Difficulty finding Balanced Input & Output Opamp schematic - Page 1
Forum section: electronics / beginners

author: jfharper
author_rank: Contributor
timestamp: March 03, 2025, 12:52:48 pm
content: I need to build a clean boost circuit for XLR inputs and outputs to increase signal level between a consumer interface and a reamp box. I can find examples of unbalanced opamp circuits, or balanced in/unbal out, or vice versa, but the only completely balanced circuit I can find discusses fully differential amplifiers. I'm not sure I need the fully differential aspects of the circuit and since I'm a beginner, trying to understand what to remove is a challenge. I think the radial reamp station may do the FDA for me, I just need to boost the signal because the consumer interface output is weaker than I need. Can someone point me in a direction of a balanced input & output opamp schematic?

author: CaptDon
author_rank: Super Contributor
timestamp: March 03, 2025, 01:38:57 pm
content: A single opamp will have the differential input you are looking for as you stated. I would perhaps use the first opamp in a normal fashion with its input acting as a differential / balanced input, perhaps using 2.2K resistors in series with the + and - signal inputs. Then set the gain you want with a negative feedback resistor from the output to the - input pin. A 22K would give a 10X voltage increase from input signal to output signal also known as a 20dB increase. THEN, wire the second half of your dual opamp as a unity gain inverter. The output from the first opamp is your + output and the output from the inverter stage is your - output. Be aware that your input signal can't go higher or lower than the + and - supply rails powering the opamps. You may want to A.C. couple the inputs using capacitors. 10uf @ 16vdc should work fine, maybe even 22uf @ 16vdc to extend the low frequency rolloff.

author: Simmed
author_rank: Frequent Contributor
timestamp: March 03, 2025, 01:40:49 pm
content: XLR is a differential signalso it is only normal to use differential opamp format

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: March 03, 2025, 02:19:51 pm
content: Hi,try SSM2141 (or THAT120x with even better CMRR) as receivers,amplify in Single-ended-mode and output via balanced driver.Good luck

author: Simmed
author_rank: Frequent Contributor
timestamp: March 03, 2025, 02:26:44 pm
content: Quote from: jfharper on March 03, 2025, 02:21:40 pmif you put a little top compressor (prevent clipping) in the DAW, go as high as you can on the channeland then see if you can increase the DAW output maybe +6 ? +10?maybe you dont need to do any of this outboard boostbut then again, i dont know how high you are pushing on the "VU"

author: jfharper
author_rank: Contributor
timestamp: March 03, 2025, 02:45:02 pm
content: Quote from: Simmed on March 03, 2025, 02:26:44 pmGood idea, I'll try that. TBH, I'm kind of looking forward to expanding my knowledge with this challenge, but easy is easy.

author: jfharper
author_rank: Contributor
timestamp: March 03, 2025, 05:08:58 pm
content: Quote from: CaptDon on March 03, 2025, 01:38:57 pmI don't think I have this correct...shouldn't the unity gain inverter output an inverted signal?

author: Simmed
author_rank: Frequent Contributor
timestamp: March 03, 2025, 06:28:44 pm
content: U3 is missing 1 more 22k resistor

author: jfharper
author_rank: Contributor
timestamp: March 03, 2025, 06:35:47 pm
content: Quote from: Simmed on March 03, 2025, 06:28:44 pmBetter?Oh, that's inverting, than it should output an inverted signal for the - output. Then I think that's it...I'll just have to play around with my supply voltages a bit. Thanks, everyone.

