That's correct. You used to have to buy a PAiA kit (or something besides an audio interface) if you wanted to output control voltages from your sequencer software. I know lathe guys also use it to trigger banding. It can also be used for automatic head drop and lift for automated locked grooves.
DC Coupled, Balanced 1/4 output?
- Thread starter AVA
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Help Support GroupDIY Audio Forum:
They stopped making DC coupled power amps decades ago for obvious reasons.
I can imagine a lot of circuits that wouldn't like a few volts of DC on their inputs.
JR
I can imagine a lot of circuits that wouldn't like a few volts of DC on their inputs.
JR
Transformers and DC are an interesting mix. Of course, "burning out" of a winding will, just according to Ohm's law, depend on the voltage (as well as current capability in some cases) involved and the DC resistance of the winding. Generally, for low-ratio output transformers, DC is below 50 Ω just to keep losses low when driving a 600 Ω load (for most Jensen types, it's 40 Ω). So, if an output was driven to the rail by DC at the input to say, a DC-coupled line amplifier, and that amplifier had ±24 V rails, current would be 24 V / 50 Ω = almost 500 mA. Assuming the line amp could supply that much current without releasing its own factory-installed smoke, the transformer winding would be dissipating 24 V x 500 mA = 12 watts of heat. It'll certainly smell for a while and eventually ruin the internal insulation, etc. But, even if the situation is corrected right away, the transformer's core will remain magnetized - which will make its even-order harmonic distortion significantly increase until it's de-magnetized. De-magnetizing a transformer is not difficult. You need only a signal generator that can output a very low audio frequency that can drive an amplifier having enough output to "saturate" the transformer core. With any transformer, this is easy to do at say 10 or 20 Hz. In the earlier example, that same line amplifier could probably do the job. If not, use a power amplifier. The idea is to start with zero level into the transformer, increasing the level until the core saturates (as seen by a soft limiting or squashing of the peaks of the sine wave on a 'scope), hold it at that level for say 30 seconds, and then turn the level back down to zero over a period of at least a few seconds. Or, if you know someone with a tape demagnetizer that does 2-inch tape, you could put the transformer in it! A static magnetic field in a transformer, whether caused by a magnetized core or by intentionally running DC in the winding (like class-A stages usually do), will shift the mix of even to odd order harmonic distortion. In preferred use (a demagnetized core and no DC across any winding), third harmonic will always be the dominant distortion and second harmonic (and other even orders) will be vanishingly small. Because even-order distortion products are an octave up, people tend to "like" them more - explaining why class-A amplifiers, transformers, and magnetic tape are often called more "musical."
Newmarket
Well-known member
I guess there's the question of whether a dc servo output is described as "DC Coupled".
No big output cap in series with the audio signal. But still a cap (and hence time constant) in the scheme.
It won't work with the DC Control Voltage thing of course.
I think I'll stay with minimum DC risk.
Very interesting post Bill , thanks
I found a short clip on youtube you made about Deane Jensens spectral contamination idea,
Simple THD measurements are a failry blunt tool compared to what the ear is capable of discerning , might be interesting to use something like Room Eq Wizard's distortion measurement capabillity to invesigate these ideas further.
I found a short clip on youtube you made about Deane Jensens spectral contamination idea,
Simple THD measurements are a failry blunt tool compared to what the ear is capable of discerning , might be interesting to use something like Room Eq Wizard's distortion measurement capabillity to invesigate these ideas further.
Differential signals arriving at a differential destination beat single ended with a dead line resistor any day. All the proponents of "balanced impedance" are producers of lower cost budget oriented stuff. The only hot signal is still a ground referenced signsl, and suffers all of the grounding foibles of shitty consumer grade single ended "RCA" interfaces. I design and build only proper galvanically isolated transformer coupled ins and outs, with zero ground connection to any portion of the signal, OR use the well designed THAT Corp balanced like receivers and transmitters. Balanced impedance single signal is a compromise that does function to keep noise pickup in the wire lower, but it does nothing to eliminate common ground problems in large systems. Just do the job right, not as cheap as possible.
radardoug
Well-known member
Impedance balancing relies on there being an impedance from hot to cold at the remote end of the line. Some balanced receivers will be just the + and - going into electronics. In this case the ground noise will transfer on the impedance balance leg, but it really is a cheap copout.
Bo Deadly
Well-known member
If the input does not actually debalance, a balanced driver vs impedance balanced doesn't help so I'm not sure what you're trying to say. And the cold resistor does provide decent ground isolation. A lot of ground loops are a small voltage causing a large current. So even 50 ohms between grounds goes a long way to knocking down those currents.
radardoug
Well-known member
The resistor is not in the ground lead. So of itself it does nothing for ground loops. If you are going into a single ended receiver, then its a crap shoot.
