Since you are getting 60 Hz hum (as well as the usual 120 Hz), it seems like either your transformer is saturating OR you are getting voltages induced from 60 Hz via electromagnetic fields.
A _steel_ chassis is well-known to get measurable eddy-current-induced (magnetic-field-induced) 60 Hz voltage differentials on it, just from the 60 Hz in the air, not to mention your transformer etc. You should be able to actually see the induced voltages by connecting your scope's probe and probe ground lead to different points on the chassis (differential probing might work better). [The following is just a guess, based on the above, but, maybe it would help to make sure that your chassis earth ground is _between_ the portion of the chassis that's most-affected by your 60 Hz wiring and circuits and the portion where any other grounds connect to the chassis, so that the ground return currents share the chassis with the least-possible 60 Hz (or whatever arrangement minimizes the 60 Hz hum, in case I guessed wrong). However (see farther below), since it is steel, it would probably be better to just not use the chassis for ground returns, and especially not for any small-signal ground returns.]
A separate issue is that using the chassis as a ground is usually OK, except that care must be taken if it's used for large-current ground returns (e.g. speakers), since larger currents will induce larger voltages between points on the chassis, which might then significantly present themselves, somewhere else, by arithmetically adding into what should be quiet ground reference points that are also connected to the chassis, depending on their relative locations. So you might normally want to try to have at least the larger-current ground returns connected very near to the point to which they must return (i.e. probably power supply caps gnd). But, in your case, for that and other reasons (see below), I would not run ANY small-signal ground returns or the star ground return through the chassis.
So, I would not connect the small-signal star ground (or any small signal ground) directly to a steel chassis, at all, since not connecting them to the chassis should prevent the possibility of picking up any eddy-current-induced 60 Hz hum from the steel chassis. (Actually, since your chassis is steel, I'd probably end up using full star grounding [i.e. All ground returns directly back to power supply caps and not through chassis], just because it would be quicker than testing to see which grounds might be OK to run through the chassis.) [Actually, I might be tempted to switch to a non-ferrous (e.g. aluminum) chassis, just to see what difference it might make.]
I haven't been following this thread as closely as I wish I had, but, you DO have each signal input 'ground' connected ONLY to the ground reference point for the stage it feeds, right??! They don't go to the star ground, and especially not to the chassis. They are not really 'ground' at all. They are simply the reference voltages for the input signals.
Also, make sure that you aren't getting any capacitive coupling, by having any 60 Hz or 120 Hz conductors anywhere near any small-signal conductors. And make sure that all supply and return pairs (for both power and signal) are as close to each other as possible, tightly twisted together if possible. You want to minimize ALL current loops' enclosed areas, to minimize field-induced junk in both directions (i.e. both received and radiated). (See: 'Faraday's Law' and/or 'Maxwell's Equations'.)
If any 60Hz or 120Hz wire MUST cross another wire or conductor, they should be at right angles to each other, and as far from each other as possible. Wires connecting to PCBs should be at a right angle to the PCB, for at least a couple of inches. The same probably applies to any wire passing through the chassis, or connecting to it.
I don't know exactly where you located your star ground point. But I would probably want it to be connected directly to the midpoint of the ground of the filter caps, as close as possible to the caps, so that the smallest length of conductor carried shared ground return currents. (But you can easily experiment, with the star point's exact location, once all of the grounds are gathered into a star point, such as a bolt. Note that most people put the higher-current ground returns nearer to the bottom of the stack that connects on the bolt. i.e. Even that can make a noticable difference.) You might also want to experiment with using the disconnect netork only for the small-signal grounds, just before the main star point, and possibly even with having two disconnect networks.
I think your photo shows a star ground with a disconnect network to the earth ground via the chassis. If that's just a small-signal star point, which did not yet get back to the filter caps, that seems wrong. Remember that the disconnect network is really just a 'safety disconnect network'. You wouldn't need to connect ANY circuit grounds to the chassis, and normally would not want to, except for safety requirements. So the star ground at least needs to be connected to the filter caps' ground point, before there's a disconnect network to the earth ground. The disconnect network's resistor (and diodes) helps to keep the circuit ground's voltage slightly higher than any noise voltages in whatever the network is connected to, so that the noise voltages at your earth ground point are less tempted to cause currents to flow into your circuit ground. So you might also want to try using regular diodes, there (if you're now using Schottkys), and/or a larger resistance value. Most people use a (beefy) bridge rectifier, instead of two diodes, giving even more voltage difference. At least I THINK that's basically how it is supposed to work. [I could be wrong, but, now that I think about it, using a safety disconnect network, at all, seems to imply that ALL circuit 'grounds' should return to the filter caps' 'ground', before the disconnect network connects them to earth ground, which seems to imply that the chassis shouldn't be used for any circuit ground returns, if a disconnect network is used and the chassis is connected to earth ground.]
By the way: Have you scoped your filament-supply conductors? I did read that they are 'DC'. But is there any 60Hz or 120 Hz in them, at all? If so, at least check whether or not they might be coupling capacitively into any small signal conductors, by being too close to them. Are the filament supply and return paths as close as possible to each other? If they are wires, twist them tightly together wherever possible. Again, you want to minimize ALL current loops' enclosed areas, to minimize field-induced junk in both directions (i.e. both into and out of them).
Most of that, and more, is explained much better, here: http://www.geofex.com/Article_Folders/stargnd/stargnd.htm
A _steel_ chassis is well-known to get measurable eddy-current-induced (magnetic-field-induced) 60 Hz voltage differentials on it, just from the 60 Hz in the air, not to mention your transformer etc. You should be able to actually see the induced voltages by connecting your scope's probe and probe ground lead to different points on the chassis (differential probing might work better). [The following is just a guess, based on the above, but, maybe it would help to make sure that your chassis earth ground is _between_ the portion of the chassis that's most-affected by your 60 Hz wiring and circuits and the portion where any other grounds connect to the chassis, so that the ground return currents share the chassis with the least-possible 60 Hz (or whatever arrangement minimizes the 60 Hz hum, in case I guessed wrong). However (see farther below), since it is steel, it would probably be better to just not use the chassis for ground returns, and especially not for any small-signal ground returns.]
