First let me thank Tom for taking such an initiative. I really appreciate this. Let me answer some questions...
First of all, the preamp still hums when turned off. (Slightly less so when plugged in but only marginally.) Since I made the grounding changes on http://home.comcast.net/~garyworld/preampfeb29.jpg the hum is as present as ever. I had some experience with this before when disconnecting the pcb end of the output ground/shield, this tended to lessen the hum slightly. Now it is about the same on or off.
The CT to filter caps and their ground to star ground are not touching as it seems in the pic. I have moved them so they do not cross or come close to each other.
Megajocke, the hum is the same with the 12X4 removed.
I have an observation, tell me what you think; as we know there are two ground tracks that circle each side of the pcb and partially disects the top. Are these supposed to be independent, that is have no continuity? I don't see how this is possible but I don't understand why the board would have two ground tracks that constantly intersect each other at solder points and screw holes.
I was under the impression that joining the in/out grounds both at the jacks and then at the pcb constitutes a loop. Please disabuse me of this idea if I am wrong and I will solder a wire across the top of the 4 in/out jacks' ground to the star ground (as described by Chrish) and remove the pcb ground wire (under the starground.) Further more should I jumper the scratched section of the pcb ground left of the filter caps and remove that separate ground connection too. I interpret Megajocke's instructions to mean that there should only be one signal connection to the isolation network which then connects only to the chassis and earth ground on it's other end.
the quote from RL refers to brown and orange wires coming from the tranny. The wires are coded as follows despite how they might appear on the photos: 2 red high voltage, 1 red/yellow center tap, 2 black AC in, 2 blue DC to rectifier. I am not aware of how to distinguish between the similarly colored wires. Any ideas? Also, I think RL might have been looking at some of the older pics since before we have made significant changes. I would hate to waste or confuse these excellent resources so I will delete all irrelevent pictures, especially those that show the tranny in the preamp.
To address RL's wiring comments, I remember Tom recommended that I reverse the wiring of the switch neutral to fuse. This had no effect. The comment about the transformer being different from the one speced is concerning. This was the one sent with the kit and returned with the kit after an attempt to diagnose it.
I like the idea of solving the hum while preamp off first.
As I said above, removing the rectifier tube does not mitigate the hum. Jumping the heater dc leads, when off but plugged in, to starground does not change the hum. It does prevent the preamp from turning on though. (I continually remind myself how new I am to this.)
Reading on, I'll bridge the bottom section (power circuit) of pcb at the filter caps farthest from the B+ and remove that ground wire.
The hum does not diminish with the cover on.
I think the loose ground referred to was from when I had the output shield snipped shy of the pcb. That has since been reconnected.
I'm not sure how to interpret 7n7is's power supply comment. Should I assume that he is thinking of the remote tranny concept we have already implemented?
I'm going to make the above starground changes and install the 4 radial replacement .33uF caps for C301 and C303. Mark suggested that the axial ones I have might be implicated.
It's weekend again!
gary
First of all, the preamp still hums when turned off. (Slightly less so when plugged in but only marginally.) Since I made the grounding changes on http://home.comcast.net/~garyworld/preampfeb29.jpg the hum is as present as ever. I had some experience with this before when disconnecting the pcb end of the output ground/shield, this tended to lessen the hum slightly. Now it is about the same on or off.
The CT to filter caps and their ground to star ground are not touching as it seems in the pic. I have moved them so they do not cross or come close to each other.
Megajocke, the hum is the same with the 12X4 removed.
I have an observation, tell me what you think; as we know there are two ground tracks that circle each side of the pcb and partially disects the top. Are these supposed to be independent, that is have no continuity? I don't see how this is possible but I don't understand why the board would have two ground tracks that constantly intersect each other at solder points and screw holes.
