[IDEA] High-End XLR connection with VGA cables

[forart.eu]

Hi everyone, this idea jumped in my mind yesterday when i'm surfing the web for "DIY High-End balanced signal cables", trying to find a valid alternative to unbalanced ones.

Check this out:



Do you think that could be a good idea ?

Constructive opinions/suggestions are always welcomed !

[infinia]

Originally posted by forart.eu
Hi everyone, this idea jumped in my mind yesterday when i'm surfing the web for "DIY High-End balanced signal cables", trying to find a valid alternative

Do you think that could be a good idea ?

Balanced audio cables in my thinking imply twisted pairs with a shield for low level and high dynamic range (ie XLR). So i suppose in a pinch you could use two of the RGB coax's leaving spare the one coax and all the wires since they arent twisted, thus only for a single channel, in my view not too economical compared to a proper XLR connection system.

[unclejed613]

my thought is, you have 15 pins in a VGA connector, therefore 5 twisted pairs and 5 shields. buy just the solder cup connectors. you can get "snake" cable with (i think) 6 individually shielded pairs.

[dmills]

The **IMPORTANT** part about a cable for balanced use is that the pairs are twisted! The screen is almost incidental most of the time, it is the twisting that resists the magnetic coupling.
In fact, you can stuff balanced line level audio over hundreds of meters of unscreened twisted pair and it will be fine if the receiver is right.

For a balanced line receiver to work well, the impedances need to be well matched (And typically the input impedance will be much higher then the output impedance to minimise the effect of tolerance of the build out resistors. It is NOT required that there be voltage balance, it is the impedance balance that does the magic.

If you are building balanced line IO into gear, please do it right, pin one goes to the case right at the connector, not to circuit 0V (That way lies hum)!

Regards, Dan.

[infinia]

Originally posted by unclejed613
my thought is, you have 15 pins in a VGA connector, therefore 5 twisted pairs and 5 shields. buy just the solder cup connectors. you can get "snake" cable with (i think) 6 individually shielded pairs.

But that wouldn't be called "VGA"! Yours would be workable using custom cable/s and DB-15 connectors but then it may as well be UJA (Uncle Jed Audio) or something else.

Originally posted by dmills
The **IMPORTANT** part about a cable for balanced use is that the pairs are twisted! The screen is almost incidental most of the time, it is the twisting that resists the magnetic coupling.
In fact, you can stuff balanced line level audio over hundreds of meters of unscreened twisted pair and it will be fine if the receiver is right.

For a balanced line receiver to work well, the impedances need to be well matched (And typically the input impedance will be much higher then the output impedance to minimise the effect of tolerance of the build out resistors. It is NOT required that there be voltage balance, it is the impedance balance that does the magic.

If you are building balanced line IO into gear, please do it right, pin one goes to the case right at the connector, not to circuit 0V (That way lies hum)!

Regards, Dan.

Light gauge twisted pairs have a Zo of about 120 ohms. So the max BW is with a difference reciever terminated with the same. If you use coax it will be 2xZo or 150 ohms using video coax.
Dan right about without shields, but you then need to max out the signal levels to keep a decent SNR depending on the ambient interference levels.
Re minimize hum (ground loops and such)
With a shield connection to the source only and left floating at the difference receiver is best for audio in my experience. The shield connection is referred to as a drain in this case. No ground loops!

[dmills]

Originally posted by infinia
Light gauge twisted pairs have a Zo of about 120 ohms. So the max BW is with a difference reciever terminated with the same. If you use coax it will be 2xZo or 150 ohms using video coax.

Why would I be even thinking about transmission line effects in an audio frequency application (unless I am the phone company)? Characteristic impedance is irrelevant for audio frequency applications over sensible cable lengths (say up to a KM or so).

Dan right about without shields, but you then need to max out the signal levels to keep a decent SNR depending on the ambient interference levels.

That has not been my experience at all, as long as the input stage was designed by somebody with some engineering chops, the screen is as near as dammit pointless most of the time (Note this means an input stage that does not rectify common mode RF and that is designed with a reasonable bandwidth limit).
I have almost 100M of unscreened cat carrying 4 differential pairs of audio to the amp room at work (it feeds >20KW of power amp), that signal actually travels via the data comms room patch bay and in the same cable bundle as Gb Ethernet, and it seems to work fine. It even works fine when you expose it to a few tens of watts at 433Mhz. I would guess those circuits average somewhere around -30dbu or so, and I dont think I have ever had an interference issue.
The screen does however make a dandy return path for phantom powered mics.

