Help Design an unbalanced consumer Line level to Balanced interface

[Whoops]

Yes, the output is impedance balanced. V1 48V is a battery, together with R9 and R10 it forms the input of a phantom supply as commonly used for mic inputs. It's only scetched for clarity.

Oh I see, so I can remove R9, R10 and V1 from the drawing as it's easier for me to understand it that way.

From my point of view there is no necessity to drive XLR Pin 3 with a complementary output voltage as seen on XLR Pin 2. Several microphone manufactures like DPA do it the same way.
C2 with + side to R3, C4 with + side to R8 and C5 with + side to R7 are electrolytic caps (voltage rating 50V).
By the way: the circuit is capable to process also +4dbu levels with only small degradation of THD...

Thank you, I will try this one out for sure.
Thanks a lot

 

[Whoops]

The cable solution wouldn't be my first choice though.

Note that some kit can have relatively high output impedance eg 1K0

I checked documentation available on the output impedance of the most common Keyboards, Synths, Drum Machines and Samplers I'm presented in Live Sound.

Most manufacturers and documentation don't refer the Output impedance, but
the few I was able to see it in the Specs are from 400r to 550r. The exception is the Roland SPD-S that is stated at 1K, so there's more exceptions for sure

 

[MidnightArrakis]

I checked documentation available on the output impedance of the most common Keyboards, Synths, Drum Machines and Samplers I'm presented in Live Sound.

Most manufacturers and documentation don't refer the Output impedance, but
the few I was able to see it in the Specs are from 400r to 550r. The exception is the Roland SPD-S that is stated at 1K, so there's more exceptions for sure

What type of -- end product -- are you looking for to house this circuit for your "LIVE" sound use? Do you want to place several of these circuits into a "stage box"? Would you prefer to place several of these circuits into a "1U rack-chassis"? Or, would you prefer to place individual circuits into an "XLR Barrel" with a Female 1/4" jack on one end and a Male XLR connector on the other end? In all of these instances, I could design - both - the mechanical chassis/enclosure/housing and/or the PCB's for you, if you would like.

Just send me whatever finished schematic that you come up with and/or decide on and let me know what you want to put the circuit into.....then.....I could whip up either the mechanical design and/or the PCB-design and then send you the fabrication CAD files so you can have things made over there in Portugal where you live. Sound like a plan? (NOTE: I have designed PCB's using only 0201 SMD components, should you want to go really, REALLY tiny)!!! HINT: A long time ago.....I used to do all of the custom PCB and mechanical rack-mount equipment designs for "one of the largest concert sound-reinforcement companies in the world"!!!

/

 

[MisterCMRR]

Sorry to be "late to the party" here. Since the unbalanced source has a low source impedance (as opposed to the high source impedance of a guitar), a transformer-based direct box is overkill. The attached circuit takes advantage of the low source impedance and the required signal attenuation to make the output microphone level. It is a 26 dB resistive pad that makes the source look impedance-balanced to the mic preamp ... and includes capacitors to block phantom voltage if it happens to be turned on. It's important that this bit of circuitry be at the source (synthesizer or other) end of the long cable and that the signal low (negative end of C2) and cable shield both be tied to the unbalanced source's signal ground. Noise rejection will depend on the CMRR of the mic preamp as well as the actual source (output) impedance of the synthesizer or other source ... the lower the better.

 

[Whoops]

Sorry to be "late to the party" here. Since the unbalanced source has a low source impedance (as opposed to the high source impedance of a guitar), a transformer-based direct box is overkill.

Thank you so much, you got it all right, you understood exactly the situation

The attached circuit takes advantage of the low source impedance and the required signal attenuation to make the output microphone level. It is a 26 dB resistive pad that makes the source look impedance-balanced to the mic preamp ... and includes capacitors to block phantom voltage if it happens to be turned on. It's important that this bit of circuitry be at the source (synthesizer or other) end of the long cable and that the signal low (negative end of C2) and cable shield both be tied to the unbalanced source's signal ground.

Well, that circuit completely wins in terms of simplicity and low component count. It's very very simple. I would not do it in a cable but in a small box.
Thank so much for sharing, I will try it also and also the other 2 active options.
But if it's able to achieve a good performance then I think it wins against the active options, it's simpler, cheaper and takes less space.

Noise rejection will depend on the CMRR of the mic preamp as well as the actual source (output) impedance of the synthesizer or other source ... the lower the better.

The Mic Preamps I use in Live Sound are all pretty good and Low Noise.
I use Consoles/Stage Boxes from Midas, DigiCo, Yamaha, Soundcraft Vi Series, all of them have great CMRR.

