Getting the full SDA Effect from Polk L800 Speakers

Matthew J Poes

Matthew J Poes

Audioholic Chief
Staff member
I know about the XPR-1, of course. Also, that was one of Gene's better reviews, which we all miss. I currently use an ATI AT3000 amplifier, which is dual-differential from input to output, as opposed to the follow-on generation AT6000, which is single differential at the input stages and dual differential in the output stages. Neither one seems to use a floating ground, as the output offset voltage is so low. As we once discussed with the McIntosh "quad balanced" amplifiers, I'm not sure all these multiple levels of differential circuitry really sounds so much better compared to more typical Class AB designs, but it certainly makes better advertising copy and, sometimes, better measurements.
While the differential output can reduce even order distortion, I actually think the primary advantage is twice the output for half the rail voltage typically needed. The rail voltages start to get so high that the power supply caps and amplifier designs lead to reliability problems and parts cost problems at such high output levels. A bunch of 100v 10,000uf caps is a lot cheaper than a bunch of 200v 10,000uf caps, often by more than a factor of two since one is a commonly used and needed part, the other is not. It's practical.
 
Matthew J Poes

Matthew J Poes

Audioholic Chief
Staff member
I did not say you missed anything or wrong, just pointed out the fact that, as Wiki stated and I quoted in my post#48. I supposed I could have used clearer wording by saying it is better not to referred to (instead of "considered" push-pull as single ended, in order not to confused it with the single ended amps are not are not of the push-pull type, i.e. the same output device, or groups of output devices take care of the full cycle.

To me, this is more about terminologies than substance. I think we can all agree with Irv's point that at the end the load is just a speaker, so differential, balanced, or whatever you want to call it, really makes no difference. If Polk Audio says it does, then there must be something I missed, would love to know what it is.

By the way, the SDAs are passive speakers right? Just in case, if they are active, then I suppose differential vs unbalance single ended could make a difference.
There seems to still be some confusion. A differential output amplifier has half the output level on each phase of the amp terminals. The SDA circuit draws from the positive phase leg. That means the signal is reduced by around 6dB. This is why it matters. It's not audiophile tomfoolery, its a real issue. This wouldn't matter with any other speaker. It matters with the Polk L800's simply because of how the SDA crossover is built.

The really tough thing about this is that the L800's were a really unusual speaker for Polk that wanted high output amplifiers and are great for 2-channel setups. For someone who is into a Sound United setup, it would be natural to go to Marantz for amplification. But the majority of integrated amps now use Hypex or Purifi modules, and so have differential outputs which are not as compatible with the Polks. Polk didn't like me claiming it was not compatible and preferred I say that it doesn't work as optimally with differential amps. Their point is that the SDA effect still works, but the XTC is reduced by 6dB. James and I found it very audible so I think its fair to say its not compatible. We found the benefit of SDA to be nearly inaudible with the Cherry amplifier.
 
Matthew J Poes

Matthew J Poes

Audioholic Chief
Staff member
I am glad you guys could resolve your dispute in a civilized way, which is very much unlike how Matthew and I settled our disagreements. It always escalated into physical violence between Matthew and myself. In those clashes, I was always partial to clubbing weapons whereas Matthew was prone to stabbing and cutting weapons. I suppose our choice of weapons was not just an expression of personality but also symbolic of our philosophical differences. You might ask who won? Well, Matthew did flee to Florida, so I would take that for a kind of answer.
Yeah because I want to get into a knife fight with @gene now!:eek:
 
Matthew J Poes

Matthew J Poes

Audioholic Chief
Staff member
I know about the XPR-1, of course. Also, that was one of Gene's better reviews, which we all miss. I currently use an ATI AT3000 amplifier, which is dual-differential from input to output, as opposed to the follow-on generation AT6000, which is single differential at the input stages and dual differential in the output stages. Neither one seems to use a floating ground, as the output offset voltage is so low. As we once discussed with the McIntosh "quad balanced" amplifiers, I'm not sure all these multiple levels of differential circuitry really sounds so much better compared to more typical Class AB designs, but it certainly makes better advertising copy and, sometimes, better measurements.
Are you saying the ATI AT3000 negative terminal is tied to ground? I'm fairly certain the XPR-1's negative terminal is not tied to ground.

