Tuesday, September 17, 2019

Balanced Quad Line Receiver

For me the "why" in AudioDIWHY is never about saving money. By the time a board gets fabbed, stuffed, tested, debugged, a front panel made and so on, the money for a Doepfer or Boss or Behringer for a consumer version of the same thing might seem cost less.

But maybe not this time?


Finished Module with FPE front panel


The back story: I had four unbalanced synthesizer ties in my home studio that needed to be amplified by about 12db and balanced. I was using two Aphex 124's for this but one went dead, no idea why.

(Note: if you need to know what this "balanced audio" thing is all about, a good quick explanation from MOTU can be found here).

Both Aphex units were getting old and took up more room in my rack than I liked, so I priced out various replacement active balanced audio line converters but all seemed a bit pricey: Intelligel's is about $260USD for quad; $40 for one channel if you want to buy a tiny Intelligel I/O widget that goes into their skiff rack; Whirlwind's offering is about $130 for 2 channels, Aphex's level shifter is about $300USD for stereo (yes, it's I/O and O/I so it will cost more) and so on.  Nothing too affordable here!

Granted some of these have extra features like level meters, but really all I need is to balance the audio and get a +12db boost. How to do this cheap?

How about this: buy some THAT line driver IC's, RAP2CV jacks and TRS 1/4ers from Ebay, get five PCB's prototypes fabbed in China for $5 USD.

Is Audio DIY getting old hat? This all feels familiar. I designed the PCB in Eagle (I had to conjure up a Neutrik TRS Eagle device--I added this to my madbean JACKS.LBR file--I can share that if you are interested in adding this TRS part to your Eagle project, comment below.....)



Is that the DHL plane flying over my house with PCBs from China? At 3AM! Could be!!! 

Unwrap the boards, pull the parts, insert, solder, power, test, repeat.

The PCB for this project uses through hole parts--if I had used surface mount THAT IC's I would have saved another $3USD.





The heart of this circuit is a THAT chip, the 1646. Small quantities are available from Mouser for about $4.50USD each; so for quad you are looking at $18. The TRS and 3.5" jacks (you may want to redesign via Eagle for RCA input? TS?) are easy finds via Ebay or maybe at a surplus place.

Everything else I used--other than the jacks and 1646 chip--were from my parts box--really all this is is an op amp buffer/gain stage in series with the 1646. Not sure it could get much easier.

Applications. Running audio long distances is best accomplished using balanced audio, and you'll get with the THAT 1646. Another common application for an audio line driver is converting a consumer audio device, like a CD player, to the professional "+4 balanced dBu" spec. If you have -10dbV gear that needs to get amped up to +4dBu I calculated that the op amp (IC1 in this circuit) needs to provide about 4x gain, along with gain provided by the THAT IC. I tried to make the design easy to mod--drop in different values in the op amp gain stage, or make it unity gain using the "BYPASS" test point hooked up to the top of R2--whatever you need.

So for -10dbV to +4dBm conversion, try 50K for R5, 250K audio taper for R6, and 100K for R2. Tweaking that on my bench got me close to +4dbU from a -10dbV sine wave test tone.

A good article on op amp only equivalent circuits for balancing and unbalancing audio can be found here. For balancing a synthesizer output, you may need to use the resistor values you see above: all 100K. That's what I used and it worked.



It only took a few hours to build 2x of these (4 channels total) and it all worked the first time.  

Still need to add bypass caps, do that after the basics work.



One more benefit: I have 2 identical boards, 4 channels total, that are easy to A-B.  I decided to put fancy and expensive Burr Brown OPA624's  (since I'm saving money here?) and 1% resistors in one, and ancient but not too horrible CA3140's and 5% resistors in the other. Will I be able to hear a difference? Try different op amps, listen to them....Update: can't hear any difference. Maybe it's just me, my ears, too much loud R&R. Perhaps it's almost always OK to use whatever you have in your junk box as long as it doesn't smoke.  

Butt Ugly!

Warning: Butt-Ugly front panel! For testing and tweaking I modified an old 2u Frac panel and used Ptouch tape for labeling. No, that's not how it will look if this device continues to get used (so far it works great so it probably will). I am ahead on budget here, so I will spring for a $45ish panel from Front Panel Express. 

Update: FPE front panel is done--see the opening photo for this post.

UPDATE 12-28-19: after finishing this board, I found out that Sparkfun has something like it: here. No op amp to adjust incoming gain though; all SMT. 

Yes, if you add up the hours I have with design, stuffing, testing etc. I think it might still be more expensive then Intellijel, but still, maybe not.  Remember that unlike Burr Brown IC's the fumes are free. The mask may not be.

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