Op Amp help needed desperately

[segasonicfan]

Hey guys, I'm new here and I'd consider myself a noob to DIY audio stuff as well since I've only been working with it about a year. Anyway, I'm almost done with an amazing (if I say so myself :P) portable amplifier system however, I would love for it to incorporate some sort of tone control A graphic equalizer would be ideal but I'll settle for treble and base. So I bought a highly regarded (from what I can tell) audio op amp-- the LM833. However, I've correctly constructed a bunch of applications for the circuit with all unsuccessfull results. My knowledge of op amps is very limited and I find all this inverting non-inverting and negative supply rail stuff a little confusing. I would love it if someone could help me out with this, I'm really stuck in a dead end.

Here's how the system works so far: it uses a LM4752 22W amp that runs of a 25v li ion battery pack. I'm also adding optional kareoke inputs and an FM automatic scan radio. The radio and LM4752 stuff is all entirely finished an functioning beautifully, like I said I just want some sort of tone control and a small mic amp maybe. I would like to construct the Tone Control, Graphic Equalizer, and Balanced Mic PreAmp applications from the LM833 datasheet here:http://cache.national.com/ds/LM/LM833.pdf
but I've been having some problems doing this. So here are my questions:

-It says the op amp requires a -15v and +15v input. Can't I simply connect the negative and positive ends of a battery for this? Or do I need 2 power supplies?

-Is the input signal on the op amp supposed to come directly from the audio source? and the output can be fed right to the inputs of the LM4752, right? That's what I'm trying to do anyway...

-Also, I'm not getting any sound when connecting a speaker to ground and the output (with the audio inputs grouned as well). But when I connect a speaker to the output and the positive supply rail I get some highly distorted sound...is this simply because the output is inverted?


Sorry for such a long post about this but I desperately need some help from someone with these op amps. I'm willing to spend whatever it takes on parts to get this working, I just need someone to help guide me through it. Thanks.

-Segasonicfan

[janneman]

Hi,

You cannot just simply connect the opamp supplies to the 25V; I mean you can but need to do a bit extra to run the opamp single-supply. You connect the + supply to the +25 and the -supply to ground. Then you bias the opamp inputs to about mid supply. Then you connect up the signal through DC blocking caps.

Assuming you use the circuit on page 12 of the data sheet: Make a +12.5 V point by using two resistors of say 10K between +25 and ground, and connect the +input to the midpoint. Decouple it with an electrolytic of a few uF to ground to get rid of hum and noise.

Then use a series cap of say a few uF on the Vi point to connect up the input signal. The output from the opamp (Vo) to the power amp should likewise be AC coupled with a series cap, but that may already be present. Use non-polar caps for 50V or more for the signal, like polyester film or, if you want to go overboard any of the designer caps. The value depends on the circuit details, but you can first try it (I have the impression you are the experimenting type, great!). If they are too small you will notice a lack of low bass.

Jan Didden

[runebivrin]

Pretty much any OP-amp application requires a split power supply, and that includes power amplifiers as well.

For the tone control circuit you'll be able to get away with a simple voltage divider with capacitor decoupling. For the power output you'll need a large capacitor in series with the loudspeaker.

Look up single supply operation in the data sheets.

Rune

[PRR]

> I'm almost done with an amazing (if I say so myself :P) portable amplifier system... I'm not getting any sound when connecting a speaker to ground and the output (with the audio inputs grouned as well). But when I connect a speaker to the output and the positive supply rail I get some highly distorted sound...

That is amazing.

The LM4752 can be used two ways. Because it is a low-cost part, they show it with a Single Supply and an Output Capacitor. But you can also use it with Dual Supply and no Output Capacitor. (The later is also how most op-amp plans are drawn.) Which way are you doing it?

[segasonicfan]

PRR--the LM4752 works perfectly...I was referring to the opamps not outputting properly. Don't mock me.

As for the Op Amp power...I forgot to mention that I was giving it 15v from the same battery (drawing it from another cell within the battery). I just don't understand why these opamps aren't working for me...in the LM4752 datasheet it says only the non inverting inputs are available and I think most of the op amp applications I'm doing have an inverting output...but I'm not sure how to make them non-inverted and how to calculate the gain and all.

Do I need to do anything to the source audio before I input it to the op amp? And I've been tested the op amp by connected a speaker to the output pin and ground...is this an improper method? I just don't want to drive it into the LM4752 unless I know it's perfect.

-Segasonicfan

[bmcevers]

I don't think he was trying to mock you, I think he was just trying to ascertain what kind of configuration you're using. You might want to try posting your schematic so people can get a better idea of what you have. Also, just a tip, you better have "thick skin" especially if you want help from some some of the more knowledgeable people on this forum, and there are some good ones (ie..john curl, nelson pass, jan didden). There are also some on this forum that like to eat 'noobies' for lunch the first time you post something even slightly controversial or incorrect. Like I said, just a tip.


As far as your circuit, I would tend to agree with Jan.Bias the input to the middle of the positive rail (V+) and ground, and AC couple it (cap in series with the o/p) to the input of the amplifier stage. Your opamp choice is good (many will disagree with me) the LM833 is a strong performer and inexpensive, you might want to check out the NE5532 (also good, maybe slightly better).