A separate issue is that using the chassis as a ground is usually OK, except that care must be taken if it's used for large-current ground returns (e.g. speakers), since larger currents will induce larger voltages between points on the chassis, which might then significantly present themselves, somewhere else, by arithmetically adding into what should be quiet ground reference points that are also connected to the chassis, depending on their relative locations. So you might normally want to try to have at least the larger-current ground returns connected very near to the point to which they must return (i.e. probably power supply caps gnd). But, in your case, for that and other reasons (see below), I would not run ANY small-signal ground returns or the star ground return through the chassis.
So, I would not connect the small-signal star ground (or any small signal ground) directly to a steel chassis, at all, since not connecting them to the chassis should prevent the possibility of picking up any eddy-current-induced 60 Hz hum from the steel chassis. (Actually, since your chassis is steel, I'd probably end up using full star grounding [i.e. All ground returns directly back to power supply caps and not through chassis], just because it would be quicker than testing to see which grounds might be OK to run through the chassis.) [Actually, I might be tempted to switch to a non-ferrous (e.g. aluminum) chassis, just to see what difference it might make.]
I haven't been following this thread as closely as I wish I had, but, you DO have each signal input 'ground' connected ONLY to the ground reference point for the stage it feeds, right??! They don't go to the star ground, and especially not to the chassis. They are not really 'ground' at all. They are simply the reference voltages for the input signals.
Also, make sure that you aren't getting any capacitive coupling, by having any 60 Hz or 120 Hz conductors anywhere near any small-signal conductors. And make sure that all supply and return pairs (for both power and signal) are as close to each other as possible, tightly twisted together if possible. You want to minimize ALL current loops' enclosed areas, to minimize field-induced junk in both directions (i.e. both received and radiated). (See: 'Faraday's Law' and/or 'Maxwell's Equations'.)
If any 60Hz or 120Hz wire MUST cross another wire or conductor, they should be at right angles to each other, and as far from each other as possible. Wires connecting to PCBs should be at a right angle to the PCB, for at least a couple of inches. The same probably applies to any wire passing through the chassis, or connecting to it.
I don't know exactly where you located your star ground point. But I would probably want it to be connected directly to the midpoint of the ground of the filter caps, as close as possible to the caps, so that the smallest length of conductor carried shared ground return currents. (But you can easily experiment, with the star point's exact location, once all of the grounds are gathered into a star point, such as a bolt. Note that most people put the higher-current ground returns nearer to the bottom of the stack that connects on the bolt. i.e. Even that can make a noticable difference.) You might also want to experiment with using the disconnect netork only for the small-signal grounds, just before the main star point, and possibly even with having two disconnect networks.
I think your photo shows a star ground with a disconnect network to the earth ground via the chassis. If that's just a small-signal star point, which did not yet get back to the filter caps, that seems wrong. Remember that the disconnect network is really just a 'safety disconnect network'. You wouldn't need to connect ANY circuit grounds to the chassis, and normally would not want to, except for safety requirements. So the star ground at least needs to be connected to the filter caps' ground point, before there's a disconnect network to the earth ground. The disconnect network's resistor (and diodes) helps to keep the circuit ground's voltage slightly higher than any noise voltages in whatever the network is connected to, so that the noise voltages at your earth ground point are less tempted to cause currents to flow into your circuit ground. So you might also want to try using regular diodes, there (if you're now using Schottkys), and/or a larger resistance value. Most people use a (beefy) bridge rectifier, instead of two diodes, giving even more voltage difference. At least I THINK that's basically how it is supposed to work. [I could be wrong, but, now that I think about it, using a safety disconnect network, at all, seems to imply that ALL circuit 'grounds' should return to the filter caps' 'ground', before the disconnect network connects them to earth ground, which seems to imply that the chassis shouldn't be used for any circuit ground returns, if a disconnect network is used and the chassis is connected to earth ground.]
By the way: Have you scoped your filament-supply conductors? I did read that they are 'DC'. But is there any 60Hz or 120 Hz in them, at all? If so, at least check whether or not they might be coupling capacitively into any small signal conductors, by being too close to them. Are the filament supply and return paths as close as possible to each other? If they are wires, twist them tightly together wherever possible. Again, you want to minimize ALL current loops' enclosed areas, to minimize field-induced junk in both directions (i.e. both into and out of them).
Most of that, and more, is explained much better, here: http://www.geofex.com/Article_Folders/stargnd/stargnd.htm
Mark, Tom
thanks for the replies. In order...
My scope is a software FFT and oscilloscope used to measure low voltage mic room resposnse. I used it to verify both 60 and 120 Hz signals coming from the preamp. I will use it to work backwards from output but I don't think it will work on high voltage.
I am still limited by the questionable circuits in my home, I have not yet checked the preamp at a reliable location. I am working at a new house, the temporary power for which comes straight from the power line with a buried ground at the box. I don't know why it hasn't occurred to me yet but this is probably an ideal testing place. Tomorrow.
I have tried several combinations with and without the R/C/diode filter, attached to the chassis and not. (I have tried connecting PCB/output/high voltage ground together but isolated from the chassis/outlet ground connection.) I took everything out of the chassis. Hum level remains constant. (Of course if I let either the output grounds or PCB ground float the hum goes crazy.)
Tom,
I'm not sure if this addresses the magnetic field point but I have tested the preamp out of the chassis with no improvement. Also, one of the things I have done to mitigate eddy currents is to mount the transformer in a remote box connected via a braided umbilical.
So far the combination that produces the least hum is with all grounds connected to the chassis. Right now the PCB ground leaves the board from the ground track immediately next to the output leads. I will try moving it to the ground track inbetween the filter caps. This will mean that the star ground will probably have to move across to that side too, right?
