Cons of using main XLR/Balanced outs for subwoofer?

Within very generous tolerances, humans are insensitive to phase shifts. He also points out that in a normal room sound is reflected many times from objects at multiple distances resulting in enormous amounts of phase shifts that dwarf the far smaller phase shifts in amplifiers (a lot of this read like Duke's take on this)

This is entirely correct; you can't sense phase at all if you're talking about a single frequency like a sine wave.


But leaving sound stage information aside (which might be messed with due to reflections) phase shift over a spectrum is perceived as a **coloration**, for example a rolloff above the audio band can be heard as a darkness in the highs if the rolloff is close enough to the audio band. In the bass its a similar phenomena, if rolling off prematurely (above 2Hz) even though your speakers are flat to 20 Hz it will be perceived as a lack of impact.


Because speakers are mechanical devices, a rolloff in them does not cause this problem.


I first became aware of how profoundly this was true a long time ago when trying to find out why an MFA Magus preamp sounded bright in the phono section. A dealer of mine wanted to sell the preamp instead of a competitor's but the brightness made it as bad as poor digitial of the day. I investigated the circuit and found that at 50KHz the circuit went from the RIAA equalization curve to flat. I removed the components responsible and the brightness was eliminated, with no sense of darkness- LPs sounded just fine after that. Now you'd think that at 50KHz there wouldn't be an issue, but phase shift can extend down to about 10th the frequency where this was happening (5KHz) so its easy to see how this was heard as a brightness. This principle is easy to demonstrate. 


I can put this another way: there is a **reason** that Jensen feels they needed to go so low (1Hz) with their subwoofer transformers. Jensen transformers are extremely well designed, are high quality, and the designer is held in high regard in home and professional audio. Anecdotally, I've tried using regular full range audio transformers made by Jensen for subwoofers and while they work OK, you can easily hear that something isn't right about the bottom octave- which is fixed using the Jensen subwoofer transformer. Preventing phase shift is is also why Stuart Hegeman of Harmon Kardon Citation fame insisted on wide bandwidth in his designs.


Since you already have the transformers I would go ahead and use them. But I would also get the subwoofer transformers and then audition them side by side- see what you think.

Duke always says the ear is very forgiving of small timing errors in the low frequencies such that the speakers + room = a minimum phase system. So I am unclear how a phase shift in the low (< 20Hz) bass is consequential. OTOH he also notes that the ear has exaggerated sensitivity to frequency response errors (non-flat curve below 20Hz) in the bass region, so maybe that is what you are trying to tell me.

Duke is right; phase shift is different from delay, and 'yes' in that order :)

@milarep Yes to both questions.
The 1Hz response will insure that you have no phase shift at all at 20Hz or even 16Hz. Phase shift in the bass will cause a lack of impact. The transformer rolling off at 10Hz will exhibit phase shift up to about 100Hz.

I'd use the Jensen transformers if I were you.
The problem you can run into is that of ground loops when interfacing single-ended and balanced equipment. The transformers sort that out by providing ground isolation. They won't rob you of any impact as they go down to nearly 1 Hz.

Question/clarification...since the ISO-Max transformer has a single ended input and a single ended output, are you using something like this Neutrik adapter to go from preamp balanced out to transformer rca input?

No.

The Jensen ISO-MAX can be supplied how you need it- for example with a balanced input (that is how mine are set up) and a single-ended output. That is specified when you order it.

two of these op-amps are to buffer the signal to the tape output and level control, the remaining four for the two balanced outputs. These integrated circuits are specified as delivering high output with low noise. "
Could we deduct something from these facts?

@c

Yes. The tape outs and volume control have nothing to do with this. From the statement I would also deduce that the RCAs are in parallel with the balanced outputs, so if you load both even though the preamp is perfectly capable of driving the load, you will probably notice an artifact and more noise could easily be it.

Will that configuration really make a big difference?

@rlb61 Did you notice a difference in the mains between not having the sub hooked up and all, and having it connected but its volume turned all the way down? If there is no difference then your RCA output is buffered from the balanced out and the ISO MAX isn't needed. If there is a difference then this is the solution.

You would run dual balanced outputs, one set to your main power amp. The other set would go to the ISO MAX units (left and right channels). Only at that point would you use RCA connections, between the ISO MAX units and your sub. So the ISO MAX units would be right by your sub so you could run a longer balanced line connection from the preamp to the ISO MAX. If you don't have dual outputs on the preamp, you would use some form of a Y adapter to make it happen. 

@cardani  If the RCA outputs are in parallel with the balanced outputs on your preamp then what you describe makes sense. If it were me I'd be using the Jensen ISO MAX subwoofer transformers to do the conversion and not use the RCA outputs of the preamp at all.

@zdw11 @imhififan  has it right. Nice setup!

@zdw11  Unless the preamp has entirely separate circuitry driving the RCA and XLR outputs, when driving both you will have a lower impedance load on one of the XLR signal pins (pin 2 or 3) than you do on the other. The preamp may make less voltage on the pin that has the lower impedance load, but at any rate the total impedance to ground will be lower on that side even if the output voltage is unaffected.

This will in turn reduce the Common Mode Rejection Ratio (CMRR) that is available to the main amplifier, which will raise the noise floor and possibly also the distortion. 

You won't hurt anything. But I think you'll find it simply sounds better using the ISO-MAX devices- that is exactly what they are for.

FWIW we were the first anywhere to make a balanced preamp for home use. One of the problems we encountered early on was subwoofers with single-ended inputs. Because our gear supports the balanced standard a.k.a. AES48, if someone tried to connect the subs in the manner you describe a buzz and humm would result. Balanced and single-ended operations are mutually incompatible; its either one or the other and never both. So if you have a balanced output on the preamp and a single-ended input for the subs, that ISO-MAX Jensen box is the elegant solution. They only started making them in the last 20 years or so.

Is there any harm in using the main XLR output to send signal to the subwoofers, and using the 1, or 2 RCA output to send signal to the main drivers?

Yes. You'll unbalance the signal going to your main amps.

The solution is to use a set of specialized subwoofer transformers made by Jensen, which are optimized for subwoofer operation (and so go down to 1Hz to prevent phase shift or any hint of a low frequency rolloff). They can be used to convert from balanced to single-ended. I use a set in my system for this purpose- MP-1 preamp has dual balanced outputs so one set goes to the main amplifier and the other set to the Jensen ISO-MAX boxes. From there I run a very short set of RCA connections that are only about a foot long to the subwoofer amplifier. Link below:


https://www.jensen-transformers.com/product/sub-1rr/