Hi all,
My first preamp project sounds great. It's a kit I installed in an old Dyna Pat4 chassis over a year ago. My problem is that ever since I built it I get a significant hum. Both low drone of 60Hz through the woofers and the buzzing 120 Hz through my mids. This was confirmed when I hooked the preamp up to my laptop and witnessed the peaks on my Signalscope FFT analyzer. The transformer is quite loud and periodically rattles the PCB especially when warming up.
I get the hum regardless of volume setting, without any source hooked up and, here's what leads me to believe this must be a grounding issue, even when the preamp is turned off. This doesn't happen with my SS preamp and I have tried several different combinations of gear swapping out cables, amps, cd players, outlets etc.
I have spent a lot of time on this site reading about possible fixes. This is what I've done:
1. Checked all connections for cold solder joints and shorts.
2. Rewired all connections.
3. Removed switcher, balance and ganged double in/outputs, all except 1 in, 1 out, 1 new 50ohm vol pot.
4. Replaced filter capacitors.
5. Experimented with numerous grounding schemes.
6. Removed tranny, turned it in every direction, mounted it on rubber, put bricks on it, put a shield around it.
7. Created a simple set up with preamp connected to amp to speakers, all powered off one bathroom GFI, home run to panel with no other appliances, dimmers etc. plugged in.
8. Replaced tubes
9. Sent it to get fixed.
10. A dozen other things that didn't work.
I have read a lot about star grounding but I am under the impression that the ground plane on the PCB is one big loop since it goes through and around the whole PCB and is attached to the chassis via aluminum seperators.
Here are links to 4 pictures and the PS and Line schematics:
http://home.comcast.net/~garyworld/
or individually at:
http://home.comcast.net/~garyworld/preamp1.jpg
http://home.comcast.net/~garyworld/preamp2.jpg
http://home.comcast.net/~garyworld/preamp3.jpg
http://home.comcast.net/~garyworld/preamp4.jpg
http://home.comcast.net/~garyworld/schematic1.jpg
http://home.comcast.net/~garyworld/schematic2.jpg
So this is my last try. If I don't fix this it will be a big defeat and a waste of a very nice sounding preamp.
All help is greatly appreciated.
gary
My first preamp project sounds great. It's a kit I installed in an old Dyna Pat4 chassis over a year ago. My problem is that ever since I built it I get a significant hum. Both low drone of 60Hz through the woofers and the buzzing 120 Hz through my mids. This was confirmed when I hooked the preamp up to my laptop and witnessed the peaks on my Signalscope FFT analyzer. The transformer is quite loud and periodically rattles the PCB especially when warming up.
I get the hum regardless of volume setting, without any source hooked up and, here's what leads me to believe this must be a grounding issue, even when the preamp is turned off. This doesn't happen with my SS preamp and I have tried several different combinations of gear swapping out cables, amps, cd players, outlets etc.
I have spent a lot of time on this site reading about possible fixes. This is what I've done:
1. Checked all connections for cold solder joints and shorts.
2. Rewired all connections.
3. Removed switcher, balance and ganged double in/outputs, all except 1 in, 1 out, 1 new 50ohm vol pot.
4. Replaced filter capacitors.
5. Experimented with numerous grounding schemes.
6. Removed tranny, turned it in every direction, mounted it on rubber, put bricks on it, put a shield around it.
7. Created a simple set up with preamp connected to amp to speakers, all powered off one bathroom GFI, home run to panel with no other appliances, dimmers etc. plugged in.
8. Replaced tubes
9. Sent it to get fixed.
10. A dozen other things that didn't work.
I have read a lot about star grounding but I am under the impression that the ground plane on the PCB is one big loop since it goes through and around the whole PCB and is attached to the chassis via aluminum seperators.
