I wonder what would happen if you replaced the output resistors (R312 in your schematic) with something rather smaller, say 20k or so. Of course, that will make your bass soft, due to the high pass filter made with C303, so if that works, you'd want to raise that capacitor's value to say, 2.2uF.
Hope you made it through all of these posts.
I have noticed that you left the RIAA equalizer out of the board.
Is this recommended by tubes4hifi? Possibly the hum is due to the empty tube sockets.
I noticed that the schematic doesn't show the equalizer. Is this a tubes4hifi schem?
Replacing the 330K with 20K would probably put an insufficient load on the tube.
I have noticed that you left the RIAA equalizer out of the board.
Is this recommended by tubes4hifi? Possibly the hum is due to the empty tube sockets.
I noticed that the schematic doesn't show the equalizer. Is this a tubes4hifi schem?
Replacing the 330K with 20K would probably put an insufficient load on the tube.
hailteflon said:
How so? Perhaps you could explain what you mean by insufficient load...
If you examine this page of tubecad's writeup on the SRPP, and backcalculate for Rl from the equation there, I think you'll see the optimal load for the circuit presented is a less than 5k. Generally speaking, the output resistor is just to assure us that the output is referenced to ground. Thus the load provided by the amplifier is typically the true load of the preamp. If the amplifier has a very high input impedance, however, then one might argue that the preamp is insufficiently loaded when the output resistor is 330k.
The input impedance of the amp stage will be in parallel with the 330K, right?
This is a cathode follower and it is not like a plate resistor in that it doesn't need about 250K-500K impedance from the next stage.
I was under the impression that a CF could drive about 50K, but not much less.
This is a queston I hope to answer when I get time. I will look at the link you posted later.
When I say insufficient load I mean not high enough.
I have a ton of questions about tubes, so I am not trying to be an authority. If the link you gave shows that a CF can drive a 10K load then this will be very good news.
This is a cathode follower and it is not like a plate resistor in that it doesn't need about 250K-500K impedance from the next stage.
I was under the impression that a CF could drive about 50K, but not much less.
This is a queston I hope to answer when I get time. I will look at the link you posted later.
When I say insufficient load I mean not high enough.
I have a ton of questions about tubes, so I am not trying to be an authority. If the link you gave shows that a CF can drive a 10K load then this will be very good news.
hailteflon said:
Yes, the input impedance of the amp stage is in parallel with the output resistor, so if the input impedance of the amp is say, 200k, then the overall impedance seen by the SRPP is 124k or so with the 330k output resistor, and about 18k with a 20k output resistor. There's very little doubt in my mind that this change should reduce the hum, when the power is off. Now, why the hum is the same with the power on as off, with no third wire on the pre, and no inputs hooked to the pre, that may be an issue that needs more work.
This circuit probably behaves a lot more like an SRPP than a CF. The SRPP is somewhat different than a grounded cathode driving a CF, but that's a story you can read at tubecad. BTW, neither a grounded cathode nor a CF needs impedance from the next stage, rather it's a necessary evil required by the next stage. In fact, for tubes with a high plate resistance, the grid leak resistor required by the next stage may be small enough to significantly degrade the overall result, particularly since it is in parallel to the plate resistor when dealing with AC. That's why you'll see people setting the DC operating point of such tubes with a CCS as a plate load and setting the AC load with the grid leak resistor of next stage.
The SRPP does work best with a particular output impedance as noted in tubecad's fine writeup. IMHO, however, it's not terribly important to get it exact, unless maybe you're driving a particularly low load, like headphones.
hailteflon said:
Roughly speaking, you can first order approximate the output impedance of a CF as 1/Gm, or rp/mu. So, for a 12au7, that number is around 350-400 ohms depending on operating point, etc. Also very roughly speaking, a load of 10x the output impedance is usually acceptable for most circuits, particularly ones without feedback.
Thanks for the good info. I'll print the ariticles and save them for the future.
I didn't know it was so easy to interface SS with tubes.
I have plans to build some tube circuits, but I want to prepare first. This is a question that needed to be answered.
The questions pile up faster than the answers.
I didn't know it was so easy to interface SS with tubes.
I have plans to build some tube circuits, but I want to prepare first. This is a question that needed to be answered.