The input grounds go first to the volume pot ground and then to the PCB ground track immediately next to where each's respective input connects.
As far as "current loops' enclosed areas," I am somewhat limited by the design of the PCB. Please take a look at the photos here and let me know how I can improve my technique:
http://home.comcast.net/~garyworld/site/
Since removing the tranny the preamp no longer contains the switch, fuse, or AC in.
If I understand you correctly I should connect output ground and PCB ground together at the filter caps and then run the filter network to tranny/outlet ground all isolated from the chassis. I'll try it and report back.
Thanks,
gary
thanks for the replies. In order...
My scope is a software FFT and oscilloscope used to measure low voltage mic room resposnse. I used it to verify both 60 and 120 Hz signals coming from the preamp. I will use it to work backwards from output but I don't think it will work on high voltage.
I am still limited by the questionable circuits in my home, I have not yet checked the preamp at a reliable location. I am working at a new house, the temporary power for which comes straight from the power line with a buried ground at the box. I don't know why it hasn't occurred to me yet but this is probably an ideal testing place. Tomorrow.
I have tried several combinations with and without the R/C/diode filter, attached to the chassis and not. (I have tried connecting PCB/output/high voltage ground together but isolated from the chassis/outlet ground connection.) I took everything out of the chassis. Hum level remains constant. (Of course if I let either the output grounds or PCB ground float the hum goes crazy.)
Tom,
I'm not sure if this addresses the magnetic field point but I have tested the preamp out of the chassis with no improvement. Also, one of the things I have done to mitigate eddy currents is to mount the transformer in a remote box connected via a braided umbilical.
So far the combination that produces the least hum is with all grounds connected to the chassis. Right now the PCB ground leaves the board from the ground track immediately next to the output leads. I will try moving it to the ground track inbetween the filter caps. This will mean that the star ground will probably have to move across to that side too, right?
The input grounds go first to the volume pot ground and then to the PCB ground track immediately next to where each's respective input connects.
As far as "current loops' enclosed areas," I am somewhat limited by the design of the PCB. Please take a look at the photos here and let me know how I can improve my technique:
http://home.comcast.net/~garyworld/site/
Since removing the tranny the preamp no longer contains the switch, fuse, or AC in.
If I understand you correctly I should connect output ground and PCB ground together at the filter caps and then run the filter network to tranny/outlet ground all isolated from the chassis. I'll try it and report back.
Thanks,
gary
gary h said:
RATHER than listening to me blathering-on, I really wish you would say that you have read the following page, in its entirety:
http://www.geofex.com/Article_Folders/stargnd/stargnd.htm
But first:
When you say "ground track", above, what does that mean, exactly? i.e. Where does it come from and where does it go? What all is connected to it? How long is it? [Maybe it would be better if you could tell me where to look for it, in one or more of your photos, and on your schematics.]
And what does the "volume pot ground" connect to? And where? Can you try running it with the pot not mounted to the chassis? If you need to have a ground for it, clip a temporary wire from it to where the input reference "ground" terminates on the board. (There might be one simple thing like that that is causing the entire problem.)
Hmmmm.....(pun intended)..... The input signal reference "ground" connects to two different grounds. I wonder if they have two separate paths back to the power supply caps. Ground loop. I wonder if one goes through the chassis. Worse ground loop.
-----
I am downloading your photos, etc, again. But I have only a 56k modem connection. So I'm still waiting. THIS time, I'll save them to my hard drive.
Can you try looking, with your PC scope, to see if there is any 60Hz or 120Hz across any two of the pot terminals, OR across the input resistor near where the input and input ground are finally connected? Do you have an actual scope probe, for that thing? What is its input impedance, at the input connector?
[Aside: For seeing somewhat-higher-voltage signals, at low (i.e. audio) frequencies, you might be able to use a simple resistive divider to get, say, a 10X "probe". Using some very large resitance, R, you could have 9R between "probe tip" (which would just be the end of the 9R string) and your measurment pick-off point (i.e. the signal wire that goes to your scope input), and 1R between there and the scope input cable's ground. Then your scope input would only see 1/10th of the voltage at the "probe tip". I'd probably try at least 1Meg for "R", there. Put the resistor string (and probably the rest of it) inside of a piece of plastic tubing, or anything that will insulate it. Maybe a BIC pen's plastic shell would work.]
The amplitude of the 60Hz or 120Hz, at the input resistor or at the pot, might be very small. But see if you can detect anything and estimate its amplitude. If it IS there, in either of those places, then their grounds are probably at fault, and might be a significant part of the cause (or the ENTIRE cause) of your problem. We probably can't get that lucky. But you never know.
OK I can see SOME of the photos, again. But many of them didn't load, for me. It looks like THIS is a better starting point, for getting your photos:
http://home.comcast.net/~garyworld/site/?/photos/
OK. HERE's one for you: Have you tried using 90% Isopropyl Alcohol and a stiff toothbrush to clean all of the solder flux residue etc off of the board? It looks like a whole lot of residue is present where a row of wires is soldered to the edge of the board, in one of your photos. Some of that stuff is slightly conductive! I'd try to eliminate all of the 'obvious' possible problems, like that, before looking for more-subtle problems.
In that same vein, have you done a thorough inspection of the PCB, by eye, using a good magnifying glass or better?
Have you reworked the solder joints? (You probably need to add just a little more flux, or solder with flux, to each joint. Be careful not to overheat any components.)
Also, as Hailteflon has mentioned several times, it's possible that some of the components that you have not installed are needed. Can you contact the manufacturer and have them find out if that could be causing your hum?
I'll try to get a better handle on things after I've finally downloaded all of your photos, and get back to you.
In the meantime, here's a nice summary that probably already contains the solution to your problem:
http://www.geofex.com/Article_Folders/stargnd/stargnd.htm
;-)
P.S. Where is C106, on your PCB Layout? It's not really only 10 uF, is it??
