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DIY TRS Balanced to RCA Unbalanced Signal Converter Project

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BillD

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Post Wed Jul 30, 2008 7:46 pm

DIY TRS Balanced to RCA Unbalanced Signal Converter Project

Awhile ago, there was some discussion on the forum regarding balanced and unbalanced inputs, and going back and forth between them. There are a number of commercial boxes that can do the whole thing (both ways) such as the ART CleanBOX. But, I wanted a little DIY project. I’d been thinking that, if I want to ever go 7.1 (versus 5.1), I could have an amp in the back or the room, and run balanced cables back to it, then convert to RCAs for input. That way, passing behind all my equipment, by power outlets, etc., picking up RFI, EMI, etc. would be eliminated when converted back to single-ended RCA outs. Then a short RCA run to the amp and another short run of wire to the speakers.

Prior to getting my Sunfire TGP-III, I had a pretty long run to the amps that ran my mains (A-500x on the bass, and C-500 on the ribbons) where the Rane AC-22 was. So, I had a pair of long balanced cables from Monoprice that I could use. They happen to be terminated with TRS (¼” stereo phone) plugs on one end (to fit into the Rane) and female XLR (that fit into the Sunfire) on the other. So, the circuit I built begins with TRS inputs rather than XLR. From the re-build of my C-4000 by James Van Dyke (jvandyke_texas), I had three OPA2134PA opamps he sent me. These are very highly regarded modern amplifiers that have extremely low noise and exceptional linearity. The initial design I had used TL072s, but James convinced me that the 2134s would be much better in what he chides me as “the world’s simplest circuit”. This circuit is shown below:
Balanced-Unbalanced.JPG
Balanced-Unbalanced.JPG (150.72 KiB) Viewed 1141 times

I had most of the resistors floating around, but they were all 5%, so I hand matched all the 1% resistors (actually, I hand matched them all to within 1 point in the third decimal place - 1 Ohm for a kOhm resistor). I had to buy the capacitors and the hardware, and a box to put it all in when I was finished.

As you can see from above, I used a wall wart for power. I found one floating around here that was used to charge batteries for a phone I no longer own. It puts out about 17.7 Volts, so my Vcc is ±8.3V. I had a green LED floating around, so I just put it in the circuit as an ON indicator.

OPA2134 is a dual opamp. So, to complete the circuit, the three of three of them I had are required. I bought a couple of DIP circuit boards that can hold up to 20-pin DIPS. On one board I put two of the sockets for the 8-pin 2134s, and on the other, just a single socket. Because I had enough real estate on that second board, I built the voltage splitter onto it, as shown below.
Bal-Unbal Output and Power Board.JPG
Bal-Unbal Output and Power Board.JPG (266.43 KiB) Viewed 1140 times

You can see the ground routing around the chip carrier.

The input board carries the 0.1µF caps and the 1MOhm resistors. The 22pF caps are carried directly on the TRS socket. Between the two chip sockets is where ground runs.
Bal-Unbal Input Board.JPG
Bal-Unbal Input Board.JPG (284.49 KiB) Viewed 1141 times

Because I ran out of real estate on both boards, I had to be creative in hooking them together. The 10kOhm resistors along with power and ground connect the two boards. Here is the unit ready for system test.
Bal-Unbal Assembly.JPG
Bal-Unbal Assembly.JPG (360.95 KiB) Viewed 1141 times


The TRS (¼” phone jack) sockets I used were THESE that I didn’t know were switched until I wired them all wrong and wondered why (for a very long time) I couldn’t get this circuit to work. The RCA jacks I’m using are THESE – both of which I got at Parts Express. Finally, I got a not so small project box at Radio Shack, primarily because the width had to accommodate those two TRS jacks side-by-side.

Here is the box running the rear channels from my Sunfire TGP-III into an M-500. I was testing it using "party" mode that I have set up to run all my speakers except the center in stereo pairs.
Bal-Unbal Final.JPG
Bal-Unbal Final.JPG (337.38 KiB) Viewed 1140 times

It might be easier to just go ahead and get a CleanBOX, but I doubt it would have the quality opamps and hand-matched resistors. However, you would have balance controls (I eliminated the pots that were in the first design for simplicity (you'd replace the 10kOhm resistors that lead to the minus side of the output opamps with a 7.5kOhm resistor and a 5kOhm pot). It was a fun little project and it's a nice little gadget to have around.
It should sound like it isn't there!
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Making life enjoyable through expensive electronics.
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engtaz

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Post Wed Jul 30, 2008 7:58 pm