The questions pile up faster than the answers.
hailteflon said:
Hi Mark,
the empty tube sockets you see are for an optional phono section. Since I don't play vinyl records I asked whether this section could be omitted. This was confirmed to be ok by Roy M at tubes4hifi, and the phono section sockets, their caps and new tubes were installed as one of the first attempts to troubleshoot the hum problem. Having them in and powered up didn't change the hum. It was done by Roy M himself when I sent the preamp to him to debug. He put some time into it but without success, (at least in my setup.) Perhaps I should take them out.
gary
This is really getting interesting.
(1) Is this the first time that they have ever suggested this omission for the phono stage?
(2) The people that sold you this board couldn't find the problem? If this is true then here is my position thus far.
I suspect that the Solen caps are defective. Something ridiculous has happened here and if he didn't see any departure from normal construction technique then the caps may have something wrong with them.
They are windings and the "non-inductive" term may not imply what it should.
I have attempted to determine if the term relates to the actual inductance of the cap or if it means that the cap cannot pick up him through the coil of foil in it.
The two factors are probably interrelated to a great degree, but it is round and that means it is a coil. Notice that in his web picture he uses the oval caps.
I would find some other caps, not necessarily the same value, and determine if they are the problem before I spent any more time otherwise.
Counterfeit components are a possibility these days. He may have gotten a "good deal" on them. M
(1) Is this the first time that they have ever suggested this omission for the phono stage?
(2) The people that sold you this board couldn't find the problem? If this is true then here is my position thus far.
I suspect that the Solen caps are defective. Something ridiculous has happened here and if he didn't see any departure from normal construction technique then the caps may have something wrong with them.
They are windings and the "non-inductive" term may not imply what it should.
I have attempted to determine if the term relates to the actual inductance of the cap or if it means that the cap cannot pick up him through the coil of foil in it.
The two factors are probably interrelated to a great degree, but it is round and that means it is a coil. Notice that in his web picture he uses the oval caps.
I would find some other caps, not necessarily the same value, and determine if they are the problem before I spent any more time otherwise.
Counterfeit components are a possibility these days. He may have gotten a "good deal" on them. M
Before suspecting the Solen caps:
Putting the other two tubes in it without the rest of the components may have done nothing.
Are the long wide tracks in the unused portion of the board part of the ground? Without the other components these tracks may be picking up the hum.
Have you taken a jumper connected to the ground and touched these unused and unterminated ground tracks?
Putting the other two tubes in it without the rest of the components may have done nothing.
Are the long wide tracks in the unused portion of the board part of the ground? Without the other components these tracks may be picking up the hum.
Have you taken a jumper connected to the ground and touched these unused and unterminated ground tracks?
Mark,
I got the impression that the kit was designed to include the phono stage and after I asked about omitting it I was told "that ought to work." It didn't seem that it was a common or even tried modification.
After I got it back it had those few phono components put in, the PAS3 balance was wired back into the circuit, some of my twisted pairs had been replaced. I was told that it had an obvious hum upon arrival and that the hour or two of work had improved it somewhat. Though this was not my observation I wouldn't expect or pay a professional to troubleshoot this for hours.
I'll get some radial caps. What value/composition do you suggest?
I'll try jumping points of the phono section ground track to the star ground and see what happens. By the way, I made an illustration of the actual PCB wiring layout, gray is the ground track on top, red is the hot on the bottom:
http://home.comcast.net/~garyworld/pcblayout.jpg
I can post a parts list for you if you want to cross reference.
But, if the problem is with the earth ground in my house I doubt that the star ground is much use, right? It is not able to directly reference earth without going through whatever loops and new/old electrical junctions that occur in the building.
Something else turned up today that was puzzling: When the preamp is unplugged it hums, let's say at volume X dBs. When I hold the chassis the hum diminishes to a tenth of X. When I plug the preamp in the hum is at 2X but diminishes very little when I hold the chassis. Turning the preamp on and off changes the volume of the hum momentarily but it always stabilizes after a few seconds. It is as if the chassis is an antenna when unplugged but irrelevent when connected to the wall. When plugged in, I get 3 Volts from outlet ground to chassis (but no short.)
gary
I got the impression that the kit was designed to include the phono stage and after I asked about omitting it I was told "that ought to work." It didn't seem that it was a common or even tried modification.
After I got it back it had those few phono components put in, the PAS3 balance was wired back into the circuit, some of my twisted pairs had been replaced. I was told that it had an obvious hum upon arrival and that the hour or two of work had improved it somewhat. Though this was not my observation I wouldn't expect or pay a professional to troubleshoot this for hours.