Here are links to 4 pictures and the PS and Line schematics:
http://home.comcast.net/~garyworld/
or individually at:
http://home.comcast.net/~garyworld/preamp1.jpg
http://home.comcast.net/~garyworld/preamp2.jpg
http://home.comcast.net/~garyworld/preamp3.jpg
http://home.comcast.net/~garyworld/preamp4.jpg
http://home.comcast.net/~garyworld/schematic1.jpg
http://home.comcast.net/~garyworld/schematic2.jpg
So this is my last try. If I don't fix this it will be a big defeat and a waste of a very nice sounding preamp.
All help is greatly appreciated.
gary
Gary,
I hear your problem, it took me ages to get the hum in my Aikido to acceptable levels. One thing to try is to remount your trafo using rubber washers or even the method used by VoltSecond - that is if your trafo is chassis mounted! - SORRY, just noticed that you've done that already.
Another problem that I encountered with the ground loop occurs with my TV and DVD-player which cause extra hum. I ended-up disconnecting signal GND for these devices. Following the same reasoning, maybe experiment by hooking-up your preamp to power amp with signal GND disconnected - maybe it'll work and you'll get sound. Incidentally, it didn't work for me, but it was a quick thing to try.
You might also try a ground lift in your preamp. I used the method advocated by Rod Elliot (http://sound.westhost.com) - look around the website for it. I only use 20R of ground lift and I got significant reduction in hum.
As for the power supply, there are a number of things that may improve things:
1. check the value of C101 against the maximum cap value for a cap-input filter for the rectifier you are using.
2. The value of C202 could easily be increased to 60uF or higher. This should reduce 120Hz ripple.
3. You could add a choke section, or even rebuild / redesign the PSU using PSUDII available from www.duncanamps.com
4. Replace heater PSU with a regulated heater PSU. There are kits available - I used the kit PSHREG from Welborne Labs. It works, but read postings on Welborne's customer service. This made the BIGGEST difference in my Aikido for the 60Hz hum.
Incidentally, I recently installed 6SL7's in the input positions of my Aikido (replacing 6SN7's). The current draw of the Aikido B+ dropped from around 25mA to around 18mA. As a result, I do not have a little 120Hz hum due to inadequate filtering. Amazingly, it was my wife who noticed it. As with the 60Hz hum, it is inaudible when the ear is moved to greater than 4 inches from the speaker - phew! Incidentally, in true choke input filters (which mine isn't) the choke requires a minimum current draw in order to function correctly. I expect that with a 5uF cap in front of my 10H choke, the PSU is more cap-input than choke-input, but not to the extent that current draw becomes insignificant.
I am sure that other readers will be able to give you more educated help than I. Anyway - Good Luck.
Charlie
I hear your problem, it took me ages to get the hum in my Aikido to acceptable levels. One thing to try is to remount your trafo using rubber washers or even the method used by VoltSecond - that is if your trafo is chassis mounted! - SORRY, just noticed that you've done that already.
Another problem that I encountered with the ground loop occurs with my TV and DVD-player which cause extra hum. I ended-up disconnecting signal GND for these devices. Following the same reasoning, maybe experiment by hooking-up your preamp to power amp with signal GND disconnected - maybe it'll work and you'll get sound. Incidentally, it didn't work for me, but it was a quick thing to try.
You might also try a ground lift in your preamp. I used the method advocated by Rod Elliot (http://sound.westhost.com) - look around the website for it. I only use 20R of ground lift and I got significant reduction in hum.
As for the power supply, there are a number of things that may improve things:
1. check the value of C101 against the maximum cap value for a cap-input filter for the rectifier you are using.
2. The value of C202 could easily be increased to 60uF or higher. This should reduce 120Hz ripple.
3. You could add a choke section, or even rebuild / redesign the PSU using PSUDII available from www.duncanamps.com
4. Replace heater PSU with a regulated heater PSU. There are kits available - I used the kit PSHREG from Welborne Labs. It works, but read postings on Welborne's customer service. This made the BIGGEST difference in my Aikido for the 60Hz hum.