When I said that these pushpull vs single ended amps typically have single ended outputs, I did not mean that no amplifier has a differential output. As I shared various times, there exists a subset of amps, including bridge tied load amps, in which a second amplifier has an inverted phase and thus the negative terminal is half the signal, rather than all of the signal being on the positive terminal and the negative terminal being tied to ground. In those amps, no terminal is tied to ground and doing so would short out the amp.
 
ryanosaur

ryanosaur

Audioholic Overlord
I am glad you guys could resolve your dispute in a civilized way, which is very much unlike how Matthew and I settled our disagreements. It always escalated into physical violence between Matthew and myself. In those clashes, I was always partial to clubbing weapons whereas Matthew was prone to stabbing and cutting weapons. I suppose our choice of weapons was not just an expression of personality but also symbolic of our philosophical differences. You might ask who won? Well, Matthew did flee to Florida, so I would take that for a kind of answer.
Wow... all because he had you help set up more heavy Speakers? :p
 
P

PENG

Audioholic Slumlord
There seems to still be some confusion. A differential output amplifier has half the output level on each phase of the amp terminals. The SDA circuit draws from the positive phase leg. That means the signal is reduced by around 6dB. This is why it matters. It's not audiophile tomfoolery, its a real issue. This wouldn't matter with any other speaker. It matters with the Polk L800's simply because of how the SDA crossover is built.

The really tough thing about this is that the L800's were a really unusual speaker for Polk that wanted high output amplifiers and are great for 2-channel setups. For someone who is into a Sound United setup, it would be natural to go to Marantz for amplification. But the majority of integrated amps now use Hypex or Purifi modules, and so have differential outputs which are not as compatible with the Polks. Polk didn't like me claiming it was not compatible and preferred I say that it doesn't work as optimally with differential amps. Their point is that the SDA effect still works, but the XTC is reduced by 6dB. James and I found it very audible so I think its fair to say its not compatible. We found the benefit of SDA to be nearly inaudible with the Cherry amplifier.
Marantz definitely has one that can do them justice:

1617912490555.png
 
TLS Guy

TLS Guy

Seriously, I have no life.
There seems to still be some confusion. A differential output amplifier has half the output level on each phase of the amp terminals. The SDA circuit draws from the positive phase leg. That means the signal is reduced by around 6dB. This is why it matters. It's not audiophile tomfoolery, its a real issue. This wouldn't matter with any other speaker. It matters with the Polk L800's simply because of how the SDA crossover is built.

The really tough thing about this is that the L800's were a really unusual speaker for Polk that wanted high output amplifiers and are great for 2-channel setups. For someone who is into a Sound United setup, it would be natural to go to Marantz for amplification. But the majority of integrated amps now use Hypex or Purifi modules, and so have differential outputs which are not as compatible with the Polks. Polk didn't like me claiming it was not compatible and preferred I say that it doesn't work as optimally with differential amps. Their point is that the SDA effect still works, but the XTC is reduced by 6dB. James and I found it very audible so I think its fair to say its not compatible. We found the benefit of SDA to be nearly inaudible with the Cherry amplifier.
I have to say that I remain highly skeptical of all this. The fact is that when in the concert hall you do not actually experience pinpoint localization. That is not natural at all, unless you are in a small concert hall listening to three or four musicians. The fact is that my rig produces an excellent facsimile of both types of venu, and over a wide area. All nine seats are good seats. So that is ideal for group listening. To me these L800 sound a real PITA. I see there is one dealer apart from Best Buy that handles Polk here. I will give him a call and see if I can audition them. If they are not a total disaster, I will see I can negotiate a loan, for some objective measurements under ideal conditions.
 
S

shadyJ

Speaker of the House
Staff member
I have to say that I remain highly skeptical of all this. The fact is that when in the concert hall you do not actually experience pinpoint localization. That is not natural at all, unless you are in a small concert hall listening to three or four musicians. The fact is that my rig produces an excellent facsimile of both types of venu, and over a wide area. All nine seats are good seats. So that is ideal for group listening. To me these L800 sound a real PITA. I see there is one dealer apart from Best Buy that handles Polk here. I will give him a call and see if I can audition them. If they are not a total disaster, I will see I can negotiate a loan, for some objective measurements under ideal conditions.
You are right, the L800s are kind of a PITA, and that is one of their drawbacks. They need a specific placement and specific listener positions and can not use a certain amplifier type. They are not a very flexible speaker. However, for those listeners who tend to listen to two-channel content by themselves in their listening rooms, you can spend a whole lot more money than 6K for something that isn't as good. For a certain kind of listener, they are a great speaker and a really good value.

I doubt that BestBuy will have L800s for audition. In the very remote case that they could be auditioned, I doubt they would be set up correctly- keep in mind a lot of professional reviewers didn't even manage to set these speakers up correctly! Matthew went the extra mile to understand these speakers, so if you do find the stamina to attempt to deal with a pair of these beasts, follow his guidelines.

