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Old 25th January 2013, 12:01 PM   #1
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Default Balanced Line Driver with DRV134

Hello,

I want to use some mono-blocks with my AVR32 Tag preamp/processor. The mono blocks require a symmetrical or balanced line input signals. Unfortunately my preamp has only normal unbalanced RCA outs.

So i thought i would build a "from unbalanced to balanced" board using the TI's DRV134 and an OPA 134 Opamp as buffer / impeadance matcher. I guess the buffer isn't really required, since my preamp has a line out impeadance of 47 ohms but i thought the buffer would make the board a bit more flexable for other scenairos.

I know circuits like this have posted many times and basically its a mixture of those I've found. The main featuers are:
-on board regulation
-buffered input

Also adj. gain would be possible here but i left it out, since the drv has some 6db gain and its prob. not necessary.

Pls. see my circuit and let me know if what u-all think. Thanx!
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Old 26th January 2013, 08:22 AM   #2
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Default Schematic update for missing connections

Hi,

just updated the schematic, since two connections on the center terminals of the trimers were missing....


Cheers,
Chris
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Old 26th January 2013, 09:04 AM   #3
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You might want to leave space for a bigger cap where you put the DRV 1uF rail decouplers, on occasion I've had to go up to 22uF to stop ringing on the outputs.
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Old 26th January 2013, 09:52 AM   #4
AndrewT is offline AndrewT  Scotland
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Pin1 does NOT connect to Audio Ground.

I suggest you add in a gain adjust giving options (switching) for +0dB and -6dB.

Otherwise attenuation of the balanced signal introduces many practical difficulties.
Attenuation of the unbalanced signal is very easy.
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Old 27th January 2013, 02:17 PM   #5
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Quote:
Originally Posted by pinkmouse View Post
You might want to leave space for a bigger cap where you put the DRV 1uF rail decouplers, on occasion I've had to go up to 22uF to stop ringing on the outputs.
Thanx for the tip pinkmouse!

I'll be sure to leave extra space on the board layout for that!

Did u use electrolytics for the 22uF's?
22 as a bypass is alot of capacitance!

Cheers,
Chris
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Old 27th January 2013, 02:51 PM   #6
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Quote:
Originally Posted by AndrewT View Post
Pin1 does NOT connect to Audio Ground.

I suggest you add in a gain adjust giving options (switching) for +0dB and -6dB.

Otherwise attenuation of the balanced signal introduces many practical difficulties.
Attenuation of the unbalanced signal is very easy.
Thanx AndrewT,

i made a correction, is that what u meant or did i get it wrong again?

I'll update my schematic for the gain and repost the schematic again.

Cheers,
Chris
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Old 27th January 2013, 03:02 PM   #7
AndrewT is offline AndrewT  Scotland
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No.
Pin1 goes to Chassis.
Read up on Pin1 Problem.
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Old 27th January 2013, 04:43 PM   #8
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On the "X1" connector, neither the pin 1 nor the shell should be connected to the circuit "GND" they should only be connected to the chassis with very short wires. Of course the circuit "GND" should also be connected to the chassis but with only one wire.

Added thoughts
The "WBTOR1" connector should be the circuit connection to the chassis. It's "S" terminal should go to circuit "GND" not to X1. pin 1.

Don't know how I skipped by AndrewT's post! I never saw it on my first read.
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Last edited by Speedskater; 27th January 2013 at 04:50 PM. Reason: added content
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Old 27th January 2013, 04:49 PM   #9
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Thanx AndrewT,

I will read up on it.

I posted a little schematic illustrating the 6db attenuation.

Maybe lower resistor values would be less noise prone, e.g. r2/r1 with 500/1k?
Not quite sure about r2, since the data sheet shows all gain examples with 1k.
It also mentions something about taking precautions on the non-inverted pin's impedance if their "parallel" resistance > 2k. Not exactly sure what is meant by parallel, e.g. just add both values?

Cheers,
Chris
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Old 27th January 2013, 04:52 PM   #10
Mooly is offline Mooly  United Kingdom
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Thats an amplifier of voltage gain 1.5, not an attenuator.
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