Cut-Throat Posted April 27, 2003 Share Posted April 27, 2003 OK, my Moondogs arrived and they are beautiful. Very nice Burled Walnut Frames! I only got to listen for them for 10 minutes, (and then I had to go on a fishing trip - someone has got to do it!) But when listening and the music stopped, I noticed a hiss, hum on my 104db Belles. Much more noticable than my 300b Billies (which is supposed to be a very quiet SET amp. ) Hum Pot was adjusted for minimium hum, but it was still noticible. Anyone done anything - mods, tubes, etc. to quiet these dogs down a bit? Thanks. Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted April 27, 2003 Share Posted April 27, 2003 Congrats, Cut T.! These guys will often make a little bit of noise when first turned on...and then quiet down when everything warms up. I also notice that my amplifiers are quieter in the evening than during daylight, which has more to do with electrical service factors than the amplifier. There is a host of things that can cuase hum, hiss, buzz, etc. in any amp. How obtrusive is it? Can you still hear it from the listening position? Does the hum pot do anything as it is turned from one side to the other? Is it only in one channel? Are your interconnects in good shape? If you have a preamp, try powering the amps up on their own. Often it's the preamp/amp interface that can mess things up. You may also have an open or poor ground connection inside somewhere, including the main chassis ground. You can take off the bottom plate to make sure that bolt is tight. BE VERY CAREFUL IN THERE, THOUGH! That big powersupply capacitor can hold a harmful charge for days. If your are not certain about what to do, take it to someone who does. Go to a supermarket or hardware store and get a cheap, three-to-two prong adapter. Put that on your preamp plug to see if it helps. Get two of those adapters, and put another one on one of the amps, as well. You may have a ground-loop problem, which this can help with. But for safety, keep one component of your system earth-grounded at the wall socket. Let us know how things go! Quote Link to comment Share on other sites More sharing options...
Randy Bey Posted April 27, 2003 Share Posted April 27, 2003 Don't Moondogs have a hum pot? Quote Link to comment Share on other sites More sharing options...
Cut-Throat Posted April 27, 2003 Author Share Posted April 27, 2003 Hi Erik and Randy, It is not a Ground Loop problem, I have had and solved those before. It does have a hum pot and I have adjusted this for minimum hum. It is only noticeable when music has stopped. It's not that bad, but it is worse than my Billies. But, obviously I want the hiss to be as little as possible. It is in both channels, exactly the same hiss level. Yes Erik, I have worked on tube amps before so I well aware of caps and their ability to store voltages. I always measure with my Multimeter before touching anything. My Interconnects are fine. My pre-amp does have some gain, so it would exacerbate the hiss problem, but I'm sure it's not causing it. I know I have read a number of posts here, where Moondog owners were trying to tame their hiss. Quote Link to comment Share on other sites More sharing options...
leok Posted April 27, 2003 Share Posted April 27, 2003 I'm using Moondogs to drive 102dB RF-7s .. and not a hint of hiss. I don't know where the hiss is coming from. If you short the inputs, is the hiss still there? If not, it's not the 'dogs. If so .. sheesh ??? Noisy 6SN7s? Hum is more complex. I've completely floated the internal audio gnd of my amps from the house ground. I use house gnd for safety only. If you're interested, I'll send a schematic that includes my grounding changes. Note that I didn't change the scheme because of a noise problem, I just wired the amp that way from the beginning. As far as the hum pots, they're pretty gross and an actual null can occur between wires on the wire-wound adjustment pot. Use of a smaller pot with surrounding fixed resistors would improve the "resolution" of this adjustment. As an engineer, however, since I know I can make it go away if I really want to, I don't worry about the small hum I have. I can talk, if you want, about that mod. leok Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted April 27, 2003 Share Posted April 27, 2003 Cut Throat: I have no idea of your electronics ability, so I advised about the storage charge as a matter of form. There was no doubt of your ability, or anything like that. Like Leo, my circuit ground is entirely floated. This has helped lots. Try doing that to see if it helps. My system is grounded at the preamp end, only. I don't have hiss problems, but a small amount of hum is pretty normal with SETs. Good luck! Erik Quote Link to comment Share on other sites More sharing options...
