# Mixer circuit pre-amp

Discussion in 'Electronics Homework Help' started by Jamesmeister, Mar 10, 2013.

1. ### Jamesmeister

8
0
Mar 5, 2013
For Mixer part, we are required to have: Three (3) high impedance microphone-level inputs (unbalanced), two (2) high impedance line-level inputs.

For microphone section:
-Input impedance (Zin): 800 ohms
-Level gain (Av dB): 42 dBu

For Line input section:
- Input impedance (Zin): 20 k
- Level gain (Av dB): 25 dBu

i have here a sample mixer circuit i saw on the internet. my problem is, how do i accomplish the requirements above. i have no ideaaaa

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,482
2,830
Jan 21, 2010
The first step is unserstanding the basics.

If I were you, I would review my notes. I would also consider what the point is of those 1k trimpots.

3. ### Laplace

1,252
184
Apr 4, 2010
I don't quite understand the specification as given: "-Level gain (Av dB): 42 dBu".

Now 42 dB is a voltage gain of about 126 but dBu gives a signal level referenced to 0.7746 volts (1 mW @ 600 ohms) so 42 dBu is 97 VRMS or 274 volts P-P. Are you really expected to design a circuit for a 274 V peak-to-peak signal level? The LM3900 is not suitable for 42 dBu.

4. ### Jamesmeister

8
0
Mar 5, 2013
ive done a little computation. 20log (x/1x10^-6 V (since ref is db(micro)) = 42
therefore, i should have an output voltage of 0.12589 mV. as far as i know, some microphones have input voltage of 1 micro volts

5. ### davennModerator

13,802
1,941
Sep 5, 2009
standard dynamic microphones dont get supplied a voltage - they generate a voltage

Electret condenser mics need a supply voltage but the preamp circuit you have shown is not set up for phantom power

Dave