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Topic: Opamp setup doubt (Read 10166 times) previous topic - next topic

Marciokoko

Jul 03, 2016, 05:21 pm Last Edit: Jul 03, 2016, 05:24 pm by Marciokoko
I've been learning about opamps for a while now (2 weeks).  I found this YouTube video and tried the  setup in one of the videos.  The setup calls for a small pc mic, a headphone set, two 9V batteries and everything else shown here but I get no sound from the mic.  Im not sure why, but Im reading the voltage between ground and the 9V+ rail and I get 15.50 instead of +9V.  I'm not sure why I would get 15.50V.

Im not using a LM324, all I could find was a TL071cp.

Here is an image of the sketch and the picture of my setup:


ChrisTenone

On your diagram, it shows Vcc- as being -9V. the potential between Vcc+ and Vcc- is 18 volts. Given that your op-amp is not rail to rail, your measurement of 15.5 volts seems entirely reasonable.
What, I need to say something else too?

DVDdoug

#2
Jul 03, 2016, 06:43 pm Last Edit: Jul 03, 2016, 07:18 pm by DVDdoug
Quote
I found this YouTube video and tried the  setup in one of the videos.
That's a TERRIBLE design!!!   ...I tried to tell you that when I replied to your other post but maybe I was too subtle.

Get a chip that's designed to drive headphones/speakers and follow the chip manufacturer's recommended schematic.  The LM386 is popular and easy to use, or there are plenty of other options.   Op-amps are great as microphone preamps, but NOT for driving headphones/speakers.


Quote
The setup calls for a small pc mic, a headphone set, two 9V batteries and everything else shown here but I get no sound from the mic.  Im not sure why, but Im reading the voltage between ground and the 9V+ rail and I get 15.50 instead of +9V.  I'm not sure why I would get 15.50V.
The batteries have to be grounded (the negative terminal of the +battery and the positive terminal of the -battery).

If the circuit is working properly the op-amp's output should be zero volts (with no input/signal).

Try taking-out the 10K pot.   Or at least try both extremes in case it's wired backwards.   A 10K pot is not going to work as a volume control with ~32 Ohm headphones, but you might  get some  signal with the pot at "maximum volume".   At "half-volume" you're not going to get any sound, because the resistance is too high compared to the headphones.

And if you do get sound, you might burn-up the op-amp because of the low impedance load.


P.S.
There is one thing from that terrible design that you will need...    The 5k resistor and 0.1uF capacitor supply power to an electret "computer microphone" and isolate that power from the op-amp while letting the audio through.   

Marciokoko

Ok

I was just trying to make use of what I have. 

Your explanation was too complicated for me to understand.

I need to read up on what a rail to rail amp is first and how to find out when an opamp is such.

One more thing, what is a pre-amplifier?

I just checked the Vout on the amp and I get -6v to about 9v depending on where the pot wiper is.

Grumpy_Mike

Quote
Your explanation was too complicated for me to understand.
Then ask about what you don't understand - that is what this forum is all about.

DVDdoug

#5
Jul 03, 2016, 11:39 pm Last Edit: Jul 03, 2016, 11:43 pm by DVDdoug
Quote
I was just trying to make use of what I have.
What you have wont' work... An op-amp can't put out enough current to drive a headphones or a speaker.   Typically you can drive a load of around 1K Ohm or more.   I don't know the specs for your op-amp but no standard op amp can drive headphones.   

You'd need to check the datasheet for the chip to find out how much current it can put-out, and then apply Ohm's Law to determine the minimum load impedance/resistance.

If you take an electronics class the first thing you learn is Ohm's Law (the relationship between voltage, resistance, and current).    You really should understand Ohm's Law before you try to understand op-amps.



Quote
I need to read up on what a rail to rail amp is first and how to find out when an opamp is such.
That won't solve your problem.   Your problem is current, not voltage.    Current and voltage are related (as described by Ohm's Law) but the problem with a regular op-amp is it's current limits.


Quote
I just checked the Vout on the amp and I get -6v to about 9v depending on where the pot wiper is.
Something's wrong.   Did you ground your batteries?  Are you measuring +9V and -9V now (with ground as your reference)?   Both inputs on the op-amp should also be approximately zero volts.   If one of the inputs is not zero,  that may help you track down the problem.

Quote
One more thing, what is a pre-amplifier?
A preamp takes a signal from a microphone or a phono cartridge (a few millivolts) and converts it to line level (about 1V).  There is a preamp built into your soundcard and preamps are built into audio mixers.    Older stereo receivers had a phono preamp but most newer receivers don't have phonograph inputs.     (Stand alone preamps are expensive specialty items.)   

A power amplifier  takes a line-level signal and boosts the voltage & current to drive a speaker.   A headphone amp is a special low-power power amplifier.


Marciokoko

Ok I have taken electronics online courses.  I know about Ohms law and I have basic knowledge of an opamp.

Thanks for the preamp and amp explanation. 

I'm looking at the data sheet but what parameter do I need to look for regarding the output current?

I did ground them and I do get +9 and -9V with reference to ground on each respective terminal out of the battery so there supply is good

Grumpy_Mike

Quote
I'm looking at the data sheet but what parameter do I need to look for regarding the output current?
Figure 6. Maximum Peak Output Voltage vs Load Resistance

Marciokoko

Ok so the headphones are 32 Ohms.  So that means the maximum voltage output would be around zero?  Is that why it fails?

I DO have the 5k ohm R and 0.1uF capacitor in the circuit.

