Polarity Reversal on LDRxB Balanced LDR Preamp

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tortugaranger

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Polarity Reversal on LDRxB Balanced LDR Preamp
« on: 14 Jun 2015, 06:31 pm »
I've been discussing the topic of phase reversal with several customers where the customers have been educating me on the topic and not the other way around. For high resolution systems, getting the correct phase can make a difference. Without getting into the weeds on this, the idea behind phase  reversal is the phase of playback on some tracks may be opposite of the way it was recorded. Especially on something percussive this may result in the speaker pulling in rather than pushing out at the start of the note or sound. Unless you have the ability to  reverse the phase during playback it's not obvious that this is even an issue. When the phase is correct the music sounds right and when it's reversed it can sound a bit off.

With balanced audio phase reversal can be accomplished by swapping the 2 out-of-phase balanced signals (pins 2 & 3). We are working on a way of doing this with our balanced LDR preamps. By wiring at least one input to two different switchable input channels you can listen to that one input in both phase states. A simple press of a button on the remote would allow you switch between phase states.

Doing this with singled ended inputs gets way more complicated since you need additional hardware (transformers or op amps) to reverse the phase.

Implementing phase reversal on our balanced LDR preamps is a matter of some additional wiring, some firmware modifications and coming up with a remote key assigned to the switching function. We are pursuing this and hope to test this very soon. This may be the final straw that kicks off a new custom remote since we would be asking yet even more from the simple Apple Remote which has already gotten a bit too complicated for all the features.

More on this once we've done some testing.

Regards,
Morten
« Last Edit: 24 Aug 2016, 05:18 pm by tortugaranger »

tortugaranger

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Polarity Reversal on LDRxB Balanced LDR Preamp
« Reply #1 on: 24 Aug 2016, 05:18 pm »
I'm pleased to report that we've successfully implemented a polarity reversal feature in our LDRxB Balanced Passive Preamp. This is something we've been working on as a side project for quite some time in close cooperation with kernelbob. While we are still tweaking how best to visually convey the reversal state, the firmware and hardware modifications have been resolved.

Polarity reversal can be provided for XLR inputs #1 and #2 only. It's not available on input #3 or the single unbalanced input.

The stock LDRxB needs to be modified to implement this feature. This involves replacement of the stock input switching modules, additional hookup wiring, and a firmware update. Reversing the polarity is a simple matter of pressing the Center button on the Apple remote when running on input #1 or input #2.

We are still finalizing the pricing for this and will be adding it as both an option for new builds and as a modification of existing units.

I admit to having been ignorant of polarity reversal. I've since learned a thing or two about it and am convinced it can make a difference. It's conceptually fairly simple. When, let's say, a bass drum kick is recorded, that initial sonic impulse comes from the drum head pushing air out into the room and into the microphone. Thus, when you're listening to said drum kick on your stereo, you'd expect your speakers to perform that same initial impulse push out into room. Said differently when the bass drum head goes out, your speaker driver goes out. More often than you'd think, this gets reversed in the course of recording, mastering and playback. Polarity reversal corrects this. How do you know if the polarity is reversed? You don't really. What you do is listen to the music both ways. Chances are, when polarity is aligned, the dynamics of music will sound better.

I've been receiving numerous inquiries by interested parties about this who are adamant that the benefits of polarity reversal are more tangible than one would imagine. An interesting read on the topic of polarity reversal can be found via this link:  http://www.positive-feedback.com/Issue1/cjwoodeffect.htm

Here is a relevant quote from the above reference article:
Quote
Polarity, in its pure state, as with one musical instrument recorded
on one channel (designated. a monaural phase effect, or MPE), is
perfectly audible, and provably so. Reproduced incorrectly, it becomes
what I call "the muffling distortion," because our ears detect an
inverted leading transient and suppress the impulse response. Thus the
pluck on a guitar string, so striking in real life, becomes dull,
muted, and inexpressive. A musician might say, "You can’t hear the
fingers, man."

kernelbob

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Re: Polarity Reversal on LDRxB Balanced LDR Preamp
« Reply #2 on: 24 Aug 2016, 11:35 pm »
I have the development LDRxB-v2 with Morten's implementation of the absolute polarity/phase reversal option.  I can tell you that sonically, the addition of that option introduces absolutely no sonic degradation.  I've owned or auditioned various components that offer the feature, but with mixed success.  The best implementations have exactly the same signal path with the inverted as with the non-inverted setting.