ALSO, since you are using an umbilical for power, you should probably have SOME amount of decoupling/bypass capacitance, where the umbilicals enter the PCB, probably at least a couple hundred uF in parallel with 0.1uF, across each input voltage/ground wire terminal pair.
P.P.S. HAVE YOU SCOPED YOUR "DC" FILAMENT SUPPLY, YET???
Gary, I have probably asked you this before, but the thread is so long that a backtrack is a long haul.
(1) Do you get hum on your scope at the preamp outputs with the PT unplugged and the preamp not connected to the amp?
I ask this because I don’t recall if you have ever connected an auxiliary wire from the preamp ground to the amp ground. Is the input of the amp grounded to the amp chassis? If so then connect the aux. wire to the amp chassis.
I recall that you used known good cables with good shields, but not sure. This issue has probably already been covered, but I wanted to be sure that your cables have shields. I bought some “RCA-Thompson” audio cables at a dollar store and they had absolutely no shielding, only a single small strand for the ground. I consequently started opening various cables and the quality of the shielding varies greatly.
Assuming that your cables are not the problem I have been wondering if the electrolytics could have anything to do with it. I keep coming up with a NO, but the hum has to be caused by something.
What I am getting at is that the dielectric absorption of an electrolytic can generate a considerable voltage for days. In this case they are still connected to the circuit and the electrons should continually drain to the circuit ground.
Anyhow, I keep wondering if there is some way that the energy in some of the electrolytics is not draining to zero and providing a voltage that somehow generates hum.
This was one of my concerns about the missing components. Possibly it provides the opportunity for the voltage in the electrolytics to remain for generating hum.
Just a wild longshot, but there is an answer to all of this.
Do you get any voltage readings (DC or AC) at any of the electrolytics after it has been turned off?
I know that you have a DC power supply circuit for the filaments. The ground for it should be connected to the main ground.
Keep digging, the answer is in there somewhere. Probably best to wait for the new caps.
I just noticed Gootee's new post, and I haven't read it completely. The flux could be a problem, but my experience has been that it is old flux that is often a problem. It changes after about 15-20 years and then can cause huge problems. It is always good practice to at least take the tip of a pocket knife and gently scratch a rut through the flux anyway. Mark
(1) Do you get hum on your scope at the preamp outputs with the PT unplugged and the preamp not connected to the amp?
I ask this because I don’t recall if you have ever connected an auxiliary wire from the preamp ground to the amp ground. Is the input of the amp grounded to the amp chassis? If so then connect the aux. wire to the amp chassis.
I recall that you used known good cables with good shields, but not sure. This issue has probably already been covered, but I wanted to be sure that your cables have shields. I bought some “RCA-Thompson” audio cables at a dollar store and they had absolutely no shielding, only a single small strand for the ground. I consequently started opening various cables and the quality of the shielding varies greatly.
Assuming that your cables are not the problem I have been wondering if the electrolytics could have anything to do with it. I keep coming up with a NO, but the hum has to be caused by something.
What I am getting at is that the dielectric absorption of an electrolytic can generate a considerable voltage for days. In this case they are still connected to the circuit and the electrons should continually drain to the circuit ground.
Anyhow, I keep wondering if there is some way that the energy in some of the electrolytics is not draining to zero and providing a voltage that somehow generates hum.
This was one of my concerns about the missing components. Possibly it provides the opportunity for the voltage in the electrolytics to remain for generating hum.
Just a wild longshot, but there is an answer to all of this.
Do you get any voltage readings (DC or AC) at any of the electrolytics after it has been turned off?
I know that you have a DC power supply circuit for the filaments. The ground for it should be connected to the main ground.
Keep digging, the answer is in there somewhere. Probably best to wait for the new caps.
I just noticed Gootee's new post, and I haven't read it completely. The flux could be a problem, but my experience has been that it is old flux that is often a problem. It changes after about 15-20 years and then can cause huge problems. It is always good practice to at least take the tip of a pocket knife and gently scratch a rut through the flux anyway. Mark
I must admit to not reading the entire thread, but I did have a look at the photos. I had a similar hum problem with a set of chip amps I built, particularly when connected to an active crossover I built. For me the solution was tying all of the signal grounds at the RCA inputs and outputs on the active crossover and on all of the RCA inputs of my four channel chipamp. The input/output RCAs were all neatly in a line, so I aligned ALL of the earth tags (L, R, input, output) and soldered a very thick copper wire to them all and then took this single ground to the active crossover board. I did the same thing in the chassis of the four channel amp. This completely solved the problem. Looking at your setup it looks like you could do a similar thing.
Too busy to take things apart to photograph at the moment, but if you are interested I can do it in the next day or so...
Hope this is of some assistance,
Chris
Too busy to take things apart to photograph at the moment, but if you are interested I can do it in the next day or so...
Hope this is of some assistance,
Chris
Do you know of anyone else with this preamp, who doesn't have a hum problem?
That ground trace that encircles the whole pcb looks like it could make a pretty nice 'loop antenna'. It's really almost the exact opposite of star grounding, too. I'd be VERY tempted to try to figure out at least one place where I could cut though it, so it wouldn't be a complete loop. Maybe you could cut it right next to the ground wire, and have two ground wires instead, going back to the filter caps. Maybe not. That would be an even bigger loop. Maybe do that, but also cut it at the far end, and jumper whatever needs it. I'd have to look at the schematic again, but I have to sleep. Is there any good reason for it to be a loop? Does it make something 'cancel out', field-wise, maybe? I kind of doubt it.
Can you measure or see any 60Hz or 120Hz between the ground points in the center of the pcb (that eventually make their way all the way around one side or the other of the loop) and the point where the ground wire leaves the pcb, OR, for completeness' sake, between the two ends of that wire, OR, between the far end of the ground 'loop' trace (or the inner grounds that connect to it, that are actually farther from the wire's connect-point), i.e. farthest from the ground wire, and the non-PCB end of the ground wire? Actually, an important one might be between where that ground wire connects to the board and the filter caps' ground. Even more important might be between the signal input ground and the filter caps' ground.