As for measurements, that isn't easy to do with a speaker like these. That was one of the reasons why I passed on doing the review, because I can't use my usual method to capture the response seeing as how the SDA system would change the response, but how do you measure the SDA response of a single speaker? Matthew put in heroic efforts to accurately capture the response of the L800s, you guys will never know how much thought he put into it. This is a really special and unique speaker, and Matthew has written the definitive review of it. Honestly, Polk was very lucky to have Matthew do a review of it, because I don't think any other reviewer has been able to fully grasp all of what is going on with them.
 
TLS Guy

TLS Guy

Seriously, I have no life.
You are right, the L800s are kind of a PITA, and that is one of their drawbacks. They need a specific placement and specific listener positions and can not use a certain amplifier type. They are not a very flexible speaker. However, for those listeners who tend to listen to two-channel content by themselves in their listening rooms, you can spend a whole lot more money than 6K for something that isn't as good. For a certain kind of listener, they are a great speaker and a really good value.

I doubt that BestBuy will have L800s for audition. In the very remote case that they could be auditioned, I doubt they would be set up correctly- keep in mind a lot of professional reviewers didn't even manage to set these speakers up correctly! Matthew went the extra mile to understand these speakers, so if you do find the stamina to attempt to deal with a pair of these beasts, follow his guidelines.

As for measurements, that isn't easy to do with a speaker like these. That was one of the reasons why I passed on doing the review, because I can't use my usual method to capture the response seeing as how the SDA system would change the response, but how do you measure the SDA response of a single speaker? Matthew put in heroic efforts to accurately capture the response of the L800s, you guys will never know how much thought he put into it. This is a really special and unique speaker, and Matthew has written the definitive review of it. Honestly, Polk was very lucky to have Matthew do a review of it, because I don't think any other reviewer has been able to fully grasp all of what is going on with them.
We do have one high end dealer in the Twin Cities who lists them. If the speakers are that touchy, I doubt 'Joe Public' will meet with much success setting them up.
If they are as good as you say, then if I could set them up in a room as good as I have here, I should be able to get a decent room response at, at least one location.
If I can't then they are not worth bothering with. I have to be honest, the theory behind all this makes very little sense to me in the totality of its content. While it may have advantages in certain aspects, I see even bigger downsides. The fact that I have never heard a Polk speaker I like just adds to my skepticism.
 
S

shadyJ

Speaker of the House
Staff member
If the speakers are that touchy, I doubt 'Joe Public' will meet with much success setting them up.
I agree, and I even mentioned to Matthew that a case could be made for the lower-end and less expensive L600s being a better speaker simply for the fact that they are much more flexible and easier to set up. I think the L800s are just too demanding of a speaker to set up correctly for the average person. However, there is no denying the sound of the L800s when they are used properly. The soundstage is astonishing.
 
G

Golfx

Full Audioholic
We do have one high end dealer in the Twin Cities who lists them. If the speakers are that touchy, I doubt 'Joe Public' will meet with much success setting them up.
If they are as good as you say, then if I could set them up in a room as good as I have here, I should be able to get a decent room response at, at least one location.
If I can't then they are not worth bothering with. I have to be honest, the theory behind all this makes very little sense to me in the totality of its content. While it may have advantages in certain aspects, I see even bigger downsides. The fact that I have never heard a Polk speaker I like just adds to my skepticism.
So with your stated bias are you capable of fairly reviewing them? Or are you going to be searching for negative findings? You seem to have soundly already made up your mind.
 
Matthew J Poes

Matthew J Poes

Audioholic Chief
Staff member
We do have one high end dealer in the Twin Cities who lists them. If the speakers are that touchy, I doubt 'Joe Public' will meet with much success setting them up.
If they are as good as you say, then if I could set them up in a room as good as I have here, I should be able to get a decent room response at, at least one location.
If I can't then they are not worth bothering with. I have to be honest, the theory behind all this makes very little sense to me in the totality of its content. While it may have advantages in certain aspects, I see even bigger downsides. The fact that I have never heard a Polk speaker I like just adds to my skepticism.
I actually had never heard a Polk speaker I liked either. And further, if not for SDA, these would be merely ok speakers. Certainly not bad speakers. They are very neutral. I just wouldn’t be that excited about them compared to the 100’s or other interesting speakers.

