Hey boys and girls,
I was playing around with some ideas for a mic pre-amp and now I have some questions:
I've seen mic pre's that put the microphone signal through a high ratio (1:5 or even 1:10) input transformer, bias the grid of the input tube to about -2 or -3V and then run with the output to a second stage, maybe add feedback, etc., etc... Now, with modern condenser mics you can get sensitivities of >10mV/Pa with maximum SPL's of around 130dB... That's 640mV (RMS) at 130dB, which translates into 3.2V RMS - ~9V peak-to-peak - after a 1:5 transformer!
So, without further ado, here's my questions:
1. Is my math correct?
2. If my math is right, -2V of grid bias isn't gonna cut it, now is it? What's a good grid bias voltage (Vgk) range for a tube mic pre with an input tranny?
3. What's the Vgk on the first stage of, say, Jakob's G9? Or Vgk on the first tube of the "insert the name of your favorite tube mic pre here"?
4. Is all this reasoning just fundamentally flawed? How so? Am I just stupid?
Thanks for your patience!
Peace,
Al.
I was playing around with some ideas for a mic pre-amp and now I have some questions:
I've seen mic pre's that put the microphone signal through a high ratio (1:5 or even 1:10) input transformer, bias the grid of the input tube to about -2 or -3V and then run with the output to a second stage, maybe add feedback, etc., etc... Now, with modern condenser mics you can get sensitivities of >10mV/Pa with maximum SPL's of around 130dB... That's 640mV (RMS) at 130dB, which translates into 3.2V RMS - ~9V peak-to-peak - after a 1:5 transformer!
So, without further ado, here's my questions:
1. Is my math correct?
2. If my math is right, -2V of grid bias isn't gonna cut it, now is it? What's a good grid bias voltage (Vgk) range for a tube mic pre with an input tranny?
3. What's the Vgk on the first stage of, say, Jakob's G9? Or Vgk on the first tube of the "insert the name of your favorite tube mic pre here"?
4. Is all this reasoning just fundamentally flawed? How so? Am I just stupid?
Thanks for your patience!
Peace,
Al.