Re minimize hum (ground loops and such)
With a shield connection to the source only and left floating at the difference receiver is best for audio in my experience. The shield connection is referred to as a drain in this case. No ground loops!

Measure of last resort IMHO, better to solve the pin one problems in the gear rather then band aiding it.

Regards, Dan.

[unclejed613]

audio cable doesn't begin acting like a transmission line until you get into lengths like 1875 meters or so (1/8 wavelength at 20khz), so the "Zo" of audio cable is not important, and neither is the velocity factor of the cable. back in the days of all passive studio mixers, having a standard set of impedances made sense, because everything was done with transformers, caps, and resistive pads, and passive mixers were designed for minimum losses. these days 99.99% of all audio is done with active devices and the old "impedance matched" methods have given way to "voltage matching" of systems.

also the shielded pairs are useful to keep common mode signals out of the system. while not absolutely necessary to have, they can be useful.

[infinia]

Originally posted by unclejed613
audio cable doesn't begin acting like a transmission line until you get into lengths like 1875 meters or so (1/8 wavelength at 20khz), so the "Zo" of audio cable is not important, and neither is the velocity factor of the cable. back in the days of all passive studio mixers, having a standard set of impedances made sense, because everything was done with transformers, caps, and resistive pads, and passive mixers were designed for minimum losses. these days 99.99% of all audio is done with active devices and the old "impedance matched" methods have given way to "voltage matching" of systems.

also the shielded pairs are useful to keep common mode signals out of the system. while not absolutely necessary to have, they can be useful.

No actually there are two sets of Zo one for low frequencies and another for high frequencies and the length and physical construction of the twisted pair determines the frequencies of transition between the two. For example using Cat5 unterminated with a length of 100 meters the frequency of transition is from 50KHz to 1 MHz going from 600 - 100 ohms. If you terminate using 100-120 ohms then you effectively swamp out the impedance transition. Yes you are correct on using active circuits esp using newer op-amps (transimpedace amlifiers) lets you drive cables with lower loads. Using Zo termination techniques free up the BW dependance of cables esp at longer. than 100 meters. Connection of shields at a single point for elimination of ground loops is good system enginering design practice! Not a bandaid solution. Microphones should connect at both ends and are not succeptible to ground loops because the power source is either batteries or from the reciever end (phantom).

[dmills]

Designing the input to comply with AES48 is good system engineering, lifting screens may help at 50/60Hz and the first dozen harmonics, but leaves you with large RF currents potentially being pushed back into the transmitting device (A conductor connected at one end has another name, an Aerial!) .

I am aware of SCIN, but it is normally minor compared to the other issues that a floating screen can cause.

There is no reason for a screen bonded at both ends to induce excessive low frequency noise and any gear that suffers in that way needs some attention.

See papers by Muncy, Whitlock, Walderon & Brown for the gory details.

With the ever increasing levels of environmental RF, it behoves audio types to consider the RF implications of what they design if we are to get satisfactory behaviour in the presence of cell phones and the like.

Regards, Dan.

[infinia]

Originally posted by dmills
Designing the input to comply with AES48 is good system engineering, lifting screens may help at 50/60Hz and the first dozen harmonics, but leaves you with large RF currents potentially being pushed back into the transmitting device (A conductor connected at one end has another name, an Aerial!) .

I am aware of SCIN, but it is normally minor compared to the other issues that a floating screen can cause.

There is no reason for a screen bonded at both ends to induce excessive low frequency noise and any gear that suffers in that way needs some attention.

See papers by Muncy, Whitlock, Walderon & Brown for the gory details.

With the ever increasing levels of environmental RF, it behoves audio types to consider the RF implications of what they design if we are to get satisfactory behaviour in the presence of cell phones and the like.

Regards, Dan.

I'm more concerned with low impedance ground currents traveling from box to box from self generated RF hash (ie diode recovery noise and other commutating circuits) as well as the proliferation of cheap SMPS than outside RF stuff getting into chassis from singly terminated drain shields. Proper unit design has RF filters on all in and outs. Now if you are in a nearby influence of a commercial RF transmitter then the band-aids are needed. The fact is if you are using balanced interconnects you are already ahead in the game, with or without shield connections.