[moved] Query about VU meters (led bargraph)

[Audioguru]

Modern opamps have good high frequency response. If your TL072 is feeding the capacitance of a shielded audio cable that feeds the power amplifier then the capacitance of the cable on the output of the TL072 causes it to oscillate at a high frequency. Add a series 100 ohm resistor from the output coupling capacitor of the TL072 to the cable connection to fix it.
Did you measure the output DC voltage of the opamp? Is it half the supply voltage?

 

[c_mitra]

Wouldnt the negative feedback be the 330k resistor from pin 2 going to pin 1? You probably didnt realize that its going to pin 1 also.

As per your drawing, pin 6 appears to be the output...

 

[Enzy]

As per your drawing, pin 6 appears to be the output...

Pin 6 is tied to pin 7 is doesnt go anywhere else I told you that pin 1 is the output just trust me on that.

In the software you can connect 2 or more lines without having to physically connect them all you need to do is give the lines the same name, in my drawing pin 1 is connected to c1 and the 330k is connected to c1 also



- - - Updated - - -

Modern opamps have good high frequency response. If your TL072 is feeding the capacitance of a shielded audio cable that feeds the power amplifier then the capacitance of the cable on the output of the TL072 causes it to oscillate at a high frequency. Add a series 100 ohm resistor from the output coupling capacitor of the TL072 to the cable connection to fix it.
Did you measure the output DC voltage of the opamp? Is it half the supply voltage?

Ill test and let you know as soon as I reach work I didnt bring the circuit home with me.

 

[Enzy]

Believe it or not I actually cleaned the circuit board with some thinner and it started to work just fine.

 

[Audioguru]

Believe it or not I actually cleaned the circuit board with some thinner and it started to work just fine.

I bought some stripboard from an Arab store when the Chinese store ran out of their very good quality stripboard. The Arab stripboard looked good but smelled very bad. I think it was made from cow dung.
It worked fine but I was careful to keep it dry because if it got wet it would probably conduct and mess up the circuit.

 

[c_mitra]

Believe it or not I actually cleaned the circuit board with some thinner and it started to work just fine.

I do believe you. The original soldering was not clean and cleaning has some effect.

Isopropyl alcohol is the recommended cleaning agent. If you cannot get it, use ethyl alcohol. Else use some common spirit (methyl alcohol). Use a old toothbrush to gently clean the board.

Once you have fixed everything, touch each joint once more (for 3-4 seconds; just enough to melt the solder) with a hot soldering iron; each point solder will melt and form a neat joint.

 

[Enzy]

Ok, Also how exactly would I calculate the gain of a preamp like that? The 330k resistor on pin 2 going to pin 1 and the 10k going to ground would set the gain. So would that make the gain be 34 in my case?

 

[Audioguru]

Ok, Also how exactly would I calculate the gain of a preamp like that? The 330k resistor on pin 2 going to pin 1 and the 10k going to ground would set the gain. So would that make the gain be 34 in my case?

Correct. 1+ (330k/10k)= 34.

I bought a 2.1 sound system for my computer and TV and its woofer became intermittent. I looked carefully at the solder joints on the pcb and they all looked good. I almost cut away a few connectors that I thought might be intermittent but instead I re-soldered every solder joint on the pcb that fixed the problem. Your cleaning the pcb might have wiggled a poor solder joint to make it connect better for a while until it becomes intermittent again.

 

[Enzy]

Ok good so having a gain of 34 is that considered a high gain? ALso the check you told me to do on the preamp checking the output voltage in dc when there is no signal and you said it should be half the ac voltage, I ac tually didnt get any reading it was 0v dc and it still worked after connecting signal, even with the signal I'm still not getting any dc out so I dont know if its my meter not being able to read it but its dc so it should be able to.

 

[Audioguru]

The Chinese amplifier has an input sensitivity of 1.6V. Then with a gain of 34 the input to the preamp must be at least 47mV which is easy from a laptop.
The DC voltage at the output pin1 of the TL072 dual opamp should be half the supply voltage.

 

[Enzy]

I will check the preamp again, also I was just playing it and checking the output of the amplifier with my hand held scope I had a speaker in 1 side (8 ohm 200w, 12" speaker) both the side with the speaker and without the speaker was giving simliar wave forms and the rms voltages were roughly the same ranging from 2vrms to about 24v rms and thats with the gain set to half and the volume nob set to half (using a phone as the input) If i remove the input with nobs in the same position the output voltage is constantly at 35.4v so I am assuming if something should happen when the amplifier is on and the input line got damaged some how for a long period of time it would damage the amplifier or burn speakers.