I was under the impression that joining the in/out grounds both at the jacks and then at the pcb constitutes a loop. Please disabuse me of this idea if I am wrong and I will solder a wire across the top of the 4 in/out jacks' ground to the star ground (as described by Chrish) and remove the pcb ground wire (under the starground.) Further more should I jumper the scratched section of the pcb ground left of the filter caps and remove that separate ground connection too. I interpret Megajocke's instructions to mean that there should only be one signal connection to the isolation network which then connects only to the chassis and earth ground on it's other end.
the quote from RL refers to brown and orange wires coming from the tranny. The wires are coded as follows despite how they might appear on the photos: 2 red high voltage, 1 red/yellow center tap, 2 black AC in, 2 blue DC to rectifier. I am not aware of how to distinguish between the similarly colored wires. Any ideas? Also, I think RL might have been looking at some of the older pics since before we have made significant changes. I would hate to waste or confuse these excellent resources so I will delete all irrelevent pictures, especially those that show the tranny in the preamp.
To address RL's wiring comments, I remember Tom recommended that I reverse the wiring of the switch neutral to fuse. This had no effect. The comment about the transformer being different from the one speced is concerning. This was the one sent with the kit and returned with the kit after an attempt to diagnose it.
I like the idea of solving the hum while preamp off first.
As I said above, removing the rectifier tube does not mitigate the hum. Jumping the heater dc leads, when off but plugged in, to starground does not change the hum. It does prevent the preamp from turning on though. (I continually remind myself how new I am to this.)
Reading on, I'll bridge the bottom section (power circuit) of pcb at the filter caps farthest from the B+ and remove that ground wire.
The hum does not diminish with the cover on.
I think the loose ground referred to was from when I had the output shield snipped shy of the pcb. That has since been reconnected.
I'm not sure how to interpret 7n7is's power supply comment. Should I assume that he is thinking of the remote tranny concept we have already implemented?
I'm going to make the above starground changes and install the 4 radial replacement .33uF caps for C301 and C303. Mark suggested that the axial ones I have might be implicated.
It's weekend again!
gary
"First let me thank Tom for taking such an initiative. I really appreciate this. Let me answer some questions...
First of all, the preamp still hums when turned off. (Slightly less so when plugged in but only marginally.) Since I made the grounding changes on http://home.comcast.net/~garyworld/preampfeb29.jpg the hum is as present as ever. I had some experience with this before when disconnecting the pcb end of the output ground/shield, this tended to lessen the hum slightly. Now it is about the same on or off."
Does the character of the hum change when turned on? Is it soft or buzzy?
"The CT to filter caps and their ground to star ground are not touching as it seems in the pic. I have moved them so they do not cross or come close to each other.
Megajocke, the hum is the same with the 12X4 removed."
Okay. This would rule out anode voltage ripple or the hum being amplified. It's either picked up capacitively by the output node, induced into it magnetically or from the heater supply. Does the hum change when all tubes are removed? If the hum sounds the same when off or on then heater supply can be ruled out too.
"I have an observation, tell me what you think; as we know there are two ground tracks that circle each side of the pcb and partially disects the top. Are these supposed to be independent, that is have no continuity? I don't see how this is possible but I don't understand why the board would have two ground tracks that constantly intersect each other at solder points and screw holes."
How are they connected now? If one side is used for heater and the other for anode supply it would make sense to have them separate but that doesn't seem like the case given the layout. The layout is really strange.
"I was under the impression that joining the in/out grounds both at the jacks and then at the pcb constitutes a loop. Please disabuse me of this idea if I am wrong and I will solder a wire across the top of the 4 in/out jacks' ground to the star ground (as described by Chrish) and remove the pcb ground wire (under the starground.) Further more should I jumper the scratched section of the pcb ground left of the filter caps and remove that separate ground connection too. I interpret Megajocke's instructions to mean that there should only be one signal connection to the isolation network which then connects only to the chassis and earth ground on it's other end. "
A loop is only a loop if it has area - tying wires together should be enough, twisting even better. In my crossover I did just that - link RCA barrels and had shields connected both ends without problems. There is no important diffence in this case between one wire and two wires very close to each other.
