Audio signal balancing: good place to start? - Page 1
Forum section: electronics / beginners

author: Humanoid
author_rank: Regular Contributor
timestamp: September 28, 2020, 12:26:56 am
content: I do a lot of audio and music work, so I'd like to learn about (and eventually build) some gear I could use.One thing I would really like to learn about and make is a device that can balance and unbalance audio signals—basically a multi-channel DI box. I have some old analog devices and digital devices with analog I/O, and some of them operate at -10dBV instead of +4dBu. Having to crank the inputs on my interface isn't ideal and some signals are still low. Some of the consumer-grade stuff is RCA I/O only so they can induce hum and such, whereas the pro-grade stuff have no hum problems and may even accept TRS cables to eliminate noise. I also want to be able to drop +4 signals to -10 with some kind of a pad (maybe adjustable) so I can output back to the old devices if i need to or just lower the gain of a hot signal.Preliminary research I've done shows that the heart of these circuits are good transformers, so it's not a component to cheap out on. I'm not sure if active DI is too complicated vs passive DI, and if the benefits of active are worth the extra work. From what I read it said passive can have much weaker signal output. Also not sure if it will be too complicated for a novice in general, but the guts of basic DI boxes I saw looked relatively simple in design—few connectors, few wires, one transformer per channel. Some had components mounted to PCB and others were point to point.I would appreciate a point in the right direction as to what I need to learn when designing and building circuits like this and what pitfalls to lookout for. I'm curious if these can be made point-to-point, or if using PCBs for certain things for more complex multi-channel applications is best and produces the cleanest signals.I also have no idea how to approach grounding/hum problems. Retail DI boxes sometimes have a ground lift on each channel, but not sure what methods and approaches to take that are more reliable than whatever tech they are using for that. I read numerous reviews of DI boxes where it seems to be hit or miss when using a ground lift switch on various units, so maybe multi-stage hum elimination is possible that can be turned on and off as needed.The long term goal is to be able to do the following:8 channels in and out assembled in a 1U rackmount unit (I could live with 2U if needed)input unbalanced -10dBV signals (RCA, TS, 3.5mm) and output balanced +4dBu (XLR/TRS)input balanced +4dBU signals (XLR/TRS) and output unbalanced -10dBV(RCA, TS, 3.5mm)optionally pass through a signal without changing the gain without having to move cables aroundphase inversion switch per channeleliminate hum per channel (button(s) or switch(es))optional pad per channel (button/switch/incremental rotary encoder?)minimize noiseminimize distortionJust thinking out loud, but for the -10dBV outputs, I wouldn't be opposed to having a single output section that has 2 or 4 RCA outs, a 3.5mm output and 2 TS outputs, since I won't need to output to -10 for a bunch of devices. Even the inputs for consumer-grade connectors could be it's own section that maybe routes to specific +4 outputs for XRL and TRS. Putting all those I/O on each channel would be overkill, cumbersome, and may not fit in a small space on a 1U device anyway. Having dedicated 1/4" inputs on every channel makes sense because old analog synths and romplers typically output to 1/4" unbalanced (and I'm always on the lookout for more toys). I could make use of XLR/TRS combo jacks for +4 outputs to save space. My current interface has 1/4" inputs that can accept TRS or TS—not sure if there's extra components inside that deal with each type or if it's a special jack.It would be cool to add additional features, of course, like some type of multiplexing to the I/O, or maybe adding in a secondary section that can optionally saturate or "color" the sounds with pleasing harmonic distortion (might be cool for synths). But that sounds like it could get complicated fast, so initially I just want to keep it simple and do multi-channel balancing and unbalancing while keeping signals as clean as possible.I'm going to mockup a design before I even get close to attempting something like this, but I would probably do all the I/O on the back of the device. Having an optional input per channel on the front would be convenient for random needs, like the commercial rackmount boxes have, but it's not needed for my daily work and I hate having cables hanging out the front of the rack (unless it's a temporary instrument cable). The front would just contain buttons/switches/knobs for pad, phase and ground lift/hum elimination so it's easy access. If active, I would also add a power switch, because I hate using devices that lack one, and a proper 3 conductor power cable on the back to ground the thing.Sorry for the long post. Just trying to be thorough. I look forward to your wisdom and advice with eagerness.Thanks!

author: BlackICE
author_rank: Regular Contributor
timestamp: September 28, 2020, 01:01:07 am
content: Take a look at thishttps://www.digikey.com/product-detail/en/sparkfun-electronics/BOB-14003/1568-1573-ND/7033191Also datasheets for That 1606 and 1646.For a passive solutions their are transformers you can use.

author: David Hess
author_rank: Super Contributor
timestamp: September 28, 2020, 04:17:24 pm
content: That seems like a mixer but without the mixing function.

author: Humanoid
author_rank: Regular Contributor
timestamp: September 28, 2020, 05:16:45 pm
content: Quote from: David Hess on September 28, 2020, 04:17:24 pmIt's not a mixer. It would be more like a Radial Pro D8 that accepts consumer connectors besides 1/4" or an expanded Rolls MB15b with extra I/O and better hardware, or maybe a merging of the 2 concepts. The Radial does allow summing on each channel by giving multiple inputs (A and B), and also has loop and through jacks, but I don't want or need summing.https://www.sweetwater.com/store/detail/ProD8--radial-prod8-8-channel-passive-instrument-direct-boxhttps://rolls.com/product/MB15bEDIT:Actually, based on the long term goals, yes, it would do mixing/summing I suppose—lettings multiple inputs feed 1 or more outputs.For an initial design though, I could do 2 inputs and outputs, however based on the relative simplicity of the schematics I've seen, I may try to do more. I would probably opt to have 2 x TS inputs, 2 x RCA inputs, and a TRS 3.5mm input, all of which went to 2 x balanced XLR or TRS outputs, so I could do dual mono or stereo. Then I would add 2 x balanced XRL or TRS inputs (probably combo jacks) that fed an unbalanced output section for the aforementioned unbalanced connector types. I'm still researching how all that would need to be connected.I attached a 3rd image that shows an existing product by Radial that does this in one direction. It has 2 x TS, 2 x RCA and a 3.5mm mini TRS and outputs to 2 x balanced XLR, with pad and ground lift.

author: electrolust
author_rank: Unknown
timestamp: September 28, 2020, 07:28:45 pm
content: Quote from: Humanoid on September 28, 2020, 12:26:56 amhttps://sound-au.com/https://sound-au.com/project35.htm

author: Humanoid
author_rank: Regular Contributor
timestamp: September 28, 2020, 08:51:38 pm
content: Quote from: electrolust on September 28, 2020, 07:28:45 pmThanks, this is great!One confusing thing in Fig 2 of the passive DI is the symbol for ground. If it's passive, there is only chassis ground, right? But his symbol is earth ground. However, maybe he meant chassis ground, because I'm seeing conflicting symbol charts where some show earth ground as the rake-like trident symbol and some show the down triangle made of lines as earth ground.Please correct me if I'm wrong, but I thought the rake-like symbol was for chassis ground.