What is happening if you just short the input of the secont tube ?
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Yes, it tells us that the hum is not coming from ripple on the B+, which is still present with the valve removed. The observation that removing the first valve causes no loss of hum tells us that the vast majority of common hum sources are ruled out.
We are all operating on some assumptions: that the phono preamp is being tested with shorted inputs, that the hum is still present with only one channel's interconnecting cable attached, and that ordinary measures are being taken to minimize safety/PE ground loops. If all those assumptions are true, and an actual 'scope picture isn't possible, then I'm leaning towards the heater supply as being a next candidate for exploring, as unlikely as it seemed at first (normally we would expect that to be far more significant to the first stage, especially with its unbypassed cathode resistor). Or parasitics - grid stops are your friend.
All good fortune,
Chris
I've changed my mind. Stephe and other folk have built this same? board and general layout successfully without issue, so we really shouldn't be looking for some exotic error in the design. Instead, we probably need to examine the details of construction, paying special attention to wiring of grounds. The rule is "one and only one ground path".
All good fortune,
Chris
All good fortune,
Chris
Yes, Stephe and alot of competent diy people have built this board with good results, but also a lot of people have had hum problems for different reasons.
Either I have done something wrong (likely) or some component is faulty.
I will buy a 6V battery today and test to run the heaters from it just to rule out or comfirm that option.
Either I have done something wrong (likely) or some component is faulty.
I will buy a 6V battery today and test to run the heaters from it just to rule out or comfirm that option.
Ok, so I did the battery test, and I am way more confused now. With a 6V led battery the hum became even louder. Also when measuring the hum with my phone with the app Spectroid there are two peaks. 50hz is the loudest, and then a 100hz peak. When I switch back to the PSU to run the heaters the 100hz peak is the loudest and the 50hz peak is also there but not as loud. Overall the hum is significantly less loud with the PSU than with the battery.
I will say this phono pre-amp will highlight bad grounding inside an amplifier. I hooked this EAR834 clone to a Wilsenton R8 and started picking up a country music radio station (and more hum than I had ever heard before). It didn't do this with any other amp I own. The fault was the grounding scheme inside that amp. Once I converted the amp to a star ground design, those issues went away.
Note in the original design, the heater is "lifted" by a voltage divider, so it doesn't shock me that using an external source that is floating would have more noise.
Note in the original design, the heater is "lifted" by a voltage divider, so it doesn't shock me that using an external source that is floating would have more noise.
Maybe it is time to throw away that box and begin anew using a metal one ...
The transformer should be put in another separate box .
If the tops of the tubes will be visible you should buy sockets with shields at least for the first two .
Ok, thanks Stephe. I don't have any other amp at home then the ones I use. It is connected to a Vincent SA-T8 preamp and a Classé Delta CA-2100 power amp.
Maybe you are right about the enclosure Depanatoru. But I have now used mumetal to shield it and hum goes away instantly when powered off. If it was a shielding problem then shouldn't the hum stay for a few seconds while the caps discharge?
Could a bad solder joint somewhere cause this?
I am wondering if this could be because of the mods? But they are made by skilled people and tested by many others. Anyway, these are the mods I made:
I don't have any other amp at home then the ones I use. It is connected to a Vincent SA-T8 preamp and a Classé Delta CA-2100 power amp.Maybe you are right about the enclosure Depanatoru. But I have now used mumetal to shield it and hum goes away instantly when powered off. If it was a shielding problem then shouldn't the hum stay for a few seconds while the caps discharge?
Could a bad solder joint somewhere cause this?
I am wondering if this could be because of the mods? But they are made by skilled people and tested by many others. Anyway, these are the mods I made:
Then there seems to be nothing wrong with the filament power.
As the hum increased. there might be some coupling between the transformer and the amp. Is it possible to
move the transformer away ? If loosened one could also move around the transformer, if any hum differences
it seems to be a coupling.
The funny thing is that if you were to draw out an actual schematic, showing actual connections, including a whole lot of nebulous options from post #118, any experienced person could pinpoint the issue. But that doesn't seem to be in the cards, and you won't be able to learn much from this exercise without that effort.
God and the Devil dwell together in the Details,
Chris
God and the Devil dwell together in the Details,
Chris