With unbalanced connections just lifting shields doesn't work as it's the signal reference as gootee has pointed out. Usually for solid state poweramps a quasi-balanced input having a gain of 1 for common mode and ~40 for differential mode can be used easing the wiring layout requirements. You interpreted my grounding scheme correct - only one wire to the disconnect network, it's not a star ground any longer. You could wait with the modifications, they might not help, try first with all tubes removed and shorting the output signal at different places as described further down.
"I like the idea of solving the hum while preamp off first.
As I said above, removing the rectifier tube does not mitigate the hum. Jumping the heater dc leads, when off but plugged in, to starground does not change the hum. It does prevent the preamp from turning on though. (I continually remind myself how new I am to this.)
Reading on, I'll bridge the bottom section (power circuit) of pcb at the filter caps farthest from the B+ and remove that ground wire.
The hum does not diminish with the cover on."
You could try (with preamp off and disconnected):
Short on the inside of RCA:s. Hum? Remove short and short over 330k resistor on the other end of internal cable. Any change?
Another thing to try is shorting over the earth disconnect network. Also with amp off and disconnected. Is there any change if mains plug is held against conducting part of case when the lid is on? What happens if RCA barrels are connected to chassis with a short wire where they enter? These tests can probably be done with a short but thick wire just touching it to the points firmly by hand.
First of all, the preamp still hums when turned off. (Slightly less so when plugged in but only marginally.) Since I made the grounding changes on http://home.comcast.net/~garyworld/preampfeb29.jpg the hum is as present as ever. I had some experience with this before when disconnecting the pcb end of the output ground/shield, this tended to lessen the hum slightly. Now it is about the same on or off."
Does the character of the hum change when turned on? Is it soft or buzzy?
"The CT to filter caps and their ground to star ground are not touching as it seems in the pic. I have moved them so they do not cross or come close to each other.
Megajocke, the hum is the same with the 12X4 removed."
Okay. This would rule out anode voltage ripple or the hum being amplified. It's either picked up capacitively by the output node, induced into it magnetically or from the heater supply. Does the hum change when all tubes are removed? If the hum sounds the same when off or on then heater supply can be ruled out too.
"I have an observation, tell me what you think; as we know there are two ground tracks that circle each side of the pcb and partially disects the top. Are these supposed to be independent, that is have no continuity? I don't see how this is possible but I don't understand why the board would have two ground tracks that constantly intersect each other at solder points and screw holes."
How are they connected now? If one side is used for heater and the other for anode supply it would make sense to have them separate but that doesn't seem like the case given the layout. The layout is really strange.
"I was under the impression that joining the in/out grounds both at the jacks and then at the pcb constitutes a loop. Please disabuse me of this idea if I am wrong and I will solder a wire across the top of the 4 in/out jacks' ground to the star ground (as described by Chrish) and remove the pcb ground wire (under the starground.) Further more should I jumper the scratched section of the pcb ground left of the filter caps and remove that separate ground connection too. I interpret Megajocke's instructions to mean that there should only be one signal connection to the isolation network which then connects only to the chassis and earth ground on it's other end. "
A loop is only a loop if it has area - tying wires together should be enough, twisting even better. In my crossover I did just that - link RCA barrels and had shields connected both ends without problems. There is no important diffence in this case between one wire and two wires very close to each other.
With unbalanced connections just lifting shields doesn't work as it's the signal reference as gootee has pointed out. Usually for solid state poweramps a quasi-balanced input having a gain of 1 for common mode and ~40 for differential mode can be used easing the wiring layout requirements. You interpreted my grounding scheme correct - only one wire to the disconnect network, it's not a star ground any longer. You could wait with the modifications, they might not help, try first with all tubes removed and shorting the output signal at different places as described further down.
"I like the idea of solving the hum while preamp off first.
As I said above, removing the rectifier tube does not mitigate the hum. Jumping the heater dc leads, when off but plugged in, to starground does not change the hum. It does prevent the preamp from turning on though. (I continually remind myself how new I am to this.)
Reading on, I'll bridge the bottom section (power circuit) of pcb at the filter caps farthest from the B+ and remove that ground wire.
The hum does not diminish with the cover on."