As for the source impedance (output of Synth,etc) well that varies a lot, as I explained earlier the lower I found was 430r and the highest was 1K.
So does that circuit maintains proper "Impedance Balance" independently of the source impedance? The resistor values don't have to be calculated depending on the source impedance like the example that was given here earlier?

Thank you so much for your help and for drawing and sharing the circuit, I highly appreciate it

 

[Bo Deadly]

Sorry to be "late to the party" here. Since the unbalanced source has a low source impedance (as opposed to the high source impedance of a guitar), a transformer-based direct box is overkill. The attached circuit takes advantage of the low source impedance and the required signal attenuation to make the output microphone level. It is a 26 dB resistive pad that makes the source look impedance-balanced to the mic preamp ... and includes capacitors to block phantom voltage if it happens to be turned on.

Noise rejection will depend on the CMRR of the mic preamp as well as the actual source (output) impedance of the synthesizer or other source ... the lower the better.

It's common for pro-sumer synths and similar to have pretty weak outputs (like a single transistor follower) followed by significant series resistance (like 1k) to isolate it. So maybe one just has to match it on the 0V leg?

It's important that this bit of circuitry be at the source (synthesizer or other) end of the long cable and that the signal low (negative end of C2) and cable shield both be tied to the unbalanced source's signal ground.

Why is that?

Funny I actually just posted a similar U-pad-in-a-cable solution here:

How effective are hum eliminators for unbalanced to balanced conversion?

but where the output was balanced and the input was unbalanced. I recommended that it would be better for the U pad to be at the unbalanced end.

Would you still recommend that the U pad be ad the source end or that it should be at the unbalanced end?

I also recommended NOT connecting the shield at the unbalanced end reasoning that it would prevent currents created by differences in device protected earths which can be a source of significant ground noise. The series resistance of the pad would be good for isolating grounds and if the source was ground sensing it would provide some rejection.

In your drawing the shields are connected at both ends. Is this still your recommendation or is the a ground-lift scenario?

 

[Whoops]

What type of -- end product -- are you looking for to house this circuit for your "LIVE" sound use? Do you want to place several of these circuits into a "stage box"? Would you prefer to place several of these circuits into a "1U rack-chassis"? Or, would you prefer to place individual circuits into an "XLR Barrel" with a Female 1/4" jack on one end and a Male XLR connector on the other end?

I was thinking of doing maybe 3 Boxes, 2 boxes with 4 circuits/channels and one box with 6 channels. The goal is to make them as small as possible, so the XLR connectors size will dictate how big it is. But for the size of one normal DI box, I want to have at least 4 circuits/channels.
I need to same space in my Trolley while traveling.

I was thinking in using one of this options and choose it after I have all the measurements (still a bit early):







I might also do some circuits in these barrels, the end is the exact size for a chassis XLR:

In all of these instances, I could design - both - the mechanical chassis/enclosure/housing and/or the PCB's for you, if you would like.

I was going to design the PCB myself, I'm able to do it but I'm really slow working with Eagle or KiCAD, so I accept your help for sure. Thanks mate.
As for the mechanical part, maybe that's not needed because I'm going to use an already made box but maybe if you help with front panels design in case needed that would be awesome.

Just send me whatever finished schematic that you come up with and/or decide on and let me know what you want to put the circuit into.....then.....I could whip up either the mechanical design and/or the PCB-design and then send you the fabrication CAD files so you can have things made over there in Portugal where you live. Sound like a plan?

For Sure mate, and thank you so much for your offer.
Just give me some time to build and test out circuits and when i have the final decision of what I'm going to do I let you know and we go from there.
Thanks

 

[MisterCMRR]

It's important that this bit of circuitry be at the source (synthesizer or other) end of the long cable and that the signal low (negative end of C2) and cable shield both be tied to the unbalanced source's signal ground.

Why is that?

In an unbalanced cable, the resistance of the "shield" (low side signal conductor) will have noisy currents (hum, buzz, etc.) flowing through it (from the chassis of one piece of gear to the other) as well as the audio signal current. That noise voltage will be added directly to the signal seen at the receive end of an unbalanced cable. The noise is coupled by the common-impedance of the cable shield (it's called common-impedance coupling).In a balanced (shielded-twisted-pair) cable, the shield is NOT part of the signal path, so ground voltage differences will have little, if any, effect in contaminating the signal. So unbalanced cables should always be as short as possible (or, better yet, non-existent).

The two biggest myths about audio cables are 1. Noise in unbalanced cables is caused by inadequate shielding. In fact, in all but very rare cases, it's caused by the end-to-end resistance of the unbalanced cable's shield.
2. That a balanced signal has equal and opposite voltage swings on the two conductors ... and this is somehow magically responsible for cancelling noise. In fact, it's the impedance balance of the two conductors with respect to ground.

