Is there enough current with a 1meg bias resistor for my transistor mic amp? The output dc voltage is 4v at the collector. The transistor is a 2n2222 or 2n3904. I'm not sure.

Also in my opamp circuit I can only get a 500mv output ac with no distortion with a line level at the input. If I increase the v in (a headphone out from a radio) the output climbs to 2v but is highly distorted. I thought I could get a 2v non distorted output with the 8v power supply. I am using a 9v battery. The ic is a TL082 or LF353 dual opamp.

 

The mic pre-amp although not the best design does work. Verified using a 2N3904.
I don't see any problem with the opamp circuit other then filter capacitors missing across the power supply pins and the + input to ground.
Are you sure the input is not distorted?
SG

 

As above, the opamp circuit needs power supply decoupling capacitors. Analog Devices recommends a 0.1 uF ceramic and a 10 uF electrolytic capacitor in parallel, with the shortest possible leads, placed as close as possible to the power pins.

Separate from that, the midpoint bias resistor divider (R4 and R5) needs decoupling also. 100 uF at 16 or 25 V, to GND.

The circuit has a gain of 5, so a 2 V signal will overdrive the output.

The mic preamp circuit is called "dangle-biasing" and is the worst possible way to bias an amplifier stage. Search for microphone preamp schematics to see many other (better) ways. One common circuit has the same number of parts, but R1 is smaller and connected differently.

What type of microphone are you using? Dynamic? Electret? Other?

ak

 

Your maximum of 500 mv distortion free might be because you do not have a negative power supply. Indeed, I am amazed that you can get more than a few millivolts distortion free.

Try -8 V (or so) for V-.

 

I am using a dynamic microphone. Will I be able to talk loud and scream into the dynamic microphone without distortion with the given bias of a 1meg resistor? Will the Mic produce too much current (let's say Mic produces 10 millivolts at a loud volume) and compromise the bias?

The input of the op-amp is not distorted. How come I can't get a large voltage swing at the output (2v peak to peak or more)? I tried a 470uF capacitor at R4 and R5 junction to ground but no change occured other than less hum with a AC to DC adapter. I don't think I have a problem with using decoupling capacitors at the power supply pins.

 

I don't think I have a problem with using decoupling capacitors at the power supply pins.

Decoupling capacitor location probably is not the cause of what you are seeing. BUT - decoupling capacitors, power source impedance, and ground management are *critical* to successful analog circuit design.

ak

 

Your maximum of 500 mv distortion free might be because you do not have a negative power supply. Indeed, I am amazed that you can get more than a few millivolts distortion free.

Try -8 V (or so) for V-.

Agree. The part is specified down to +/-6 V rails, probably for a reason. Headroom is +/- 1.5 V typical, +/-3 V max, so on paper there can be a usable output with a single 8 V rail. But not much.

ak

 

Will the Mic produce too much current (let's say Mic produces 10 millivolts at a loud volume) and compromise the bias?

The mic circuit as is has a gain of about 150 with a max signal output of about 3.5 volts peak to peak.
10 mv from the mic will get you 1.5 volts p-p so well below the clipping level.
I couldn't find a LM353 so used a LM358 for the mixer circuit. With a 9 volt supply was able to get 5.5 volts output p-p before clipping.
The 10mv from the mic will overdrive the the opamp unless you reduce the gain somewhere.
As far as only getting .5 volts output you need to check what chip you are actually using and tell us where you got the chip.
EDIT: The LM353 or TL082 are dual opamps, use one side for the mic pre-amp and the other for the mixer thus eliminating the transistor circuit.
SG