Successful implementations include DACs where the normal/inverted setting for a DAC is implemented in software.  So the alternate settings have the same signal path.  I've heard preamps with the feature, but the implementation is usually via a switch and sometimes even with one of the alternate settings going through additional wiring, even through an additional stage.  My experience has been that the simpler path always sounds better, but not because it is the correct phase/polarity setting, but because there are fewer components in that path than the alternate path.  To me, it's also mandatory that the selection between the two settings needs to be able to be performed from a remote at the listening position.  Having to walk over to the DAC and switching the setting and then returning to the listening position to evaluate the difference gets old... really fast.

Also, if you rely on using a DAC with the feature you may be all set, but if you use a turntable or another source that doesn't go through the DAC, you're out of luck.

Morten has managed to add this feature without increasing the complexity of the signal path and without having to introduce the dreaded switches that the LDRxB (and the other Tortuga controllers) have completely eliminated.  It works with any type of input whether it is a DAC, turntable, or tape.  Quite a nifty piece of engineering.

Currently in my LDRxB, the normal/reversed setting is indicated by a single pixel at the lower right corner of the right channel level display.  The only problem I have is that my listening position is about 18 feet from the Tortuga and it is almost impossible to see if the pixel is off (normal) or on (reversed).  It isn't all that big of a deal since I'm not obsessive enough to go through my recordings and identify the correct settings for each.  Besides if you're listening to a streaming source, a movie on TV, etc., then you have to choose the correct setting on the fly.  Easy enough to do since the change between the alternate settings is easy to hear and is consistent across recordings (the difference that is, not the phase/polarity setting).  This post is getting long enough already, so I'll save the description of the differences for a follow up.

As Morten indicated, the only lingering issue, at least for me, is to determine if there is a way to more obviously indicate whether the Tortuga is set for normal or reversed without mangling the simple elegance of the Tortuga display.  We've had several discussions (thanks Morten for your patience), which always seem to come down to a neat way to indicate the difference versus the complexity to program it into the system. Whatever becomes the final implementation, I suggest keeping that single pixel normal/reversed indicator in addition to any other indicator since some of the display options would allow the user to define a display option opposite from another user (e.g dimming a bit the left or right LED brightness level).

Again, thanks Morten for a sonically flawless implementation of this feature.

Best,
Robert

craig sawyers

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Re: Polarity Reversal on LDRxB Balanced LDR Preamp
« Reply #3 on: 27 Aug 2016, 03:38 pm »
The phase inversion subject is very interesting.  I'm lucky enough to have two Stax E/S headphones (a 1980's Lambda, and a three year old SR007), a clone of the outrageous SRM-T2, and a DAC that does phase inversion via a front panel switch. The headphones, or "ear speakers" as Stax call them, have a common feature with all electrostatic transducers - exceptional transient response and very low distortion http://www.innerfidelity.com/images/StaxSR007.pdf .

I actually also have an old Koss ESP9 on another system, but I have not done a detailed phase inversion comparison with that.

Using that Stax set up, I have to admit that I cannot hear a phase reversal, regardless of music type.

There had to be a however, and this is it.  With a dynamic loudspeaker system using conventional cones in a room, phase reversal (at least to my ears) is clearly audible. A possible conclusion is that it is more to do with loudspeaker, or loudspeaker/room interaction that the ear/brain sensitivity to absolute phase. My previous speakers were actually also electrostatic (Quad ESL57's) but, darn it, I did not do a phase audibility measurement while I had them.

Given that my main system uses dynamic, open baffle loudspeakers, I'm really interested in a phase reversal option for my balanced Tortuga.

tortugaranger

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Re: Polarity Reversal on LDRxB Balanced LDR Preamp
« Reply #4 on: 28 Aug 2016, 12:48 pm »
Given that my main system uses dynamic, open baffle loudspeakers, I'm really interested in a phase reversal option for my balanced Tortuga.

To implement polarity reversal you need some way to swap the 2 balanced signals in each channel. We're able to do this with 2 different balanced inputs in our LDRxB model because it has 2 unused left/right unbalanced inputs in the input switching boards that are designed to handle up to 6 inputs whereas the stock design only uses 4 of these 6.

