Hi,
I tried to use a mini digital oscilloscope (Zeeweii DSO3D12) to visualize hum, but it's kinda act weird. Not sure if I hook up correctly.
I've just recently built a tube preamp with recommendation from this thread (https://www.diyaudio.com/community/threads/6j1-china-preamp-thoughts.309876/). It sounds fine but it has some hum, louder on the left channel. I connect probe ground to signal ground and probe to left output. Despite loud hum from the speaker, the scope show no signal. However, if I float the probe ground, it show 50hz signal at few Vpp. Using a DMM, it also show 50Hz.
What should I adjust my setting? Please suggest.
AP
I tried to use a mini digital oscilloscope (Zeeweii DSO3D12) to visualize hum, but it's kinda act weird. Not sure if I hook up correctly.
I've just recently built a tube preamp with recommendation from this thread (https://www.diyaudio.com/community/threads/6j1-china-preamp-thoughts.309876/). It sounds fine but it has some hum, louder on the left channel. I connect probe ground to signal ground and probe to left output. Despite loud hum from the speaker, the scope show no signal. However, if I float the probe ground, it show 50hz signal at few Vpp. Using a DMM, it also show 50Hz.
What should I adjust my setting? Please suggest.
AP
How sensitive is the DSO3D12? Can you set it on a mV scale? It doesn't take many mV to be audible on a speaker.
Don't float the probe ground, that's just making the thing into an antenna and tells you nothing.
Don't float the probe ground, that's just making the thing into an antenna and tells you nothing.
I followed the thread and video by TheStuffMade (
) and Tha(t)oneGuy (https://www.diyaudio.com/community/threads/6j1-china-preamp-thoughts.309876/post-7539158) so credit goes to them.
Here's the schematic by Tha(t)oneGuy post. I have all the mods except power supply bleeding resistors.
My test setup is using this pre-amp with a power amp to the speakers. I used 10X probes. I tried floating power amp input and it was quiet. It only hum when pre-amp is connected, regardless of pre-amp input is connected to source (battery-powered tablet) or not.
My questions are;
1. If I follow the signal path, I should be able to "trace" the hum? I meant from "Audio in" to "Audio out". I connect probe ground to input ground.
2. I don't see hum. What should I look for? I expected some frequency at 50Hz with some amplitude, but I see nothing at "Audio output" yet I can hear it from the speaker.
3. I haven't tried FFT yet. I'll try.
Thanks for any recommendation.
Here's the schematic by Tha(t)oneGuy post. I have all the mods except power supply bleeding resistors.
My test setup is using this pre-amp with a power amp to the speakers. I used 10X probes. I tried floating power amp input and it was quiet. It only hum when pre-amp is connected, regardless of pre-amp input is connected to source (battery-powered tablet) or not.
My questions are;
1. If I follow the signal path, I should be able to "trace" the hum? I meant from "Audio in" to "Audio out". I connect probe ground to input ground.
2. I don't see hum. What should I look for? I expected some frequency at 50Hz with some amplitude, but I see nothing at "Audio output" yet I can hear it from the speaker.
3. I haven't tried FFT yet. I'll try.
Thanks for any recommendation.
How is the preamp powered - from the mains through a plupack with 12Vac output? Have you tried another beefier plugpack, as the loading on the plugpack transformer includes a dc bias from the heater supply.
Perhaps connect the preamp to power amp, with the preamp power connected to mains but with one 12Vac lead disconnected from C1 and C5 (ie. no +/-28dc).
Perhaps connect the preamp to power amp, with the preamp power connected to mains but with one 12Vac lead disconnected from C1 and C5 (ie. no +/-28dc).
Can you post a photo of your O-scope screen when it's showing nothing?
Also: do you know what your trigger voltage is set to?
Also: do you know what your trigger voltage is set to?