Why a 50 Ohm preamp? There is already a 50 Ohm load on the mixer (which probably causes huge volume drop) so you should really follow it with a high input impedance pre-amp. The 3.3mH/1uF LC filter will only work properly (~3KHz LPF) feeding a high impedance anyway.
Why the 1M potentiometer on pin 4 of the TDA7052A? Pin 4 has no internal connection in the IC.
[Edit - if it is TDA7052B rather than TDA7052A, pin 4 can be used as a volume control in which case you should remove the other control completely and bypass the potentiometer with a 1uF capacitor]
Brian.
You probably do want some DC leakage to ground (fig 2, R2) after the mixer to prevent the coupling capacitors charging up and blocking the mixer diodes but I would be inclined to add a simple single stage common emitter amplifier before the main audio amp if you need more gain.
You must bypass pin 4 if you want to control the volume that way.
Brian.
Instead of it.Where shall I add it. before the shown 50R amplifier or after it?.
Instead of it.
I would:
1. Add a resistor of about 1K after the 3.3uH choke to ground. I think there should be a DC path there or charge on the audio coupling capacitor could bias the mixer module.
2. Add the single stage amplifier as you described.
3. Take the output from the collector, through a capacitor to the top of the volume control.
4. Leave pin 4 of the TDA7052 disconnected - you don't need two volume controls and isolating it will eliminate ground current ingress.
5. I can't see clearly but there should be 220uF and 100nF capacitors across the TDA7052 supply pins close to the IC.
Brian.
Change 1 had no effect
Change 2, 3, 4, 5 Did not improve things at all.
I guess there is sufficient gain, both on the previous solution and with the new one, but the power amplifier does oscillate so the gain has to be limited, which of course has as a result lower output volume.
So the thing is how to keep the gain high and the output volume high?
I have not found a stable enough circuit on my experiments.
Any ideas?
I think 1. should be there anyway, not for stopping oscillation but to make sure the mixer works properly.
What kind of oscillation is it?
If you disconnect the signal from the top of the volume control does it still oscillate?
All the gain is in audio stages so I would suspect a layout or power wiring problem is allowing amplifier current to pull down the supply on earlier stages. Things to double check are that the speaker is not grounded on one side, for example if you use a headphone socket and it is mounted down to grounded chassis. The speaker wires must both float for the TDA7052 to work properly and it will draw excessive current if one output is grounded. Make sure the speaker wires are twisted, it helps to stop magnetic coupling. Ensure the power supply is fed in at the amplifier end of the circuit so drops along the supply wiring are minimized.
Perhaps a full schematic would help.
Brian.
My preference would be the LM386 for it's simplicity and single ended output. When you have a differential loudspeaker signal in a high gain amplifier, new problems of signal coupling start to arise and you have to be very careful with the route the wires take. Accepting the simplicity of the design, your underlying problem has to be the layout, use a ground plane under the audio amp and keep the power rails closely decoupled. Be especially careful of the speaker and volume control grounding to minimize any voltage drop between them.
Brian.
Your layout makes it difficult to be sure but without using a PCB, my approach would be to make a single 'star' point for the bottom of the volume control, the ground pin of the amplifier, the RC network (if you use one - recommended you do) across the amp output and the loudspeaker. Use that point also as the negative supply input point.
Feed the positive supply directly to the LM386 and add a 100nF and 100uF capacitor across the supply pin and the star ground. The idea is that the star point is as near to a common 'zero signal' point as possible. Your next task would be to give the star point as good as possible connection to the PCB as a ground plane. I'm not sure what the other parts in your photograph are (the ones before the mixer) but they look like ICs of some kind. I would take the power feed to them from the audio stage through a small resistor (10 - 100 Ohms) and decouple each IC to the ground plane close to them to reduce signal conduction along the supply wires. If you can use a screened cable from the volume control to the LM386 it would help, use the screen to ground the bottom of the potentiometer (and connect nothing else to it) and at the amp end connected to the star point as I described above.
Brian.
I also got similar oscillations when I built an audio amplifier (maybe LM377 or 383). It occurred at a certain range when adjusting the volume control potentiometer. I had to test various ways to hook it up. (Maybe I installed an extra resistor or capacitor.) Finally I got the oscillations to stop. I never figured out what caused them in the first place.
I agree Nik_2213 but this seems to be more of a low frequency instability problem due to ground loops and maybe being too ambitious with high gain amplifiers. I think ferrites may come later when sorting out RF issues.
Neazoi, adding resistors at the inputs of the regulators might help. Do not add them after the regulator or you lose the regulation!. As long as there is no risk of the voltage dipping below the regulator minimum input voltage you can use as high a value as convenient. Add the regulator input capacitor as close to the regulator as you can and directly between the Vin and GND pins. The idea is that the resistor and input capacitor form a low pass filter but the output side of the regulator stays at low impedance and constant voltage. Any voltage dropped across the resistor is just less dropped across the regulator but it forms a signal barrier along the supply that otherwise would be conducted to ground where you least want it.
Brian.
Well, don't forget that the output of the mixer will be no higher than (signal input - mixer losses) which will only be a few uV at most. This is why distributed amplification stages are normally used rather than lots of gain in one place. Remember that in conventional receivers there is generally 10dB or more gain ahead of the mixer then two or more 10dB gain stages after it before the detector, you don't have them in your design.
I still think you should be able to run an LM386 with a single stage pre-amp without any problems though, it is what is done in many domestic products without problems. Try dropping the LM386 gain a little, you might find you get more audio with the volume turned up higher and the gain reduced. With more than one million times amplification in a tiny space you really have to be careful.
Brian.
Your layout makes it difficult to be sure but without using a PCB, my approach would be to make a single 'star' point for the bottom of the volume control, the ground pin of the amplifier, the RC network (if you use one - recommended you do) across the amp output and the loudspeaker. Use that point also as the negative supply input point.
Feed the positive supply directly to the LM386 and add a 100nF and 100uF capacitor across the supply pin and the star ground. The idea is that the star point is as near to a common 'zero signal' point as possible. Your next task would be to give the star point as good as possible connection to the PCB as a ground plane. I'm not sure what the other parts in your photograph are (the ones before the mixer) but they look like ICs of some kind. I would take the power feed to them from the audio stage through a small resistor (10 - 100 Ohms) and decouple each IC to the ground plane close to them to reduce signal conduction along the supply wires. If you can use a screened cable from the volume control to the LM386 it would help, use the screen to ground the bottom of the potentiometer (and connect nothing else to it) and at the amp end connected to the star point as I described above.
Brian.
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