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Old 29th December 2012, 04:42 PM   #1
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Default Can't seem to get OpAmps to work

Hello!

I can't seem to hook up an opamp without horrible distortion.

I designed some circuits based off many great posts on this site and using a quad opamp but everything I do turns into distortion. It sounds like the amp is being overdriven. I'm testing the circuit using audio from an iPod and putting the output into some headphones.

I finally pulled the opamp onto a separate board and put it into a simple unity gain configuration and I still get overdriven sounding distortion. What could I be doing wrong with a unity gain circuit to cause this?

I've tried this with a cheap opamp from rat shack, and an opa4134 with multiple power supplies.

Any suggestions?
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Old 29th December 2012, 06:02 PM   #2
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Well, most opamps can't drive headphones to any significant level. You need at least a small power amp or chip amp.
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Old 29th December 2012, 06:07 PM   #3
Mooly is offline Mooly  United Kingdom
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Can you post the circuit you are using ?
Following on from what Conrad has said you could try a series resistor to the headphones. Something like 100 ohm.
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Old 29th December 2012, 06:47 PM   #4
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Thanks!

I've never done line level before so I didn't even think that it couldn't drive a pair of headphones. When it's complete, though, hooking it directly to an amp from the output will work?

I'm not great at drawing schematics, but I've attached my planned final circuit. It's the same board, left is top down and the right side is from the bottom.
Moving counterclockwise, it's a summing amplifier, an inverting amplifier, and 2 Sallen Key filters. The first is variable frequency and the second is static.

Does this look reasonable? The two single pots will be replaced with a dual gang linear pot when all is done.
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Old 29th December 2012, 06:57 PM   #5
Mooly is offline Mooly  United Kingdom
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That's not easy to follow tbh. Much better to see it in proper schematic form if you can. It doesn't have to be neat, just drawn on paper is fine.

I see you are running on a single supply so that means the design has to be AC coupled and that you need some means of biasing the opamps to half of the supply voltage for it to work. I can't tell if you have that or not tbh by looking at that. The output pin of each opamp should be at 6 volts on a 12 volt supply. If any isn't you have a problem. So that's something to check.
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Old 29th December 2012, 07:10 PM   #6
sreten is offline sreten  United Kingdom
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Hi,

Use an essentially identical chip but one that is for driving headphones.

Typical op-amps cannot drive headphones, small amplifiers can.

rgds, sreten.
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Old 29th December 2012, 08:35 PM   #7
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haha, no, it wasn't meant to follow, it was meant to help build it...

I attached the schematic. I left the pots out. They are wired as variable resistors, so assume they are at 0ohm

I know I didn't take into consideration any biasing of the opamps. If I changed to 4 individual opamps on a common power supply, would this still need any consideration? I'm trying to keep this as simple as possible.

Side question:
Are there any kits or premade products that would do this?
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Old 29th December 2012, 08:36 PM   #8
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The figure on the right shows 12V connected directly to pins 4 and 11. As Mooly stated, the op amp needs to be biased at half the supply voltage, to allow for full swing at the output.
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Old 29th December 2012, 08:38 PM   #9
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Sofaspud - I'm not entirely sure what that means. Could you explain a bit further or link me to some reading?
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Old 29th December 2012, 09:18 PM   #10
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If the op amp has no -V voltage, it cannot reproduce the negative portion of the output. So in your inverting circuit, the positive half of the input signal is being chopped off.
An op amp cannot create an output more negative on the low side, or more positive on the high side, than the supply voltage.
Here's a basic tutorial.
Here's a bit more advanced TI application note on single-supply op amp design.

I should add that you can make a split supply from your 12V by using a voltage divider of 2 equal value resistors. By making the junction of the 2 resistors the reference ground point, one end of the divider will be +6V and the other end will be -6V. This and other methods to achieve the same should be easy to find.
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Last edited by sofaspud; 29th December 2012 at 09:27 PM.
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