I strongly suggest reading some of the background research before dismissing the concept. XTC is a part of a lot of cutting edge sound reproduction research and for a reason.

the tech doesn’t make the imaging more pinpoint. It causes the boundaries of the stage to expand far beyond the speakers. By eliminating crosstalk it eliminates the source cues that tell us we are listening to two speakers. Again, the stereo image is just a facsimile of the actual performance. How speakers reproduce sound has very little to do with the way we would hear the original performance.

just because I like the way these speakers present a soundstage does not mean I think your speakers or all others are wrong. The positivity of the review is not a personal attack on your speakers or preferences. I’ve never heard your system. Id love too. The review simply reflects my opinion that this idiosyncratic speaker presents the best and largest image I have ever heard.

it feels a bit like you are assuming that if this is right what you love must be wrong. Therefor this must be wrong. But I don’t see it that way. There are a lot of ways to get realistic sound. But nothing is Uncompromised. Everything we listen to is flawed. Every system we listen to is flawed.

there are real flaws with the binaural approach. While it tends to give the most accurate soundstage/image of any approach. The encode process typically imparts coloration. The decode side is usually headphones which have an “in your head“ sound and tend to cause positional reversals of the image. Things in front sound behind. Switching to a binaural decoding speaker fixes that but has the problem of still having some compromises in timbre. It imparts Its own issues.

ambisonics is currently the best thing I have ever heard. But at this time it is not a consumer friendly technology. This speaker reminded me a lot of what the Ambisonics system did so well, though certainly not as good. It is my sincere hope that we get there some day.

even though I now own these Polks, I kept my Gedlee Abbeys and intend to setup both. I still like the Gedlees for some things better. The Polks for other things.

mad for setup, they are finicky. They don’t work that great in small rooms with strong reflections. Or at least that was my experience. Such spaces need careful setup and some absorption. Having said that, their optimal placement is consistent with how many people place their speakers anyway. So if someone has a living room that is at least 15’ wide, this might actually be an easier speaker to place. I get about 3-5 clients a month through Poes Acosutics. I bet about half or more have their speakers in rooms at least 15’ wide, pushed against the front wall, and only 6-8 feet apart. They are usually sitting on a couch, pushed against a wall 10-12 feet away. I actually think these Polks would work better for those folks than the speakers they use. More traditional designs really should be pulled out from the wall and spread much farther apart to sound their best. I would normally argue that such people probably don’t spend 5k on speakers, but I have been surprised what some have. There are folks spending that and more and still placing them badly.
 
Matthew J Poes

Matthew J Poes

Audioholic Chief
Staff member
I agree, and I even mentioned to Matthew that a case could be made for the lower-end and less expensive L600s being a better speaker simply for the fact that they are much more flexible and easier to set up. I think the L800s are just too demanding of a speaker to set up correctly for the average person. However, there is no denying the sound of the L800s when they are used properly. The soundstage is astonishing.
Don’t make me come back to Illinois James! You know what you say are lies!
 
TLS Guy

TLS Guy

Seriously, I have no life.
I actually had never heard a Polk speaker I liked either. And further, if not for SDA, these would be merely ok speakers. Certainly not bad speakers. They are very neutral. I just wouldn’t be that excited about them compared to the 100’s or other interesting speakers.

I strongly suggest reading some of the background research before dismissing the concept. XTC is a part of a lot of cutting edge sound reproduction research and for a reason.

the tech doesn’t make the imaging more pinpoint. It causes the boundaries of the stage to expand far beyond the speakers. By eliminating crosstalk it eliminates the source cues that tell us we are listening to two speakers. Again, the stereo image is just a facsimile of the actual performance. How speakers reproduce sound has very little to do with the way we would hear the original performance.

just because I like the way these speakers present a soundstage does not mean I think your speakers or all others are wrong. The positivity of the review is not a personal attack on your speakers or preferences. I’ve never heard your system. Id love too. The review simply reflects my opinion that this idiosyncratic speaker presents the best and largest image I have ever heard.

it feels a bit like you are assuming that if this is right what you love must be wrong. Therefor this must be wrong. But I don’t see it that way. There are a lot of ways to get realistic sound. But nothing is Uncompromised. Everything we listen to is flawed. Every system we listen to is flawed.

there are real flaws with the binaural approach. While it tends to give the most accurate soundstage/image of any approach. The encode process typically imparts coloration. The decode side is usually headphones which have an “in your head“ sound and tend to cause positional reversals of the image. Things in front sound behind. Switching to a binaural decoding speaker fixes that but has the problem of still having some compromises in timbre. It imparts Its own issues.

ambisonics is currently the best thing I have ever heard. But at this time it is not a consumer friendly technology. This speaker reminded me a lot of what the Ambisonics system did so well, though certainly not as good. It is my sincere hope that we get there some day.