 

[Audioguru]

both the side with the speaker and without the speaker was giving simliar wave forms and the rms voltages were roughly the same ranging from 2vrms to about 24v rms and thats with the gain set to half and the volume nob set to half

24v RMS into an 8 ohm speaker is 72W and into a 4 ohm speaker is 144W. It is good that the knobs are at half so you can turn up the volume when you have low level signals.

If i remove the input with nobs in the same position the output voltage is constantly at 35.4v.

If it is 35.4VDC then the amplifier is defective and will burn speakers and itself because the output should be 0VDC.
If it is 35.4VAC then either the input wires are picking up mains hum or the amplifier is oscillating at a high frequency that might destroy the amplifier and any tweeter connected to it.

There is no schematic or parts list of the Chinese amplifier so maybe they designed or built it wrong.

 

[Enzy]

its 35.4v AC, it might be mains hum then since im using a Iron core transformer and not a toroidal.

- - - Updated - - -

It will be used only for bass playing 2 600rms and 2 1500rms speakers do you think it will affect them?

 

[Audioguru]

Mains hum is picked up by unshielded amplifier input cables or by a breadboard with messy wire "antennas" all over the place. It is 50Hz or 60Hz and since you measured 35.4V then the speaker will produce its sound with 157W or 313W which will be very loud and be obvious mains hum.
All my amplifiers use an iron core transformer, not a toroidal, and they do not play mains hum.
Maybe your positive or negative supply has a defective main filter capacitor or maybe the rectifiers are wired wrong.

 

[Enzy]

I think the capacitors are ok but the cables arent shielded cables probably thats my issue I use single core wires to do all the wire runs

 

[Audioguru]

Audio systems ALWAYS use shielded audio cables between devices and they NEVER use ordinary wires that act like antennas that pickup mains hum and all kinds of other interference.
My "VU meter" that has an LM3915 IC driving its LEDs has an electret mic as its input and the mic has a shielded audio cable only 4cm long connecting it to the input of the preamp circuit so that it does not pickup mains hum or other interference.

 

[c_mitra]

I think the capacitors are ok but the cables arent shielded cables probably thats my issue I use single core wires to do all the wire runs

You can, as an emergency measure, use a tightly twisted pair that will reduce hum very significantly. You must ensure, if you are using a shielded cable, ground the shield only at the amplifier side and not at both ends.

Your circuit layout is also critical.

- - - Updated - - -

mic has a shielded audio cable only 4cm long connecting it to the input of the preamp circuit so that it does not pickup mains hum or other interference.

If a 4cm long run needs shielding, the circuit is badly laid.

 

[Enzy]

I got some shielded cables just now and replaced the single core wires from input connectors to preamp, then from preamp to amplifier input, Im not sure how I would ground the cable, the cable only has a layer of rubber over 2 coated wires and bare copper wire, I only used the 2 insulated wires in the cable. When I tested the amplifier it still had the 35v coming out when I remove the input signal cable the voltage only exists when the nob isnt at 0 once I turn it up a bit it starts to put out 35v

 

[Audioguru]

If a 4cm long run needs shielding, the circuit is badly laid.

The good shielding is needed and it works. The gain of my mic preamp is very high at 1820 so that it is very sensitive. The project has a plastic enclosure not metal. My circuit has AGC that cuts loud levels to 1/10th.

- - - Updated - - -

I got some shielded cables just now and replaced the single core wires from input connectors to preamp, then from preamp to amplifier input, Im not sure how I would ground the cable, the cable only has a layer of rubber over 2 coated wires and bare copper wire, I only used the 2 insulated wires in the cable. When I tested the amplifier it still had the 35v coming out when I remove the input signal cable the voltage only exists when the nob isnt at 0 once I turn it up a bit it starts to put out 35v

Your cable might have a pair of wires shielded by a foil layer and a bare copper wire allows connections to the shield. You used both wires but you did not use the very important shield that should connect to the signal ground at both ends.
Does the cable have a foil shield surrounding the two insulated wires? is the bare copper wire touching the foil shield along the entire length of the cable?
Is the amplifier output 35VAC or 35VDC and does it make a speaker hum loudly?

Here is a photo of a typical shielded audio cable that comes with every amplifier, record player, tape player, CD player and MP3 player. My cable TV recorder and its replacements also came with shielded audio cables. I have a pretty big box full of them.

 

[Enzy]

It has the rubber layer on the outside, no foil over wires just plastic insulation over coper wires (stranded copper) and around both insulated wires is a stranded layer of copper wire which based on the diagram would be the shield, so I used 1 wire for positive audio and 1 for negative so that means added to that wire I used for negative I should also connect the shield into the ground input of preamp and output of preamp going to the amplifier and also connect the sheild wire in the ground connection of the amplifier also?

- - - Updated - - -

I just realized I didnt ground the case either, I am gonna run a wire from the ground pin from the ac in and just attach it to the body of the case.