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Old 1st August 2006, 09:51 AM   #1
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Default Op Amps for Balanced Line Converter

Hi there,

I'm planning on creating an iPod interface for my Saab 9-3 that utilises a balanced line converter (as per http://www.aberrant.org/~dkane/myPictures/iPodMod/). Rather than buy a balanced line converter, I want to build my own. My converter will be based on the attached circuit diagram, which calls for 4560 op amps.

I'm pretty much a novice when it comes to building circuits and I don't know much about op amps. However, I've read through a bunch of threads and it seems that the 4560 op amps may not be what I'm looking for. What I'm looking to do is build a converter that has an excellent signal to noise ratio and a good frequency response (20 - 20,000 Hz).

Will the 4560 op amps enable me to acheive my goals? What about the 4580, NE5534, LM833, and LM4558? I've been trying to find a place that sells that the 4560 and 4580 op amps, but I can't seem to find any retailers in Australia that sell the bloody things. I can get the NE5534, LM833, and LM4558 op amps at JayCar, but I don't know if they're any good.

Any suggestions or recommendations?

Cheers,
Doug
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Old 1st August 2006, 01:39 PM   #2
SY is offline SY  United States
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Have you looked at the SSM line from Analog Devices? The 2142 would seem to be suitable.
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Old 1st August 2006, 02:05 PM   #3
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Doug,

I concur with SY. The SSM2142 was specifically made for balanced line transmission, and casn readily handle loads down to 600W. Check it out. Also, have you considered a DC-DC converter to power the audio chips? You might consider posting in the Power Supply Design forum for that. LOTS of good help there.

Regards,

Steve
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Old 1st August 2006, 05:23 PM   #4
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fwiw -- i think that in this app you will get better performance if you use a bipolar supply with your opamps and the SSM2142 is ideally suited -- as N-channel suggests you can use a 9V wall wart and one of the very low noise switcher chips from Linear Technology -- LT3439 -- to derive +/- rails -- take a look at Linear Tech Design Note 316
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Old 1st August 2006, 05:36 PM   #5
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Doug,

Is this going in your car, or is this for home? If home, try Jackinnj's suggestion, but for the car, try looking at Rod Elliot's website for a (+/-) DC-DC converter for powering op-amps and other low-powered audio electronics in the car. Here's the link:

http://sound.westhost.com/project69.htm

Cheers,

Steve
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Old 1st August 2006, 05:53 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
look down at performance and ESP quotes 2.5mA for the TL072.
He has forgotten about the output current which adds to the quiescent current. The same applies to all opamps.

If your loads are 10k then output currents are quite low, but if you approach the performance abilities of the balanced configuration then output currents will be much higher. Any capacitance on the output will also increase the output current.

If you just use the -ve side of the DC to DC you can double your current supply capability.

I would avoid rail to rail opamps if possible.
Many of their specs are compromised to achieve outputs approaching either rail and even more when both rails.
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Old 1st August 2006, 06:08 PM   #7
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Quote:
Originally posted by N-Channel
Doug,

Is this going in your car, or is this for home? If home, try Jackinnj's suggestion, but for the car, try looking at Rod Elliot's website for a (+/-) DC-DC converter for powering op-amps and other low-powered audio electronics in the car. Here's the link:
For a pre the SG3525 should be accompanied by a pair of post regulators -- LM317LZ and LMLM337LZ -- and perhaps some nice shielding -- when I have used the SG3525 and SG3524 you can hear the harmonics on an a.m. radio.

for the shielding you can take some surplus PCB and solder the sides together to form a little box -- or use a Mento's container.
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Old 1st August 2006, 06:35 PM   #8
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Altoids cans work great, too!
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Old 2nd August 2006, 12:24 PM   #9
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Thanks for the advice! I wasn't expecting such a rapid response! Anyway . . .

This device is going in my car. It will take the unbalanced output from my iPod and convert it to a balanced output. The balanced output will then run to a relay based switch circuit (designed, but not yet built) that will allow me to switch between my Saab's factory CD Changer and the iPod. For some reason, the bloody Saab CD Changer has a balanced audio output, which is why I need to build the converter to begin with.

I took a quick look at the DC-DC converter page, but I haven't had time to digest it yet. What I saw looked good.

As for the SSM2142, I don't really understand where that would fit in. Give me a day or two to read up on it and maybe I'll get it. As I said in my original post, I'm pretty much a novice when it comes to this, but I want to learn.

And yes, Altoids cans are great! I used one to build a battery backup for my iPod.

OK, thanks again!

Cheers,
Doug
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Old 2nd August 2006, 12:27 PM   #10
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I am so pleased to see at last balanced connections being used in a car. Hats off to Saab.

However, if you just want to plug your iPod into where the CD changer would normally go, there is no need to build a converter. All you need to do is short the GND and COLD pins on the headunit input then connect the iPod as normal to GND and HOT pins. Obviously you would have to fully disconnect your changer with the shorting taking place.

There is no need also to build a switching converter that will be 3 times as big as the rest of your circuit. The simply supply splitter on the ESP site will be more than adequate for your needs, or your could just AC couple the whole thing and not need any additional PSU stuff.
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