edster00 Posted April 27, 2003 Share Posted April 27, 2003 Cut Throat, I am pretty much electrically challenged. I did have some pretty significant hum when I first got my Moondogs but was able to eliminate nearly all of it by using cheater plugs on the power cords. After adding the cheater plugs and adjusting the hum pots I had to be very close to my K-horns to hear any hum with no music playing. After Erik tweaked the ground wiring on my Moondogs I now have to really put my ear next to the speakers to discern any hum. I have never had a hiss. Quote Link to comment Share on other sites More sharing options...
jazman Posted April 27, 2003 Share Posted April 27, 2003 Cut-Throat, Congratulations again on your Moondogs. Of course you already know I'm using 104db Belles with my Moondogs and I think you will enjoy this combination. What are you using as your preamp? Which rectifiers and output tubes are you using? Is the hiss/hum present if you don't turn up the volume? Klipsch out. Quote Link to comment Share on other sites More sharing options...
tubelion Posted April 27, 2003 Share Posted April 27, 2003 Hi, I am interested to learn how to mod the floating ground, schematic is appreciated. I do not have the skill cut throat has, but will try, I do have hizz if put he ear to speaker, but can't hear it even at 3 feets from Belle, is it OK? Thanks in advance Tubelion Quote Link to comment Share on other sites More sharing options...
leok Posted April 27, 2003 Share Posted April 27, 2003 tubelion, Without pretty good soldering skills, you probably don't want to make the mod. The note above about not using the ground pin on the power does almost the same thing except there is no safety gnd. Based on your description, it's not going to get any quieter than it is anyway. Mine makes as much noise as you describe. leok Quote Link to comment Share on other sites More sharing options...
tubelion Posted April 28, 2003 Share Posted April 28, 2003 Hi leok, Yes, I probably won't do it myself. But it will be great to learn something new and interesting. Apart from quiet, floating the ground and reduce the distortion from ground contamination. Here in Hong Kong, we live in the 16 floor building, the ground is noisy. Can you share your schematic? I can find someone to do the job for me. Thanks in advance Tubelion Quote Link to comment Share on other sites More sharing options...
leok Posted April 28, 2003 Share Posted April 28, 2003 tubelion, Sure. It will take me a day or two to get it prepared with the grounding information. leok Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted April 28, 2003 Share Posted April 28, 2003 To add briefly to Leo's post: Probably every house has its own oddities in terms of ground-related issues. The house we were in 5 years ago had very little; our new home has had lots of problems. I also added two extra copper-clad ground rods to the main house ground, and 'daisy-chained' them to the original with very heavy ground cable. But removing the circuit from the chassis ground on my amplifiers did improve the situation. Doing the same on the preamp, however, created more noise problems. YOu might also find the twin-axial cable might work well for interconnects rather than coax. Do you know which you have? You can build a pair of cables with twin-axial (two conductor plus shield)cable, and ground the shield at only one end. This way you get some shielding, minus some of the noise and other hash that can be picked up by conventional coaxial interconnects. Try some different tubes for that hiss problem...maybe that would help. Quote Link to comment Share on other sites More sharing options...
