DC Coupled, Balanced 1/4 output?

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AVA

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Hi All,

For whatever reason, I was measuring the signal that came off the TRS cable that was plugged into my PreSonus Audiobox 96, while it put out a 1kHz sine wave.

The tip, as expected, carries the signal.
But the Ring doesn't...
I expected the Ring to carry a reversed polarity version of the signal.
PreSonus replied saying this is normal because the output 'is DC coupled' and this blew my mind...

I've been thinking wrong about coupling all this time!

Can someone explain this to me? How can this be a balanced line then?


Thanks in advanced! Apologies if this is a dumb question, but I'm struggling to get it right. I've scoured google and this forum, to no avail.
 

gyraf

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..you probably spoke to the marketing department, not the tech staff :)

I guess they mean it's "impedance balanced" (a common solution when it has to be cheap) - so not really balanced other than for marketing. But works ok for reducing induced noise..

/Jakob E.
 

AVA

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Hey Jakob, thanks for the reply.
Haha, you're probably right! They did seem to have that dismissive marketing like quality about them

But even if they meant impedance balanced, I should still see some kind of signal on the ring, no?

Sorry if this is a noob question...I am after all somewhat inexperienced. Though, my tenacity makes up for it
 

emrr

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lots of resources to read about the way balancing is done, plenty of existing threads detailing the methods here.
 

Newmarket

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Hey Jakob, thanks for the reply.
Haha, you're probably right! They did seem to have that dismissive marketing like quality about them

But even if they meant impedance balanced, I should still see some kind of signal on the ring, no?

Sorry if this is a noob question...I am after all somewhat inexperienced. Though, my tenacity makes up for it
No - there's no audio signal on the ring in an "Impedance Balanced" system as the term is generally applied. It connects to "Signal 0V" via a similar impedance to that of the audio signal (typically defined by a resistor in series with an op amp output - typically between 47R to 100R on pro kit. It's a moot point whether any capacitor needs to be duplicated given likely capacitor tolerances. Both lines are (to a degree) affected in the same way by any interference/noise and this is rejected at the receiving balanced / differential input.

Loads of info available on this - eg on this site and Douglas Self does a fairly comprehensive guide to various balancing / ground sense options.
Link below to Graham Blyth (Soundcraft) on this - used extensively on Soundcraft / Spirit desks.
https://cosmomusic.ca/media/W1siZiI...Balanced_Connections.pdf?sha=35ec48ee7c33be99
 
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AVA

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Hey emrr, thanks for the reply.

I know the theory behind balancing, and how it's generally done in practice. But the way I learned it is that the ring carries an inverted polarity of the signal that the tip does...
I'm sure it's obvious to a lot of you as to why the ring in my case appears to be also ground, but my research only returns loads of information that I don't yet understand.
Hence why I'm hoping someone could shed a little clarity on the topic.
 

AVA

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No - there's no audio signal on the ring in an "Impedance Balanced" system as the term is generally applied. It connects to "Signal 0V" via a similar impedance to that of the audio signal (typically defined by a resistor in series with an op amp output - typically between 47R to 100R on pro kit. It's a moot point whether any capacitor needs to be duplicated given likely capacitor tolerances. Both lines are (to a degree) affected in the same way by any interference/noise and this is rejected at the receiving balanced / differential input.

Loads of info available on this - eg on this site and Douglas Self does a fairly comprehensive guide to various balancing / ground sense options.
Link below to Graham Blyth (Soundcraft) on this - used extensively on Soundcraft / Spirit desks.
https://cosmomusic.ca/media/W1siZiI...Balanced_Connections.pdf?sha=35ec48ee7c33be99
That's perfect! Thanks so much. Ill study up on this.

Cheers!
 

AVA

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This is right in the money. Thanks a million folks! I appreciate the time you guys are putting into my ongoing education.

John, my apologies for not finding this info in the forum, I didn't realize that 'impedance balance' was what I was actually searching for. It's a new concept to me. One that seems so obvious now. My circuits have all been instrument level until now.

Before learning the circuitry in detail, I toured for years, and the word on the road is that a balanced input is simply two opposite waveforms, but never goes much deeper, so I didn't even stop to think that it was any different.

It turns out that this actually saves a lot of headache as far as design is concerned.

Thank you all. These links are super informative reads for me...dare say, I'm actually pumped about this haha!

Cheers
 

Bo Deadly

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Actually they could very well be DC coupled. Some interfaces have DC coupled outputs now. I'm not sure why exactly but it might be for control voltages for synths and such. Many of the newer MOTU interfaces are DC coupled now.

Presumably they are also impedance balanced but DC coupled and "balanced" of any kind are somewhat contradictory. So it would be interesting to know for sure if there is a resistor on pin 2. Assuming pin 1 is connected to 0V, I suppose one might be able to measure this with a simple meter measuring resistance between pin 2 and 1 and also for pin 3 and 1 (with the unit off). If they both measure the same in the 50-100 ohms range, that's fairly conclusive.
 

MisterCMRR

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ALL the advantages of a "balanced" signal accrue from the impedances, with respect to ground, of the two lines being the same. Signal symmetry (the equal and opposite signal) is NOT a requirement - contrary to years of misinformation and BS "explanations," sometimes even in otherwise respectable books! The reason for using a balanced interface with no signal on the low side, is that it can be plugged into an unbalanced input (where the low side is grounded) without misbehaving in any way ... some would call it "musician-proof." It's a perfectly legitimate balanced output - but it has the added advantage of compatibility!