 

[MisterCMRR]

Would you still recommend that the U pad be ad the source end or that it should be at the unbalanced end?

I also recommended NOT connecting the shield at the unbalanced end reasoning that it would prevent currents created by differences in device protected earths which can be a source of significant ground noise. The series resistance of the pad would be good for isolating grounds and if the source was ground sensing it would provide some rejection.

In your drawing the shields are connected at both ends. Is this still your recommendation or is the a ground-lift scenario?
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Lifting the ground at either end of an unbalanced cable will almost always increase ground noise! If the cable ends are grounded, any ground voltage difference will be reduced, if only slightly, by the "loading" of the source caused by the shield resistance. If you disconnect either end, now the full "open circuit" ground voltage difference will be added to the signal. Imagine if the signal source, say a synth, had a 2-prong AC power connection. It's chassis could be floating at a large fraction of mains voltage ... and that would be seen at the receive end of the unbalanced cable! Disconnecting a cable shield at one end is only an option (and often a good one) with balanced cables.
See pages 60 to 65 of my seminar handout at
https://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf

 

[MidnightArrakis]

I was thinking of doing maybe 3 Boxes, 2 boxes with 4 circuits/channels and one box with 6 channels. The goal is to make them as small as possible, so the XLR connectors size will dictate how big it is. But for the size of one normal DI box, I want to have at least 4 circuits/channels.
I need to same space in my Trolley while traveling.

I was thinking in using one of this options and choose it after I have all the measurements (still a bit early):

View attachment 103000
View attachment 103001




I might also do some circuits in these barrels, the end is the exact size for a chassis XLR:

View attachment 103002







I was going to design the PCB myself, I'm able to do it but I'm really slow working with Eagle or KiCAD, so I accept your help for sure. Thanks mate.
As for the mechanical part, maybe that's not needed because I'm going to use an already made box but maybe if you help with front panels design in case needed that would be awesome.



For Sure mate, and thank you so much for your offer.
Just give me some time to build and test out circuits and when i have the final decision of what I'm going to do I let you know and we go from there.
Thanks

For your "multiple circuit" version, I would go with an "Extruded Aluminum" enclosure, primarily because they have the grooves on the inside to slide a PCB into. This eliminates having to use more hardware. Something similar to this:



For your "single circuit" version, using the small "XLR" connector enclosure certainly makes sense. I recently bought 2-dozen of them from AliExpress just before Christmas.

[Just give me some time to build and test out circuits] -- Take all of the time that you need. I'm in no hurry!!!


[when i have the final decision of what I'm going to do I let you know] -- With all of the new and recent input you have received from the other forum members here, it looks as though you now have some really good options to choose from!!!

[we go from there] -- "Standing By.....".....

/

 

[Cranehazard]

I know you said no transformers but. It could be passive and not rely the house phantom. I bet those synths can drive a transformer. I just love the way a transformer handles long cables with line level. If the levels match good a 1:1 could do it, if the source is to hot step it down 2:1 or 4:1. A more expensive solution but you won't be sorry about having some transformers around as an audio engineer. It is overkill but I believe it will interface better with a broader range of gear and save you hassles with phantom in the long run.

 

[Bo Deadly]

Why is that?

In an unbalanced cable, the resistance of the "shield" (low side signal conductor) will have noisy currents (hum, buzz, etc.) flowing through it (from the chassis of one piece of gear to the other) as well as the audio signal current.

Well I didn't say anything about using an unbalanced cable. What you are advocating is like this:



where the source is the TS is the source and XLR is balanced, the source has a builtin U pad and the shields are connected at both ends.

But if the source was the XLR and the destination was TS, where would you put the pad and why?

 

[Bo Deadly]

Lifting the ground at either end of an unbalanced cable will almost always increase ground noise!

I'm sure it would (and without the "almost").

But again, I wasn't recommending that anyone use unbalanced cables.

What about a scenario like this where the source is balanced and the destination is unbalanced:



Would you recommend that the shield be connected at both ends?

Although I must admit I never thought much about connecting pins 1 and 3 together at the source as shown in this graphic which was "borrowed" from the Rane website:

RANE Commercial - Knowledge Base

I would be interested on your thoughts on that as well. If the source is THAT 1646, would you recommend connecting 1 and 3? I assume by "cross-coupled" they mean ground sensing like THAT 1646 or transformer.