Incidentally, I recently installed 6SL7's in the input positions of my Aikido (replacing 6SN7's). The current draw of the Aikido B+ dropped from around 25mA to around 18mA. As a result, I do not have a little 120Hz hum due to inadequate filtering. Amazingly, it was my wife who noticed it. As with the 60Hz hum, it is inaudible when the ear is moved to greater than 4 inches from the speaker - phew! Incidentally, in true choke input filters (which mine isn't) the choke requires a minimum current draw in order to function correctly. I expect that with a 5uF cap in front of my 10H choke, the PSU is more cap-input than choke-input, but not to the extent that current draw becomes insignificant.
I am sure that other readers will be able to give you more educated help than I. Anyway - Good Luck.
Charlie
Hi Gary,
It sounds like you've tried many of the logical fixes without success. However something you said gives a very important clue. And that is that it hums even when it's turned off. This is very curious. But your schematic doesn't depict the on/off circuit.
It would be helpful to know if the hum stays or goes when you pull the AC power plug out. (when it's turned off) If it disappears, the hum is from leakage somewhere in the power supply. If it remains even with the plug pulled, you have extraneous pickup. Perhaps from a faulty ground.
You also say that the power transformer is very loud and sometimes rattles the PC board. This should not be and is another clue. If the circuit is correct and there's no defective parts, (you said it sounds great) I'd question the transformer itself.
I once repaired a tube headphone amp that was always dead quiet. Then, all of a sudden, it developed a low level hum. This hum appeared immediately at turn on, even before the tubes got warm. It turned out to be the power transformer. No amount of reorientation helped. The fix was to remove the transformer altogether and place it in a box with an umbilical to the amp. A replacement transformer wasn't available.
I also once had a case where one diode in a bridge was open. This unbalance caused the transformer to hum and act weird.
Your problem can be corrected, It just takes perseverance and a little thinking out of the box.
Victor
It sounds like you've tried many of the logical fixes without success. However something you said gives a very important clue. And that is that it hums even when it's turned off. This is very curious. But your schematic doesn't depict the on/off circuit.
It would be helpful to know if the hum stays or goes when you pull the AC power plug out. (when it's turned off) If it disappears, the hum is from leakage somewhere in the power supply. If it remains even with the plug pulled, you have extraneous pickup. Perhaps from a faulty ground.
You also say that the power transformer is very loud and sometimes rattles the PC board. This should not be and is another clue. If the circuit is correct and there's no defective parts, (you said it sounds great) I'd question the transformer itself.
I once repaired a tube headphone amp that was always dead quiet. Then, all of a sudden, it developed a low level hum. This hum appeared immediately at turn on, even before the tubes got warm. It turned out to be the power transformer. No amount of reorientation helped. The fix was to remove the transformer altogether and place it in a box with an umbilical to the amp. A replacement transformer wasn't available.
I also once had a case where one diode in a bridge was open. This unbalance caused the transformer to hum and act weird.
Your problem can be corrected, It just takes perseverance and a little thinking out of the box.
Victor
It looks like your standoffs are aluminum. If they are then your ground is the chassis. The little thin track around the perimeter of the board is not a sufficient ground by itself.
A question that I have never been able to decide upon is whether to connect the chassis to the earth of the house wiring.
In case you don't know, the "grounded neutral" in modern house wiring has the ground (third pin on wall plug) strapped to the neutral (wide spade on polarized wall plug) at the breaker box. This looks to me like it would have impedance, so it is probably best to actually use the third pin on the wall plug.
It is the impedance of the ground that couples the hum and a star ground is IMHO not as good as a very low impedance ground plane, such as a chassis.
Some equipment grounds are connected to the earth and some aren't. Again, I have never been able to decide which way is best.
You have a power transformer isolating the B+ from the line voltage so I don't think there would be a shock hazard if the chassis is connected to the earth.
I see that you have DC on the filaments so that is ruled out as a source of hum.
My point is that the relationship of the chassis with the power system ground may have something to do with the hum. Hope I haven't been confusing, Mark
A question that I have never been able to decide upon is whether to connect the chassis to the earth of the house wiring.