The reason for DC-coupled outputs is the same as for eliminating coupling capacitors wherever possible in an audio signal chain - to eliminate the high-pass filter effect, which rolls off low-frequency response, and more importantly, causes phase distortion that alters the timbre of instruments like kick drums. To reduce phase-distortion, coupling capacitors (when they must be used) should be chosen for a -3 dB frequency under 1 Hz (rather than the widely-used criteria of -3 dB at 10 Hz or 20 Hz). This is the same reason Jensen transformers are intentionally designed to have extended low-frequency response, often as low as 0.5 Hz.
 

AVA

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ALL the advantages of a "balanced" signal accrue from the impedances, with respect to ground, of the two lines being the same. Signal symmetry (the equal and opposite signal) is NOT a requirement - contrary to years of misinformation and BS "explanations," sometimes even in otherwise respectable books! The reason for using a balanced interface with no signal on the low side, is that it can be plugged into an unbalanced input (where the low side is grounded) without misbehaving in any way ... some would call it "musician-proof." It's a perfectly legitimate balanced output - but it has the added advantage of compatibility!

The reason for DC-coupled outputs is the same as for eliminating coupling capacitors wherever possible in an audio signal chain - to eliminate the high-pass filter effect, which rolls off low-frequency response, and more importantly, causes phase distortion that alters the timbre of instruments like kick drums. To reduce phase-distortion, coupling capacitors (when they must be used) should be chosen for a -3 dB frequency under 1 Hz (rather than the widely-used criteria of -3 dB at 10 Hz or 20 Hz). This is the same reason Jensen transformers are intentionally designed to have extended low-frequency response, often as low as 0.5 Hz.
Thanks for the info on the DC coupling outputs...I would have spent ages before stumbling on to that little gem!

Lol musician proof...I like that. As a former touring monitor guy, I can go on for days about that very thing.

I've been reading on this impedance thing all day, I'm so glad you all have informed me. Now it seems so bloody obvious ...of course the impedance is the key element!! What a fool I was!


Thank you for your time, I really appreciate it!!

Cheers
 

Bo Deadly

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The reason for DC-coupled outputs is the same as for eliminating coupling capacitors wherever possible in an audio signal chain - to eliminate the high-pass filter effect, which rolls off low-frequency response, and more importantly, causes phase distortion that alters the timbre of instruments like kick drums.

That might be a reason but the manufacturer advertised reason for DC-coupled outputs is for compatibility with software that can use digital audio interfaces to emit control voltages for synths and other stuff that receives control voltages:

MOTU.com - Testing analog outputs for control voltage compatibility

At least that probably what the PreSonus rep meant by the output 'is DC coupled'.
 

JohnRoberts

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DC coupled audio paths are a recipe for disaster if that DC content gets carried through to outputs and presented to loudspeakers.

Back before some of you were born Crown made a DC coupled power amp (DC 300). It was a notorious speaker eater, if something upstream leaked DC into the audio input path.

JR
 

MisterCMRR

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"DC-coupled" in any conventional use means no coupling capacitor is present. DC voltages can be a real a problem in audio interfaces because switching or connecting/disconnecting signal paths results in clicks and pops. "Volta" is apparently a scheme dreamed up by MOTU for hijacking signal interfaces to carry variable DC voltages (like variable phantom power) to control analog synthesizers. To quote from a page I found trying to figure this out, "If you use analog synth gear and a Mac-based DAW, you need Volta." 1. That's pretty far from mainstream audio gear; 2. I never heard of it until today; and 3. I actively participate in several AES standards committees and it's never been mentioned. I don't understand how Volta would have anything to do with a normal analog interface, notwithstanding PreSonus support.
 

PermO

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I was thinking about this also.

I have a Class A MosFet amplifier and with a little DC at the inputs it will become a powersupply... a great recepy for destroying expensive studiomonitors.

And how about transformers ? .. is there enough current to ruin your audiotransformers ?

Very cool that this can do all that, but a little user error, or loading the wrong template in your DAW and poof !
 

JohnRoberts

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That old Crown DC300 dc coupled power amp had been put into emergency service on occasion to replace a broken console power supply for a short term field fix, the show must go on. Still a bad idea for conventional audio paths to input or output DC. DC is not music, or audio.
===
Back in the 70s I briefly considered multiplexing a DC control voltage on mic/line inputs to remote control gain in external preamp boxes (I abandoned that idea along with numerous others).

JR
 

Bo Deadly

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I don't understand how Volta would have anything to do with a normal analog interface, notwithstanding PreSonus support.

It's not just MOTU. There are like 4-5 manufacturers that are doing it. The "Volta" thing is just the software that comes with their stuff. There are other software packages that utilize it. You haven't heard of it because it's all about synth stuff. If someone wants to bridge the computer to a physical analog synth, that's how it's done.

And incidentally I can't really find fault with it. It's highly unlikely that a few volts DC is going to hurt anything. If you have to find fault with a DC coupled output or a DC coupled power amplifier, I will find fault with the DC coupled power amplifier every time!
 
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