You could try (with preamp off and disconnected):
Short on the inside of RCA:s. Hum? Remove short and short over 330k resistor on the other end of internal cable. Any change?
Another thing to try is shorting over the earth disconnect network. Also with amp off and disconnected. Is there any change if mains plug is held against conducting part of case when the lid is on? What happens if RCA barrels are connected to chassis with a short wire where they enter? These tests can probably be done with a short but thick wire just touching it to the points firmly by hand.
Great News...
It would seem the star grounding has worked; the 120 Hz hum is now absent from the preamp out and amp out when preamp is off. There is still 100mV 60Hz hum when they are on but I see this as an important victory and feel very happy. It was previously theorized that this hum was the sum of several issues possibly compounding each other.
Here is a pic of the current state. Once again, thanks for the constant support and advice:
http://home.comcast.net/~garyworld/preampmarch1.jpg
Here is a close up of the starground:
http://home.comcast.net/~garyworld/starground.jpg
Please let me know if this can be improved to make it final.
Here is where you guys are going to get annoyed with me...I made the grounding changes prescribed most recently by megajocke. I tested the system without a successful outcome and proceeded to install the 4 output caps and scrub all the pcb connections with alcohol and toothbrush. Then I realized that the rca output cable was still plugged into the old, now floating, output jack. So I can't conclude which one of the three changes caused the improvement. I'm assuming it's the ground scheme. Mark, I'm sorry about this. I will gladly replace the old caps if you are interested in a conclusive answer. Let me know.
Divide and conquer. Tom, I'm going to order resistors to step down the scope probe signal to my computer. What is the next step?
You guys are the best. Talk to you on Monday.
gary
ps, I cleaned up the picture site: http://home.comcast.net/~garyworld/site/?/photos/
I will open up the tranny box and take pictures, I get the feeling there are some improvements I can make with the fuse and switch wiring.
It would seem the star grounding has worked; the 120 Hz hum is now absent from the preamp out and amp out when preamp is off. There is still 100mV 60Hz hum when they are on but I see this as an important victory and feel very happy. It was previously theorized that this hum was the sum of several issues possibly compounding each other.
Here is a pic of the current state. Once again, thanks for the constant support and advice:
http://home.comcast.net/~garyworld/preampmarch1.jpg
Here is a close up of the starground:
http://home.comcast.net/~garyworld/starground.jpg
Please let me know if this can be improved to make it final.
Here is where you guys are going to get annoyed with me...I made the grounding changes prescribed most recently by megajocke. I tested the system without a successful outcome and proceeded to install the 4 output caps and scrub all the pcb connections with alcohol and toothbrush. Then I realized that the rca output cable was still plugged into the old, now floating, output jack. So I can't conclude which one of the three changes caused the improvement. I'm assuming it's the ground scheme. Mark, I'm sorry about this. I will gladly replace the old caps if you are interested in a conclusive answer. Let me know.
Divide and conquer. Tom, I'm going to order resistors to step down the scope probe signal to my computer. What is the next step?
You guys are the best. Talk to you on Monday.
gary
ps, I cleaned up the picture site: http://home.comcast.net/~garyworld/site/?/photos/
I will open up the tranny box and take pictures, I get the feeling there are some improvements I can make with the fuse and switch wiring.
Wooo Hooooooo!!!!!
That's great news, Gary!!!!!
-----
No, I'm not annoyed about the errant cable versus caps question. I'm just happy that you fixed the thing (AND that you found the errant cable)!
I "might" be "annoyed" if you tell me how long that cable has been like that...!
)) Just kidding!!
-------
*Coincidentally*, THIS was recently posted in sci.electronics.design, for us:
-----
By legg:
On Fri, 29 Feb 2008 23:28:49 GMT, Rich Grise <r...@example.net> wrote:
>Your preamp has a loose ground on its output, and is acting like an antenna.
Yes, it really sounds like a lousy patch cord with an open ground on one end. - the extreme case of 'ground loop'.
RL
-----
And, for completeness' sake, here's another recent post by legg/RL:
------
Looking at the schematic, it seems that the output impedance of the preamplifier is extremely high.
