Torben
Well-Known Member
Hi all,
I'm getting an interesting result from a simple mic preamp I'm building. The schematic should be attached.
First off, I'm using VA's (Visual Analyzer--freeware soundcard scope program) frequency analysis to check the response. At low to medium gains and with good amounts of input, the response line looks really good--a nice clean line from ~20kHz (the highest you can see on this soundcard) which starts dropping at around 300-400Hz and looks like it would hit unity right around DC (can't see down to DC on this thing either).
However, in silence or near-silence, the response graph goes all over the place. When I provide sound into the electret the graph line cleans right up. Of course, providing too much input overdrives it and the line goes to hell again, but I expect that.
Note about the schematic: I don't actually have a TL081 in there at the
moment; it's a 5534--hence the 47pF compensation cap.
For reference, this is for an acoustic guitar mic--I've got the response pretty much tailored to where the mic element is sitting inside my guitar at the moment. No lack of bass there. I can, however, overdrive the thing by playing hard enough. It actually sounds pretty good through my mixer and monitors, but I can hear a bit of noise still.
Questions:
1) Is it normal to see the response graph go crazy with little or no input? Or is the thing just basically unstable, needing input to stabilize it?
2) Currently I am using 5% resistors across the board, but matched the 47K divider resistors with a meter. I understand that there are other reasons to prefer precision resistors--temperature stability, degradation over time, higher Johnson noise, etc--so I've got some 1% ones to try. Figure it's worth it?
3) Could I make it harder to overdrive this thing by upping the power supply current? At the moment I've got it set up with a 9V battery for power, but if I'm calculating it right, this thing should really only last about 5-6 hours on a regular 9V. Anyway, initial testing with VA seems to show that I don't really get more resistance to distortion by upping the supply to, say, 15 or 18 V. But I've read that you get better headroom with increased supply. I guess I'm not really understanding what that means.
Thanks,
Torben
I'm getting an interesting result from a simple mic preamp I'm building. The schematic should be attached.
First off, I'm using VA's (Visual Analyzer--freeware soundcard scope program) frequency analysis to check the response. At low to medium gains and with good amounts of input, the response line looks really good--a nice clean line from ~20kHz (the highest you can see on this soundcard) which starts dropping at around 300-400Hz and looks like it would hit unity right around DC (can't see down to DC on this thing either).
However, in silence or near-silence, the response graph goes all over the place. When I provide sound into the electret the graph line cleans right up. Of course, providing too much input overdrives it and the line goes to hell again, but I expect that.
Note about the schematic: I don't actually have a TL081 in there at the
moment; it's a 5534--hence the 47pF compensation cap.
For reference, this is for an acoustic guitar mic--I've got the response pretty much tailored to where the mic element is sitting inside my guitar at the moment. No lack of bass there. I can, however, overdrive the thing by playing hard enough. It actually sounds pretty good through my mixer and monitors, but I can hear a bit of noise still.
Questions:
1) Is it normal to see the response graph go crazy with little or no input? Or is the thing just basically unstable, needing input to stabilize it?
2) Currently I am using 5% resistors across the board, but matched the 47K divider resistors with a meter. I understand that there are other reasons to prefer precision resistors--temperature stability, degradation over time, higher Johnson noise, etc--so I've got some 1% ones to try. Figure it's worth it?
3) Could I make it harder to overdrive this thing by upping the power supply current? At the moment I've got it set up with a 9V battery for power, but if I'm calculating it right, this thing should really only last about 5-6 hours on a regular 9V. Anyway, initial testing with VA seems to show that I don't really get more resistance to distortion by upping the supply to, say, 15 or 18 V. But I've read that you get better headroom with increased supply. I guess I'm not really understanding what that means.
Thanks,
Torben