The idea there is that 60Hz and/or 120Hz current, returning from somewhere and going back to the filter caps, creates a voltage drop all along the ground conductors. If your input ground shares the same conductors, then it will not be at 0v, but, instead, will be changing at 60Hz and/or 120Hz, which has the same effect as applying a 60Hz or 120Hz signal to your input.
That's the main idea behind star grounding, of course. Your umbilical, with its one long ground return wire, apparently makes it even more important, and... more of a pain. It seems like you actually would need all of the separated star grounds to run all the way back, with the umbilical. However, if you add bypass caps where the power rails enter the PCB, maybe you won't need that. But I don't really know. (Too tired to think, right now.)
By the way, for the input and pot ground return to the power supply caps' ground, I would use a whole extra wire, just for them, in parallel with your umbilical. I would ALSO cut all other grounds that connect to them, and jumper around them if that's needed for the other grounds to stay connected.
Where does that PCB ground wire go, anyway? Does it go to the power supply filter caps or to the chassis and then to the filter caps, or to chassis then earth ground then filter caps? I would probably want it to go to the caps first, then directly to your earth ground (which is at the transformer?), probably with a safety-disconnect network. The chassis would then have its own separate connection to the earth ground, and wouldn't be used by any circuit ground returns.
Sorry if I'm repeating myself. G'night.
That ground trace that encircles the whole pcb looks like it could make a pretty nice 'loop antenna'. It's really almost the exact opposite of star grounding, too. I'd be VERY tempted to try to figure out at least one place where I could cut though it, so it wouldn't be a complete loop. Maybe you could cut it right next to the ground wire, and have two ground wires instead, going back to the filter caps. Maybe not. That would be an even bigger loop. Maybe do that, but also cut it at the far end, and jumper whatever needs it. I'd have to look at the schematic again, but I have to sleep. Is there any good reason for it to be a loop? Does it make something 'cancel out', field-wise, maybe? I kind of doubt it.
Can you measure or see any 60Hz or 120Hz between the ground points in the center of the pcb (that eventually make their way all the way around one side or the other of the loop) and the point where the ground wire leaves the pcb, OR, for completeness' sake, between the two ends of that wire, OR, between the far end of the ground 'loop' trace (or the inner grounds that connect to it, that are actually farther from the wire's connect-point), i.e. farthest from the ground wire, and the non-PCB end of the ground wire? Actually, an important one might be between where that ground wire connects to the board and the filter caps' ground. Even more important might be between the signal input ground and the filter caps' ground.
The idea there is that 60Hz and/or 120Hz current, returning from somewhere and going back to the filter caps, creates a voltage drop all along the ground conductors. If your input ground shares the same conductors, then it will not be at 0v, but, instead, will be changing at 60Hz and/or 120Hz, which has the same effect as applying a 60Hz or 120Hz signal to your input.
That's the main idea behind star grounding, of course. Your umbilical, with its one long ground return wire, apparently makes it even more important, and... more of a pain. It seems like you actually would need all of the separated star grounds to run all the way back, with the umbilical. However, if you add bypass caps where the power rails enter the PCB, maybe you won't need that. But I don't really know. (Too tired to think, right now.)
By the way, for the input and pot ground return to the power supply caps' ground, I would use a whole extra wire, just for them, in parallel with your umbilical. I would ALSO cut all other grounds that connect to them, and jumper around them if that's needed for the other grounds to stay connected.
Where does that PCB ground wire go, anyway? Does it go to the power supply filter caps or to the chassis and then to the filter caps, or to chassis then earth ground then filter caps? I would probably want it to go to the caps first, then directly to your earth ground (which is at the transformer?), probably with a safety-disconnect network. The chassis would then have its own separate connection to the earth ground, and wouldn't be used by any circuit ground returns.
Sorry if I'm repeating myself. G'night.
Gootee, I haven't read what you have posted yet....astigmatism slows me down.
You are bringing up issues that have probably already been covered. No criticsm intended here and I know you are writing essentially for Gary, but conventional attempts to cure this problem have all failed.
This circuit hums with the transformer disconnected. He has taken the PCB out of the chassis. The only thing that stopped the hum was bypassing the two large black Solen caps at the output. This is why he has ordered new ones and is waiting for their arrival.
This is a commercial setup for a Dynaco steel chassis that evidently has no known design flaws, but Gary may be the only one that didn't put all of the parts in the board. He sent it back to them and they couldn't fix it.
I did just notice that there is no obvious way for the B+ to bleed down after turnoff. Looking at the schematic Gary posted. It is only part of the circuit, the RIAA section is omitted. He said he will post the whole thing eventually.
The plate seems to stay energized after turn-off. Possibly there is a bleeder resistor in the RIAA section that is not on the board.
You are bringing up issues that have probably already been covered. No criticsm intended here and I know you are writing essentially for Gary, but conventional attempts to cure this problem have all failed.
This circuit hums with the transformer disconnected. He has taken the PCB out of the chassis. The only thing that stopped the hum was bypassing the two large black Solen caps at the output. This is why he has ordered new ones and is waiting for their arrival.
This is a commercial setup for a Dynaco steel chassis that evidently has no known design flaws, but Gary may be the only one that didn't put all of the parts in the board. He sent it back to them and they couldn't fix it.
I did just notice that there is no obvious way for the B+ to bleed down after turnoff. Looking at the schematic Gary posted. It is only part of the circuit, the RIAA section is omitted. He said he will post the whole thing eventually.
The plate seems to stay energized after turn-off. Possibly there is a bleeder resistor in the RIAA section that is not on the board.
hailteflon said:
Even worse, I have been forgetting that it hums even when powered off! Sorry!
Gary, you can forget most of what I've said so far (as far as this particular problem is concerned, anyway).
