HOW TO CONNECT BALANCED INPUTS AND OUTPUTS TO UNBALANCED INSERT(S)
|The Balanced-Insert cable no company offers
CAVEAT: This article will describe a method for making an unbalanced Insert-cable for use with balanced input and output connections like you find on most professional audio signal processing. This Insert-cable will not work with unbalanced inputs and outputs, both input and output must be balanced type circuits.
I'm going to break this story into three parts.
Part One will look at and consider "What is an Insert-Point"? You can't use it if you don't know what it is. If you know, then skip to Part Two.
Part Two will explain how to make the "Balanced Insert" cable which will allow you to connect your outboard processing using the unbalanced insert jack on a mixing console. This is a non-standard way of wiring the plugs which connect to the balanced input and output on your outboard processing.
Part Three will explain why this wiring works.
PART ONE: THE UNBALANCED INSERT POINTS ON YOUR MIXER
What is an Insert Point?
An Insert point is a location in the signal path where you can "insert" some outboard signal processing, maybe a higher performance EQ or compressor or a whole external signal chain of gadgets. An insert usually comes after the preamp stage and before the channel fader on most mixing consoles.
Technically the Insert is a normalized, unbalanced, TRS 1/4” jack; it has been used by Mackie, Soundcraft, Allen and Heath, Amek, Midas, Behringer, Peavey, Phonic, Crest, Tascam, Studiomaster, Carvin, and Yamaha. Normalizing allows the audio to flow through the jack when no plug is inserted in the Insert jack. When you insert a TRS plug the normalized connection between the ring and the tip is opened and the audio can flow out to an outboard signal processor and then return to that individual console channel for further mixing.
Before you implement the Balanced-Insert cable, which I'm going to describe, you should check the technical specifications for YOUR mixer (or insert point) to be sure the send and return (output and input) are wired the way I'm going to describe. Just reverse the Tip and Ring connections on the TRS insert plug if your mixing console Insert point is backwards to what I describe, there is no real industry standard for this connection (that I'm aware of) so the most important thing is to get the inputs and outputs identified correctly.
|My field notes for Balanced-Insert cable
The “Tip” connection of the insert jack is the unbalanced HOT (+) signal output which will feed to your outboard-gear “Input”. You're supposed to use the sleeve of the TRS insert jack for the shield of your two-conductor, insert cable, output.
|Typical molded Insert cable
The “Ring” of the insert jack is the “Return” input connection for bringing an external audio signal back into the console for further mixing. Again, you're supposed to use the sleeve of the TRS insert jack for the shield of your cable (the shield is common between the Ring and Tip connections).
If you buy any brand of premade “insert”cable you'll get a 1/4” TRS male plug which feeds to two 1/4” unbalanced male plugs. Those cable makers intend for you to use the two 1/4” unbalanced male plugs to feed the input and output of your external signal processor, the TRS end goes in the insert hole on your mixer.
If you use this “typical” insert-cable on your mixer with a piece of outboard equipment, you'll hear hum in your output because these cables are wired in such a way they compromise your audio and electrical grounds. You could try using AC ground lifts to get rid of the hum but that is dangerous, it is less than 50% effective at eliminating hum and as you'll see, unnecessary.
MAKING INSERT CABLES THAT DON'T HUM
The easy way to get a correctly wired cable is to buy a premade insert cable, cut the unbalanced connectors off the TIP and RING input/output connections, solder on some XLR plugs the way I'll show you, plug in and enjoy your outboard processing.
|Hosa Insert cable available in variety of lengths
This project requires four little soldered connections per cable, two per plug. If you can't solder buy the parts and get them ready, then ask a friend to complete the soldering. It's very easy, maybe this would be a good time to finally sharpen up your own soldering skills. Here is a step by step guide to soldering right here on High on Technology.
