# I have to design an audio preamplifier as a project, need help with OrCad simulation

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#### CrisuCris

##### New Member
I have received an example schematic of how an audio preamplifier should look like. Based on that and a set of data(input voltage(7V), input resistance(60 ohms), output resistance(8 ohms) and amplification(2.1) ) I have to design my own schematic(which shouldn't be a lot different at the end of the day), but I don't quite understand how to do it.

Note: this is the 1st time I am designing a circuit, so I really don't know much about it.

My questions are:

*First, why is an AC power supply used as an input for the circuit?
*Which exactly are the values of the input and output voltage in this image(so I can calculate the amplification); The reason I am asking this is because I see those "0V" values
*Is there any simple way to explain how changing the circuit(adding resistors, changing a resistor to a very high value etc.) influences the circuit?
I am assuming the 81.54mV is the ouput voltage(still need an answer). All my attempts to modify the circuit (using my parameters and other transistor(we have to use a different one from BC109C) ) led to attenuation instead of amplification.

https://i.postimg.cc/hvD9zrQz/Untitled.png

#### kubeek

##### Well-Known Member
AC source is used to simulate the input signal, as it will be some audio signal which is AC.
All those voltages are DC voltages, so it shows you the DC operating point. To find AC magnitudes you would need to run AC or transient simulation.

How can you have such an assignment to design it, if you don´t know how to calculate the design on paper? The simulation should only by used to verify your calculated results.

#### audioguru

##### Well-Known Member
Your "set of data" makes no sesnse:
1) The input voltage of a preamp is in mV, not 7V. The 7V is probably the DC power supply voltage.
2) The input resistance of a preamp is never 60 ohms but might be 600 ohms or 10k ohms.
3) The output resistance of a preamp is probably not 8 ohms unless it uses a high current amplifier.

4) R111 attenuates the input signal. Why?
5) R1 does nothing and should be removed.
6) C5 and R80 do nothing and should be removed.
7) R9 does nothing and should be replaced by a piece of wire.

#### CrisuCris

##### New Member
Your "set of data" makes no sesnse:
1) The input voltage of a preamp is in mV, not 7V. The 7V is probably the DC power supply voltage.
2) The input resistance of a preamp is never 60 ohms but might be 600 ohms or 10k ohms.
3) The output resistance of a preamp is probably not 8 ohms unless it uses a high current amplifier.

4) R111 attenuates the input signal. Why?
5) R1 does nothing and should be removed.
6) C5 and R80 do nothing and should be removed.
7) R9 does nothing and should be replaced by a piece of wire.
I think I expressed myself poorly. That was an example given by the professor, which used a set of data we don't have information of.

I have my own set of data(the one I wrote in the original post) and I have to build a preamplifier with it and it should look somewhat similar with the original one, which means I can do all those changes you suggested more or less.

Also I think I may have written the wrong scale for the unit of measure. Like it's probably 7mV input.

#### audioguru

##### Well-Known Member
A preamp is used for an old magnetic phono cartridge or a microphone. Your "data" with an input resistance of only 60 ohms will short circuit the input signal.
A magnetic phono cartridge used a preamp with an input resistance of 47k (47 thousand) ohms. The input resistance of a preamp for a microphone is 600 ohms to 33k ohms.

Since the input signal is only 7mV then your "data" gain (amplification) of only 2.1 times is almost 100 times too low.

A preamp does not drive an 8 ohm speaker, instead it drives the fairly high input resistance of a power amplifier that has a very low output resistance to drive a speaker.

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