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Audio preamp problem

vinod chandran

Jun 21, 2011
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Hi,
I built this pre-amp circuit. (please look the image).It's working great without any noise. But there is not enough bass in the output. Somebody please help. Thanks in advance
Vinod
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Increase the value of C1 and C2
 

duke37

Jan 9, 2011
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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.
 

BobK

Jan 5, 2010
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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
 

duke37

Jan 9, 2011
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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.
 

vinod chandran

Jun 21, 2011
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Hi,
Thanks to all. I just replace the c1 and c2 with 1uF 63V . Bass is ok now. but there is a little noise. So i think i need more help. I request you to draw a circuit instead of posting text. After all one picture is worth than thousand words. Once again thanks to all.
Regards
Vinod.KC
 

BobK

Jan 5, 2010
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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
 

duke37

Jan 9, 2011
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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.
 

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vinod chandran

Jun 21, 2011
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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

Hi BobK,
Yes you are right. it's not from the amp. I can explain about the input. The first thing is China made mp3 and FM board (VIRE-01F V 3.0). And the output of this board goes to an old bass treble board (with a few transistors and lot of capacitors). Output of this bass treble board is connected with my 5.1 home theater (Universal 3300). But there i feel some lack of gain. So i decided to connect a preamp in between Home theater and bass treble board and made the above said preamp with 2N3904. It was resulted in bass cut. then i changed the cap value to 1uF. There comes the noise. I don't know anything related to impedence of both these home theater and bass treble board. I hope you can help me .
Vinod
 

vinod chandran

Jun 21, 2011
192
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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.
Hi duke37,
Thank you for the reply. I am a beginner in electronics. so i don't know the maths about gain and impedance. I always making the circuits without hesitating what a particular component is for. I am using TINA but it is not reliable . because it shows strange result in some conditions. Besides, i don't know how to simulate audio circuits in TINA. I am searching for a good tutorial video. After all there is no component value in your circuit.
-Vinod.
 

duke37

Jan 9, 2011
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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.
 

vinod chandran

Jun 21, 2011
192
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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.

Hi duke37,
Thanks. At last i replaced that old B&T board with a new one. Now the gain is ok. so the problem solved. The power supply is smoothed with a 1000uF 25v cap. And the 5V regulator is connected with another 1000uF. By the way could you please suggest me some good TINA tutorials ?.
-Vinod
 
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duke37

Jan 9, 2011
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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.
 
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