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Old 5th July 2012, 10:06 AM   #1
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Default OPA AMP

Hi guys posting for my second time,

Attached is an amp im trying to modify to my needs, its rated for 5v but I'm needing it for 12V, i have minimal understanding in this area so any help would be great...

The Opa! Amp - iPod Amp for Aux-In

Basically i want to simplify the circuit to have a input 3.5mm headphone jack (including bypass caps) to the Opa amp (Opa2227) then a output 3.5mm headphone jack.

Again it needs to be used with a 12v power source.

Thanks in advance guys!
Luke
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Old 5th July 2012, 10:30 AM   #2
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Hi,

The OPA2227 in this design can be powered by 12V, no problem (anything from 5 to 36V DC will work). You will need coupling caps and you will need to bias the input to ~6VDC and set the gain. That's pretty much it...

To get decent performance make sure the gain setting resistors are not to large (10K range should do nicely), and make sure you filter the DC supply with a LC filter.
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Old 5th July 2012, 11:02 AM   #3
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Thanks Mark,

Thankyou for confirming the 12V issue,

As ive said i have little to no experience in circuits, could you possibly quickly make a schematic for what i need. i have no idea. unsure on where to put the LC filter and all that.

Appreciate the help and i know im asking alot!

Thanks again,
Luke
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Old 5th July 2012, 11:22 AM   #4
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Can't really make a drawing right now, don't have the tools. But let me try to explain.

An LC filter is simply a coil and a capacitor, coil in series with the + power supply of the opamp and from the same pin of the opamp a capacitor to ground. This should clean up your power supply. A good starting point would be something like 10uH + 10uF.

The coupling caps are in the signal path (your audio "+" pins) use something like 10uF. Both on the input and the output!

The biasing is done with a voltage divider (2 resistors in series from + power supply to ground), use 2 resistors of 47K for this.
The gain is set with the 2 remaining resistors.

Edit: made a paint drawing, hope that helps..
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File Type: jpg OPA amp.JPG (4.7 KB, 334 views)
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Last edited by Mark.Clappers; 5th July 2012 at 11:28 AM.
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Old 5th July 2012, 12:40 PM   #5
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ok now i understand abit more, thanks for explaining, how am i to know what pins of the opa amp chip go where?

and how would the circuit described be 2 channel?

Thanks
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Old 5th July 2012, 12:46 PM   #6
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The datasheet with pinouts is available here: http://www.ti.com/lit/ds/symlink/opa2227.pdf

For a 2 channel version just build the circuit 2 times, you will only need one OPA2227 (2 opamps inside)
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Old 6th July 2012, 11:29 AM   #7
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Ok im trying to do a schematic at the moment but i cant find an opa2227 chip,

Im trying to make this amp as a part of another mod (LS1GTO.com Forums - View Single Post - AUX input on a factory stereo - my way)

Trying to cut in aux audio but the mod above requires the input device to be turned up high and the headunit itself turned up also, so the idea is to place this opa amp in between the above mod (LS1GTO.com Forums - View Single Post - AUX input on a factory stereo - my way) to eliminate that problem and improve audio quality!

If you get a chance could you help me find the part for eagle cad or help design the idea, id even consider paying you if it all works out

Thanks for the help!
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Old 6th July 2012, 11:49 AM   #8
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If you use a program like Eagle for your layout there should be a generic footprint that fits the OPA (DIL8 or something like that). Professionally I used Altium designer, never used Eagle..

If you find the OPA2227 hard to find you can always consider a different opamp for this application. There are many possible options but a few that come to mind are the OPA2134 and the NE5532. Basically any decent quality opamp which is unity gain stable should be able to satisfy your needs.

On a side note: do you know how much gain you need? you will need to figure this out before you build the amp (the resistors set the gain).

I am always willing to help and comment on your layouts, but don't have the time to do it all for you. I also have some projects going on myself
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Old 7th July 2012, 09:34 AM   #9
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Heres what ive done in eagle let me know where im going wrong

unsure of R4 resistance also

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Old 7th July 2012, 01:51 PM   #10
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Quote:
Originally Posted by kudos0743 View Post
Heres what ive done in eagle let me know where im going wrong

unsure of R4 resistance also


Not much that make sense there, unfortunately.

Start with this circuit:
http://tangentsoft.net/audio/cmoy-tu...angent-sch.pdf

Or this one that shows the pin-outs of the chip:
CMoy Headphone Amplifier

In your case there would be only one 12 V supply instead of the two 9 V batteries. So read "+6 V" and "-6 V" where +9 V and -9 V is shown.

Note that the circuit in post 4 does not have any gain. That circuit will also push any power supply noise into the amplifier.
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