In case you don't know, the "grounded neutral" in modern house wiring has the ground (third pin on wall plug) strapped to the neutral (wide spade on polarized wall plug) at the breaker box. This looks to me like it would have impedance, so it is probably best to actually use the third pin on the wall plug.
It is the impedance of the ground that couples the hum and a star ground is IMHO not as good as a very low impedance ground plane, such as a chassis.
Some equipment grounds are connected to the earth and some aren't. Again, I have never been able to decide which way is best.
You have a power transformer isolating the B+ from the line voltage so I don't think there would be a shock hazard if the chassis is connected to the earth.
I see that you have DC on the filaments so that is ruled out as a source of hum.
My point is that the relationship of the chassis with the power system ground may have something to do with the hum. Hope I haven't been confusing, Mark
Hi,
I use a star ground at the minimum impedance point (where the power transformer CT connects) and couple the power ground to earth ground via a 100 ohm resistor.
If you still get a loop, coupling to earth via 2.2 Meg resistor bridged by 0.047uF X2 cap works with all but real leaky transformers.
Cheers!
I use a star ground at the minimum impedance point (where the power transformer CT connects) and couple the power ground to earth ground via a 100 ohm resistor.
If you still get a loop, coupling to earth via 2.2 Meg resistor bridged by 0.047uF X2 cap works with all but real leaky transformers.
Cheers!
You might try disconnecting the red/yellow wire from the PCB and splicing a little more wire to it, then connect that directly to the (-) terminal of the first filter cap. Yes, it's SCHEMATICALLY the same point in the circuit. But you may be re-routing the capacitor ripple current through a path that DOESN'T flow past signal circuits.
(1) Have you tried twisting the leads for the high volt? I notice that you twisted all of the others.
(2) I recall a rule concerning shielded wiring. The shield should be connected to the source. I notice that your shield goes near a tube plate and is then connected at the board. Having it connected at both ends creates a ground loop. I recall that the ground on the input to the shielded wire should be connected to the system ground, usually the chassis ground.
(3) The vibrating transformer is strange. Is this a Chinese transformer? The PA number sounds like a Dynaco part.
(4) The transformer could be setting up an eddy current in the chassis. It may be inducing hum in the ground tracks at the edge of the board. Eddy currents could account for the vibration.
(5) Have you tried taking a clip lead and jumping the board ground to the chassis? Even though the chassis is connected to the board ground tracks (assuming the standoffs are aluminum) there is still the impedance of the tracks before the connect to the chassis.
Be sure and let us know what you find.
(2) I recall a rule concerning shielded wiring. The shield should be connected to the source. I notice that your shield goes near a tube plate and is then connected at the board. Having it connected at both ends creates a ground loop. I recall that the ground on the input to the shielded wire should be connected to the system ground, usually the chassis ground.
(3) The vibrating transformer is strange. Is this a Chinese transformer? The PA number sounds like a Dynaco part.
(4) The transformer could be setting up an eddy current in the chassis. It may be inducing hum in the ground tracks at the edge of the board. Eddy currents could account for the vibration.
(5) Have you tried taking a clip lead and jumping the board ground to the chassis? Even though the chassis is connected to the board ground tracks (assuming the standoffs are aluminum) there is still the impedance of the tracks before the connect to the chassis.
Be sure and let us know what you find.
SY said:
Tom Bavis said:
Both above are very important points, another to keep in mind particularly true in the case of steel chassis (eddy currents) is that you want only one single ground connection between the audio circuitry and the chassis.
Make sure that the board is not grounded to the chassis, and that non of the jacks are either (inadvertently or deliberately) both the board ground and the jack grounds should come back to just one common point which may be connected to chassis through a parallel combination of a 20- 33 ohm resistor and a pair of anti-parallel 1N4002 (or similar ) diodes. Sometimes a small ceramic disk cap (say 0.01uF) may be added in parallel as well for lower impedance at RF.
For the 60Hz issue make sure that the transformer is not too close to the board, and for the 120Hz issue check that the filament supply is ripple free. Ripple sawtooth is really high dvdt and couples very effectively through the cathode/filament insulation and radiates (electrostatically) into surrounding circuitry.