I mean 330K? Get real. Although the reciever may have an input that's equally high, the idea is always to get the source impedance lower than the receiver, if noise is a concern.
I think these tube could drive 33K and still develop a 3V signal with only a 2% change in cathode current. (coupling cap 0.12 for 40Hz)
Hell, why isn't it 3K3? ......1uF /240V coupling still gives -3dB at
50Hz
RL
------
Gary,
Not that I'm 'a glutton for punishement', or something like that, but, can your HEAR the 100mV of hum? If so, how much of a problem is it? i.e. How badly do you want to get rid of it?
Anyway, CONGRATULATIONS!!
- Tom
That's great news, Gary!!!!!
-----
No, I'm not annoyed about the errant cable versus caps question. I'm just happy that you fixed the thing (AND that you found the errant cable)!
I "might" be "annoyed" if you tell me how long that cable has been like that...!
)) Just kidding!!-------
*Coincidentally*, THIS was recently posted in sci.electronics.design, for us:
-----
By legg:
On Fri, 29 Feb 2008 23:28:49 GMT, Rich Grise <r...@example.net> wrote:
>Your preamp has a loose ground on its output, and is acting like an antenna.
Yes, it really sounds like a lousy patch cord with an open ground on one end. - the extreme case of 'ground loop'.
RL
-----
And, for completeness' sake, here's another recent post by legg/RL:
------
Looking at the schematic, it seems that the output impedance of the preamplifier is extremely high.
I mean 330K? Get real. Although the reciever may have an input that's equally high, the idea is always to get the source impedance lower than the receiver, if noise is a concern.
I think these tube could drive 33K and still develop a 3V signal with only a 2% change in cathode current. (coupling cap 0.12 for 40Hz)
Hell, why isn't it 3K3? ......1uF /240V coupling still gives -3dB at
50Hz
RL
------
Gary,
Not that I'm 'a glutton for punishement', or something like that, but, can your HEAR the 100mV of hum? If so, how much of a problem is it? i.e. How badly do you want to get rid of it?
Anyway, CONGRATULATIONS!!
- Tom
gary h said:
Wouldn't it be easier to just put the rca output cable back in the old, now-floating output jack? If the old hum problem returns, then that cable was probably the main problem, or at least the 'last' major problem.
How long HAS that cable been like that, anyway?
Or, rather, since what set of modifications has it been that way?I guess if you really want to know what's going on, you might have to undo everything back to that point!
I think I would just 'go with it', and proceed from this point, to try to remove the remaining hum.
However, going back two paragraphs, it would be worth at least analyzing what has been done since the time when you think the output was left in an 'open' state, just so we don't enhance the possibility of missing something that we thought didn't work, that might be a clue to fixing the continuing hum problem.
So we are talking about Friday night after post #123.(http://home.comcast.net/~garyworld/preampfeb29.jpg which I have since removed so as not to confuse.) I had cut the pcb upper and lower ground tracks before and after the filter caps, connected the CT to the now small section of pcb ground track at those filter caps and made a ground connection for it and the larger remaining section of ground track serving the other 300 degrees of the board. I had also connected the output shield back to the ground spot near the output and tied the signal leads together into a bundle. The rca cable was connected to the live output at this point.
I was still suspicious that connecting a single ground at the jacks would create a loop but megajocke assured me that this was not a problem. He was right. I now understand that it is area of loop that impacts hum, not necessarily schematic loop.
The time during which I had the ouput plugged into the wrong jack was for the 15 minutes it took me to replace the output caps. I soldered on that copper wire across the 4 in/out jacks, (moving the active output up to the adjacent inputs,) neglected to reconnect the rca cable respectively, tested, had hum, put in the caps, then realized my mistake. So it could be the caps that made the difference, but most likely it is the single ground wire soldered across the jacks, replacing all the other pcb ground connections. Like I said, I am happy to put the old caps back in so we all know the truth. Let me know if you have the slightes itch to know for sure, it would only take me a few minutes. Again, the current ground is pictured here:
http://home.comcast.net/~garyworld/preampmarch1.jpg
http://home.comcast.net/~garyworld/starground.jpg
So...the 60 Hz hum from the unplugged preamp is now below 1 mV. This is the same for the amp with nothing plugged in. When connected together it is 100 mV to the speakers, audible. I've tried changing interconnects without success. Is there something else at play here that I can fiddle with? Otherwise I'm not going to look a gift horse in the mouth.
gary
I was still suspicious that connecting a single ground at the jacks would create a loop but megajocke assured me that this was not a problem. He was right. I now understand that it is area of loop that impacts hum, not necessarily schematic loop.