 

[MisterCMRR]

Inasmuch as a cross-coupled output approximates the floating character of a transformer secondary, its low side can be referenced to a remote ground point, like the unbalanced input low side. However, this should only be done with either a real transformer or a high-performance cross-coupled output stage like THAT 1646. Earlier versions of this circuit, like those by SSM, become very unstable (prone to ultrasonic oscillation) if either output leg is grounded remotely, so it must be grounded near the IC (as advised by the Rane note). But when pins 1 and 3 are tied at the source end, the entire interface is degraded to unbalanced with essentially zero noise rejection. With a true floating output (transformer or THAT), noise rejection can be as high as 70 dB, depending on frequency. I recommend that XLR to RCA adapters (which tie pins 1 and 3 for either input or output versions) never be used for the same reason: they convert the entire interface to unbalanced when "free" noise rejection is available by using either a "floating" output or a balanced input. A three conductor (shielded-twisted-pair) should be used in any case.

 

[KA-Electonics.com]

MisterCMRR wrote: Earlier versions of this circuit, like those by SSM, become very unstable (prone to ultrasonic oscillation) if either output leg is grounded remotely, so it must be grounded near the IC

Jung added "stability" caps (C1 and C2) to preload the outputs of the SSM2142 to mitigate "varying cable lengths" in this diagram published in Op Amp Applications Handbook. It's the only SSM2142 drawing where I've seen them added. Do you suppose this remedy addressed the 2142's remote grounding issue?

 

[MisterCMRR]

Possibly. It's been decades since I tested the SSM2142, but as I recall, there was a range of output capacitive loads that made the IC very unhappy! In THAT's 1646, the HF gain rolloff to prevent the instability is included inside the IC ... along with fixing its awful behavior when driven to clipping even briefly. Gary Huber of THAT Corp is a very clever guy ...

 

[MisterCMRR]

Regarding interfaces in general, one has to carefully consider disconnection of a cable shield at either end. The existence of equipment having 2-prong (ungrounded) AC power connections poses the very real possibility of having huge common-mode voltages develop in the signal interface. These voltages (typically half the mains voltage) will completely overwhelm the common-mode voltage range (and rejection) of active input or output circuits. Although a transformer can withstand such voltages without damage, even it will not have enough common-mode rejection to eliminate the noise added to the signal. For example, Rane diagram 13, a few posts back, would have this problem if the unbalanced signal source was a piece of ungrounded gear. In my seminars, I often recommend grounding such gear by changing its power plug to a 3-prong type and connecting the plug ground to the chassis of the gear by spiraling a wire around the existing power cord.

Balanced outputs on commercial gear present a real problem to those who put systems together. Most manufacturers don't offer a clue on their data sheets about whether one leg can be grounded or not. I'm a member of AES standards committee SC0505 and we're currently working on a document "standard" to pressure manufacturers to put this, and other essential information, on their data sheets. Otherwise, it's time-wasting guesswork to put together a "plug-n-play" system!

 

[Whoops]

Thank you so much for all your help and input Bill,
it's a priviedge to be able to have someone with your knowledge and experience here in this forum.

In my seminars, I often recommend grounding such gear by changing its power plug to a 3-prong type and connecting the plug ground to the chassis of the gear by spiraling a wire around the existing power cord.

Better just to replace completely the 2 wires original power cord, it's easy and very cheap

 

[Newmarket]

Well I didn't say anything about using an unbalanced cable. What you are advocating is like this:

View attachment 103099

where the source is the TS is the source and XLR is balanced, the source has a builtin U pad and the shields are connected at both ends.

But if the source was the XLR and the destination was TS, where would you put the pad and why?

Your text says shield connected at both ends but drawing show NC shield at TS end.

 

[Whoops]

Sorry to be "late to the party" here. Since the unbalanced source has a low source impedance (as opposed to the high source impedance of a guitar), a transformer-based direct box is overkill. The attached circuit takes advantage of the low source impedance and the required signal attenuation to make the output microphone level. It is a 26 dB resistive pad that makes the source look impedance-balanced to the mic preamp ... and includes capacitors to block phantom voltage if it happens to be turned on. It's important that this bit of circuitry be at the source (synthesizer or other) end of the long cable and that the signal low (negative end of C2) and cable shield both be tied to the unbalanced source's signal ground. Noise rejection will depend on the CMRR of the mic preamp as well as the actual source (output) impedance of the synthesizer or other source ... the lower the better.

Hi Bill,
I find the 26dB's attenuation to be too much for these duties.
As I wrote previously I used quite a few times Synths connected directly to the Live Sound's Mic Preamps (TS jack to XLR, Pin3 grounded) without attenuation and that never presented a problem, with some synths with higher output I might use the Mic Preamps PAD but that no problem because I will always have a PAD in the Mic Preamp in case needed. But overall the Mic Preamps have enough headroom to accept these signals without overloading.

So without affecting the impedance balance is it possible to reduce the U PAD attenuation in the circuit you posted, for example going from 26dB's to 15dB's or 10dB's only?

Thank you