I see that you did use aluminum standoffs.
You have two basic design philosophies to choose from. Some want to use the chassis as an effective shield in conjunction with a star ground. A star ground often creates more problems than it solves.....................loops.
It has been my observation that the low impedance ground plane is the best.
You don't have a ground plane on your board, you have a thin high impedance track around the board.
The chassis is thick and is a good ground plane. You are already connected to it, but the eddy currents in the chassis may be setting up potentials at the aluminum spacers.
If you strap the ground track on the board to the chassis in a few places where the components tie to ground this will elimninate the track impedance.
The advice from other posters about the resistors, diodes strapped with the caps, and the star ground may be the way to go, but don't think that the chassis ground is necessarily a mistake.
What you probably have here is several ground problems that are reinforcing each other. The vibrating transformer is the main clue here. It is putting out excessive current due to positive feedback of 60 and 120Hz.
The theory behind the ground plane is that if the ground has near zero impedance then the flaws are nulled.
Your untwisted high volt from the transformer is probably the main source of energy.
I have built solid state with the chassis ground and it is very quiet. But, I have never built a tube circuit this way.
I'm not trying to argue with anyone, only looking for facts and understanding.
It is a lot easier to go ahead and use the chassis as a ground plane first. The star ground is going to be a lot of work.
Strap the grounds at the jacks to the chassis too. If you get everything grounded to the chassis it may become quiet.
Wow!
Thanks for the wealth of information. My hope is renewed. Since it is the most common and easiest advice, the first thing I will do is remove the transformer from the chassis and isolate it outside, retwisting the high voltage leads (R/RY/R wires.) This raises the question: What is the safest and most effective method to do this? Solder longer leads and then shrink wrap them? Wire nuts? Insulated connectors?
I will get to each of the other recommendations in the order of increasing difficulty/parts lead time and report back.
Thanks again for all the help, you guys are the best.
gary
Thanks for the wealth of information. My hope is renewed. Since it is the most common and easiest advice, the first thing I will do is remove the transformer from the chassis and isolate it outside, retwisting the high voltage leads (R/RY/R wires.) This raises the question: What is the safest and most effective method to do this? Solder longer leads and then shrink wrap them? Wire nuts? Insulated connectors?
I will get to each of the other recommendations in the order of increasing difficulty/parts lead time and report back.
Thanks again for all the help, you guys are the best.
gary
Hi Gary,
Solder on extension wires and twist them tightly as you did the others. A braid is what is usually used, but just twist them if you don't know how to braid it. it doesn't hurt to use ferrite suppressor beads.
(1) Does your power switch break the hot or the neutral-------do you have a polarized wall plug?
(2) Does the PT vibrate with B+ disconnected?
(3) Is your filament supply connected to the same ground as the B+?
(5) are you sure that the rectifiers for the filament supply are all good?
(4) I have a SS Dynaco amp and the input jacks are grounded to the chassis. Is yours the same way? It also has a steel chassis. My chassis grounds have always been aluminum.
I am very curious about what the problem(s)? are. Keep us posted, Mark
Solder on extension wires and twist them tightly as you did the others. A braid is what is usually used, but just twist them if you don't know how to braid it. it doesn't hurt to use ferrite suppressor beads.
(1) Does your power switch break the hot or the neutral-------do you have a polarized wall plug?
(2) Does the PT vibrate with B+ disconnected?
(3) Is your filament supply connected to the same ground as the B+?
(5) are you sure that the rectifiers for the filament supply are all good?
(4) I have a SS Dynaco amp and the input jacks are grounded to the chassis. Is yours the same way? It also has a steel chassis. My chassis grounds have always been aluminum.
I am very curious about what the problem(s)? are. Keep us posted, Mark
Mark,
cheers for the help.
1. I think it breaks the neutral. That is the lead next to the AC in on the board.
2. I'll try that tonight, I'm going to have to hook the tranny back up, I disconnected it to extend the leads but I'm waiting on some hook up wire. I ordered 22 gauge, same that comes out of the tranny, does that sound right?
