I have mainly built integrated amps so I don't really have experience with separate pre and power amps. Here is my issue...
I built a pre-amp using #27 tubes from a schematic mrcurwen posted last year some time. Looking at the output on my scope it looks very good when using my signal generator.
I just built the shishido 12ax7 -> 2a3 dc coupled amp. When the preamp is off, the hum is very very slight. I've nulled it out as best as I can.
Now... when I turn on the pre-amp, I get a very loud hum almost like you would have if there were a ground loop between your source and amplifier.
Would it be a bad idea to connect the two ground bus segments together? I would have thought that they would have been tied together with the interconnect cables.
Looking for input.
Thanks,
John
I built a pre-amp using #27 tubes from a schematic mrcurwen posted last year some time. Looking at the output on my scope it looks very good when using my signal generator.
I just built the shishido 12ax7 -> 2a3 dc coupled amp. When the preamp is off, the hum is very very slight. I've nulled it out as best as I can.
Now... when I turn on the pre-amp, I get a very loud hum almost like you would have if there were a ground loop between your source and amplifier.
Would it be a bad idea to connect the two ground bus segments together? I would have thought that they would have been tied together with the interconnect cables.
Looking for input.
Thanks,
John
The interconnects are not intended to be the ground connection between two devices because they should never carry current. You apparently have a difference of potential between the two grounds of the preamp and the amp.
The Shishido amp has an input sensitivity of about 1VRMS and there for should be used without a preamp. Your preamp is not just adding hum, it is also adding noise and distortion.
The Shishido amp has an input sensitivity of about 1VRMS and there for should be used without a preamp. Your preamp is not just adding hum, it is also adding noise and distortion.
I think that I may have found the problem.....
after turning on the pre-amp, I noticed that when I move my had near the plate ccs, the tone of the hum changed. Also, if I touch the grid chokes, the hum becomes louder. I just grounded the grid choke frames and that helped a lot.
So.... touching the #27 envelopes, the tone changes. Touching the grid chokes (which seem to be vibrating) the hum will get louder. Getting my hand near the plate ccs, the hum starts to sound like a thereman. Very odd.
I think that shielding is in order for the ccs and better placement for the grid chokes (talking about the pre-amp).
I am listening to the amp at the moment and it is "listenable" however I know the hum is there so I do hear it in the back ground.
after turning on the pre-amp, I noticed that when I move my had near the plate ccs, the tone of the hum changed. Also, if I touch the grid chokes, the hum becomes louder. I just grounded the grid choke frames and that helped a lot.
So.... touching the #27 envelopes, the tone changes. Touching the grid chokes (which seem to be vibrating) the hum will get louder. Getting my hand near the plate ccs, the hum starts to sound like a thereman. Very odd.
I think that shielding is in order for the ccs and better placement for the grid chokes (talking about the pre-amp).
I am listening to the amp at the moment and it is "listenable" however I know the hum is there so I do hear it in the back ground.
Both the core and the outer of the coaxial interconnect carry current.
These will be the signal current and the interference current.
That cannot be avoided in an unbalanced interconnect.
Lex,
The interference current can be bad, due to poor wiring.
This interference cannot be eliminated, but it can be attenuated.
First
ensure the amplifier grounding is correctly implemented.
Measure the output noise and output offset using a DMM voltage scale that can resolve 0.1mVac and 0.1mVdc.
With the input shorted and with the input open, with no load on the outputs. That's four measurements for the power amplifier alone.
Second
when connecting an unbalanced interconnect, recheck the output offset and output noise when One interconnect is added, with the far end open and with the far end shorted.
Then swap to the other channel and repeat the four measurements.
Then add on a second interconnect and repeat all eight measurements.
Finally connect the barrels/outer of the far end of the two interconnects and repeat the eight measurements.
Post all your results and we should be able to help identify your problem/s.
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Does the tone change or does the level change? If the tone changes it is not hum
Did you try connecting the grounds of the two chassis together?
Jan
I did connect the two chassis grounds together but that was no help.
Tone change or level change: Both.
I did discover that grounding the frame of the grid chokes (pre-amp) helped a bit. I touched the top of the grid choke and it is vibrating. I am going to go back to the original schematic and replace the grid chokes with resistors. The #27 tubes that I am using are microphonic and I think that the vibrations of the grid chokes are making the noise worse. The chokes will get a soak in transformer varnish.