I agree that he should try to see how long the thing stays energized. I would also want to try temporarily adding a resistor in parallel with each set of filter caps, if the circuit can tolerate that. A few kOhms to a few tens of kOhms should be OK, if the rest of the circuit doesn't mind. Otherwise, discharge the caps with a resistor, after powering it off, as a test.
I will re-read the thread.
Tom, Gary should have his new caps in a short while. Then we will have something to go on.
Don't give up. The original configuration of the ground has been altered through the troubleshooting process.
When the cause of the hum is found Gary may need some advice about how to get the ground right again. I have built SS with an aluminum chassis, but have no real experieince with high-voltage and a steel chassis.
Bleeding the caps down will make it quiet when off, but probably won't change much in the ON state.
There is no high current because this is only a preamp, but the steel chassis needs special consideration.
There is a link to tubes4audio.com (or something similar) that shows some pictures of the Dyanco chassis used with the PCB. Mark
Don't give up. The original configuration of the ground has been altered through the troubleshooting process.
When the cause of the hum is found Gary may need some advice about how to get the ground right again. I have built SS with an aluminum chassis, but have no real experieince with high-voltage and a steel chassis.
Bleeding the caps down will make it quiet when off, but probably won't change much in the ON state.
There is no high current because this is only a preamp, but the steel chassis needs special consideration.
There is a link to tubes4audio.com (or something similar) that shows some pictures of the Dyanco chassis used with the PCB. Mark
Tom, I don't consider your posts errors. I plan to read them when I get time.
The threads get so long that it is impractical to totally catch up before posting and I understand this.
You appear to have a good understanding of ground problems and I hope that you will hang around until Gary gets back.
A steel chassis, as an isolated concept, should dissipate the eddy currents as heat, but in reality the currents may couple to the circuit and extract energy such as to become stronger.
I did save the article that you posted on star grounds.
No need to apologize, you appear to have sufficient understanding to help Gary get rid of this hum. I hope to build some tube equipment in the future and this sort of understanding is quite valuable. Mark
The threads get so long that it is impractical to totally catch up before posting and I understand this.
You appear to have a good understanding of ground problems and I hope that you will hang around until Gary gets back.
A steel chassis, as an isolated concept, should dissipate the eddy currents as heat, but in reality the currents may couple to the circuit and extract energy such as to become stronger.
I did save the article that you posted on star grounds.
No need to apologize, you appear to have sufficient understanding to help Gary get rid of this hum. I hope to build some tube equipment in the future and this sort of understanding is quite valuable. Mark
But I will certainly try to be much more careful, from now on.Gentlemen,
all I can say is that I am thrilled to be getting so much help. I was ready to give up long ago. Now I feel like I am in some strange mystery plot that must be solved before I go insane. (Insaner that is.) I am starting to understand how so many of the members here might be aptly described as...obsessed...
I brought my test set up to the job site where I work today, plugged everything in right off the pole, had the electician confirm that I was plugged in feet from the earth ground. Guess what...hum. So at least now I feel that I can cross my home wiring off the short list of culprits. (Perhaps it will languish in mediocrity somewhere at the middle of the list.)
Tom, it is actually refreshing for me to go over your ideas, even though Mark, and others, have introduced them in one way or another. It helps remind myself of what we've done and why. I wouldn't expect anyone to go through the whole thread. I like the idea of scratching the ground track (I see you found the layout diagram here:http://home.comcast.net/~garyworld/pcblayout.jpg) so that it is not a loop. This couldn't hurt things right? My understanding is that this design allows all the components to easily return to ground. It was originally delivered with aluminum standoffs so the whole chassis was looping to the PCB in 7 spots.
Mark, I'll scan and post the entire layout diagram including the phono section and the phono schematic presently. Sorry I forgot about that. As much as I am chomping at the bit to slay this dragon, tonight I must tend to the SAR. My wife is starting to see the preamp as competition. I fear that one day I will come home and find a kitchen knife buried in it. (I hope she knows to use one with a plastic handle.)
I will get the scope running and post some measurements tomorrow.
Chris, I would love to see a pic or diagram of your ground scheme should you find time.
Thanks again, have a great weekend
gary
all I can say is that I am thrilled to be getting so much help. I was ready to give up long ago. Now I feel like I am in some strange mystery plot that must be solved before I go insane. (Insaner that is.) I am starting to understand how so many of the members here might be aptly described as...obsessed...
I brought my test set up to the job site where I work today, plugged everything in right off the pole, had the electician confirm that I was plugged in feet from the earth ground. Guess what...hum. So at least now I feel that I can cross my home wiring off the short list of culprits. (Perhaps it will languish in mediocrity somewhere at the middle of the list.)
Tom, it is actually refreshing for me to go over your ideas, even though Mark, and others, have introduced them in one way or another. It helps remind myself of what we've done and why. I wouldn't expect anyone to go through the whole thread. I like the idea of scratching the ground track (I see you found the layout diagram here:http://home.comcast.net/~garyworld/pcblayout.jpg) so that it is not a loop. This couldn't hurt things right? My understanding is that this design allows all the components to easily return to ground. It was originally delivered with aluminum standoffs so the whole chassis was looping to the PCB in 7 spots.
Mark, I'll scan and post the entire layout diagram including the phono section and the phono schematic presently. Sorry I forgot about that. As much as I am chomping at the bit to slay this dragon, tonight I must tend to the SAR. My wife is starting to see the preamp as competition. I fear that one day I will come home and find a kitchen knife buried in it. (I hope she knows to use one with a plastic handle.)
I will get the scope running and post some measurements tomorrow.
Chris, I would love to see a pic or diagram of your ground scheme should you find time.
Thanks again, have a great weekend
gary
Gary and Mark,
Here is a thread that contains some posts that sound like they *might* be relevant to the problem at hand:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=112359
I haven't studied it in enough detail to comment. But I am talking mostly about posts 13 through 17, or so, although your should probably read the entire (short) thread, from the beginning.
Here is a thread that contains some posts that sound like they *might* be relevant to the problem at hand:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=112359
I haven't studied it in enough detail to comment. But I am talking mostly about posts 13 through 17, or so, although your should probably read the entire (short) thread, from the beginning.
