Not sure of that. Perhaps, since it is done without input transformer
there is ground reference for instrumentation amp that is usually
connected with higher value resistor (and small cap) to a chassis.
Signal is still differential and I cannot understand why would it reduce
effect that cable brings. Cable capacitance, inductance and dielectric
absorption are still there. Do you think that extremely dirty copper
would sound wonderful in balanced cable?
The way balanced line eliminates cable artifacts is twofold. First, ground is ignored, so the shield is not part of the sound (nor the is noise to which its exposed; in a single-ended system the shield is part of the signal path).
Second, the source impedance is low and so is able to swamp the inductance and capacitance of the cable.
In fact in our tests and auditions we have frequently used an aged (60 year old) microphone cable that used rubber dielectrics and tin-plated multi-stranded copper, with obvious corrosion on the shield. When driven by a balanced passive control (which did not meet AES file 48) the cable literally sounded broken compared to other cables (Purist, Kimber, Audioquest and also Mogami)- broken enough that you wondered what the heck was wrong with the stereo. When we then ran the same set of cables with a source that supported file 48, the differences between the cables, including the one with the 'dirty copper' vanished.
IOW, what the technology is **for** is to eliminate a problem that audiophiles are well aware of- which is that cables sound different and often not as musical as one would hope. Put another way, if you can hear a difference between cables in an audition, it means that **both** cables are wrong, as everyone knows full well that next year someone (likely the manufacturer of the better cable) will have a newer cable that sounds amazingly better. This has been going on decades (one would think an audiphile would just like to finally get to where that is all going and not have to search for the holy grail anymore), yet somehow recordings made in the 1950s (using balanced lines) just sound better and better as the playback equipment improves...
It does not make sense. I understand how even harmonics are eliminated
but don't know of any mechanism that would remove odd harmonics in fully
balanced amp. Remember we're talking about Fully balanced amps in
general - not only your designs.
I get that it does not make sense for you. But that does not mean it does not make sense. The idea here is to remove distortion sources (now this is strictly my opinion). If you can't use feedback to get rid of distortion, how do you get rid of it? Eliminate distortion sources! A common complaint about tubes: 2nd harmonic (ask any solid state guy). OK- fully balanced differential design gets rid of the even orders. Now we are left with the odd orders. To reduce them, we set bias points in the voltage amplifier such that it cancels the odd orders. Then design the circuit to use as few stages of gain as possible (in our amps there is only one stage of gain, making them a simpler signal path than an SET). Use triodes throughout. Get rid of the output transformer (which may or may not add distortion). Take care to avoid obvious diode issues (proper metallurgy) in component selection. Stuff like that.
Kijanki, you have made one of the most reasonable cases for the sonic
advantages of the single ended circuit and its simpler design. This
positive perspective is rarely seen. Well done gentlemen!
For the most part I agree, however just for the record as I pointed out above, our amps have only one gain stage with no phase inverter circuit. They are a simpler signal path than most SETs. This should be possible with any **fully** differential circuit- it does not have to be an OTL.
BTW despite what Ralph contends phono cartridges are NOT balanced as
there is no ground, no ground pin connection, only 2 lines out.
This statement has long been debunked. Herman, take this from someone who has apparently made a career of balanced line operation, who also understands single-ended (I've built many SE amps; type 45, 2A3 and 300b):
Any source that employs a magnetic pickup, like a phono cartridge, dynamic microphone or tape head, is an inherently balanced source. The proof of this is simple: reverse the connections to the device and all that happens is you reversed the phase. If it really was a single-ended source, reversing the connections from it would get you a whole lot of hum and buzz!
The reason is simple- all of those sources are simply a winding.
Now I think I see where the disconnect is happening: you probably think that balanced line requires three connections, and that somehow something like a cartridge would too. It does not work that way. If you refer back to my initial post in this thread, you will see that a balanced line **ignores ground** (and so uses two connections, not three, as ground is only used for shielding).
To put that in perspective, let's take the example of a transformer driving a balanced line. I suspect you think that such a transformer would use a center tap for the ground, but that is not the case. The reason is simple: you want the best Common Mode Rejection Ratio you can get. If you use a center tap of a transformer, the CMRR will be degraded because the center tap can never be placed perfectly in the center of the winding). Instead the winding of the transformer floats- its output (or input) is merely tied to pins 2 and 3 of the XLR and nothing else. No ground connection- that is only for shielding and the chassis and is not tied to the transformer, cartridge, tape head or dynamic microphone.
So only two connections are needed, and in many cases you can operate a balanced line without the shield (sometimes its actually lower noise this way, particularly in shorter runs of less than 3 feet). IOW you don't need three connections to be balanced.
One thing here I like to point out:
Running balanced lines is an entirely different matter from having an amp or preamp that has a fully balanced differential circuit! By this I mean that you can have a single-ended circuit but run balanced lines outside the box and retain all the advantages of balanced line as it pertains to cables (a good example of this is the venerable Ampex 351 tape electronics, which is almost entirely single-ended, but runs balanced line inputs and outputs; many fabulous recordings exist in many audiophile's libraries that were recorded on those electronics).
IOW its important to not conflate the two.
Now as it happens, there are advantages to running a fully balanced circuit. I think that one of them is the simplified signal path. For example in our preamps we have three stages of gain from low output moving coil input to the line stage output (which can drive headphones). In the amps we have one stage of gain. That's about as simple as a signal path can get!
I too prefer higher efficiency loudspeakers with an amplifier of simple design and signal path. So that is one thing that Herman and I have in common.