The time during which I had the ouput plugged into the wrong jack was for the 15 minutes it took me to replace the output caps. I soldered on that copper wire across the 4 in/out jacks, (moving the active output up to the adjacent inputs,) neglected to reconnect the rca cable respectively, tested, had hum, put in the caps, then realized my mistake. So it could be the caps that made the difference, but most likely it is the single ground wire soldered across the jacks, replacing all the other pcb ground connections. Like I said, I am happy to put the old caps back in so we all know the truth. Let me know if you have the slightes itch to know for sure, it would only take me a few minutes. Again, the current ground is pictured here:
http://home.comcast.net/~garyworld/preampmarch1.jpg
http://home.comcast.net/~garyworld/starground.jpg
So...the 60 Hz hum from the unplugged preamp is now below 1 mV. This is the same for the amp with nothing plugged in. When connected together it is 100 mV to the speakers, audible. I've tried changing interconnects without success. Is there something else at play here that I can fiddle with? Otherwise I'm not going to look a gift horse in the mouth.
gary
Gary,
Yes, there is definitely more that can be done.
The filament-supply grounding comes to mind, as one thing that should probably be changed.
Edit: Wait. You said 60 Hz hum. Hmmmm.... It may be something else besides the filament supply, then, I guess, since that would probably be 120 Hz. (But what about the rectifier tube's AC filament supply, as I think hailteflon mentioned a while back? Could that be a problem? If so, is there any good way to filter that?)
-----
To megajocke, hailteflon, Gary, and everyone else who is interested,
There are some very interesting new developments in the Usenet threads about this (use links, below). I hope that someone else can start helping to answer their questions. I am getting very busy with other stuff. And they are asking more questions, including some that I cannot answer.
http://groups.google.com/group/sci....b16bf44591d/31b59326f5f760e8#31b59326f5f760e8
http://groups.google.com/group/rec....55b8584b92a/d55b93b16c56b0f4#d55b93b16c56b0f4
Yes, there is definitely more that can be done.
The filament-supply grounding comes to mind, as one thing that should probably be changed.
Edit: Wait. You said 60 Hz hum. Hmmmm.... It may be something else besides the filament supply, then, I guess, since that would probably be 120 Hz. (But what about the rectifier tube's AC filament supply, as I think hailteflon mentioned a while back? Could that be a problem? If so, is there any good way to filter that?)
-----
To megajocke, hailteflon, Gary, and everyone else who is interested,
There are some very interesting new developments in the Usenet threads about this (use links, below). I hope that someone else can start helping to answer their questions. I am getting very busy with other stuff. And they are asking more questions, including some that I cannot answer.
http://groups.google.com/group/sci....b16bf44591d/31b59326f5f760e8#31b59326f5f760e8
http://groups.google.com/group/rec....55b8584b92a/d55b93b16c56b0f4#d55b93b16c56b0f4
hi guys,
megajocke, the hum is present only when the preamp is turned on. It doesn't diminish at all with rectifier or/and 12AU7 tubes removed.
I'm sure you all are wondering, is this hum a problem or is he just fussing over something insignificant. I wish it were the case. Although the buzzing is now gone it's absence reveals a rich and equally uninvited lower hum. My speakers are flat down to the low 30s and don't waste this opportunity to remind me. The Adcom has no attenuation and is full on. The hum is still independent of the volume control so it's always there. It really is still unacceptable.
I didn't quite understand all the discussion about my probe misrepresenting the measurements. I thought that the hum occurring only when the preamp was on and connected to the amp would be a clue regardless.
