Hello fellow DIYers,
I have a bit of a riddle for you: I've just completed a 4 channel 1290-style microphone preamp, using a PCB I designed/derived from the original BA283/284 cards and schematics. The PCBs were also designed to accommodate the 2006 Group Buy Carnhill transformers (VTB2168), meaning that the PCBs (on which the input TX is mounted) incorporate the changes needed to use the differently-labeled input transformer properly, as CJ intended.
Upon my first listening tests, all is well. They sound darker than some of my other preamps, but that isn't necessarily a problem. What I found unusual, however, was when I did some square-wave testing on them. Using a BK Precision 4040 function generator, and feeding a 1kHz square wave to the preamps (with a 150 Ohm resistor across pins 2 and 3 of the input XLR to simulate a microphone), I get some strange results. While feeding the same signal to my REDD47, Syteks, and Ampex 351, I got nearly perfect square waves on their outputs. But, when going into the 1290-style preamps, I got this:
https://photos.app.goo.gl/LFzMq8RnzstgXCAT8
All channels exhibit an identical "bass-cut" square wave profile, so it doesn't appear to be a simple bad component. Plus, using my scope probe to measure the output of my function generator, I get a perfect square wave, but as soon as I attach the function generator to the XLR of the preamps, I get the skewed square wave (even measured prior to where the signal hits the input transformer!).
For further testing, I tried feeding the preamps a 10kHz square wave, then a 3kHz square wave, respectively. Both exhibit a much flatter "horizontal" portion:
https://photos.app.goo.gl/EHJUDrfB4hwQmNdv7
https://photos.app.goo.gl/QR4kJxZg2SFc2Vzg7
Does anyone have any clue what is happening here? Is this what a typical Neve-style preamp will produce? Is it possible that the input transformer is loading the input signal in some unusual way, such that a square wave is caused to become skewed even when measured at Pin 2 on the back of the input XLR?
Thanks!
I have a bit of a riddle for you: I've just completed a 4 channel 1290-style microphone preamp, using a PCB I designed/derived from the original BA283/284 cards and schematics. The PCBs were also designed to accommodate the 2006 Group Buy Carnhill transformers (VTB2168), meaning that the PCBs (on which the input TX is mounted) incorporate the changes needed to use the differently-labeled input transformer properly, as CJ intended.
Upon my first listening tests, all is well. They sound darker than some of my other preamps, but that isn't necessarily a problem. What I found unusual, however, was when I did some square-wave testing on them. Using a BK Precision 4040 function generator, and feeding a 1kHz square wave to the preamps (with a 150 Ohm resistor across pins 2 and 3 of the input XLR to simulate a microphone), I get some strange results. While feeding the same signal to my REDD47, Syteks, and Ampex 351, I got nearly perfect square waves on their outputs. But, when going into the 1290-style preamps, I got this:
https://photos.app.goo.gl/LFzMq8RnzstgXCAT8
All channels exhibit an identical "bass-cut" square wave profile, so it doesn't appear to be a simple bad component. Plus, using my scope probe to measure the output of my function generator, I get a perfect square wave, but as soon as I attach the function generator to the XLR of the preamps, I get the skewed square wave (even measured prior to where the signal hits the input transformer!).
For further testing, I tried feeding the preamps a 10kHz square wave, then a 3kHz square wave, respectively. Both exhibit a much flatter "horizontal" portion:
https://photos.app.goo.gl/EHJUDrfB4hwQmNdv7
https://photos.app.goo.gl/QR4kJxZg2SFc2Vzg7
Does anyone have any clue what is happening here? Is this what a typical Neve-style preamp will produce? Is it possible that the input transformer is loading the input signal in some unusual way, such that a square wave is caused to become skewed even when measured at Pin 2 on the back of the input XLR?
Thanks!