Opamp audio circuit problem

[FireAce]

Hi fellows, I found a great audio mixer circuit online

http://www.all-electric.com/schematic/simp_mix.htm

with an image of four simple audio mixer circuits. I'm having a problem with the inverting opamp part, as in, I cant get audio to come through. As soon as I remove the opamp the audio finds its way through the 10k res and works.

I tried using both the TL082 and the NE5532P running at 9v from an adjustable voltage regulator.

Any ideas? This guy wasnt born yesterday and it must have worked for him.

Thanks

 

[ccurtis]

For one thing, right off the bat, the op amps require positive and negative power supplies in that design; not a single-ended 9v power supply.

 

[FireAce]

aaaaah, was wondering about that. I'll find a decent circuit and give that a try, thanks!

The thing is, I've run Op Amps in the past without a neg power supply, and it worked. Maybe it could have worked better.......

 

[crutschow]

You can use an op amp with a single supply and AC signals, but you need to AC couple the signals and bias the op amp input at 1/2 the supply voltage (pseudo ground) to avoid clipping the bottom half of the waveform. Also the single supply must be above the minimum required for the particular op amp you are using (which isn't always given in the data sheet but typically is about 9-10V).

 

[ccurtis]

aaaaah, was wondering about that. I'll find a decent circuit and give that a try, thanks!

The thing is, I've run Op Amps in the past without a neg power supply, and it worked. Maybe it could have worked better.......

You're welcome. Maybe some audio will come through with the op amps installed but it won't sound pretty. If no audio, maybe the op amps are "latching up" or getting damaged because the input is swinging too far below ground (the - power input terminals to the op amps) without a negative power source. I'm not disparaging the circuit so much, just that particular design requires a split power supply.

 

[FireAce]

Ok, went ahead and found I already had the +_ voltage I was needing in an amp circuit also included in my project, I hooked everything and WOLLAAAAA, it didnt work. Nothing, just a very faint distorted signal. So I dug up a center tapped transformer and did up the simple circuit as I found online, and tried that, STILL NOTHING. I must really be stupid, this has to work, thousands have done these circuits and I know they work. What am I missing? This is the simplest of audio circuits.

 

[ccurtis]

Check the simple stuff first. Actually measure that the supply voltage is there, is correct magnitude and polarity, and is connected to the correct pins. Measure from ground to each of the supply voltage pins. Check all of the connections, also looking for connections that may be there that shouldn't be there. Make sure the load is not too great for the op amps. They cannot drive a speaker, for example. Make sure the input source voltage is not too high, or is not too low. The circuit is not designed to amplify the input signals. Connect your input signal directly to the load you are driving and check that you hear something. If you don't, the problem is not with the circuit. It would help to know what your source(s) and load are to better help you. What was the idea behind the center tapped transformer, anyhow?

 

[crutschow]

Make sure the common of the power supplies is connected to your circuit common.

 

[audioguru]

Why not post a schematic showing exactly how you connected everything showing the input source and its voltage swing, opamp part number, load resistance and supply voltages.

 

[FireAce]

Greetings, I'll try to be more specific, and try to troubleshoot as much as I can by myself.

First, I'm not happy with the center tapped transformer circuit I tried, its too simple (just a transformer and 4 diodes) and seems like a cheap way to go.

The other circuit from the amp that I tapped into is powering two 4558D Op Amps, but suddenly I'm only getting 4V on the pos side, and 9v on the neg, I thought I had 9v on both sides, or my tester was drunk, dunno.

A link to a recommended and tested +_ circuit would be appreciated. I'm using TL082ACP's and NE5532P Op Amps.

Thanks fellows, been too busy and havent had time to upload a decent schematic, at the moment I'll try to get a good power supply wired up

 

[ChrisP58]

Here is a good place to look for power supply design. https://www.electro-tech-online.com/threads/power-supply-design-for-beginners.113613/

What voltage and current do you need?

 

[FireAce]

I need a bipolar power supply (I think thats what they call it) 12v would be good. I can find many online, just wondering if anyone has personal favorites for Op Amps

 

[audioguru]

I have used many TL071 single, TL072 dual and TL074 quad audio opamps. They are the same as a TL08x but are selected for low noise. Like most opamps, their maximum positive swing is about 1.5V less than the positive supply voltage and about 1V less than the negative supply voltage. Their minimum load is 2k ohms.

 

[FireAce]

ok, I have a good bipolar power supply hooked to the NE5532P opamp, 13v each side.

So far so good, now I need a good earphone amp

 

[audioguru]

Is each earphone 8 ohms, 32 ohms or 600 ohms?

 

[FireAce]

audioguru, the circuit has to be flexible. I'm building an in-ear monitoring system for our band, and I dont know what earphone are gonna show up.

Currently I did a test with the NE5532P opamp and some Bose headphones and was pleased with the results. Dont know if this is the best way to go, it may bite me down the road. Any ideas?

Also I have the concern of "popping" noises from bad cables or guitars unplugging, hurting peoples ears. I may have to do up some limiter circuits, unless theres a simpler way.

 

[audioguru]

Like most good audio opamps, the NE5532 is low noise, low distortion, wide bandwidth and has a minimum total supply of only 6V.
But its minimum peak output current is only 20mA so its max output is:
1) 600 ohms= 170mW RMS (extremely loud with a 30V total supply or 95mW RMS (very loud with a 20V total supply).
2) 32 ohms= 9mW (not loud and the total supply must be more than 7.5V).
3) 8 ohms= forget it.

To properly drive 32 ohm headphones or less then a transistor output stage is needed.

Again, a limiter is designed for the impedance of the headphones. A headphone is loud at 50mW. 50mW RMS into a 32 ohm headphone is a voltage swing of 1.27V RMS which is 1.8V peak. Three diodes in series (then another string inverted and in parallel) will do it when paralleled with a 32 ohm headphone

 

[FireAce]

Makes perfect sense. Attached is a class A mosfet follower, is that what your referring to? Or would you recommend another that you have had good success with?

 

[audioguru]

A class-A amplifier makes a lot of heat and a small amount of output power.
Nearly all pro and hifi amplifiers are class-AB or class-D which do not make nearly as much heat and sound PERFECT.