Think it over. A xfmr or cross-coupled output stage senses the voltage at the receiver's ground, hence provides hum cancellation.
A simple impedance-balanced output does not.
A ground-sensing impedance balanced output (as per Doug Self) also provides hum cancellation.
No. Actually a balanced connection with an infinite load is close to ideal.
What do you mean? Ground noise will be transferred to both legs equally, which is the reason for balancing.
Bo Deadly
Well-known member
It might be. It depends on how pin 3 is wired at the remote end. If pin 3 is wired to earth ground and pin 1 is floating at the remote end, then the resistor would provide isolation. If both pin 3 and pin 1 are connected to earth ground at the remote end, then it would not and you could get significant ground loop currents.
This is probably the number one annoyance with impedance balanced outputs. In practice, when coupling balanced to unbalanced, the type of gear is frequently guitar pedals running on batteries or some earth-less device that connects to a floating instrument like a guitar or mic. In that case, it needs earth ground. So the tendency is for pin 3 and pin 1 to be connected to earth ground. I used to have different cables wired to the bay that were wired both ways. But I eventually just ended up grounding 3 and 1 at the remote end because there was always some device that needed earth ground.
PermO
Well-known member
No, as a hobbyist I can tell you this situation is much more common,.. and a problem then you think.
It's not just the occasional guitar pedal on a battery...
Budget outboard FX ...unbalanced, all synths, drummachines and keyboards, unbalanced.
No trow in a budget mixer that is 'dubiously balanced' .... buy your standard pre soldered jacks and you keep wondering "how come does my stuff sound like shit ?"
I have figured it out now
It's manufacurers cheaping out, that's why !
It's not just the occasional guitar pedal on a battery...
Budget outboard FX ...unbalanced, all synths, drummachines and keyboards, unbalanced.
No trow in a budget mixer that is 'dubiously balanced' .... buy your standard pre soldered jacks and you keep wondering "how come does my stuff sound like shit ?"
I have figured it out now
It's manufacurers cheaping out, that's why !
I spent 15 years at Peavey trying to make products that customers couldn't misuse, but that turned out to be all but impossible.
In decades of experience I would often see new companies making mistakes that experienced companies already learned from (by making the same mistakes years ago).
I am repeating myself, but IMO impedance balanced is not only cost effective but it is fine when used with proper (3 circuit) wiring and feeding a decent differential (balanced?) receiver.
Live sound reinforcement was like the MASH surgery of the sound business, the show must go on with whatever gear you have, or is still working. The studio side on the other hand has the luxury of time to get their wiring and gear sorted. Thats why high end live sound reinforcement liked transformers, they are harder to screw up.
Modern active balanced circuitry is far more forgiving than early attempts, and cheaper than transformers. Impedance balance is cheaper yet... I used to joke that those $0.02 resistors were the best bang for the buck a mixer marketer could ever get, allowing the sales pukes to claim "balanced", without lying. Don't hate it because its cheap.
JR
In decades of experience I would often see new companies making mistakes that experienced companies already learned from (by making the same mistakes years ago).
I am repeating myself, but IMO impedance balanced is not only cost effective but it is fine when used with proper (3 circuit) wiring and feeding a decent differential (balanced?) receiver.
Live sound reinforcement was like the MASH surgery of the sound business, the show must go on with whatever gear you have, or is still working. The studio side on the other hand has the luxury of time to get their wiring and gear sorted. Thats why high end live sound reinforcement liked transformers, they are harder to screw up.
Modern active balanced circuitry is far more forgiving than early attempts, and cheaper than transformers. Impedance balance is cheaper yet... I used to joke that those $0.02 resistors were the best bang for the buck a mixer marketer could ever get, allowing the sales pukes to claim "balanced", without lying. Don't hate it because its cheap.
JR
That is a concern only when the system designer insists on +24dBu capability. It is important in digital systems where 100%fs =+24dBu. That was based on a 20dB headroom over +4 (0VU). This does not make much sense in a world of 5V (or even 3.3V) converters. More and more digital systems offer customizing 100%fs to whatever value between +18 and +26.
Indeed!
