Balanced vs RCA preamps

atmasphere

It is a fact in high end audio though that most producers of balanced line products pay no attention to AES48 (the balanced line standard) ... That is part of the reason there is a balanced vs RCA debate.

Not really, the benefits of balanced designs are pretty well known. Designing to the AES spec is just one approach to balanced design.

...the Topping DACs; The 40 Ohm value is actually the two 20 Ohm output impedances put in series when really they should be in parallel, which would be about 10 Ohms. WRT to the ARC stuff they are doing the same thing.

The Topping and Audio Research schemes are different than your designs. That doesn’t make your design right or "proper" any more than it makes the ARC design "improper," as you’ve claimed. They each take a different approach to balanced amplification, either method offers improved CMRR. (Similarly, some speaker designers use sealed boxes, some use ported boxes and some use no boxes at all. Each approach can be valid and its success will depend on implementation.)

The standard for balanced line studio line level input impedance is 600 Ohms ... Our MP-1 preamp can drive 600 Ohms directly ... Most tube preamps will fall flat on their face trying to drive loads like that. In fact most solid state RCA preamps will too.

Most home users have no need to drive 600 ohm loads.

You have to be careful about things you read like the specs above! I own the two of the Topping DACs; The 40 Ohm value is actually the two 20 Ohm output impedances put in series when really they should be in parallel, which would be about 10 Ohms.

@atmasphere  One should be very careful when interpreting the impedance of a true balanced XLR cable. The two conductors (pin 2 and pin 3) carry both positive and negative signals from the source to the load simultaneously, behaving like a parallel connection. However, because they are of opposite polarity, the signals travel in differential (v.s. common) mode. The resulting voltage is the so-called differential voltage, which is twice the amount of the individual phase voltage.  This explains why the voltage from an XLR port is twice that of its RCA counterpart.

Similarly, the resulting impedance, known as differential impedance, is also twice the impedance per phase.  The manufactural reports either impedance per phase or the differential impedance.  Hope this helps.

The resulting voltage is the so-called differential voltage, which is twice the amount of the individual phase voltage. This explains why the voltage from an XLR port is twice that of its RCA counterpart.

@lanx0003 Actually this isn’t true if the source is AES48 compliant.

An output transformer is a good way to look at this if you understand how they work. In the case of a an output transformer driving a balanced line, there is a simple secondary with no taps. One end of the secondary output is tied to pin 2, the other end to pin 3. Pin 1 is typically tied to chassis.

Now if you want to run this output single-ended, pin 3 is tied to pin 1. You’ll note that the actual output Voltage is unchanged as is the output impedance. IOW either way the Voltage and impedance of the source is the same, which is to say there’s no 6dB increase when running balanced as opposed to single-ended. The transformer doesn’t care if one side is grounded or not. Your conclusion in the quote above is false WRT AES48 compliant sources, such as our MP-1 or MP-3.

Now if you have a non-compliant source (such as a Topping DAC or ARC preamp) then your comment is true.

But that isn’t how balanced lines are supposed to work. Here is a link to a Neumann microphone. Note the output impedance and the load its expected to drive (1KOhm). This mic is set up to drive balanced lines properly. A device that behaves as you described above does not.

Put another way, the idea that the output impedance of a balanced source is twice as high is true only if the balanced source isn’t designed properly to drive balanced lines!

To understand this issue better take a look at this page on the Rane website- its a quick read. If you scroll down to the ’Absolute Best Right Way To Do It’ you’ll see I’m not making this up. You might also read portions of this book by Bill Whitlock who designed most of the Jensen transformers. It gets more relevant to this discussion on page 13; if you look at the diagrams which show balanced connections, you’ll see no connection to ground except when it goes single-ended (such as driving an opamp in the case of an input transformer). Page 15 shows a microphone input. Page 16 shows a line output. I think you’ll see what I’m talking about.

atmasphere

... the idea that the output impedance of a balanced source is twice as high is true only if the balanced source isn’t designed properly to drive balanced lines!  ..

That is your opinion, and you're certainly entitled to it. You express it here frequently, as is your right.

But it is just as opinion, and one which many of the world's foremost manufacturers of audio equipment reject. That appears to trouble you greatly, perhaps because you seek to promote your trademarked and patented Balanced Differential Design® components. And that's fine, too. But when you claim that those whose circuit topologies don't embrace your favored methods somehow aren't "proper," you really make yourself look kinda silly, imo.

I have a lot of respect for you Ralph, as do many others on this forum. But for just about everything in life, there simply is no One Absolutely Right Way, and that includes the design of balanced circuits.

I'm telling you this Ralph because I think you do yourself a disservice by denigrating the designs of others.