3. Here is a layaout diagram I drew up in Illustrator:
http://home.comcast.net/~garyworld/pcblayout(1).jpg
4. Can I use a multimeter to check to see if the diodes are good? How else if not?
5. I disconnected all the old jacks and posts. Tonight I plan on disconnecting the board end of the input and output grounds and coupling them together in series at the jacks per your instructions.
I will report back soon.
gary
cheers for the help.
1. I think it breaks the neutral. That is the lead next to the AC in on the board.
2. I'll try that tonight, I'm going to have to hook the tranny back up, I disconnected it to extend the leads but I'm waiting on some hook up wire. I ordered 22 gauge, same that comes out of the tranny, does that sound right?
3. Here is a layaout diagram I drew up in Illustrator:
http://home.comcast.net/~garyworld/pcblayout(1).jpg
4. Can I use a multimeter to check to see if the diodes are good? How else if not?
5. I disconnected all the old jacks and posts. Tonight I plan on disconnecting the board end of the input and output grounds and coupling them together in series at the jacks per your instructions.
I will report back soon.
gary
1. I think it breaks the neutral. That is the lead next to the AC in on the board.
-------Be sure. Do you have a polarized wall plug? If so, the wide terminal is the neutral. Try to find out now why it hums with the power switch off. If you are breaking the neutral it could explain the hum with switch off. There is also the possibility that you have some sort of primary/secondary short in the PT.
2. I'll try that tonight, I'm going to have to hook the tranny back up, I disconnected it to extend the leads but I'm waiting on some hook up wire. I ordered 22 gauge, same that comes out of the tranny, does that sound right?
------------that ought to do it., leave plenty of length to twist it.
4. Can I use a multimeter to check to see if the diodes are good? How else if not?
-------------See if it has a rectifier symbol on the function switch. If so, the diode should read open circuit in one direction and about .6V in the conductiing direction. Otherwise just use the ohms function to check it.
5. I disconnected all the old jacks and posts. Tonight I plan on disconnecting the board end of the input and output grounds and coupling them together in series at the jacks per your instructions.
---------not sure what you mean here. I don't think I told you to series anything.
-------Be sure. Do you have a polarized wall plug? If so, the wide terminal is the neutral. Try to find out now why it hums with the power switch off. If you are breaking the neutral it could explain the hum with switch off. There is also the possibility that you have some sort of primary/secondary short in the PT.
2. I'll try that tonight, I'm going to have to hook the tranny back up, I disconnected it to extend the leads but I'm waiting on some hook up wire. I ordered 22 gauge, same that comes out of the tranny, does that sound right?
------------that ought to do it., leave plenty of length to twist it.
4. Can I use a multimeter to check to see if the diodes are good? How else if not?
-------------See if it has a rectifier symbol on the function switch. If so, the diode should read open circuit in one direction and about .6V in the conductiing direction. Otherwise just use the ohms function to check it.
5. I disconnected all the old jacks and posts. Tonight I plan on disconnecting the board end of the input and output grounds and coupling them together in series at the jacks per your instructions.
---------not sure what you mean here. I don't think I told you to series anything.
gary, from looking at your power supply schematic, and the hum that happens on power-up, that is sometimes caused by an inrush current (loading up the capacitors upon startup) and should disappear after a few seconds. This can be checked by turning it off and then on again after 2-3seconds - if no hum, then this might be the problem.
I would be surprised though if this is the problem, as tube rectifiers have a higher impedance than solid state so that limits the current.
If the transformer continues to hum, it might just be too small for the current you are asking it to deliver....
and, your filament supply is rectified and then filtered with a pretty small cap (C106). IMHO, rule of thumb is 1000uF per amp if you want a not-so-bad DC. If you don't have to have DC, why not leave out the rectifier altogether and power the filaments with AC? (that obviously depends on the secondary voltage of the transformer winding)
hope that helps
alfred
I would be surprised though if this is the problem, as tube rectifiers have a higher impedance than solid state so that limits the current.