Randy Bey Posted April 28, 2003 Share Posted April 28, 2003 Kevin, I had hum problems with my 300B Laurels, I think that it's still there, and the high efficiency speakers exacerbate the problem. However, Ron W. always recommends removing the ground with a cheater plug to see if that helps or not. I even at one point in time removed the ground connection to the top plate (on Ron W.s advice). This helped but the biggest help was removing the antenna from the system. Then I got a ground isolation hoober for the coaxial antenna lead and it's at an acceptable level. I also corresponded with others who advised to essentially "grin and bear it" as the hum sound is inaudible when music is playing and I should learn to think of it as "music about to happen." Quote Link to comment Share on other sites More sharing options...
tubelion Posted April 28, 2003 Share Posted April 28, 2003 Hi, It seems to be a COMMON problem. So for 104db speaker we have, what would be the distance that we start to hear no hum? In my case, 3 feet. Just want to know what is considered accetable or normal to one, but not to others like cut-throat. I doubt there is anyone who doesn't have a hizz with the ear near the tweeter, am I right? Tubelion Quote Link to comment Share on other sites More sharing options...
Randy Bey Posted April 28, 2003 Share Posted April 28, 2003 Sze, ha ha, a pun. At my listening position I can barely hear a hum. It is more pronounced the closer I get to the speaker, hard to put an exact number on it. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted April 28, 2003 Share Posted April 28, 2003 Very few tube amps Moondogs or other wise don't have a slight hiss with the volume 100% and your ear to the speaker. I have had about 40% of the scotts dead silent. My 299B is one of them. My ST-70 is pretty much dead silent by itself with the inputs shorted so is my Scott 208 power amp. With either of my preamps they both have a slight hiss. Its tube circuits they all have some hiss. Hum I will not stand for it drives me nuts. But some amps hum and there is nothing you can do about it but minimize it ! Some HF-81's are this way and a few others even some Scotts I have worked on. It can usually be traced back to the things mentioned above or a pesky over stressed main power transformer in the vintage gear. Craig Quote Link to comment Share on other sites More sharing options...
leok Posted April 29, 2003 Share Posted April 29, 2003 My Moondogs are really pretty quiet. Be sure that the measurement is the Moondog only. To do this, insert a shorted RCA plug into the input. I suspect the hiss people are hearing is the preamp. I mentioned granularity with the hum pots, but they can get rid of mos of the hum. With the inputs shorted: I have to disconnect the speaker to one amp while I adjust the other. This way I can get the hum way down. I listen at very low levels and am not particularly tolerant of hum. I suspect, if you have excessive hum it's either a pot tweak or grounding. tubelion, until I get that info to you, you canachieve almost the same thing by disconnecting the ground on the plug as has been described above. The result is almost the same. leok Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted April 29, 2003 Share Posted April 29, 2003 On the inside of the Moondogs, you've got three things connected to chassis ground: 1. Safety ground on the IEC connector 2. Negative side of the 6SN7 DC heater supply 3. and B- and other circuit grounds. On my amps, the only ground I have on the chassis is the safety ground -- everything else is chassis grounded at the preamp end through the interconnects. Some schools of thought also separate the negative side of the input/driver heater supply from B-, but I honestly don't notice too much difference one way or the other. Through some experimentation with series resistance on the input, and battery-powered 6SN7s heater supply, I was able to reduce hum to the point where I had to try very hard to hear anything with my ear up to the speakers -- a slight hiss on the tweeter is normal for me. But the amp didn't sound open and clean anymore as far as how it played music. All that junk was taken off, I incorporated the Ultrapath output stage, and everything sounds very good now. Leo has taken this further and now uses parallel output on the Moondog -- a great idea, I think. But! Let me go check the schematic real quick to make sure I got all those grounds on the chassis -- I'm just doing it from memory...and that fails me once in awhile! Erik Quote Link to comment Share on other sites More sharing options...
leok Posted April 29, 2003 Share Posted April 29, 2003 1) In the Moondogs, there is a single wire that runs from the signal gnd to the gnd lug on the chassis toward the input side. Cut it and insert a 500KOhm to 1MOhm resistor. 2) Remove the connection between speaker output and gnd .. replace it with a similar resistor to the chassis gnd. Now there remains a safety gnd to the chassis, but the circuit is floating. Quote Link to comment Share on other sites More sharing options...
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