With our DIY components or kits the input boards only handle 3 inputs. You would have to designate 2 of the 3 to a single input (say input 1) and wire that input both ways, let's call it 1A and 1B. That would leave 1 remaining unbalanced input but without the reversal. 

kernelbob

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Re: Polarity Reversal on LDRxB Balanced LDR Preamp
« Reply #5 on: 28 Aug 2016, 01:43 pm »
Following up with the differences between the alternate polarity settings that I hear in my system.

There's no way of knowing whether the "normal" or the "inverted" setting in a playback system corresponds to the original performance's polarity.  There are many components in the recording and reproduction chains.  So, I'll refer to the differences that I hear between the alternate settings as Left and Right.  The easiest and fastest way to identify which is which in your system for any particular recording is the switch the polarity setting using your remote from the listening postion.  I consistently identify a set of characteristics of the alternate phase/polarity settings that span recordings, labels, and genre as follows.

1) Bass.  in the Right setting, the bass has better weight.  It also extends more deeply-- this is more evident on some labels than others since some labels just don't have much really deep bass.  Hall ambiance is well defined and more distinct.  The bass is tighter and clearer.  Pianos have more weight and power.  Orchestral recordings are especially easy to distinguish the differences between Left and Right.

2) Lower midrange.  Bass harmonics are richer.  The tenor voice of a piano has a better balance with the Right setting and can even go missing with the Left setting.  Transients are more integrated.  Check out the sound of plucked bass in jazz recordings.

3) Midrange to upper midrange.  There's an odd quality that appears in the Left setting.  The middle to upper midrange starts to sound a bit to forward relative to the rest of the frequency range.  Also in the Left setting, the upper midrange starts to take on a degree of hardness.  Both of these qualities are corrected in the Right setting.

4) Treble.  With the Right setting, all the improvements in the bass to upper midrange are combined with a more balanced treble.  Much of the treble information are the harmonics of fundamentals in the bass and midrange.  So getting the bass and midrange right allows the treble to sit comfortably on top.

One point to keep in mind.  Our hearing isn't symmetrical when it comes to the compression and rarefaction components of a sound wave.  It's actually a bit more sensitive to rarefaction.

Best,
Robert


ArchOlson

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Re: Polarity Reversal on LDRxB Balanced LDR Preamp
« Reply #6 on: 16 Sep 2016, 02:00 pm »
Hi...i am a new user here. In my case currently in my LDRxB, the normal/reversed setting is indicated by a single pixel at the lower right corner of the right channel level display.  The only problem I have is that my listening position is about 18 feet from the Tortuga and it is almost impossible to see if the pixel is off or on .  It isn't all that big of a deal since I'm not obsessive enough to go through my recordings and identify the correct settings for each.  Besides if you're listening to a streaming source, a movie on TV, etc., then you have to choose the correct setting on the fly.  Easy enough to do since the change between the alternate settings is easy to hear and is consistent across recordings. This post is getting long enough already, so I'll save the description of the differences for a follow up.

pcba
« Last Edit: 7 Dec 2016, 11:15 pm by ArchOlson »

tortugaranger

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Re: Polarity Reversal on LDRxB Balanced LDR Preamp
« Reply #7 on: 12 Dec 2016, 03:41 pm »
Hi...i am a new user here. In my case currently in my LDRxB, the normal/reversed setting is indicated by a single pixel at the lower right corner of the right channel level display.  The only problem I have is that my listening position is about 18 feet from the Tortuga and it is almost impossible to see if the pixel is off or on .  It isn't all that big of a deal since I'm not obsessive enough to go through my recordings and identify the correct settings for each.  Besides if you're listening to a streaming source, a movie on TV, etc., then you have to choose the correct setting on the fly.  Easy enough to do since the change between the alternate settings is easy to hear and is consistent across recordings. This post is getting long enough already, so I'll save the description of the differences for a follow up.

pcba

I just now saw this post so apologies to the very late response. Yes, the pixel (decimal point) reverse indicator is woefully insufficient from a distance. An alternative and less ambiguous approach would be to turn off the left display altogether when reversed. There are limited options here with only 7 segment displays but changing the left display in some way is one of them. Open to suggestions.

craig sawyers

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Re: Polarity Reversal on LDRxB Balanced LDR Preamp
« Reply #8 on: 16 Dec 2016, 11:49 pm »
Since they are 7-segment, would it not be possible to change one display (left or right) to something non-numeric?

ohlins

Re: Polarity Reversal on LDRxB Balanced LDR Preamp
« Reply #9 on: 18 Dec 2016, 09:50 am »
How about using a mirror image reflection of the volume display .... 33 will look like EE