Tom, thanks for representing on the other forums. I'll make some time to explain myself over there.
gary
megajocke, the hum is present only when the preamp is turned on. It doesn't diminish at all with rectifier or/and 12AU7 tubes removed.
I'm sure you all are wondering, is this hum a problem or is he just fussing over something insignificant. I wish it were the case. Although the buzzing is now gone it's absence reveals a rich and equally uninvited lower hum. My speakers are flat down to the low 30s and don't waste this opportunity to remind me. The Adcom has no attenuation and is full on. The hum is still independent of the volume control so it's always there. It really is still unacceptable.
I didn't quite understand all the discussion about my probe misrepresenting the measurements. I thought that the hum occurring only when the preamp was on and connected to the amp would be a clue regardless.
Tom, thanks for representing on the other forums. I'll make some time to explain myself over there.
gary
gary h said:
Hi Gary,
I have been following this every night and am up to date on your progress. I have not commented because others were and I didn't want to confuse the issue. Too many cooks can spoil the broth so to speak.
With regard to your statment above, this only leaves one thing. The power transformer. Here is what I'd like you to do.
Remover the mounting bolts from the transformer so that you can turn the transformer 90º. And also lean it backwards so that it is lying down.
I think that this will not really help much but it must be tried first to see if there is any change in the hum level.
When this doesn't cure the hum, the next step is to remove the transformer from the preamp altogether. Then, either using alligator jumper wires or extenting the transformer leads by soldering on long wires, reconnect it to the circuit but keep it at least one foot away from the preamp.
If the hum stops, you have a tramsformer that may otherwise work ok, but is producing hum. No shielding will help either. I've had this problem before with hummy tramsformers and the only cure was to isolate the transformer from the chassis or replace it with a different unit.
All this assumes that you get hum the instant you throw the preamp power switch on. That's what it sounds like from your statment above. Correct me if I'm wrong and/or report the outcome.
Victor
Hi Victor,
if I'm not mistaken you have previously recommended this very option in this thread. If it wasn't you then thanks anyway for the advice. The truth is I followed this advice and mounted the transformer in a separate rat shack box via a 20" braided umbillical.
Sorry to say it didn't yield significant improvement. That's not to say it wasn't worth while and I enjoy the fact that the preamp now looks like it's on leave from the gadget ICU.
gary
if I'm not mistaken you have previously recommended this very option in this thread. If it wasn't you then thanks anyway for the advice. The truth is I followed this advice and mounted the transformer in a separate rat shack box via a 20" braided umbillical.
Sorry to say it didn't yield significant improvement. That's not to say it wasn't worth while and I enjoy the fact that the preamp now looks like it's on leave from the gadget ICU.
gary
So Victor has suggested that if the preamp hums immediately on switch on, which it does, with the tubes removed, it is likely that the transformer could be bad. I'm under the impression that this is hard to test. Does anyone else have thoughts on this? If I am to replace the transformer what would be a more worthy substitute?
gary
gary
Yo Gary,
Please correct me if I'm mistaken here. I did suggest this a few pages back. As I understand things, there were two hum condidtions. One was hum with the preamp turned off but connected to the amp. This has now been corrected...yes?
Now you have a second hum when the preamp is turned on. If you disconnected the power transformer before the first hum problem was fixed, it may have seemed like doing so didn't help. If this is true, then moving the power transformer again to check for lower hum may be necessary. Do not just replace it until you are sure it needs replacing.
As I type this, I don't remember if you're using tube or solid state rectifiaction. If tube, it applies. If SS, there may be other causes that need to be eliminated first since SS rectification provides power virtually instantaneously.
Victor
Please correct me if I'm mistaken here. I did suggest this a few pages back. As I understand things, there were two hum condidtions. One was hum with the preamp turned off but connected to the amp. This has now been corrected...yes?
Now you have a second hum when the preamp is turned on. If you disconnected the power transformer before the first hum problem was fixed, it may have seemed like doing so didn't help. If this is true, then moving the power transformer again to check for lower hum may be necessary. Do not just replace it until you are sure it needs replacing.
