Audio preamp problem

R2 produces negative feedback so the input impedance of the transistor is low. I would guess that it is about 1k. This will give a high pass filter with the 100n input capacitor with a cut below 1kHz.

If you have a low impedance source, you could just increase the capacitance.

To reduce the feedback, R2 could be split with a capacitor to earth at the junction. A two transistor amplifier could provide high impedance input with feedback to give low distortion.

 

The 1K resistor from the input to V+ looks odd to me. Does anyone know what its purpose is? It certainly lowers the input impedance. Also, negative feedback should raise input imedance no?

Bob

 

The input resistor also looks strange to me, perhaps it is to provide power to the previous stage.

Negative feedback can increase or decrease input impedance. In this case, the input Z of the transistor will be low also the feedback in anti-phase will reduce it further. If the gain of the transistor is high or an op-amp is used, the input appraches a virtual earth with zero impedance.
If the negative feedback was fed to the emitter, then the input Z would be raised. To do this, two transistors would be required to invert the signal twice.

 

What kind of noise 50/60 Hz hum?

Bob

 

It is probably not the amp itself, it is probably there on the input. You only didn't hear it before because of the bass cut. What is the input?

Bob

 

You have not said what R1 is for.
You have not said what gain you want.
You have not said what the source impedance is.
You have not said what the load impedance is.

I attach a diagram of a two transistor circuit which will have a high input impedance and a fairly low output impedance. The gain of the basic circuit will be 1+R1/R2.. The resistors affect the DC operating point so the gain is probably limited to 5.
The gain can be increased by placing R3 across R2 with a capacitor in series to stop DC current.
You could spend many happy hours on a simulator with different components and impedances.

For 50Hz hum, try shorting the input and see if it disapears. If it stays, it is probably the power supply.

 

I have not given values of components since the requirements were not given.
Perhaps TINA is reliable, I can assure you that strange results can be obtained in practice!
If your existing circuit is working just use it.

Getting rid of hum can be very difficult. All power supplies should be smoothed properly and all leads should be short and perhaps screened.
One way of getting hum is if there is an earth loop which picks up a magnetic signal from the mains. The way of getting round this is to have one common earth point for the system.

 

I have never used TINA, I used ECA, a DOS program, many years ago. I used it mainly for heat transfer and vehicle suspension simulation.
I have used 5spice once recently but would not be able to tutor someone in its use.