I'll get some radial caps. What value/composition do you suggest?
I'll try jumping points of the phono section ground track to the star ground and see what happens. By the way, I made an illustration of the actual PCB wiring layout, gray is the ground track on top, red is the hot on the bottom:
http://home.comcast.net/~garyworld/pcblayout.jpg
I can post a parts list for you if you want to cross reference.
But, if the problem is with the earth ground in my house I doubt that the star ground is much use, right? It is not able to directly reference earth without going through whatever loops and new/old electrical junctions that occur in the building.
Something else turned up today that was puzzling: When the preamp is unplugged it hums, let's say at volume X dBs. When I hold the chassis the hum diminishes to a tenth of X. When I plug the preamp in the hum is at 2X but diminishes very little when I hold the chassis. Turning the preamp on and off changes the volume of the hum momentarily but it always stabilizes after a few seconds. It is as if the chassis is an antenna when unplugged but irrelevent when connected to the wall. When plugged in, I get 3 Volts from outlet ground to chassis (but no short.)
gary
It is significant that the hum diminished when you touch the chassis. It is probably the capacitance of your body. This is an indication that some sort of stray field is inducing this hum.
I don't think much of the star ground as I stated several days ago. I see it as unnecessary confusion. I am an advocate of the chassis ground with the chassis connected to the earth.
The pic you posted is similar to one from a few days ago. I assume it has some modifications and I will look at it later.
I don't know exactly how your board is grounded. Have you disconnected all of the ground contact to the chassis, the input/output jacks and all paths through the screws on the ground track?
I don't know that your amps are not somehow aggravating the problem. Eliminating questions helps. Did you ever get to a more modern wiring system?
The one obvious factor is that you have a board that does not have all of the components in it.
For now I would put the same value caps in it. I scanned the TDJ (suggested by another poster) article briefly and if this is a SRPP amp then the component values are critical to performance. At least this is what I thought I saw written in the brief scan, so for now, use the same values. Anything will work experimentally. A small cap will alter the frequency response, but if it is the right voltage it won't hurt anything. There are four of this type of cap.
Have you shorted the inputs and outputs. It may eliminate hum. Jump the plugs with clip leads.
Does the hum diminish when you plug a CD player into it?
One thing that I keep in mind is what I learned when I took a piece of PVC plumbing pipe and put about 30 wraps of insultated wire around it and plugged it into a DVM. I walked around the house and also the back yard and picked up considerable AC voltage through this coil. There were places in the back yard with very high hum fields coming out of the ground. So, I assume that any house has a slurry of hum fields lurking around and certain inductive properties in this board may be picking them up and somehow resonating them.
I don't start something that I don't plan to finish. Just keep eliminating factors and sooner or later we will find the cause. This is the only way to gain this type of knowledge. IT IS PRICELESS. Mark
I don't think much of the star ground as I stated several days ago. I see it as unnecessary confusion. I am an advocate of the chassis ground with the chassis connected to the earth.
The pic you posted is similar to one from a few days ago. I assume it has some modifications and I will look at it later.
I don't know exactly how your board is grounded. Have you disconnected all of the ground contact to the chassis, the input/output jacks and all paths through the screws on the ground track?
I don't know that your amps are not somehow aggravating the problem. Eliminating questions helps. Did you ever get to a more modern wiring system?
The one obvious factor is that you have a board that does not have all of the components in it.
For now I would put the same value caps in it. I scanned the TDJ (suggested by another poster) article briefly and if this is a SRPP amp then the component values are critical to performance. At least this is what I thought I saw written in the brief scan, so for now, use the same values. Anything will work experimentally. A small cap will alter the frequency response, but if it is the right voltage it won't hurt anything. There are four of this type of cap.
Have you shorted the inputs and outputs. It may eliminate hum. Jump the plugs with clip leads.
Does the hum diminish when you plug a CD player into it?
One thing that I keep in mind is what I learned when I took a piece of PVC plumbing pipe and put about 30 wraps of insultated wire around it and plugged it into a DVM. I walked around the house and also the back yard and picked up considerable AC voltage through this coil. There were places in the back yard with very high hum fields coming out of the ground. So, I assume that any house has a slurry of hum fields lurking around and certain inductive properties in this board may be picking them up and somehow resonating them.