Headroom, and (if you're fond of bundling multiple channels of unshielded cables) crosstalk are the ONLY things compromised in balanced interfaces that don't drive symmetrical signals. I've had this argument a thousand times! When we proposed a change in the IEC's CMRR test (in IEC 60268-3) back in 2000, we got exactly this pushback, largely from Europeans who seemed to have an intellectual investment in the idea that equal-but-opposite signals are essential to noise rejection (as is wrongly taught in many textbooks as well). The revised standard, using a method I proposed, has an "informative annex" to explain that only the impedance balance matters in a balanced interface! In my seminars, I've offered $1,000 to anyone who can (using science, not screaming) prove that signal symmetry has anything whatsoever to do with noise rejection. Remember that the reason we use balanced interfaces in the first place is to reject noise (from ground-voltage differences between send and receive devices and/or for voltages induced in cables by electric or magnetic fields). You could make line drivers that deliver symmetrical signals on the two lines. doubling headroom and cancelling crosstalk, but it won't reject noise unless the impedances of the lines (with respect to "ground") are matched - and that includes driver, receiver, and the line itself since they're effectively paralleled. If you can visualize a balanced interface as a Wheatstone bridge, with each of the balanced lines one leg of the bridge, you'll see the relationships required to "null" the bridge (reject noise). For many engineers who attend my seminars, this is the "aha moment!" The same misconception about signal symmetry leads some to "fiddle" the matched pairs of resistors in a simple diff-amp line receiver, too. The misconception is very widespread
Impedance balanced but asymmetrical are not "illegitimate" - but they allow for very flexible interoperability in equipment that can't afford a transformer or a high-performance "cross-coupled" output stage like the THAT 1646. Inexpensive gear is still being made with balanced outputs that are simply two single-ended amplifiers - shorting the low-side output to ground will cause release of smoke at worst and system crosstalk issues at best. Balanced asymmetrical outputs prevent that. I'm a member of the AES standards group working on project X152, which is an effort to, at least, disclose on their data sheets just what kind of balanced output their equipment has. Currently, the user has to guess or experiment to, for example, figure out how to connect the balanced output to an unbalanced input.
And to Jen - while I applaud your preference for "proper" interfaces like transformers or well-designed circuits like THAT makes, the signal on a balanced line is not ground referenced, it's strictly differential. Look at the secondary of a transformer output - any voltage you see on one line will have only a parasitic relationship to ground - as anyone whose ever tried to use one line to feed an unbalanced input will quickly find out. The other end of the winding will have only some small capacitance between it an ground, making that unbalanced input get a signal that's being coupled through that tiny capacitance (i.e., no bass). In an ideal world, balanced lines would completely "float" with respect to ground. Thinking of a balanced line as two ground-referenced signals is what leads thinking down the rabbit hole!
Impedance balanced but asymmetrical are not "illegitimate" - but they allow for very flexible interoperability in equipment that can't afford a transformer or a high-performance "cross-coupled" output stage like the THAT 1646. Inexpensive gear is still being made with balanced outputs that are simply two single-ended amplifiers - shorting the low-side output to ground will cause release of smoke at worst and system crosstalk issues at best. Balanced asymmetrical outputs prevent that. I'm a member of the AES standards group working on project X152, which is an effort to, at least, disclose on their data sheets just what kind of balanced output their equipment has. Currently, the user has to guess or experiment to, for example, figure out how to connect the balanced output to an unbalanced input.
And to Jen - while I applaud your preference for "proper" interfaces like transformers or well-designed circuits like THAT makes, the signal on a balanced line is not ground referenced, it's strictly differential. Look at the secondary of a transformer output - any voltage you see on one line will have only a parasitic relationship to ground - as anyone whose ever tried to use one line to feed an unbalanced input will quickly find out. The other end of the winding will have only some small capacitance between it an ground, making that unbalanced input get a signal that's being coupled through that tiny capacitance (i.e., no bass). In an ideal world, balanced lines would completely "float" with respect to ground. Thinking of a balanced line as two ground-referenced signals is what leads thinking down the rabbit hole!
radardoug
Well-known member
As soon as you introduce the resistor on one leg impedance balanced approach, you are dealing with two ground referenced lines. And that is the problem. So the ideal balanced line is one with a perfect transformer at each end. The next best is That drivers on both ends. The rest of the solutions are compromises which may or may not work.
How is it THE problem?
The basic analysis of CMR is the Wheatstone bridge. Indeed, if the two bottom impedances (the common-mode impedances) were infinite, the top impedances would not matter much, but it's never the case, there are always stray capacitances, and no elecronic circuit has infinite impedance.
The solution that consists in balancing the bridge as well as possible is not a compromise, it's a perfectly valid practical answer to a practical problem.
Thrasformers are not perfect either, their leakage capacitance cannot be perfectly balanced.
Perfect is the word = impossible practically.