author: jfharper
author_rank: Contributor
timestamp: March 03, 2025, 08:59:54 pm
content: I've one more challenge to this setup...on the interface, apparently it is not a true balanced output, pin3 goes to ground thru a resistor (https://sound-au.com/project87.htm#cheat), so I was going to run an additional tl072 (https://www.ti.com/lit/ds/symlink/tl072.pdf?ts=1741018002790&ref_url=https%253A%252F%252Fwww.mouser.com%252F) according to the attached image. The OP said this:"(Inverting amp) on the output of the (input follower) buffer to output to pin 3 through cap." and later "tl072 with 22K x3"On my interface, the IC is an ad8694, OUT A (pin 1) (https://www.analog.com/media/en/technical-documentation/data-sheets/AD8691_8692_8694.pdf) is going up through a 22k to the tl072 input- (pin 6 closest to our POV), then we see another 22k resister from the output (pin 7) back to the input- for the feedback (or gain) loop thing, but the 3rd 22k which comes off the input+ (pin 5) is a mystery...I cannot figure where it goes from the image unless it is going to ground or vcc- which is pin 4. The other side of the TL072 is for the other XLR channel (right and left I can't tell which). Can anyone help me figure where this 3rd 22k (pin 5) may be going? Also the OP said the output is going through a cap, I cannot see a cap, but was wondering what kind and what value? Thanks.EDIT: After taking a look at this site, post 8, (https://www.diystompboxes.com/smfforum/index.php?topic=114916.0) maybe that 22k pin 5 is getting the bias voltage from pin 8 on the tl072...but if so, why is there also vcc+ applied to pin 8. I'm confused.EDIT2: Actually, I think I figured it out...attached is the schematic. I don't know what voltages the OP used or the output cap value, but can probably test/simulate and figure them out.

author: John B
author_rank: Frequent Contributor
timestamp: March 03, 2025, 11:37:27 pm
content: I would strongly recommend not to waste time, energy or PCB space trying to create rudimentary and poorly performing differential receivers or drivers.For receivers I've used the INA1650, which is a stereo version, and for drivers I've used the OPA1632. You pay a little more upfront for the parts, but save time and achieve the best performance.You can do all your analogue processing, gain etc, using regular single ended signals and opamps. The NE5532 is more than plenty for this

author: jfharper
author_rank: Contributor
timestamp: March 04, 2025, 11:28:42 am
content: Quote from: John B on March 03, 2025, 11:37:27 pmThat looks like the next step for me in my journey...thank you.

author: jonpaul
author_rank: Super Contributor
timestamp: March 04, 2025, 12:23:51 pm
content: no needed: Use a transformer , 1:1 audio, INHERNTLY balanaced in balanced out ONE part, no power, Simple, cheaphttps://www.amazon.com/NEUTRIK-NTE1-TRANSFORMER-AUDIO-piece/dp/B00Q7VSK0SJon

author: jfharper
author_rank: Contributor
timestamp: March 04, 2025, 01:59:39 pm
content: Quote from: CaptDon on March 03, 2025, 01:38:57 pmI'm a little stuck understanding the voltage supply. Do I need vbias on both inputs and should input- be vbias-? Should the - supply rail be something lower than zero, i.e. not ground?EDIT: Yeah, I'm not getting something right...here's how it simulates:EDIT2: I got this working, going a different route. I came across this schematichttps://sound-au.com/project87.htm#cheatwhich matched the configuration of my interface, understanding that I didn't need to have "true" balanced signals. From this, I was able to use references and videos to get a clean boost opamp working. It works but has a bit of noise understandable which I'll now try and add a low pass filter to fix. This would be a good opportunity for me to learn bode plots with my 1104x-e and sdg1032x. Granted, my problem maybe with the tl071 where I should use an opamp like what was recommended in this thread.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 06, 2025, 07:02:26 pm
content: You can DIY one. About $10 gets you the main components and perf board, not including the XLR sockets obviosly.One resistor sets the gain. A switched gain setting arrangement would make sense. +3 +6 +9 +12dB etc. Pots are crap.You need decent (10k) resistor matching for best balance. Use a precision resistor array for this, it'll work better and be cheaper in the end.Most opamps will do as long as they meet your noise specs and have plenty of output drive. Most audio opamps will.You don't have to run it from +/-12V but I wouldn't go below +/-6V. Many audio opamps will let you go as high as +/-18!Error alert. Updated schematic