Here is a shot of the inputs to my active crossover. Not one of my neater projects. Built it in an MDF box when I was experimenting with it and have never bothered to build a better enclosure. Input RCAs are the two on the very right, the others are the outputs. Left most pair of outputs are unused at the moment. Setup in my multi-channel chip amp is the same. This fixed the hum problem I was having between pre-amp, active crossover and amps. Hope this is of some assistance...
Chris
Chris
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Here are three links that can also be found at the bottom of http://home.comcast.net/~garyworld/site/?/photos/
Mark, hopefully these will answer some questions about the phono section. I can provide a parts list to identify the components if necessary:
http://home.comcast.net/~garyworld/pcblayoutphono.jpg
http://home.comcast.net/~garyworld/phonoschematic.jpg
Here's a sketch I made to illustrate the current ground scheme. Please suggest alternatives, if we have already tried them it won't hurt to try them again, after all I have ring connectors on the end of all of these grounds so they are easy to switch:
http://home.comcast.net/~garyworld/groundscheme.jpg
Judging by Chris's post I should open up my amp to check it's grounding. The SS Adcom I use that loves to amplify the preamp's hum has a 3-prong plug. My test amp (that also loves to amplify the preamp's hum) has 2 poles. Is it recommended that I fit it with an earth ground connection? Reading through the recommended reading recently posted can I assume that it is double insulated? Please remember that I have used three amps to reproduce the hum. The other is a Harmon Kardon integrated with a 2 pole plug. None of these preamp/amps hum together or with other components/sources attached. By the way, everything is now plugged into a new isolation transformer that claims to remove AC line noise. Although I can't hear any difference this is one more potential variable that can be benched.
I am going over the last string of posts and will address the points as I generate measurements.
cheers,
gary
Mark, hopefully these will answer some questions about the phono section. I can provide a parts list to identify the components if necessary:
http://home.comcast.net/~garyworld/pcblayoutphono.jpg
http://home.comcast.net/~garyworld/phonoschematic.jpg
Here's a sketch I made to illustrate the current ground scheme. Please suggest alternatives, if we have already tried them it won't hurt to try them again, after all I have ring connectors on the end of all of these grounds so they are easy to switch:
http://home.comcast.net/~garyworld/groundscheme.jpg
Judging by Chris's post I should open up my amp to check it's grounding. The SS Adcom I use that loves to amplify the preamp's hum has a 3-prong plug. My test amp (that also loves to amplify the preamp's hum) has 2 poles. Is it recommended that I fit it with an earth ground connection? Reading through the recommended reading recently posted can I assume that it is double insulated? Please remember that I have used three amps to reproduce the hum. The other is a Harmon Kardon integrated with a 2 pole plug. None of these preamp/amps hum together or with other components/sources attached. By the way, everything is now plugged into a new isolation transformer that claims to remove AC line noise. Although I can't hear any difference this is one more potential variable that can be benched.
I am going over the last string of posts and will address the points as I generate measurements.
cheers,
gary
ok,
I have some scope measurements. I will still get to all the other questions but while this is fresh in my mind here goes:
http://home.comcast.net/~garyworld/preunplugged.jpg
This is taken from the preamp out, not plugged in, not connected to the amp. I see a 60Hz with 120Hz spikes and a lot of junk. It is hard to see but amplitude peak is about 20mV.
http://home.comcast.net/~garyworld/preplugged.jpg
This is what I get when I plug the preamp in but it is turned off. It looks to me like there is nothing there.
http://home.comcast.net/~garyworld/bothpreunplugged.jpg
This is measured at the amp speaker out. The preamp is connected, plugged in but not turned on. We see a handsome 60.08 Hz sine wave. The peak voltage is now 61mV. This would be the hum at it's lowest.
http://home.comcast.net/~garyworld/amponly.jpg
This is a measurement at the amp speaker out with the preamp NOT connected. This is clearly both 60 and 120 Hz waves, peaking at 140mV. It looks terrible, right? The interesting thing is that I cannot hear this, it doesn't manifest itself as hum and I cannot feel the speaker move. (?)
http://home.comcast.net/~garyworld/preampon.jpg
This is what I get at the preamp out, not connected to the amp, powered on. The blue cursors show a 119Hz period. I don't know how to interpret this but it looks bad. This is at 300mV. And when I plug it in to the amp...
http://home.comcast.net/~garyworld/bothon.jpg
Voila, a veritable city of hum. This is measured at the amp speaker out, the cursors show a 120 Hz period at 340mV.
I realize that the voltage measurements are probably relative, I made these measurements with a simple aligator clip running into my computer's audio in and to this nifty program called SignalScope. I bought it for measuring speakers in rooms. I would recommend it (around $60 I think.)
What do you guys think? I'll get back soon with answers to the other questions.
Hope you had a nice weekend. It rained here in San Francisco.
gary
I have some scope measurements. I will still get to all the other questions but while this is fresh in my mind here goes:
http://home.comcast.net/~garyworld/preunplugged.jpg
This is taken from the preamp out, not plugged in, not connected to the amp. I see a 60Hz with 120Hz spikes and a lot of junk. It is hard to see but amplitude peak is about 20mV.
http://home.comcast.net/~garyworld/preplugged.jpg
This is what I get when I plug the preamp in but it is turned off. It looks to me like there is nothing there.
http://home.comcast.net/~garyworld/bothpreunplugged.jpg
This is measured at the amp speaker out. The preamp is connected, plugged in but not turned on. We see a handsome 60.08 Hz sine wave. The peak voltage is now 61mV. This would be the hum at it's lowest.
http://home.comcast.net/~garyworld/amponly.jpg
This is a measurement at the amp speaker out with the preamp NOT connected. This is clearly both 60 and 120 Hz waves, peaking at 140mV. It looks terrible, right? The interesting thing is that I cannot hear this, it doesn't manifest itself as hum and I cannot feel the speaker move. (?)
http://home.comcast.net/~garyworld/preampon.jpg
This is what I get at the preamp out, not connected to the amp, powered on. The blue cursors show a 119Hz period. I don't know how to interpret this but it looks bad. This is at 300mV. And when I plug it in to the amp...
http://home.comcast.net/~garyworld/bothon.jpg
Voila, a veritable city of hum. This is measured at the amp speaker out, the cursors show a 120 Hz period at 340mV.