If the transformer continues to hum, it might just be too small for the current you are asking it to deliver....
and, your filament supply is rectified and then filtered with a pretty small cap (C106). IMHO, rule of thumb is 1000uF per amp if you want a not-so-bad DC. If you don't have to have DC, why not leave out the rectifier altogether and power the filaments with AC? (that obviously depends on the secondary voltage of the transformer winding)
hope that helps
alfred
Here's an article that I happened to find, yesterday, about star grounding, mainly for tube-based amplifiers (with an example):
http://www.geofex.com/Article_Folders/stargnd/stargnd.htm
Incidentally, that link was found at the site below, which seems to have quite a wealth of audio-related circuits and information:
http://www.messaggeri.it/xaudio costruire.htm
http://www.geofex.com/Article_Folders/stargnd/stargnd.htm
Incidentally, that link was found at the site below, which seems to have quite a wealth of audio-related circuits and information:
http://www.messaggeri.it/xaudio costruire.htm
Thanks guys,
Mark,
I disconnected the transformer and ran it via umbilical. Pics here:
http://home.comcast.net/~garyworld/preamp5.jpg
http://home.comcast.net/~garyworld/preamp6.jpg
Unfortunately it didn't change anything. I tried reversing the swith wiring to no effect. I have polarized plugs in my apartment, no ground though and I'm very suspicious of the old knob and tube wiring.
Regarding the input shielding: I disconnected the input shield/ground from the board. I left it in place for, I presume, RF interference protection. I connected another ground wire to the input jack ground and ran it to terminal riveted to the chassis. My thinking is that this will become a star ground to which I will eventually connect all grounds. I tried doing this with the output wiring too but the hum went crazy. So neither of these attempts changed anything.
Diodes seem to be fine.
Regarding hum while preamp is off: The hum is present with no source attached, switched on or off, AND when the preamp is unplugged. The hum varies when I unplug or throw the switch but returns to the same level after about 10 seconds. I assume this is the capacitors charging/discharging. Does this mean that the problem is entirely a low voltage ground issue and nothing to do with the PS?
Tom,
thanks for the links, I'll check them out soon.
Thanks all,
gary
Mark,
I disconnected the transformer and ran it via umbilical. Pics here:
http://home.comcast.net/~garyworld/preamp5.jpg
http://home.comcast.net/~garyworld/preamp6.jpg
Unfortunately it didn't change anything. I tried reversing the swith wiring to no effect. I have polarized plugs in my apartment, no ground though and I'm very suspicious of the old knob and tube wiring.
Regarding the input shielding: I disconnected the input shield/ground from the board. I left it in place for, I presume, RF interference protection. I connected another ground wire to the input jack ground and ran it to terminal riveted to the chassis. My thinking is that this will become a star ground to which I will eventually connect all grounds. I tried doing this with the output wiring too but the hum went crazy. So neither of these attempts changed anything.
Diodes seem to be fine.
Regarding hum while preamp is off: The hum is present with no source attached, switched on or off, AND when the preamp is unplugged. The hum varies when I unplug or throw the switch but returns to the same level after about 10 seconds. I assume this is the capacitors charging/discharging. Does this mean that the problem is entirely a low voltage ground issue and nothing to do with the PS?
Tom,
thanks for the links, I'll check them out soon.
Thanks all,
gary
Hi all,
this is a follow up to my recent post trying to find a solution to my ground problem. I took almost all of the advice given, (many thanks) and sadly have no improvement. This is what I have done since:
1. Removed transformer and put it in a remote box separated by 2' umbilical of braided 18 gauge wire. The fuse and switch followed the transformer. Pics here:
http://home.comcast.net/~garyworld/umbilical1.jpg
http://home.comcast.net/~garyworld/umbilical2.jpg
2. Installed star ground. Pics here:
http://home.comcast.net/~garyworld/starground2.jpg
http://home.comcast.net/~garyworld/starground1.jpg
Installed as recommended inbetween chassis and star ground are parallel 100 Ohm resistor/.01 uF cap/50V 1amp antiparallel diodes.