Marciokoko

I think the opamp tl071 can be used as a split supply amp because in the reference sheet on page 4 it states at the bottom the Vcc- and Vcc+ power supply voltages:

http://www.ti.com/lit/ds/symlink/tl071.pdf

Doesnt that make it a rail-to-rail opamp?  So the problem is as Doug stated, that the current output is too low, not so much as ChrisTenone said that:

Quote
On your diagram, it shows Vcc- as being -9V. the potential between Vcc+ and Vcc- is 18 volts. Given that your op-amp is not rail to rail, your measurement of 15.5 volts seems entirely reasonable.
Right?

Grumpy_Mike

#10
Jul 04, 2016, 07:15 am Last Edit: Jul 04, 2016, 07:22 am by Grumpy_Mike

Rail to rail means the output of the op-amp can be set to be a fraction of a volt of each rail. This is not true for all op-amps, look at the maximum voltage output figure in the spec.
Page 7 shows the maximum output swing is only 12V when you have a supply rail of 15V, so this is not a rail to rail op-amp by a long way


As stated before, that op-amp can not drive such a low impedance load. If you want it to then you have to put a transistor in an emitter follower configuration in the negitave feedback loop.

Marciokoko

#11
Jul 04, 2016, 04:51 pm Last Edit: Jul 04, 2016, 06:49 pm by Marciokoko Reason: tried to answer my own questions...
ok so my first confusion stemmed from the fact i thought:

single supply = normal opamp

split supply = rail-to-rail opamp

I read here (http://electronics.stackexchange.com/questions/182636/what-is-rail-to-rail-supply) that rail-to-rail opamp meant that the output voltage could swing from V supply + to V supply -.

Now I understand that a rail-to-rail opamp can be identified from the fact that, as Mike stated, its Max Output Voltage swing is +/-12V which is not a fraction of the supply voltage of +/-15V.  So I looked over at the LM324 which is the one the afrotechmods author uses and I found its supply voltage can be 32V or +/-16V (http://www.onsemi.com/pub_link/Collateral/LM324-D.PDF).  I guess the Output Voltage High Limit and Low Limits on page 4 is what I should be comparing with.  But if Im reading it correctly itll output 3.5 if supplied with 5V, thats still 7/10 of the supply voltage.

I have 3 questions:

1. Im still not sure what it means for it to be a rail-to-rail opamp because Mike says:
Quote
Rail to rail means the output of the op-amp can be set to be a fraction of a volt of each rail.
but that link says that it means the voltage can swing anywhere from Vsupply + to -.  So maybe both explanations are the same but Im not understanding it that way.  I guess Im trying to read the datasheet and try to determine why the LM324 IS a rail-to-rail and the TL071 IS NOT rail-to-rail?

2. What do you mean when you say the TL071 cant drive a low impedance load?  Im sure this is related to my next question but it may be the critical piece I have yet to connect.  If I plug in 9V = 500Ohms * mA I get 18mA.  This means thats how much current the opamp can produce at 9V?  If the headphones are 32Ohms, then 9V/32Ohms, would require 281mA?  Is this why I would need the bjt?

3. As for the comment about the output current, I tried looking for both these specs on the datasheets and couldnt find them.  I saw the Figure 6 for TL071 (that Mike suggested) but Im not sure how to interpret it.  If Im supplying it +/-9V, I should be able to drive a 0.5kOhm load?  On the other hand the LM324 does have an output current parameter of 20-40mA.




ChrisTenone

Rail to rail means the output goes from V- to V+. Non rail to rail op-amps, like your lm324 typically have a range 2 or 2.5 volts less than the range of input.

For use with Arduino, a rail to rail op-amp that goes from 0 to 5 volts is especially useful.
What, I need to say something else too?

Marciokoko

Ok I may have caused confusion because although the diagram shows a lm324, but as I mentioned in the OP, I actually only have a tl071.

Anyways, you say rail-to-rail means it's output goes from -V supply to + V supply.  Ok so for lm324 I can see that Vsupply is 30V and maximum output swing is up to 28V.  So this is not rail to rail, great got that.

Now I'm using a tl071, it appears Vsupply is 15V and max output is 13.5V.  Ok so I see how neither of theses is a r2r opamp.  Thanks.

With that out of the way, a few questions remain:

1.  How does not being r2r affect my circuit example?  Is it because it can't reach a high enough voltage to produce the current necessary to drive the headphones?

2.  Is my interpretation of Fig 6 (from Grumpy_Mike's post) in my post #11, correct about the resistance calculation correct or at least makes sense?  Or am I way off and if so, how do I interpret what he said about the tl071 not being able to produce enough current?

ChrisTenone

Not being rail to rail isn't a problem at all, just realize that you need to supply slightly more voltage than you need out of it.

Op-amps also have a maximum current they can supply. I'm not familiar with the tl071, but many op-amps are designed to amplify tiny signals, and so cannot supply sufficient current to drive an inductive load such as your headphones. I trust Mike's knowledge that your op-amp does not output enough current. Some amplifiers are made for driving audio, so that is the kind you should be looking at. I believe DVDoug mentioned the LM386, a basic audio amplifier. If you want to use a bare chip, that one will do it.

Using an op-amp and a transistor should work, but that would not be the best way to do it. If your intent is to learn about op-amps, then sure you could do it that way, and it would be instructive. But if your goal is to make an small amplifier for audio use, then get an audio amplifier. Not sure where you are located, but both Sparkfun and Adafruit have little breakout boards (for less than $10) that have an audio amp, and all the supporting circuitry on them. Should make powering your headphones a quick job.
What, I need to say something else too?

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