on the line as it makes a sound when there is more noise than normal, say, if a blow-drier is being used. The PC is built to be completely silent, and the PSU was one of the best switching PSU's I could find that could be quiet. The fan only spins up when there is a large load, and that is only if I am in one of my CAD programs or a game or somthing. I have plugged the PC in way at the other end of the place with a loooong extension cord to see if that helped, but no.
There is actually less hum when the PC is plugged into the same spot as my Ultimate Outlet. Common ground seems to be very important. Most of the hum is gone but there is a little bit. Way under audible now, but it still bugs me. I can sometimes make it go away if I un-plug and plug things in to test. I have not yet figured out the "secret". What I know is that because all the digital is through coax s/pdif, there is no true electrical isolation between the PC and the DAC, or the stereo it-self. I use a hagUSB for S/pdif out with ASIO, and then connected to the DIP. The DIP will turn just about anything into a top notch digital source, even the hagUSB, which is supposed to already be one of the better options. There is no toslink though! I can't output toslink to my DAC. Even toslink into the DIP sounds better than coax, which is why I am thinking of a Trends UD-10.1, or somehow modding the hagUSB to output in toslink.
Anyway, your work on the C350 is really cool because I would love to go back to the integrated if it sounded as good (or better) than the c160/poweramp combination. I have a DAC that does not freak out with a line stage anymore, and would love to go back to one unit.
The C160/C350 combo is great usually, except for that hum. I basically built a C37x (C370 or 72, whatever, I think they are mostly the same except the c162 has a better phono stage than the c160, so I guess this would apply to the c372)
I basically built a C370, but with a non Power Envelope amp. I had a 2255 poweramp restored. That would be re-capped and re-adjusted. After all those years I didn't trust it was even still biased correctly. So now it is a really solid amp. For some reason, the modern "classic" NAD stuff gets warm compared to the old stuff. This 2255 can put out an amazing amount of power and hardly get warm (it is usually metalic-cold to the touch). The preamp gets much hotter than the poweramp (the pre is pretty standard class A SS). I don't know why, but my guess is the newer NAD amp sections, while class AB, they are biased towards class A as much as possible, making them get pretty warm. I suppose that is one reason they might sound so good. Take the proven tech of the NAD designs of the past, learn from the mistakes of the PE era, and take full advantage of SS components designed for the heat, and you get that extremely clean, powerful sound of the new "classic" NAD amp design.
I did not get a separate pre knowing there would be such a difference. My DAC at the time put out too little voltage for just a line stage, or passive pre, so I found a good deal on a C160, giving me a nice separates system using the C350 as an amp. I was worried that there was something wrong with the pre, and with the brown "glue", I was even more concerned because I thought it could be a sign the caps were leaking. The 2255 has a warmer sound than the C350, but it takes more load than I have in any of my speakers lying around to thin it out. The C350 as an amp seems like it could do even better, but it is very revealing. When used an an integrated, the C350 is a bit dark, a little cold and clinical. Everything is there, just behind a little veil. The pre/power configuration was like having that veil lifted, it was full of life, at least the amp section was. Since I had a pre now, I began to play with other amps just to have some fun, and the 2255 became a bit of a hobby that turned into my main feature. I kept saying I would use the C350 for 2nd system. I have more than enough to build a 2nd NAD system. I was thinking C350, the CDP they were selling around that time, and I have a C420 or something like that for a tuner. It would be perfect for the bedroom. Out here, I pull in radio (92mhz and below) with a fixed up Sansui. I like being able to turn the dial and fine tune a station. In the bedroom, all the NAD stuff would work on the same remote, which would be great. Still that line stage bothers me, and the 2255's place out here is no sure thing. With this new DAC, the pre/power combo using the C350 as an amp blew me away the other day when I was playing a FLAC rip of a well recorded jazz album.
Whew. As you can tell, This audiophile has been bitten by the bug of DIY. I have always obsessed about the usual audiophile crazyness, but this somewhat new, as well, it is something that seems rewarding.