I don't start something that I don't plan to finish. Just keep eliminating factors and sooner or later we will find the cause. This is the only way to gain this type of knowledge. IT IS PRICELESS. Mark
I looked at the wiring diagram and the 39.7 and 36.2 volts ac to the board ground is a bit strange. I assume that you measured this with a DVM with 10Megohm input resistance. This is evidently a hum reading. Since the meter doesn't draw any current the voltage is quite high. This is evidently a magnetic coupling from the transformer primary to the secondary ground. The fuse and wires near the ground track are not helping things.
I would guess at this point that the steel chassis and the unused ground tracks are forming some sort of reactor (either inductive and/or capacative) and picking up hum. Many factors that would be relatively harmless if the phono stage was in it are showing up as a big problem.
If this is the case then taking the board out of the chassis should eliminate at least most of the hum.
I would guess at this point that the steel chassis and the unused ground tracks are forming some sort of reactor (either inductive and/or capacative) and picking up hum. Many factors that would be relatively harmless if the phono stage was in it are showing up as a big problem.
If this is the case then taking the board out of the chassis should eliminate at least most of the hum.
Mark,
I'll respond in order of your thoughts:
1. Star ground. Please check my methodology for me even though you are not a fan, this way I know if I have at least installed it correctly. (I installed the star ground only because several other posters made a convincing argument for one. You'll notice I also installed the cap/resistor/anti-parallel diode combination that was prescribed on page 1.)
The chassis is still the ultimate ground plane but the PCB is connected to it exclusively via one connection. Both output jacks' grounds go to the SG (the output leads are shielded but this shielding does not connect to the PCB) , the ground from the TF (red/yellow) goes to the SG, the PCB is connected to the SG via one connection near the PCB output. These 4 points meet at a screw connected to a vinyl standoff. The resistor/cap/diodes then jump to the other side of the standoff where it is screwed to the chassis.
Pic here: http://home.comcast.net/~garyworld/starground2.jpg
So my understanding of this was that the chassis is the low impedence ground plane but there are no loops within the circuit that join it in more than one place, sort of like spokes on a hub with no rim. Is correct SG design that the chassis is bypassed and system ground goes directly from SG to outlet earth ground? I think that since we are discussing my chassis being a possible perpetrator here and I have put so much time into it, I should at least create a valid SG before eliminating it.
2. I'll be bringing the amp tonight to a friend's house who has modern wiring. I'll report on that soon.
3. I'll order 4 radial caps tonight.
4. I have shorted both inputs and outputs. No luck.
5. Sources connected don't improve things, that includes my ipod on battery power.
6. I hear you when you say this learning is priceless. I can't imagine how happy I will be when/if this is fixed. Another expression comes to mind though, "what doesn't kill you makes you stronger." I don't feel like I'm getting any stronger.
7. I just confirmed those TF voltages with the new umbillical set up (to chassis.) 35 Volts hot, 54 V neutral. Incidentally, I get 35 Volts when the preamp is turned off too. I don't understand this. Here is a pic of the wiring, the black rectangle coming off the main neutral is the back of the fuse case.
http://home.comcast.net/~garyworld/trannybox.jpg
You'll notice that the TF doesn't have a ground in, just hot and neutral. Should I wire the mains ground to the chassis? I would only have to feed another wire through the umbillical.
8. I won't be able to do it tonight but I will definitely remove the preamp from the chassis. Should this prove to make a difference what else can I put it in that will shield the preamp?
9. As much as I admire the sound of vinyl played through tube gear I'm afraid that it is not in my nature. I've ruined, lost, and obsessed over far too many records and can't start collecting all over again. I didn't get the phono parts because they accounted for over a third of the cost of the project. I would be doing it only to troubleshoot the hum problem. If this is the case I will be disgruntled to say the least, I was assured this would be fine when I got the kit.
Let me try the caps and removing the chassis and report back.
Thanks again,
gary
I'll respond in order of your thoughts:
1. Star ground. Please check my methodology for me even though you are not a fan, this way I know if I have at least installed it correctly. (I installed the star ground only because several other posters made a convincing argument for one. You'll notice I also installed the cap/resistor/anti-parallel diode combination that was prescribed on page 1.)
The chassis is still the ultimate ground plane but the PCB is connected to it exclusively via one connection. Both output jacks' grounds go to the SG (the output leads are shielded but this shielding does not connect to the PCB) , the ground from the TF (red/yellow) goes to the SG, the PCB is connected to the SG via one connection near the PCB output. These 4 points meet at a screw connected to a vinyl standoff. The resistor/cap/diodes then jump to the other side of the standoff where it is screwed to the chassis.
