So I just finished assembling the first channel of what's hopefully going to be a functioning pair of BAD preamps in the near future. Since I'm a complete newbee and this is my first real project apart from some minor modifications and guitar pedals, I expected to encounter some problems along the way - and sadly I was right.
First off, I measured the voltages on all the points that are marked in the BAD schematic and they all seem to be correct. The phantom power led is working and I can measure the phantom power on the XLR-connector as well. The pad and the phase switch seem to be working fine as well. Also the wiring from the XLR-connectors to the board seems to be correct.
The problems:
1. There seems to be a grounding issue but I can't really figure out what it stems from. I have a lot of 50hz hum as well as white noise, which gets even worse when I touch the body of a plugged in mic or the xlr connector. And it is quieter when I touch the back of my case, where the power connector and xlr connector are screwed in. I recorded a sound sample with an SM57 where you can hear me narrating over the different types of noises that thing makes ;D : https://janniseckert.de/preamp/preamp_noise.wav Please note that the sound samples can be pretty loud so turn the volume down if you have your headphones on
Also the OEP A262A3E input transformer seems to be picking up a lot of noise when no input is connected, even though it is in a shielding can. Just moving my fingers closer to it while the unit is powered on makes a pretty significant noise and when I touch the wires to the variable z-pot, the pot itself or the frontplate, you can here a clicking noise.
2. I sometimes get weird random tones around 800 Hz and I have no idea where that could be coming from. Just RF-noise maybe? Anyway, there is a sound sample as well: https://janniseckert.de/preamp/800hz_noise.wav
3. When I engage the phantom power the preamp produces really loud low frequency oscillation that ramps up in frequency and amplitude over time, and which masks pretty much any sound coming in. In the sound sample at around 14 sec you can hear me humming into the plugged condenser mic if you want to get a feel for that nice SNR: https://janniseckert.de/preamp/condenser_48v.wav
This is the BAD schematic for reference: http://www.jlmaudio.com/Baby%20Animal%20Dual%20Mic%20Pre%20Schematic.pdf .
Any help would be greatly appreciated!
Cheers
Jannis
First off, I measured the voltages on all the points that are marked in the BAD schematic and they all seem to be correct. The phantom power led is working and I can measure the phantom power on the XLR-connector as well. The pad and the phase switch seem to be working fine as well. Also the wiring from the XLR-connectors to the board seems to be correct.
The problems:
1. There seems to be a grounding issue but I can't really figure out what it stems from. I have a lot of 50hz hum as well as white noise, which gets even worse when I touch the body of a plugged in mic or the xlr connector. And it is quieter when I touch the back of my case, where the power connector and xlr connector are screwed in. I recorded a sound sample with an SM57 where you can hear me narrating over the different types of noises that thing makes ;D : https://janniseckert.de/preamp/preamp_noise.wav Please note that the sound samples can be pretty loud so turn the volume down if you have your headphones on
Also the OEP A262A3E input transformer seems to be picking up a lot of noise when no input is connected, even though it is in a shielding can. Just moving my fingers closer to it while the unit is powered on makes a pretty significant noise and when I touch the wires to the variable z-pot, the pot itself or the frontplate, you can here a clicking noise.
2. I sometimes get weird random tones around 800 Hz and I have no idea where that could be coming from. Just RF-noise maybe? Anyway, there is a sound sample as well: https://janniseckert.de/preamp/800hz_noise.wav
3. When I engage the phantom power the preamp produces really loud low frequency oscillation that ramps up in frequency and amplitude over time, and which masks pretty much any sound coming in. In the sound sample at around 14 sec you can hear me humming into the plugged condenser mic if you want to get a feel for that nice SNR: https://janniseckert.de/preamp/condenser_48v.wav
This is the BAD schematic for reference: http://www.jlmaudio.com/Baby%20Animal%20Dual%20Mic%20Pre%20Schematic.pdf .
Any help would be greatly appreciated!
Cheers
Jannis
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