Sound Box design - Please, help.

[ToniGodoy]

Hello to everybody.

Son of a technician, I learnt how to solder when I was only seven years old. Then, I rerouted my live as a software man and only adventured myself in doing/repairing very simple and small circuits. So, I'm very rookie in building such big circuits as the one I'm describing here and will be very grateful for your help and advices. I really need this box and I'm always trying to find an excuse for returning to the past and learn some electronics, once and for all.
(Excuse my English too, if it's not as correct as it should be.)


GOAL:
To build a circuit with 2 functions:
+ Pre-Amplifier of electret microphones with output to the PC's microphone input.
+ Mixing and Amplifying of the PC's line output plus the microphones' signal, and sending them to a 32 ohms Headphones.


CHARACTERISTICS:

• Preamplifier's output of about 20 mVpk (40 max) to be adjusted at setup time.
• Mixer/Headphones amplifier's output of about 1 Vpk, by a load of 32 ohms.
  (Actually, I'd wish to be able to connect 2 headphones, eventually, so I'm thinking of putting 2 jacks connectors in series to avoid the voltage dropping).
• I don't care whether the initial adjustment of the trimmer R11 affects the amplitude of the input to the mixer/amplifier. What I must avoid is that the adjustment of the volume potentiometers (R3,R15) affects the microphone's output. Due to that, I must isolate these two stages by means of a Buffer (U1A).
• Although I don't want bad sound in the headphones, the HIGHEST priority of the circuit is the good output of the microphones to the PC's sound card, for recording. Here I need the best amplification with the minimum noise.
• I must avoid to a maximum the cross-feed between the 2 channels of the microphones, because the main reason for building this circuit is to replace the horrible recording/monitoring features of my new motherboard's sound card in an application for directional localization of sounds. For the same reason, I should also avoid the cross-feeding between the two output channels to the headphones while I am monitoring or listening to the recorded audio.

DESCRIPTION:

• The circuit is split into two stages. The first one is an electret microphone Pre-Amplifier with the output MIC_OUTPUT (top of the diagram). The second stage is a Mixer/Amplifier of the first stage and the line input LINE_INPUT, for driving a 32 ohms Headphone.
• The circuit will be fed by an external power supply of +9V. For simplicity, instead of using a supply splitter (virtual ground) for the op-amps, I'm using a Reference-Voltage of 4.5V.
• In order to keep the amplified noise to a minimum, for the first stage I'm using a transistor instead of an op-amp.
• To isolate the impedance between stages, I'm using one half of the op-amp (U1A) as a Buffer.
• To be able to present a low impedance to the low load of the headphones, I'm also using one half of another op-amp (U2B) as an output buffer.
• In the first stage, R20 keeps under 7V the voltage applied to the electret microphone (the maximun uses to be 10V in most of the datasheets I've read).
• To simulate the electret microphone, I'm using an AC Current signal generator of 1uA and to simulate the sound card's line output I'm using a AC Voltage signal generator of 0.4V.
• The variable resistor (trimmer) R11 is soldered to the PCB and will be used to adjust the microphones' sensibility at setup time. After that, it will remain fixed.
• The Volume potentiometers R3 and R15 are dual audio-logarithmics, external to the PCB. Maybe I'll replace them by linear ones with a resistor, in order to simulate the exponential curve.
• For simplicity, I'm not using neither an active mixer nor a panner to mix the microphone and the line outputs. I could add, instead, a global volume by means of a potentiometer to ground, just before the output buffer.
• With the same configuration as in the image, I'm getting in the simulation values of 21 mVpk at the microphone output and 1.1 Vpk (24 mA rms) at the headphones output.
• The circuit depicts a single channel. Both channels will share the same power supply but no any other element, save the dual volume potentiometers (R3,R15) and the stereo jacks.
• Still to be implemented, there will be a Led and a Switch between the power supply and the circuit.

DOUBTS:

• Will it even work?
• What are the correct values for the resistors (R1,R2) and capacitors (C1,C4) at the Reference-Voltage output? ¿May they be electrolytic ones or should they be Film (polyester) ones? I haven't found any examples for voltage reference, but for supply splitters only.
• If I simulate the electret microphone with an AC Current source, everything is fine but if I simulate it with a JFET transistor, all the signals drop near to ground. Why?
• At the Microphone output and at the Headphones output, I'm putting resistors (R6,R7) in order to simulate the impedance of the sound card and the headphones. Although all those external elements will be connected most of the time, should I put some real resistors there, in case some of those elements are disconnected from the circuit? May be 5K and 500 ohms resistors?
  In the case of the headphones, it's really simple because I can put a 32 ohms resistor between the detection pins of the jack connector. But the output from the microphone preamplifier will be a soldered cable, in order to minimize noise.
• To avoid the crossfeeding between the 2 channels of the microphones, I'm avoiding the use of the same dual op-amp for different channels. So, the buffer at the end of the preamplifier and the main microphone amplifier are both one half of the same op-amp (U1A, U1B). There will be any problem for doing the same with the line amplifier and the buffer to the headphones (U2A, U2B)? Would it be better to use the same op-amp for both amplifiers and the same op-amp for both buffers?
• The capacitors C5,C6,C8,C10, should they be electrolytic, film type, or ceramic ones (at least for C5)?

Thank you, very much.
I have attached some screenshots and a zip file with the whole project (Spice netlist, images and the circuit file for MultiSim). The oscilloscopes image shows the final output signals (DC coupled) and the op-amps outputs before being mixed (DC coupled, shifted -4.5V).
Note: To watch the images at full resolution, download them or left click first, then right-click and choose "View image".

[ToniGodoy]

Humm. I think I'm watching a big mistake here.

The first buffer (U1A) is AC coupled to the preamp by C2 but, because the power supply in not giving negative voltages, this op-amp should be clipping the output signal. But it's not! The simulation depicts a shift of aprox. +1 volt DC in the input and, because of that, it is working.

Is it possible or Is this a bug in the simulator?
Shouldn't I feed the non-inverting input of U1A with DC_Ref too?

[ToniGodoy]

Well. I think I have now correctly biased the Buffer U1A.

• I have connected its non-inverting input to DC_REF (+4.5V) through R17.
• I had to increase C2 value to 200nf for a good response at 100Hz.
• Output signals keep almost the same.

Have I done it right?

[tooki]

Following, since I’m working on an extremely similar project!

(The problem I ran into was crosstalk from the line in onto the mike preamp output to the computer. I got some advice on here, but haven’t yet gotten around to trying it all.)

[ToniGodoy]

Thank you, tooki.
Do you think that crosstalk  is possible despite the buffer which isolates the amplifier and the preamplifier?
I'm glad to have you there but I warn you that I'm a very rookie on this matter.

[ToniGodoy]

These are the changes made to the values of some capacitors and resistors, following the advices of some external people:

The value of these capacitors were drastically increased, to minimize the cut-off at 20Hz:
C2, C3, C5, C6, C8, C10
All of them are now polarized (electrolytic or film types?).

The values of these resistors, at the mixing point, were drastically increased, to not overload the op-amps:
R18 and R19

Signals at the outputs remain the same above 1KHz but are drastically improved in the range of 20 to 100Hz.
I'm still studying some advices about the biasing of the transistor.
But it's past time to go to bed here, in the Canaries...

Attached you have the new images and circuit files.
Note: To watch the images at full resolution, download them or left click first, then right-click and choose "View image".

[ToniGodoy]

This is the result of an AC Sweep: only -3db at 20Hz. It's not too bad, is it?
(Please, can someone at least to answer my doubts in the first post? I think I should move this thread to another board of this forum.)