Pic here: http://home.comcast.net/~garyworld/starground2.jpg
So my understanding of this was that the chassis is the low impedence ground plane but there are no loops within the circuit that join it in more than one place, sort of like spokes on a hub with no rim. Is correct SG design that the chassis is bypassed and system ground goes directly from SG to outlet earth ground? I think that since we are discussing my chassis being a possible perpetrator here and I have put so much time into it, I should at least create a valid SG before eliminating it.
2. I'll be bringing the amp tonight to a friend's house who has modern wiring. I'll report on that soon.
3. I'll order 4 radial caps tonight.
4. I have shorted both inputs and outputs. No luck.
5. Sources connected don't improve things, that includes my ipod on battery power.
6. I hear you when you say this learning is priceless. I can't imagine how happy I will be when/if this is fixed. Another expression comes to mind though, "what doesn't kill you makes you stronger." I don't feel like I'm getting any stronger.
7. I just confirmed those TF voltages with the new umbillical set up (to chassis.) 35 Volts hot, 54 V neutral. Incidentally, I get 35 Volts when the preamp is turned off too. I don't understand this. Here is a pic of the wiring, the black rectangle coming off the main neutral is the back of the fuse case.
http://home.comcast.net/~garyworld/trannybox.jpg
You'll notice that the TF doesn't have a ground in, just hot and neutral. Should I wire the mains ground to the chassis? I would only have to feed another wire through the umbillical.
8. I won't be able to do it tonight but I will definitely remove the preamp from the chassis. Should this prove to make a difference what else can I put it in that will shield the preamp?
9. As much as I admire the sound of vinyl played through tube gear I'm afraid that it is not in my nature. I've ruined, lost, and obsessed over far too many records and can't start collecting all over again. I didn't get the phono parts because they accounted for over a third of the cost of the project. I would be doing it only to troubleshoot the hum problem. If this is the case I will be disgruntled to say the least, I was assured this would be fine when I got the kit.
Let me try the caps and removing the chassis and report back.
Thanks again,
gary
Gary, It is either the thrill of victory or the agony of defeat.
Short the signals leads to ground on both input and output. I do suspect that the absence of the phono stage is the problem, unless the round caps are bad. When you shorted across the Solen cap it went almost quiet. This is very significant.
The loops are only part of the story. The ground wires have impedance and it is impedance that picks up hum. The large thick chassis has much lower impedance than a wire. It should work OK. Even sloppy technique should work OK. It isn't as though the star ground is bad. Even if the whole thing was a disgraceful mess it shouldn't have a hum problem like this.
This whole situation is strange and the only really strange thing about your board is the missing parts. It is quite common for boards to be incomplete as an option so this may be a strange conincidence. It may have something to do with the two large electrolytics in the phono section. They are large rolls of aluminum foil.
I thought you had the parts for the phono stage.
I see that you have the star ground connected to the chassis through the anti-parallel rectifiers and RC filter as prescribed earlier. This is surely sufficient. It should be OK for all practical purposes. The preamp works OK so you evidently have the circuit wired properly
The main question that I have now is whether you get hum on your oscilloscope with the circuit unplugged and not connected to your amp. If the answer to this is yes then I would say that the phono section is the problem.
Do you have the schematic for the phono section? If so please post it.
Also, have you tried touching a jumper connected to the chassis to varies places in the phono section. If you momentarily ground out the source of the hum then it should go quiet.....if this is the problem. Try grounding the positive terminals of the electrolytics.
I'll read your post again later and reply accordingly.
What kind of caps are you going to order?
Post the phono schematic if you have it and I will look for something.
Klatu, Barada ,Nikto, I must get back to my sci-fi thriller. Mark
Short the signals leads to ground on both input and output. I do suspect that the absence of the phono stage is the problem, unless the round caps are bad. When you shorted across the Solen cap it went almost quiet. This is very significant.
The loops are only part of the story. The ground wires have impedance and it is impedance that picks up hum. The large thick chassis has much lower impedance than a wire. It should work OK. Even sloppy technique should work OK. It isn't as though the star ground is bad. Even if the whole thing was a disgraceful mess it shouldn't have a hum problem like this.
This whole situation is strange and the only really strange thing about your board is the missing parts. It is quite common for boards to be incomplete as an option so this may be a strange conincidence. It may have something to do with the two large electrolytics in the phono section. They are large rolls of aluminum foil.