Going to pull the chokes now..... I will report back when I have that done.
Tone change or level change: Both.
I did discover that grounding the frame of the grid chokes (pre-amp) helped a bit. I touched the top of the grid choke and it is vibrating. I am going to go back to the original schematic and replace the grid chokes with resistors. The #27 tubes that I am using are microphonic and I think that the vibrations of the grid chokes are making the noise worse. The chokes will get a soak in transformer varnish.
Going to pull the chokes now..... I will report back when I have that done.
Grid chokes removed and replaced with 470K resistors. That did eliminate some of the noise. I also replaced the mesh plate #27 tubes and put in a pair of solid plate #27 sylvania tubes. The "theremin" effect was gone.
It seems that the tubes that I was using (mesh plates) were pretty but also REALLY prone to any outside electrical interference.... including even getting your hand near them.
I do have hum... but this is actual hum which I can work on with improving the grounding and also shielding the heater wires. I have copper tape that I can wrap the heater wires with and ground one end and see if that helps if changing around the grounding proves to not help that much.
I also think that I need to mount the power transformer on rubber grommets as it is vibrating. Touching the outer case of it dampens the noise so there may be interference coming from that as well.
Lots to work on.
The 2A3 amp is pretty quiet once it finishes warming up.
It seems that the tubes that I was using (mesh plates) were pretty but also REALLY prone to any outside electrical interference.... including even getting your hand near them.
I do have hum... but this is actual hum which I can work on with improving the grounding and also shielding the heater wires. I have copper tape that I can wrap the heater wires with and ground one end and see if that helps if changing around the grounding proves to not help that much.
I also think that I need to mount the power transformer on rubber grommets as it is vibrating. Touching the outer case of it dampens the noise so there may be interference coming from that as well.
Lots to work on.
The 2A3 amp is pretty quiet once it finishes warming up.
Well hum is always the line frequency. If you say the tone changes (if that is what it does) it looks more like an oscillation. Are you sure the tone changes, in the sense that the pitch or frequency changes?
Ja
Tonight I did the following:
1) wrapped all filament wires in copper foil and grounded the foil
2) grounded any input/output panel (rca jacks and pot are mounted on aluminum)
3) tried shielding the tube with grounded tin foil (ala #26 tubes)
These didn't do anything to eliminate the hum. It dawned on me that the source of the hum could be coming from the stw11nm80 heat sinks. I didn't put a mica insulator between the to-247 device and the heat sink and the back of the part is not isolated. So, it would pick up general junk and amplify it, or at least put it in the output signal.
I ordered some islet insulators and some silicone pads. We will see. If this doesn't work I will lay out the plate ccs on a pcb and etch it so that I can shield it. I kind of need to do that anyway before I build this into a permanent case.
1) wrapped all filament wires in copper foil and grounded the foil
2) grounded any input/output panel (rca jacks and pot are mounted on aluminum)
3) tried shielding the tube with grounded tin foil (ala #26 tubes)
These didn't do anything to eliminate the hum. It dawned on me that the source of the hum could be coming from the stw11nm80 heat sinks. I didn't put a mica insulator between the to-247 device and the heat sink and the back of the part is not isolated. So, it would pick up general junk and amplify it, or at least put it in the output signal.
I ordered some islet insulators and some silicone pads. We will see. If this doesn't work I will lay out the plate ccs on a pcb and etch it so that I can shield it. I kind of need to do that anyway before I build this into a permanent case.
I found a bunch of to220 ceramic insulators and I installed those. Now you have to turn the volume up more than half way to hear the noise over the 120hz that is coming from the 2a3 amp. Next up, build them on pc boards and shield them. I'm also thinking about better filtering of HV going into the boards.... common mode chokes, etc... I will have to play around with that.
I did the same things to the 2a3 amp that I did to the pre..... wrapped all of the filament wires in copper foil tape and grounded it, grounded all mounting plates, etc... I have the hum from there nulled out pretty well, but I want to do better. Working on a better power supply for the filaments.....
ps... my grounds are star grounds, so those are ok. I really can't improve on those very much.
I did the same things to the 2a3 amp that I did to the pre..... wrapped all of the filament wires in copper foil tape and grounded it, grounded all mounting plates, etc... I have the hum from there nulled out pretty well, but I want to do better. Working on a better power supply for the filaments.....
ps... my grounds are star grounds, so those are ok. I really can't improve on those very much.
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