B
Ban
- Jan 1, 1970
- 0
The schematic
http://rapidshare.com/files/21272377/mic_amp_2.jpg
looks pretty simple, but it still needs work. I found a couple of gotchas:
1. When you switch on the phantom power the Vbe of the transistors gets
reversed momentarily(+17V instead of -0.7V), degrading beta and Vos. This
will slowly destroy the input devices. This happens always in normal
operation with or without a mike.
2. The power supply rejection is very poor(-20dB) especially at higher
frequencies. Here current sources might improve the situation. A lot of
additional filtering is also needed.
3. When saturating the opamps will return to normal operation in a staggered
way, creating spikes in the O/P signal.
4. The offset voltage varies with the gain, making it sensitive to
variations in gain setting.
I have attached a link to a commercial product, just to show that the art of
making a good preamp is not *that* simple.
http://rapidshare.com/files/21831341/mic_pre_02.png
http://rapidshare.com/files/21272377/mic_amp_2.jpg
looks pretty simple, but it still needs work. I found a couple of gotchas:
1. When you switch on the phantom power the Vbe of the transistors gets
reversed momentarily(+17V instead of -0.7V), degrading beta and Vos. This
will slowly destroy the input devices. This happens always in normal
operation with or without a mike.
2. The power supply rejection is very poor(-20dB) especially at higher
frequencies. Here current sources might improve the situation. A lot of
additional filtering is also needed.
3. When saturating the opamps will return to normal operation in a staggered
way, creating spikes in the O/P signal.
4. The offset voltage varies with the gain, making it sensitive to
variations in gain setting.
I have attached a link to a commercial product, just to show that the art of
making a good preamp is not *that* simple.
http://rapidshare.com/files/21831341/mic_pre_02.png