PART TWO: MAKE YOUR OWN INSERT CABLE, STEP BY STEP
For this example I'm going to modify off-the-shelf HOSA brand insert cables. They're easy to find and the wire is such that it is not difficult to solder. You are free to modify whatever brand conforms to the same specifications electrically. I like working with Switchcraft XLR plugs but you can use Neutrik or whatever brand you want. You could also use TRS male plugs instead of XLR, solder the (+) to the Tip lug and the shield wire to the Ring lug. If you don't know what that means then use the XLR plugs as shown. You will need one XLR male plug and one female XLR plug for each insert cable.
STEP BY STEP
Determine which is the corresponding tip and ring connections on the insert cable you're going to modify. I put a piece of red electrical tape on the ring lead wire (Red is for Ring and Ring is for return, my little way of remembering).
Cut the unbalanced connectors off the ends of the cable which correspond to the Tip and Ring connections. (Don't cut off the TRS plug).
Carefully open up the jacket on the Tip and Ring wires, separate them and prepare them for soldering. In my experience the Hosa brand insert cables have nice copper inside (it solders easily), there is not lots of extra so don't waste a single strand. Take your time preparing these connections.
Prepping for Soldering
Identify all your connections and prepare your connectors. The Tip connection has two electrical conductors, an inner hot wire and the outer shield. The Tip connection is an audio output so it will have a male XLR plug attached. The Ring connection has two electrical conductors, an inner hot wire and the outer shield. The Ring connection is an Input so it will have a female XLR plug attached.
Identify the pins on your XLR plugs. They are numbered 1, 2 and 3.
1 = ground (Not Used), 2 = hot (+), 3 = cold (-)
Pin 2 = (+) hot lead, Pin 3 = (-) shield wire
Solder the inner wire from the Tip connecting cable to pin #2 on the XLR male plug
Solder the shield wire from the Tip connecting cable to pin #3 on the XLR male plug
Solder the inner wire from the Ring connecting cable to pin #2 on the female XLR plug
Solder the shield wire from the Ring connecting cable to pin #3 on the female XLR plug
Check your connections to be sure there are no whiskers between the XLR pin connecting points before carefully reassembling the XLR plugs. Note: there is no connection to pin #1 in any of the XLR plugs.
I use my digital voltmeter to check the cable for shorts before plugging it in and using it.
|The modified cable should look like this
UNBALANCED BUS INSERTS TOO
Many mixers use this unbalanced insert point scheme on the output buses, this is where you can insert bus compression and more EQ. The insert cables I've described work great in these connections too.
My current mixing desks are all Soundcraft GB series which have these unbalanced insert points on every input and output. Each of my four stereo bus compressors uses two of these modded insert cables. Currently all of the following in my system are wired using insert connections like I've described.
Audio-Scape Bus Comp (active balanced inputs and outputs)
UA 1176 (transformer balanced inputs and outputs)
UA LA2a (transformer balanced inputs and outputs)
WA76 (transformer balanced inputs and outputs)
KT76 (transformer balanced inputs and outputs)
Neve 1073spx (line in to line out) (transformer balanced)
API 2500 stereo bus comp (transformer balanced)
SSL Bus+ stereo bus comp (active balanced inputs and outputs)
WA EQP tube EQ (transformer balanced inputs and outputs)
KT EQP tube EQ (transformer balanced inputs and outputs)
WA2 tube comp (transformer balanced inputs and outputs)
WA Stereo Bus Comp (active balanced output, optional transformer balanced output)
DBX160, 166 compressors (active balanced inputs and outputs)
Midas 500 modules (EQ and Comp) (active balanced inputs and outputs)
Various graphic equalizers (active balanced inputs and outputs)
PART THREE: WHY THIS WORKS
Transformers are the undisputed kings of differential input and output connections, nothing beats them for electrical isolation and common mode rejection. This means transformers can pass audio and disconnect ground loops at the same time.