Re: DIY TRS Balanced to RCA Unbalanced Signal Converter Project

Cool
engtaz


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OBI56

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Post Wed Jul 30, 2008 8:34 pm

Re: DIY TRS Balanced to RCA Unbalanced Signal Converter Project

This is great Bill!!! Which reminds me that someone was jsut askign about connecting a C-1 to a PM series amp and he needed appropriate cabling for them. This would work beautifully!
Why let facts or common sense get in the way of your opinions.
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BillD

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Post Thu Jul 31, 2008 12:30 am

Re: DIY TRS Balanced to RCA Unbalanced Signal Converter Project

Actually Marc, going the other way is a little easier. You only need two opamps.
Unbalanced-Balanced.JPG
Unbalanced-Balanced.JPG (105.59 KiB) Viewed 1103 times
I think I'd be tempted to put each channel on a single opamp if I were going to build this circuit, but I'd probably use two of the little DIP circuit boards to get enough real estate for the resistors, capacitors and power.

I also forgot in the other post the pin layout on the opamps, just for completeness.
OPA2134PA.jpg
OPA2134PA.jpg (80.81 KiB) Viewed 1096 times
It should sound like it isn't there!
There is a difference between hearing and listening...
Making life enjoyable through expensive electronics.
_________________
Carver: C-4000 & C-1 preamps, PSC-60 preamp/tuner, TX-11a tuner, M-400 (2), C-500, M-500, M-500t, M-500t Mk.II, A-500x, AL-III loudspeakers (2 pr.)
Sunfire:Theater Grand III processor, Ultimate Receiver, Cinema Grand Signature 400 ~ seven, True Subwoofer Mk. II, D-10 Subwoofer
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BMKoenig

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Post Sun Jan 18, 2009 9:00 pm

Re: DIY TRS Balanced to RCA Unbalanced Signal Converter Project

Having just purchased a PM-175 amp and a C-5 preamp I am interested in the topic of balanced to unbalanced connections as I have never had a configuration such as this.

According to the PM-175 operators manual, the balanced XLR and TRS inputs allow for unbalanced input operation without switches or adapters, just with properly wired cables. If so, is there any advantage to a converter or would it be best to go with the operators manual cable configurations?
Brian Koenig
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Dreamer

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Post Sun Jan 18, 2009 9:12 pm

Re: DIY TRS Balanced to RCA Unbalanced Signal Converter Project

BMKoenig,

The main problem with those converter cables is that they can sometimes induce hum into your rig, depending on how your gear's grounds are wired and how the XLR pinouts work. These little converter boxes will keep things quiet, and also give you a true "balanced" signal over those XLR/balanced lines, so they don't puck up any RFI or hum on long runs...
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BMKoenig

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Post Sun Jan 18, 2009 11:00 pm

Re: DIY TRS Balanced to RCA Unbalanced Signal Converter Project

Thanks for the explanation!

I am not much for DIY circuitry, so are there any recommended converter boxes?

I have been looking at the Rolls Buzz Off: http://www.rolls.com/product.php?pid=HE18

But are there devices which might work better?
Brian Koenig
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radioeng2

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Post Mon Jan 19, 2009 7:54 am

Re: DIY TRS Balanced to RCA Unbalanced Signal Converter Project

Rolls stuff is the worst of the worst. Cheap shit all!!

You can't put cheap op-amps, cheap electrolytic capacitors in the signal path and not expect a negative impact! I'd suggest avoiding them at all costs...it'd be money wasted if your pursueing high quality audio.

Balanced to unbalanced can be simple and can be a bit complex. The reason why is that it may or may not be just a simple adapter thing. It may be level, impedance, and a Dreamer said, have grounding involved.

If somebody has the specs of the two pieces involved, it could be easy to determine this particular situations specifics....

Mark
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BillD

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Post Mon Jan 19, 2009 11:55 am

Re: DIY TRS Balanced to RCA Unbalanced Signal Converter Project

I use quality polyethylene blocking caps , Burr-Brown OPA2134 opamps, 4% silver solder and hand matched 1% resistors in my projects.
It should sound like it isn't there!
There is a difference between hearing and listening...
Making life enjoyable through expensive electronics.
_________________
Carver: C-4000 & C-1 preamps, PSC-60 preamp/tuner, TX-11a tuner, M-400 (2), C-500, M-500, M-500t, M-500t Mk.II, A-500x, AL-III loudspeakers (2 pr.)
Sunfire:Theater Grand III processor, Ultimate Receiver, Cinema Grand Signature 400 ~ seven, True Subwoofer Mk. II, D-10 Subwoofer

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