All aluminum standoffs are replaced with nylon standoffs isolating the PCB from the chassis. I experimented with differentground leads to the chassis and found no significant differences so ended up with 1 from each output jack, CT ground direct from transformer, 1 lead from PCB ground track. I tried disconnecting input shielding from ground track without any noticeable difference.
So I still have hum with the preamp off and unplugged. Now the hum gets worse when I plug it in and doesn't improve when turned on.
Any other suggestions? At this point have we ruled out:
1.bad transformer
2.bad diodes
3.bad filter caps
4.ripple
? I make this assumption because the hum exists without current going to or from the transformer.
There is a schematic for the the PS and line stage and a PCB diagram here:
http://home.comcast.net/~garyworld/
For those who noticed C106, it doesn't exist. Instead there are the caps shown in the H+ schematic on the line section page. Are these adequate?
What's the story on isolation transformers, the kind they use in hospitals to keep RFs from interfering with scopes etc.? I see them on eBay for $100. Do these lift the ground to whatever is plugged into them?
Thanks for the continued help.
gary
this is a follow up to my recent post trying to find a solution to my ground problem. I took almost all of the advice given, (many thanks) and sadly have no improvement. This is what I have done since:
1. Removed transformer and put it in a remote box separated by 2' umbilical of braided 18 gauge wire. The fuse and switch followed the transformer. Pics here:
http://home.comcast.net/~garyworld/umbilical1.jpg
http://home.comcast.net/~garyworld/umbilical2.jpg
2. Installed star ground. Pics here:
http://home.comcast.net/~garyworld/starground2.jpg
http://home.comcast.net/~garyworld/starground1.jpg
Installed as recommended inbetween chassis and star ground are parallel 100 Ohm resistor/.01 uF cap/50V 1amp antiparallel diodes.
All aluminum standoffs are replaced with nylon standoffs isolating the PCB from the chassis. I experimented with differentground leads to the chassis and found no significant differences so ended up with 1 from each output jack, CT ground direct from transformer, 1 lead from PCB ground track. I tried disconnecting input shielding from ground track without any noticeable difference.
So I still have hum with the preamp off and unplugged. Now the hum gets worse when I plug it in and doesn't improve when turned on.
Any other suggestions? At this point have we ruled out:
1.bad transformer
2.bad diodes
3.bad filter caps
4.ripple
? I make this assumption because the hum exists without current going to or from the transformer.
There is a schematic for the the PS and line stage and a PCB diagram here:
http://home.comcast.net/~garyworld/
For those who noticed C106, it doesn't exist. Instead there are the caps shown in the H+ schematic on the line section page. Are these adequate?
What's the story on isolation transformers, the kind they use in hospitals to keep RFs from interfering with scopes etc.? I see them on eBay for $100. Do these lift the ground to whatever is plugged into them?
Thanks for the continued help.
gary
gary h said:
Hokay, have you tried this?
With the preamp unpluged from the wall and with the output connected to the main amp (turned on), use a grounded clip lead and begin grounding out signal points starting at the output plug itself and working back toward the input. This is to see if at some point along the way the hum stops.
If I read you correctly, you're getting hum from a preamp that isn't powered on and not connected to the AC mains. This has to be a case of external pick-up. Nothing else makes sense unless your interconnect has an open ground.
Victor
Gary, I am currently working on a tube tester so I don’t have time to open the links you posted. Three questions:
(1) What happens if you pull the tubes out?
(2) Is there any primary to secondary shorting in the PT....... is there any continuity from the wall plug to the secondary?
(3) Did you check for hum with the transformer disconnected from the circuit?
Don’t give up on this problem. Solving it will give a unique understanding. Back Later Mark.
(1) What happens if you pull the tubes out?
(2) Is there any primary to secondary shorting in the PT....... is there any continuity from the wall plug to the secondary?
(3) Did you check for hum with the transformer disconnected from the circuit?
Don’t give up on this problem. Solving it will give a unique understanding. Back Later Mark.
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