Or... I could just forget it all and save up for that all tubed amp or integrated. If the tube buffer or the tubed section of my DAC has taught me anything, it is tubes rock, and rolling them is a lot of fun. I haven't yet because, for some reason, I just like the NAD "house" sound.
Yes, the C350 preamp section veils the sound, I was astonished how much so. The amp sounds remarkably like tube amps I've owned. It is liquid, sweet, decidedly on the warm side of neutral, but with excellent detail and imaging, and its one of the most musical sounding pieces of anything I've owned.
Many have gotten great results using it with a tubed pre, like the Bottlehead Foreplay.
Like you, I had the 350 in another setup for cable TV, where it did very well. I ran into hard times, and sold my Cary 300B amp and CDP. I was going to try and replace those, but decided to start tweaking the NAD and one thing led to others.
Good luck in the finding the hum source. Note that AC neutral lines are connected to earth ground at breaker panels, so adding a third wire earth ground to a component may introduce a ground loop. That's why I didn't put an IEC on the NAD.
It hums nowhere near as bad as it did. This is with the pre. When I get time I am going to pull the C350 out and test it as an integrated. I remember it having virtually the exact same hum sound, so I want to see what grounding will do. The difference will be that it won't be separates anymore.
Anyway, if I am introducing a ground loop, wouldn't the hum and distortion increase?
I tried cheater plugs on various other things, and still got the same results. I am not sure where to go from here. Other than messing with the C350 for fun (which, I probably will do), as far as the hum goes, it seems to get better or worse seemingly randomly, but it all depends on the order I turn things on, or one day will go away if I just un-plug one component and plug it back in. It is strange. I have no glass output to test the idea of some sort of problem with the PC, and once I even put the entire PC and monitor on a cheater just to see, and still same thing. If the pre or integrated were not grounded, it was hum city. While grounded I get much less, 90% less, but there is still some. It is still above the noise floor.
I have tried moving things around, using different inputs, cheaters, another DAC, electrical isolation for the coax, and even for the DAC to the pre (via a tube buffer), same thing.
I think the only thing I can tell without having testing equipment is that, one, my place has some grungy ass power, and two, there could be DC on the line. The transformer is outside my door, actually, it takes power from the lines, and distributes it to several units via underground cable for us bottom dwellers and cable through pipe for the folks on the 3nd floor. I am not sure how many people that thing distributes to, but I don't think it is more than 4-6.
I hear it vibrating sometimes and have to go outside and give it a tap to stop the metal on metal vibrations. I have to wonder what would do that to a very large distribution transformer. I figure someone has something plugged in their unit that is mucking things up, or we are all drawing too much power.
I know there is DC on the line, I don't know how much though, but I know it's there. I confirmed it with some testing and also the super and I talk about various different things because he knows I know a bit about this stuff. I was allowed access to the breaker bank with my apartment's (as in my place) breakers on it, and I did not see anything out of place. I am pretty darn sure that it's a true earth ground.
Anyway, if I was introducing ground loop, the trade off is worth it because I am removing 90% of my problem. So if I am injecting a small amount, it is nothing compared to when I have the pre/integrated un-grounded.
What confuses me is how many things can be a source of hum. I have ruled out things like simple induction, like say, from a power transformer. The newer NAD stuff have pretty good PSU sections anyway, and I can tell you, in the case of the C160 and the C350, the PSU is well shielded and uses a toroidal transformer. The 2255 uses a normal kind, so does the tubed buffer, but the hum is there even when that is off. My DAC has a toroidal transformer but it hums just the same as one that I have with a normal kind. It all seems to point to how the DAC behaves with the PC. It appears like the PC, through the DAC, is causing some kind of loop through whatever you plug sources into, then back to the PC. I have the PC connected as well via sound card for movies and games, unhooking that does nothing.
So basically it is a loop that has nothing to do with my separate amp, and nothing to do with any other source it seems. It is like it's going through the s/pdif into the DAC and back out the line level and into the preamp section, separate or not. I don't know how to fix that.
OK, well I'm stumped. Try posting again in this forum. Surely there is someone here who has been through the same thing.
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