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Need a compact +/-12 volt source with 12V input

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spiff72

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Hello all,

I am a first time poster, and really don't know much about electronics, but I created a project for myself last year that I want to upgrade.

What I am doing is using an dual opamp (TL082 from Radio Shack) as a voltage follower in a project box in my car, between a Harman Kardon Drive+Play ipod interface, and a ground loop filter. I am doing this at someone else's suggestion because of an apparent impedance mismatch between the output of the HK unit and the GLI filter. This mismatch creates a high-pass filter, so it cuts all of the bass response out of my audio.

I am using the GLI filter because my car (a 2007 Camry Hybrid) seems to have an inherent problem with the 3.5mm aux input if ANYTHING is plugged into it, as well as being plugged into power (12v accessory power). There is an audible high pitch whine that you can hear at even low or moderate volume levels. The filter DOES remove the noise, but as I said above, the filter also removes bass.

The opamp circuit is being fed the output of the HK unit (1V max) audio signal, and I am powering the opamps with a pair of 9v batteries arranged to provide me with a +/-9 volt reference (and ground at the the zero point). I also included a 12v relay in the box to disconnect the 9v batteries when the ignition is turned off. I would like to eliminate the batteries, since I seem to get nasty distortion if the batteries are even slightly discharged.

I searched these forums and found a reference to a surplus site that has a 5v input, +/-12v output. This is compact, but I think I would need a regulator to drop the input voltage from the level of the cars accessory voltage to 5v. Is this difficult? I don't know much about regulators, but I suspect I can read up on them and figure out how to get regulated 5v from 12v.

What I am wondering about is the dc/dc converter. Should it work for my application? Here is a link:
https://www.mpja.com/prodinfo.asp?number=14592+PS

Also, does anyone see any issue with the opamp I am using? I am not sure how to read the datasheets, but the input to the opamp is only 1v, while I am powering the opamp with +/-9v (or +/-12v if I do what I want to above). Was this opamp a poor choice? I have checked the battery voltage when I hear distortion, and it is only about +/- 8.5v. Should this be expected?
 
A TL082 might have hiss. A TL072 is a TL082 selected for low noise and it costs the same at normal electronic parts suppliers.

With an 18V total supply, its output will clip with a high resistance load when it reaches 15Vp-p which is 5.3V RMS and is a little louder than 1V RMS.
 
audioguru said:
A TL082 might have hiss. A TL072 is a TL082 selected for low noise and it costs the same at normal electronic parts suppliers.

With an 18V total supply, its output will clip with a high resistance load when it reaches 15Vp-p which is 5.3V RMS and is a little louder than 1V RMS.

Thanks for the response, but I am not sure I understand. I assume that I should be OK with the TL082 other than the fact that it isn't low noise?

The input is only 1V (I assume p-p), which is well within the rail voltages of the +/-9V supply, but I still hear distortion when the voltage drops much below 9v on each battery. Is there a lower limit to the supply voltages of this opamp (is 9V near the lower threshold, and thus a +/- 12V supply would benefit me)?

EDIT: The Ground Loop Interference filter that the Opamp is feeding into (before going to my aux port in the car) is the last item on this page:
**broken link removed**

The one in the image is a bit different than mine (in appearance, but is the same model number).

Thanks!
Jeff
 
Last edited:
The ground loop isolation transformers don't have spec's about their input impedance. Maybe it is low and is causing your TL082 opamps to limit the current which causes distortion.

Try an NE5532 dual opamp that has 3 times the output current and lower noise.
 
Perhaps you can derive midpoint and use a single supply instead of seeking +/-12V, provided the situation permits working at +/-6V as your need is in automobile, it is better to have a level of preregulation instead of direct use of voltage from battery. the voltage reaches rather highlevels and will contain noise.

try to see the single voltage applications, for op amps like TL072 /082
 
audioguru said:
The ground loop isolation transformers don't have spec's about their input impedance. Maybe it is low and is causing your TL082 opamps to limit the current which causes distortion.

Try an NE5532 dual opamp that has 3 times the output current and lower noise.

Thanks, I will order up a couple of those opamps and try them out. I used a socket for the opamp, and it looks like it is pin compatible with the one I used. I think I am going to order one of the DC/DC converters I mentioned too, along with a 5V regulator so I can avoid the use of batteries.

Thanks for your help.
 
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