Simple preamp, noise


2010-10-19 6:28 pm
I've built k1803 preamp to run it with tda7375. I had to make it, because when I connect it to my mp3 it isn't loud, but with pc is ok. I just used NE5534, later I also tried LM748 in place of lm741. The problem is that it's making noise like old radio. I don't know why, I run it with 9V battery so it isn't ground loop problem, I tried to shield it, put 10nF between 4 and 7, nothing helped. Everything is connected as it should, it works, makes sound louder, but it makes unacceptable noise. Maybe this preamp isn't suitable to use it with tda7375, if so I hope you can suggest good, simple preamp with asymmetrical power supply that will work (make sound from mp3 louder without noise).




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Replacing R4 with a Zener diode (reverse-biased) can help. If your power supply voltage is affected by the speaker's demand for current, you can couple the power supply variations into the signal path. I see the 10uF capacitor as a filter, but you can still get low-frequency oscillations (this has happened to me). The Zener diode will regulate the bias voltage at the + input.

The Zener diode can be a source of noise. I don't think it will matter in your circuit, but you could put a resistor between the Zener diode and the + input of the op-amp, then bypass the + input of the op-amp to ground with the 10uF cap. This will filter any audible noise from the Zener.

You may experience some noise as you adjust the pot, because you have DC across the pot. If you block the DC _before_ the potentiometer, this can reduce scratchy sounds as you move the pot.

The input impedance of this amplifier is 2k2. I'm not sure what's driving your preamp, but if its output is significantly higher than 2k2 then you have signal loss.

If you use a 748, it may need a compensation capacitor. The circuit appears to have a high gain, but this is only true if the input of the circuit is being driven by a low-impedance source. Otherwise, the op-amp is operating at a lower gain and a 748 may need compensation.



2010-10-19 6:28 pm
Well, I use my mp3 player to run preamp. I guess the difference in loudness between mp3 player and PC is because of different impedance. When I connect my amplifier without the preamp to PC it's very loud at max, while connecting to mp3 player even at max is quiet. Changing R2 and C3 didn't help. I'll try to add 47k later, I have to cut trace to do so. If it fails I will put zener diode, any more details about that diode? I used 33pF as a compensation capacitor for 748, which helped just a bit, i think I will put NE5534 again and when I get rid of that noise I'll put better opamp.
your opamp circuit is wrong.
You have a single ended supply feeding half supply voltage as a bias into the +IN pin.
The input is isolated from this bias with the input capacitor.
The output is isolated from DC with a capacitor.
So far OK.

the +IN and the -IN are at almost the same voltage.
The output will also be at almost the same voltage.
i.e. the output is sitting at ~half supply voltage just like +IN & -IN.

The VR=1k0 will pass ~4mA to 0V when supply is ~8Vdc.

The opamp will be forced into using just one side of the output pair and will run hot and in addition the output voltage swing when feeding a 1k0 load will drop massively below the expected 1/3 * Vsupply.
A 741 running on 9Vdc should be able to send a maximum signal of ~3Vac to a 50k load.
You have crippled the 741 with this circuit.
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- R1 should go from pin2 to ground, not from output to ground.
- There should be a capacitor in series with R1 to avoid that input offset voltage is amplified and appears at the output.
- You have to add a resistor to ground after C5 to set the output high-pass filter frequency.
- The most common recommendation for the high-pass is set to it somewhere between 100kHz and 300kHz. It is also better to have the low-pass before the amp input, not in the feedback loop. Are you sure about the 38,862 kHz?
- The high-pass filter setting depends on the application and your taste. Take into account that this is a preamp and what is once filtered out, cannot be brought back. You should set the filter to a roll-off lower than or equal to the high-pass filter in the power amp.


2010-10-19 6:28 pm
- R1 should go from pin2 to ground, not from output to ground.
right, thx

Ok, I made it, it works and it doesn't. Connected to tda7375 - no sound, only vibrating low tone loudspeaker. When connected to headphones sound is quiet. It should be louder with it not quieter!

I give up, how can I make my speakers with tda7375 connected to mp3 player play as loud as connected to pc?