[janneman]

Originally posted by segasonicfan
PRR--the LM4752 works perfectly...I was referring to the opamps not outputting properly. Don't mock me.

As for the Op Amp power...I forgot to mention that I was giving it 15v from the same battery (drawing it from another cell within the battery). I just don't understand why these opamps aren't working for me...in the LM4752 datasheet it says only the non inverting inputs are available and I think most of the op amp applications I'm doing have an inverting output...but I'm not sure how to make them non-inverted and how to calculate the gain and all.

Do I need to do anything to the source audio before I input it to the op amp? And I've been tested the op amp by connected a speaker to the output pin and ground...is this an improper method? I just don't want to drive it into the LM4752 unless I know it's perfect.

-Segasonicfan

Please be clear, are you trying to hang a speaker on the opamp ('833) output?? That won't work! If it did, you wouldn't need the power amp.

Do I read you right: the power amp part works, but the (pre) opamp part not? What exactly ARE you trying to do? Didnt you say you wanted to use the tone control schematic? Please draw a schematic and we all can move forward.

If you have 25V single ended, that will be fine for the opamp. If you limit the opamp to 15V and the power amp to 25V you will not get full output (the opamp will limit it). Please re-read my previous post.

Jan Didden

[segasonicfan]

here's a schematic of what I'm trying to do. all the stuff on the right is already finished and can't be changed (the amp itself)


I just want to configure the LM833 as a active tone control for my amp.

-Segasonicfanhere

[PRR]

> configure the LM833

Essentially the same as that power-amp, except the PA chip handles some dull details that you have to DIY with the universal op-amp.

You have +25V and Gnd. (I'm going to call it +24V for math simplicity.) An audio amp has to handle both polarities of audio. However, the audio signal reference does not have to be the DC ground. It could be almost any voltage between the +24V and Gnd supply pins. Half-way is often best: +12V.

We don't have a +12V supply. Neither does the 4752, so it makes its own with that box called "bias". If you probe pin 5, it sits at half the supply voltage. And if you probe the output pins, you will find them near half the supply voltage (because the feedback gain-set networks reference the Bias voltage). The output pins swing from +12V to +22V to +2V and back. Co is charged to 12V at start-up, and after that will cancel 12V, making the voltage at the speaker 0V, +10V and -10V. Swings both ways, even though the chip only does positive voltages.

Do the same with the op-amp. You "could" even use the same Bias supply, tapped at pin 5. In general, I hate to try small fine work while hitched to a hard-working Power stage. I'd probably use two resistors and a cap, as shown in the attachment, to make a Bias supply for (all) the opamp(s). The two resistors split the voltage; the capacitor makes it clean for audio reference.

Opamp power is still +24V and Gnd. But the DC reference for the inputs must be changed to the +12V Bias source. For this inverting tone control, not connected to anything else, if you set the "NonInverting" input to +12VDC, the output will also lay at +12VDC. So will the input to the tone network, so you can not connect it to a source directly. You need an input cap.

My volt-notes in purple are (roughly) the voltages you should observe. Not critical: it "should be" +12.5V for 25V battry, but +/-10% deviation for resistors makes no difference. Measuring some of those points with a typical voltmeter will load-down the bias a tenth-volt or so.

> does the signal have to be inverted again?

No. Audio works with any number, odd or even, of inversions. Switch the speaker leads, sounds the same. (For stereo, flip both speakers or you get a hole in the middle and lame bass.)

Or "I say no", with a weasel. On a few recordings, for some ears, for some speakers, there is an "Absolute Phase": a big bass drum's initial transient is (often) positive air pressure at the audience, and in the right situation an inversion makes a difference. Audio producers don't obsess about it: boxes are usually non-inverting but there is a Phase Invert switch on the input of every mike channel, and every patchcord is potentially wrong-phase.

> Your opamp choice is good (many will disagree with me) the LM833 is a strong performer and inexpensive

Compared to most any chip power amp, it is a jewel. National must have been really lazy to get nearly-no market penetration with the 833. Here, its low noise is a strong point: that power amp has gain=50, quite a bit.

5532 is another go-to, but with these tone-pot values you get a lot of DC error (more than 833, which is hardly DC-error-free). I suppose that may be moot now that we jacked the whole bias up 12V.

The other go-to, TL072, can drive the 4752 well, and has zero input current so zero DC error in a 100K pot. Often used for EQ knobs in a certain class of Pro gear. But 2uV input noise times a gain of 50 in the output is marginal; the BJT chips will be a whisper quieter.

[segasonicfan]

Hey, thank you SO much for all the help. You even put in the schematic for me =) I tried everything you said and tested all the voltage places. I got all the correct volt readouts on everything with the op amp and the LM4752 is functioning perfectly. However, the output (when going through the opamp) is very very soft and you can barely hear it. The bass knob is working perfectly but the treble only sounds right when it's at full or off. In between it gets very fuzzy and sometimes intercepts the radio..heh. The only thing I changed from your schem was I used 3k resistors instead of 2.7k on the bias input. I don't think this makes any diff though,

Maybe my op amp is messed up though from previous testing. I'm going to try another one tomorrow and see what happens!

-Segasonicfan