As I type this, I don't remember if you're using tube or solid state rectifiaction. If tube, it applies. If SS, there may be other causes that need to be eliminated first since SS rectification provides power virtually instantaneously.
Victor
Hi Victor,
That is correct
I haven't replaced the transformer since I put it in the box. I wired the box with switch and fuse, added an EMI outlet so I could run an earth ground to the preamp. Here is a link which has a picture of the preamp and remote transformer on a table:
http://home.comcast.net/~garyworld/site/?/photos/
Victor, the above link also has schematics and diagrams so you can see the preamp's architecture.
thanks,
gary
HollowState said:
That is correct
I haven't replaced the transformer since I put it in the box. I wired the box with switch and fuse, added an EMI outlet so I could run an earth ground to the preamp. Here is a link which has a picture of the preamp and remote transformer on a table:
http://home.comcast.net/~garyworld/site/?/photos/
Victor, the above link also has schematics and diagrams so you can see the preamp's architecture.
thanks,
gary
Hi all,
for those who followed this thread with interest and/or helped me throughout, (special thanks to Hailteflon, Tom Gootee, megajocke, and HollowState for outstanding support,) I have finally solved this hum issue.
To summarize, I built a triode preamp which manifested both 120 Hz and 60 Hz hum at the output. The 120 Hz was solved after a comprehensive grounding scheme was employed. The 60 Hz hum proved to be ripple on the high voltage rail. It has been fixed by changing resistor values at the filter caps.
http://home.comcast.net/~garyworld/site/?/photos/
The link above has schematics, for those who are still interested R101 and R102 (1.5K Ohm 1W) were replaced with 10K Ohm 2W resistors. This reduced the 100 mV pp hum at output to 500 uV. After amplification this presents an audible yet acceptable level of hum, and the source material sounds great with the warm and living character I was hoping to find in the first place. TG put me in touch with someone at another forum who ran the preamp schematics through a spice simulation to arrive at the suggested fix. I'm sure that there is a price to pay somewhere but I will credit that to my next project.
Thanks once again, if there is a consensus for a compiled and sensible chronology of events I would be happy to generate one and post it as a separate thread.
g
for those who followed this thread with interest and/or helped me throughout, (special thanks to Hailteflon, Tom Gootee, megajocke, and HollowState for outstanding support,) I have finally solved this hum issue.
To summarize, I built a triode preamp which manifested both 120 Hz and 60 Hz hum at the output. The 120 Hz was solved after a comprehensive grounding scheme was employed. The 60 Hz hum proved to be ripple on the high voltage rail. It has been fixed by changing resistor values at the filter caps.
http://home.comcast.net/~garyworld/site/?/photos/
The link above has schematics, for those who are still interested R101 and R102 (1.5K Ohm 1W) were replaced with 10K Ohm 2W resistors. This reduced the 100 mV pp hum at output to 500 uV. After amplification this presents an audible yet acceptable level of hum, and the source material sounds great with the warm and living character I was hoping to find in the first place. TG put me in touch with someone at another forum who ran the preamp schematics through a spice simulation to arrive at the suggested fix. I'm sure that there is a price to pay somewhere but I will credit that to my next project.
Thanks once again, if there is a consensus for a compiled and sensible chronology of events I would be happy to generate one and post it as a separate thread.
g
Hi Gary,
I'm very glad to read that it's finally working correctly and you're enjoying it. I'll sleep a little better tonight too. With only two 22ufd caps as filters, I guess it makes sense in retrospect. And the current drain must be low enough as to not cause a big voltage drop with the 10Ks. Now I'm wondering...is the power transformer back in the chassis or still on the umbilical?
I'm very glad to read that it's finally working correctly and you're enjoying it. I'll sleep a little better tonight too.
With only two 22ufd caps as filters, I guess it makes sense in retrospect. And the current drain must be low enough as to not cause a big voltage drop with the 10Ks. Now I'm wondering...is the power transformer back in the chassis or still on the umbilical?- Status
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