Speaking of having a variac, or in my case not having one... looking at the cheap ones online what would be the minimum specs to have for a variac
Would this be ok??
0.5KM, 5A , 500VA Variac, Variable Transformer Input 110V 60hz AC, output 0-130V AC Amazon.com: 0.5KM, 5A , 500VA Variac, Variable Transformer Input 110V 60hz AC, output 0-130V AC: Industrial & Scientific
Would this be ok??
0.5KM, 5A , 500VA Variac, Variable Transformer Input 110V 60hz AC, output 0-130V AC Amazon.com: 0.5KM, 5A , 500VA Variac, Variable Transformer Input 110V 60hz AC, output 0-130V AC: Industrial & Scientific
On start, off set all over place, beginning to settle. I will let it warm up and have my JBL "mule" amp hooked up to speakers.
Once warm I will see if bias is down to normal, or attempt to turn it to normal. I dont think right now I would ever trust this preamp to just turn on and use, may have to be like B1, eternally on. If it still hums I am giving up on this one, something is wrong with my build. Case and PS can be re purposed. Or wait for kit to hit store and try again.
Does anyone use this, just turn it on from cold and go? or just leave it on?
Russellc
Once warm I will see if bias is down to normal, or attempt to turn it to normal. I dont think right now I would ever trust this preamp to just turn on and use, may have to be like B1, eternally on. If it still hums I am giving up on this one, something is wrong with my build. Case and PS can be re purposed. Or wait for kit to hit store and try again.
Does anyone use this, just turn it on from cold and go? or just leave it on?
Russellc
Well, it still seems to have way more DC offset than I am comfortable with. sometimes single digit mV, other times 40-70 mV. I know I could put a cap on the output, but this just shouldn't have that much if all is right.
Hum is still there, maybe not quite as bad as in the case less format, but clearly audible. I have a CL-60 coming from the Browsky power supply going to chassis ground, the ground wire from the power cord in at same spot, as well as the Antek transformers shield wire. No other grounding, yet.
I must of messed up somewhere, but this sort of result is a "dog that ain't gonna hunt" for me. I will think about it for a while, try a ground change or two, then done.
It seems to sound fine, but after playing for 30 mins or so, to mute and measure the offset and see that much just isnt acceptable. If I can make no headway on the excessive hum/offset I may try again when the store gets the unit board there. Otherwise, I'm thinking this will be a great chassis/supply for Iron pre when ever it makes appearance in the store.
Russellc
Hum is still there, maybe not quite as bad as in the case less format, but clearly audible. I have a CL-60 coming from the Browsky power supply going to chassis ground, the ground wire from the power cord in at same spot, as well as the Antek transformers shield wire. No other grounding, yet.
I must of messed up somewhere, but this sort of result is a "dog that ain't gonna hunt" for me. I will think about it for a while, try a ground change or two, then done.
It seems to sound fine, but after playing for 30 mins or so, to mute and measure the offset and see that much just isnt acceptable. If I can make no headway on the excessive hum/offset I may try again when the store gets the unit board there. Otherwise, I'm thinking this will be a great chassis/supply for Iron pre when ever it makes appearance in the store.
Russellc
Russellc, I know you have built many amplifiers so do not give up yet. Pictures, and what kind of regulator are you using? I am using just a cheap Chinese one from ebay but I did add some more filtration, much as Nelson has done on other projects that use a switcher. I suspect the switcher is the problem. Try another one before throwing in the towel and add some more caps after the switcher. Measure the voltage out of the switcher and see if it is varying for 15 minutes or longer. This is too nice a linestage designed by one of the best in the business to give up. The reason I say switcher is because the problem is in both channels.
I tend to think I damaged something, but it is in both channels. No one seems to be overly concerned about the offset, but I have always been told that DC on a preamps output is a bad thing...especially with an amplifier that is direct connected. Perhaps this is not correct, but if all was well, I wouldn't have this much. I don't tolerate this much on a power amp!
I don't like the idea of the cap on the output, ok as a just in case measure, but not as a bandaid. The hum I can probably figure out.
As to my power supply/regulator, it is the Browsky regulator from Tubecad that 6l6 used in this thread. It is excellent and steady as a rock.
Grounds used are, CL-60 from power supply ground to chassis. Power cord ground goes to same spot, as does transformer shield. This usually does it.
Russellc
I don't like the idea of the cap on the output, ok as a just in case measure, but not as a bandaid. The hum I can probably figure out.
As to my power supply/regulator, it is the Browsky regulator from Tubecad that 6l6 used in this thread. It is excellent and steady as a rock.
Grounds used are, CL-60 from power supply ground to chassis. Power cord ground goes to same spot, as does transformer shield. This usually does it.
Russellc
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Most amps are difficult to get much below 50 mV DC offset. To get stable below 10 mV you need a very stable PSU (it seems you have that) and you need to be able to control the temperature of the line stage so it's temp is stable within….maybe 1 degree. That is difficult. When you measure offset with top off the temp goes down and when you put top on temp goes up and offset will drift. Be happy with let us say 20 mV if your power amp is not DC coupled and will amplify DC.
Regarding hum is it important to know if hum is coupled magnetic/electrical to signal wires or from PSU or caused by ground loop. Is it 60 or 120 Hz hum? ....you can measure using Arta software and Focusrite (same method when building BA3-pre
).
Regarding hum is it important to know if hum is coupled magnetic/electrical to signal wires or from PSU or caused by ground loop. Is it 60 or 120 Hz hum? ....you can measure using Arta software and Focusrite (same method when building BA3-pre
).Any ideas? I would have more ideas if it were a power amp. This method has always worked before. I used exactly the same switching (tubecad) as we both used in BA3 FE build. Same power switch to!
I even used the same volume control, ebay Chinese ladder pot, 25K.
I guess its time to get out the wired alligator clips and playing around. I'm at a loss, dont think hum problem has affected others with this particular build. Guess I could start with bypassing the CL-60, but that seems unlikely.
Russellc
I even used the same volume control, ebay Chinese ladder pot, 25K.
I guess its time to get out the wired alligator clips and playing around. I'm at a loss, dont think hum problem has affected others with this particular build. Guess I could start with bypassing the CL-60, but that seems unlikely.
Russellc
![IMG_4717[1].jpg](/community/data/attachments/677/677391-9d25e9ba387b4b0528996e2e1fc046fb.jpg)
![IMG_4716[1].jpg](/community/data/attachments/677/677404-bd458d8a967322593fae53b6b4f9516c.jpg)
Hi Russellc, is this fuze holder on the primary side of transformer?
If it is, please remove this away from the metal bottom.
It is danger!
Checking continuity from the main ground bolt, I seem to get the continuity buzzer on all places on all boards where it should be. I do note that the only panel with continuity is the floor pan. The anodizing prevents it on front and side panels, if that makes a difference.
This is the first preamp or phono preamp I have built with a hum. Well maybe a slight one on some tube pres...
Russellc
This is the first preamp or phono preamp I have built with a hum. Well maybe a slight one on some tube pres...
Russellc
Not sure if I understand this one. I have never seen a fuse holder that wasnt! Even if one of those twist off types that mounts on the back panel, it usually has continuity to the floor pan as well, or soon will as soon as the mounting screws wear through the anodizing.
The fuse is in the hot side, as all First Watt power supplies show.
Maybe I misunderstand, how do you mount your fuse holders?
Russellc