If you use a stock, unmodified Hosa insert cable to connect a signal processor like a DBX 166 compressor to a console Insert point, you are creating a ground-loop connected at the sleeve of the Insert-TRS jack, that's where the hum is entering your signal path. In this example lifting the AC power ground on the DBX 166 won't eliminate the hum, though it will reduce it by about 6 dB. Lifting AC power grounds is a desperate move which creates unnecessary safety issues and it should be avoided.
The grounded AC power plugs on your mixing console and your outboard equipment should all be connected to electrically grounded outlets in accordance with N.E.C. standards. Anything else compromises your safety and increases the chance of death by electrocution.
PROCESSORS WITH TRANSFORMER BALANCED INPUTS AND OUTPUTS
The modified Hosa insert cable takes advantage of the galvanic isolation provided by the audio transformers used on the input and output of your outboard signal processors.
For example, an 1176 style FET compressor has nice transformer balanced inputs and outputs. Pin #2 (+) and #3 (-) in the XLR plugs of the modified insert cable take advantage of the galvanic isolation provided by the inbuilt audio transformers. Even though the Insert sleeve connection is shared by the input and output they are isolated thanks to the input/output transformers and no connection to pin #1 in the XLR plugs (pin #1 is a ground connection). This makes an 1176 or any other processor with transformer balanced connections a perfect choice for inserting on your unbalanced TRS insert points. You get all the benefits of transformer isolation without any additional circuitry adding noise and distortion.
PROCESSORS WITH ACTIVE BALANCED INPUTS AND OUTPUTS
When you use the modified Hosa insert cable to connect a piece of gear with electronic balanced connections to a TRS insert jack you're relying on the electronic balanced input/output connections to isolate the audio and electrical grounds. We've been told for years that "electronically balanced" is just as good as transformer balanced (but a whole lot cheaper for the manufacturer). When connected like I've shown in this article it turns out that active balanced inputs/outputs can perform with virtually identical isolation to transformers.
WHY HOSA CABLE?
I buy my Hosa cables just like you, HoT does not accept compensation for product mentions. Hosa cables were used as an example because they're easy to obtain, reasonable in cost and easy to work with. You could just as easily use cables by any brand that is wired similarly or you can feel free to make your own from scratch.
Once upon a time in 1998 I made 32 of these insert cables to bring out the inserts on a Mackie 8 bus mixer to a patch bay, I used Mogami console wire and Switchcraft connectors. It is not fun trying to pack all the required wires and connections into a Switchcraft 297 TRS plug for the Insert connection. After years of stubbornly making my own Insert-cables I started modding premade Hosa cables for my own studio use.
BETTER SOUNDING INSERT
Using your unbalanced mixer-Insert-point with balanced outboard gear can actually provide a better sounding audio connection compared to mixers with active or transformer balanced insert points. If a mixer manufacturer includes the necessary active circuits or transformers to give you balanced connections on the insert points that means more op amps or more transformers for your audio to go through. Many mastering engineers have been known to remove audio transformers from gear to reduce distortion and cloudy reproduction.
I have read many posts on forums which degrade, denigrate and disparage unbalanced insert points as inferior or somehow less than professional. I think one of the problems which leads to this way of thinking is that you need to know what you're doing with your signal path and not be afraid of a simple solder job to make the right cable wired the right way.
When connected correctly unbalanced mixer-Insert points provide identical frequency response to the rest of the mixing console. Likewise headroom and noise floor are not compromised on unbalanced insert points unless you are grossly mismatching levels on the outboard processor. Inserting a compressor or Dimension D chorus (or both) after the preamp stage in a console makes it easy to gain stage the signal correctly which results in better sound.
Try one of these modified insert cables with your balanced outboard gear and discover how well it can function.
Good Music To You!
©June 2023 by Mark King, All rights reserved. It's not OK to copy or quote without written permission from the author. Thank you for reading High on Technology.
|TRS Insert plug feeds to and from XLR 3-pin connectors