I realize that the voltage measurements are probably relative, I made these measurements with a simple aligator clip running into my computer's audio in and to this nifty program called SignalScope. I bought it for measuring speakers in rooms. I would recommend it (around $60 I think.)
What do you guys think? I'll get back soon with answers to the other questions.
Hope you had a nice weekend. It rained here in San Francisco.
gary
Hi Gary,
First I want to mention that it might be a good idea to at least make a shielded-cable 'probe', just so you don't pick up as much from fields in the air.
You can probably find out if you need to do that or not, by connecting a small resistor to you alligator leads and seeing what you get on the scope with the equipment on and the leads in different locations. You should probably also try a larger-value resistor, and with and without your hands on the leads.
(At least I THINK that that type of testing will give you an idea of whether or not there's a possible problem with using plain alligator leads. At any rate, trying a shielded cable is so cheap and easy that it's probably worth doing. Then you could do the same tests and see if there's a difference, at least.)
I don't know what your soundcard's input impedance might be. But, for these frequencies, you might be able to just modify a short piece of cheap 50 Ohm or 75 Ohm coax, or some other type of shielded cable. On one end, fit a plug that will work with your soundcard's input jack, making sure that the cable's shield is tied to the jack's ground. It might be easiest to use coax that has a braided-wire shield instead of foil, if you use coax (or a cable with a foil shield but also a 'drain' wire). On the 'probe' end, you can solder an alligator clip to the shield (or drain wire), after removing and inch or more of it so it's not too close to the tip, with maybe just a few inches of plain wire between the shield and the alligator clip. With coax, you can just bare about 1/4-inch of the center conductor, to make a 'probe tip'.
If that is all wrong, or partially wrong, I hope that someone else will speak up.
Then, you might want to also probe at some strategic points inside the preamp and amp circuits. Your second-last scope photo, of the preamp output when powered but not connected to the amp, looks 'a bit suspicious'. (By the way, what is the value of the smoothing capacitor, in your lower-voltage supply? The schematic I saw had 10 uF. That couldn't be right, could it? If that's for the heaters, would a three-terminal adjustable regulator be worth trying?)
I know that the problem occurs even with the preamp unpowered. But I don't know if the scope photos that you already got, for that condition, include junk that was picked up by the alligator-clip leads, through the air.
This might have been covered, already, but, for the case when the preamp is off, you might want to scope it for two cases, one when it's been off for a long time and one when it's only been off for a short time, since there was some question about whether or not some of the capacitors might be staying charged.
Also, I don't remember of you ever tried discharging both sets of smoothing caps, in your pre's power supplies.
This might be just a shot in the dark, too, but, it might be interesting to probe all of the pre's psu rectifier inputs and outputs, when it's plugged in but turned off, just in case something totally-unexpected is happening. But NOTE that it probably would not be safe for your soundcard (to put it mildly), to probe the HV supply, if the caps are charged. Check them with a voltmeter, first, at least!
First I want to mention that it might be a good idea to at least make a shielded-cable 'probe', just so you don't pick up as much from fields in the air.
You can probably find out if you need to do that or not, by connecting a small resistor to you alligator leads and seeing what you get on the scope with the equipment on and the leads in different locations. You should probably also try a larger-value resistor, and with and without your hands on the leads.
(At least I THINK that that type of testing will give you an idea of whether or not there's a possible problem with using plain alligator leads. At any rate, trying a shielded cable is so cheap and easy that it's probably worth doing. Then you could do the same tests and see if there's a difference, at least.)
I don't know what your soundcard's input impedance might be. But, for these frequencies, you might be able to just modify a short piece of cheap 50 Ohm or 75 Ohm coax, or some other type of shielded cable. On one end, fit a plug that will work with your soundcard's input jack, making sure that the cable's shield is tied to the jack's ground. It might be easiest to use coax that has a braided-wire shield instead of foil, if you use coax (or a cable with a foil shield but also a 'drain' wire). On the 'probe' end, you can solder an alligator clip to the shield (or drain wire), after removing and inch or more of it so it's not too close to the tip, with maybe just a few inches of plain wire between the shield and the alligator clip. With coax, you can just bare about 1/4-inch of the center conductor, to make a 'probe tip'.
If that is all wrong, or partially wrong, I hope that someone else will speak up.
Then, you might want to also probe at some strategic points inside the preamp and amp circuits. Your second-last scope photo, of the preamp output when powered but not connected to the amp, looks 'a bit suspicious'. (By the way, what is the value of the smoothing capacitor, in your lower-voltage supply? The schematic I saw had 10 uF. That couldn't be right, could it? If that's for the heaters, would a three-terminal adjustable regulator be worth trying?)
I know that the problem occurs even with the preamp unpowered. But I don't know if the scope photos that you already got, for that condition, include junk that was picked up by the alligator-clip leads, through the air.
This might have been covered, already, but, for the case when the preamp is off, you might want to scope it for two cases, one when it's been off for a long time and one when it's only been off for a short time, since there was some question about whether or not some of the capacitors might be staying charged.
Also, I don't remember of you ever tried discharging both sets of smoothing caps, in your pre's power supplies.
This might be just a shot in the dark, too, but, it might be interesting to probe all of the pre's psu rectifier inputs and outputs, when it's plugged in but turned off, just in case something totally-unexpected is happening. But NOTE that it probably would not be safe for your soundcard (to put it mildly), to probe the HV supply, if the caps are charged. Check them with a voltmeter, first, at least!
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