I thought you had the parts for the phono stage.
I see that you have the star ground connected to the chassis through the anti-parallel rectifiers and RC filter as prescribed earlier. This is surely sufficient. It should be OK for all practical purposes. The preamp works OK so you evidently have the circuit wired properly
The main question that I have now is whether you get hum on your oscilloscope with the circuit unplugged and not connected to your amp. If the answer to this is yes then I would say that the phono section is the problem.
Do you have the schematic for the phono section? If so please post it.
Also, have you tried touching a jumper connected to the chassis to varies places in the phono section. If you momentarily ground out the source of the hum then it should go quiet.....if this is the problem. Try grounding the positive terminals of the electrolytics.
I'll read your post again later and reply accordingly.
What kind of caps are you going to order?
Post the phono schematic if you have it and I will look for something.
Klatu, Barada ,Nikto, I must get back to my sci-fi thriller. Mark
Mark,
these are the caps I was going to order:
http://www.panasonic.com/industrial/components/pdf/ABD0000CE45.pdf
Concerning the thrill of victory, I don't know why I have only now discovered this, but I got the hum to completely disappear when I shorted out the output jacks. This result occurs both plugged in and switched on.
So, what does this tell us? Victor suggested doing this, I thought I had without success and assumed that I needed to be connected to earth ground and gave up until I had the chance to take my preamp elsewhere. Perhaps this was before all the business with the starground.
I'll post the phono schematic when I scan it in tomorrow.
Enjoy your thriller.
gary
these are the caps I was going to order:
http://www.panasonic.com/industrial/components/pdf/ABD0000CE45.pdf
Concerning the thrill of victory, I don't know why I have only now discovered this, but I got the hum to completely disappear when I shorted out the output jacks. This result occurs both plugged in and switched on.
So, what does this tell us? Victor suggested doing this, I thought I had without success and assumed that I needed to be connected to earth ground and gave up until I had the chance to take my preamp elsewhere. Perhaps this was before all the business with the starground.
I'll post the phono schematic when I scan it in tomorrow.
Enjoy your thriller.
gary
I've seen that movie so many times that I watch it as an historical ritual. I get a kick out of Klatu helping the professor with his space/time theory and the little old lady comes in and tries to erase all that junk.
The Panasonic caps look OK.
Shorting the outputs means you sent the signal to ground. It doesn't mean anything. Shorting the inputs is significant.
QUOTE]Is correct SG design that the chassis is bypassed and system ground goes directly from SG to outlet earth ground? I think that since we are discussing my chassis being a possible perpetrator here and I have put so much time into it, I should at least create a valid SG before eliminating it.[/QUOTE]
Actually, from a safety standpoint the chassis should be connected to the earth. Your chassis is not the “perpetrator.” The chassis is a large piece of ferrormagnetic material. That is, it can be magnetized so it is more active in an electro-magnetic situation than aluminum for example. Hum is caused by electro-magnetic fields.
The real concern about grounding is usually high frequency. RF circuits (radio freq) use a large plate of copper as a ground plane and the components are often soldered to it. What is good for RF is good for audio because good hi-freq suppression means low noise. That is, hi-freq (supersonic) noise in the circuits taxes it such that it degrades the circuit performance.
A circuit doesn’t know it is playing music. It does one thing all the time. It trys to maintain the derivative (rate of change) of the voltage. Supersonics in the circuit keep the circuit busy trying to control signals it was not designed for.
With tubes there is a high voltage situation that causes more hum, so special consideration is necessary. Even so, hum should be an audible nuisance such as in a guitar amp, but this noise in your circuit is strange to say the least.
Perhaps the star ground is the best for a steel chassis and tubes.
This is not an invention of your own. This has been sold to many customers and if it didn’t work properly they would have ceased production by now.
Redesigning anything should not be necessary. Once again, I suspect that you need to find the source of the hum in the phono section and ground those tracks.
Your ground is not really connected to your chassis. The two diodes are surge (crackle and pop) protection as when a freezer turns off and the RC filter (resistor and cap for hum suppression) are not a direct connection. The diodes don’t conduct until about .3 volt. Thus, your readings from primary to chassis may be deceptive. Take your readings from the PCB ground and see if they are different. But, the voltage to the chassis may be more significant. Jump the star ground to the chassis (bypassing the diodes) and see what you get. If you are using a modern DVM the readings don't mean much anyway because the input impedance of the meter is so high.
