I'd call that balanced, strictly speaking. It's the impedances that decide whether it's balanced or not (and going to reap the benefits), whether or not you've got signal on it. It's also an invaluable mod for audio sources. I much prefer it with signal on the cold, though, and it's not going to cut it if you need distortion cancellation, which could happen as late as the speakers. But anything that gets rid of the hateful RCA phono plug is good in my book. It's an appalling bit of cheapskatery that has done the hi fi industry decades of harm.In my experience in the mixer business (decades ago) "impedance balanced" was the terminology used to describe that very specific topology where you add one equal value resistor in series with the - output lead connected to ground to convert an unbalanced output to (cough) balanced.
There have been voluminous discussions about the different flavors of "balanced" right here. I recall years ago digging out my copy of the IEEE S-100 standards dictionary. That S-100 search was not very rewarding (I expect a search here could reveal some of our past discourse on this topic).
JR
yes the double cap is good idea if one is looking for exactly the same impedance to ground, still, I think it is one of the things you can get away with if necessary. I always felt like it was a complete waste to get a high-valued expensive electrolytic just to parallel the resistor to ground, but it does work.+1. Some also advocate replicating any coupling cap'. I've done both options on DIY bits. Doubtful wrt cap tolerance matching but may be close if from same batch? And then only one sees any signal voltage and that should be small. Maybe overthinking it
I don't know if it beats a $1,500 pre, but it is simply an INA217. I used to give that as a project to my students back when I taught at an audio school. They didn't find assembling it as enjoyable as I thought they would. Also, as far as I've seen, the INA217 is no longer available in DIP8, but who cares? I prefer the THAT 1510 anyway.
Those are not really op-amps in the INA217 or the THAT1510, they are complete integrated preamps, you can consider them Instrumentation Amplifiers (actually that is what the INA in the INA217 stands for: INstrumentation Amplifier).
No, it is not just 3 op-amps inside or whatever, everything inside is in integrated form at the transistor level and I am sure there are at least 70-100 of them, it has interconnections, integrated resistors and so on, the op-amps are also not discrete, but integrated at the transistor level ; we don't say that a Microprocessor is 3 billion transistors or whatever. It can't be replaced with just three op-amp's. You can make a discrete version of it, but it will not be exactly the same.
So you know there are actually not just 3 op-amps in it as the diagram shows. Anyway, the point is that these are not just parts that can be swapped with op-amps as you say. Or am I missing something that you are trying to convey? perhaps you can be clearer.
Yes, it would ground the signal. Why do I prefer it? In short, it gave me the biggest single audio improvement in an amplifier I have ever experienced. And it was a direct A/B. About 15 years ago I was working on a small, single supply chip amp which used a LM3875 (now discontinued) in a differential topology. The preamp part was 4 op amps whose functions were, Differential Input, Volume Control, LP filter and Output, and then Inverter (and output). Though there was always a place for the 4th op amp, when I built the prototype I omitted the 4th amp as I already had far more gain than I needed and just used a second 27R resistor. This seemed to be confirmed as a good decision when the volume control (which I wouldn't do the same way now) ended up with a lovely taper that felt very classy. (It was a linear taper pot with a resistor in parallel to mimic a log taper and it seemed I had chosen the right values.) Anyway, it sat that way for about 18 months until one night I decided on a whim to put the final opamp in, to squeeze out the last bit of S/N. As I have said, the difference was quite stunning.
Having a signal on the Cold could improve the probablity of success. I'd have to think this through more carefully, but maybe if the inversion happens in the second half of a dual op amp, you've virtually guaranteed it.
Yes, it would ground the signal. Why do I prefer it? In short, it gave me the biggest single audio improvement in an amplifier I have ever experienced. And it was a direct A/B. About 15 years ago I was working on a small, single supply chip amp which used a LM3875 (now discontinued) in a differential topology. The preamp part was 4 op amps whose functions were, Differential Input, Volume Control, LP filter and Output, and then Inverter (and output). Though there was always a place for the 4th op amp, when I built the prototype I omitted the 4th amp as I already had far more gain than I needed and just used a second 27R resistor. This seemed to be confirmed as a good decision when the volume control (which I wouldn't do the same way now) ended up with a lovely taper that felt very classy. (It was a linear taper pot with a resistor in parallel to mimic a log taper and it seemed I had chosen the right values.) Anyway, it sat that way for about 18 months until one night I decided on a whim to put the final opamp in, to squeeze out the last bit of S/N. As I have said, the difference was quite stunning.
I don't think the improvement could have been just S/N, though 6dB is substantial. There could have been some vagaries in the chip but most likely is that it overcame some deficiency in the grounding scheme. That alone is a good reason to do it, especially as grounding and layout must be at the top of the list of perfectly good circuits failing to perform sonically. (Or, if you like, failing to meet distortion specs.) On this particular amp, where pre and power shared an SMPS, though each part had its own regulation inside the amp, a pointer might be that I remember having problems getting the sometimes very expensive pre amp regulators to perform to spec. So it's not just about CMR, but getting the right reference to its destination, which I suspect I failed to do across just 5" of wire!
So, that's why I prefer it. Mainly my personal experience. But some of the stuff I'm working on at the moment actually needs a signal as the two halves have separate paths right through the box and don't meet anywhere to do their CMR thing or any distortion cancellation. Besides, who in audio electronics doesn't love symmetry?
Wouldn't use what circuit type? You asked why I preferred signal on the cold leg. Those were my reasons, though there are more reasons than just those, especially with the possibility of distortion components cancelling in the loudspeakers. And obviously some of those distortions are amplitude related so you need a signal for that,Not making a lot of sense to me tbh. Unless you are saying you prefer conventionally balanced / symmetrical outputs. In which case you wouldn't use this circuit type
I would strongly advise to include at least a capacitor to block DC. As I mentioned previously, my students built many of these, a student built one with no caps and claimed to have fried one of the inputs of his DAW. Of course, one can't be sure this was the real reason, or whether a secondary issue in the preamp was the cause. But I still would recommend to include a cap just in case.
Also, I would advice to use at least 50 V rated caps, it is not uncommon that people plug the outputs of these preamps to prosumer audio interfaces, which have the same input for line and mic signals. If for some reason phantom power is engaged, a cap rated for less than 50 V will snap. Which is yet another reason why it is wise to add a cap at the output of the pre. In that case, it is also a good idea to either use bipolar or two series electrolytics back-to-back.
Excellent points. It is sadly true that today we live in a rfi cesspool what with wifi, GSM, and bluetooth by the truckload. Back in the 70s the worst you had to contend with was a dodgy taxi parked outside the studio.