You don’t need the whole phono stage. If it is the problem then you will only need the few resistors that will bridge some gaps and terminate the open tracks.
A situation like this feeds upon itself. When you jumped the Solen cap it may have disrupted the situation.
This is a situation that I refer to as “phantom.” That is, it wasn’t meant to exist. Mark
[
The Panasonic caps look OK.
Shorting the outputs means you sent the signal to ground. It doesn't mean anything. Shorting the inputs is significant.
QUOTE]Is correct SG design that the chassis is bypassed and system ground goes directly from SG to outlet earth ground? I think that since we are discussing my chassis being a possible perpetrator here and I have put so much time into it, I should at least create a valid SG before eliminating it.[/QUOTE]
Actually, from a safety standpoint the chassis should be connected to the earth. Your chassis is not the “perpetrator.” The chassis is a large piece of ferrormagnetic material. That is, it can be magnetized so it is more active in an electro-magnetic situation than aluminum for example. Hum is caused by electro-magnetic fields.
The real concern about grounding is usually high frequency. RF circuits (radio freq) use a large plate of copper as a ground plane and the components are often soldered to it. What is good for RF is good for audio because good hi-freq suppression means low noise. That is, hi-freq (supersonic) noise in the circuits taxes it such that it degrades the circuit performance.
A circuit doesn’t know it is playing music. It does one thing all the time. It trys to maintain the derivative (rate of change) of the voltage. Supersonics in the circuit keep the circuit busy trying to control signals it was not designed for.
With tubes there is a high voltage situation that causes more hum, so special consideration is necessary. Even so, hum should be an audible nuisance such as in a guitar amp, but this noise in your circuit is strange to say the least.
Perhaps the star ground is the best for a steel chassis and tubes.
This is not an invention of your own. This has been sold to many customers and if it didn’t work properly they would have ceased production by now.
Redesigning anything should not be necessary. Once again, I suspect that you need to find the source of the hum in the phono section and ground those tracks.
Your ground is not really connected to your chassis. The two diodes are surge (crackle and pop) protection as when a freezer turns off and the RC filter (resistor and cap for hum suppression) are not a direct connection. The diodes don’t conduct until about .3 volt. Thus, your readings from primary to chassis may be deceptive. Take your readings from the PCB ground and see if they are different. But, the voltage to the chassis may be more significant. Jump the star ground to the chassis (bypassing the diodes) and see what you get. If you are using a modern DVM the readings don't mean much anyway because the input impedance of the meter is so high.
You don’t need the whole phono stage. If it is the problem then you will only need the few resistors that will bridge some gaps and terminate the open tracks.
A situation like this feeds upon itself. When you jumped the Solen cap it may have disrupted the situation.
This is a situation that I refer to as “phantom.” That is, it wasn’t meant to exist. Mark
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Mark,
Until the replacement caps arrive, I jumped the chassis straight to the outlet ground. This had a significant effect: There is almost no hum now when the preamp is not on. Unfortunately, when the preamp is turned on the hum returns to full volume, easily audible from 10' away. Although jumping various phono section tracks to ground has had no success I will try again.
gary
Until the replacement caps arrive, I jumped the chassis straight to the outlet ground. This had a significant effect: There is almost no hum now when the preamp is not on. Unfortunately, when the preamp is turned on the hum returns to full volume, easily audible from 10' away. Although jumping various phono section tracks to ground has had no success I will try again.
gary
Hi Gary, As I recall you have a scope. If so, start at the output and trace the hum backwards through the circuit.
When you jumped the chassis to the outlet ground was it the ground that is of questionable quality, the one you originally used?
Did you ever take your system to another house?
When you connected the chassis to ground was the anti-parallel rectifier device still between the star ground and the chassis? Have you tried it with the anti-parallel disconnected and the chassis to outlet ground? That is, the PCB and star ground isolated from the chassis and the chassis connected directly to outlet ground? Mark
When you jumped the chassis to the outlet ground was it the ground that is of questionable quality, the one you originally used?
Did you ever take your system to another house?
When you connected the chassis to ground was the anti-parallel rectifier device still between the star ground and the chassis? Have you tried it with the anti-parallel disconnected and the chassis to outlet ground? That is, the PCB and star ground isolated from the chassis and the chassis connected directly to outlet ground? Mark
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