even though I now own these Polks, I kept my Gedlee Abbeys and intend to setup both. I still like the Gedlees for some things better. The Polks for other things.

mad for setup, they are finicky. They don’t work that great in small rooms with strong reflections. Or at least that was my experience. Such spaces need careful setup and some absorption. Having said that, their optimal placement is consistent with how many people place their speakers anyway. So if someone has a living room that is at least 15’ wide, this might actually be an easier speaker to place. I get about 3-5 clients a month through Poes Acosutics. I bet about half or more have their speakers in rooms at least 15’ wide, pushed against the front wall, and only 6-8 feet apart. They are usually sitting on a couch, pushed against a wall 10-12 feet away. I actually think these Polks would work better for those folks than the speakers they use. More traditional designs really should be pulled out from the wall and spread much farther apart to sound their best. I would normally argue that such people probably don’t spend 5k on speakers, but I have been surprised what some have. There are folks spending that and more and still placing them badly.
It seems to me that this is in many aspects analogous to the dummy head recordings for headphone listening. This is very effective, except their is limited content available and I dislike headphone listening. I hardly ever listen to phones, unless I'm editing analog tape, or on a plane.

As I have alluded to in another post, when it comes to creating a realistic sound filed, speakers are only half of the loaf. The other half is the microphones and how they are placed.

That is why I spent so many volunteer hours recording for the public radio station over many years. Apart from learning a lot, it was pivotal in my development as a designer of speakers. I mainly used two techniques, Blumlein and the matrix C-S equivalent. I also used spaced omnis, the width of a head, with a perspex baffle separating them, to substantially reduce cross talk. This latter technique was particularly effective for chamber groups, like trios and quartets.

Now it is true at their best, given optimal source and set up a pair of stereo speakers will only give you a window on the venue. The image in the main is between the speakers. One of the biggest errors in setup is having the speakers too close together. I see this again and again in pictures placed here. Optimal placement is 12 to 14 feet, 14 being about optimal in my view. However I realize problem speakers may not be able to sustain a sound stage that wide, and create the "hole in the middle" problem. However good speakers will, and also have good imaging depth. Given a good recording excellent speakers will accurately image between the speakers and provide good image depth.

In my view a big game changer has been upixers and especially the latest Dolby upmixer. Good recordings using minimal miking techniques are able to recreate the original acoustic to incredibly faithful degree. Again I refer you the the recordings done by Tom Scott of his brother Jonathan and also when he joins in at the piano.
There are lots of these on YouTube. I think they have given over 30 concerts during the pandemic. They have been releasing about two a month. They also have a lot of material pre pandemic. The musical content is of the highest order and the realism incredible. You should sample them. So although not from a stereo pair of speakers, these upmixers can allow a multichannel system properly set up do a very passable job of imposing the acoustic of the recording venue on your listening room. This includes a wide soundstage.

I can understand how this SDA technique could create a soundstage wide of the speakers. Given proper spacing and positioning of a single listener. This in itself would rule them out for me. However I would wonder how really accurate the soundstage is. In addition, this technique seems to me to be riddled with opportunities for creating frequency response errors. I can see how this effect could attract initial attention, but I have to wonder how accurate it really is. After all a wide soundstage is only one small slice of the pie, and of itself alone not highly significant in the totality of what is required for long term highly pleasurable reproduction in the home.
The fact you are also intending to use another set of speakers as well, just adds another notch to my skepticism.
 
P

PENG

Audioholic Slumlord
nope that is NCORE based. It has full bridged outputs. Those are differential so it will have the same problem.
Really, then they may be the most expensive ncore based amp!! They do have RCA unbalanced outputs, don't they count?
 
P

PENG

Audioholic Slumlord
Their point is that the SDA effect still works, but the XTC is reduced by 6dB. James and I found it very audible so I think its fair to say its not compatible. We found the benefit of SDA to be nearly inaudible with the Cherry amplifier.
This 6 dB reduction thing really bug me as I cannot visualize how/why that could be the case. I assume you don't have access to the XO schematics but can you and/or Gene sketch a scenario that can show us (those who can read electrical schematics) how the 6 dB can be lost just because the signal to the two/four/six? terminal speakers is between the +ve and -ve legs instead of just one signal leg to ground?

Again, I am assuming the L800 is a passive speaker so there should be no ground connections to begin with anyway, except for may be a common point of connection for every piece (for the LF, MF, HF drivers etc.) of the crossover network. I trust Polk Audio what they are talking about, just curious to know the how/why.
 
P

PENG

Audioholic Slumlord
I actually think the primary advantage is twice the output for half the rail voltage typically needed.
Interesting, how is it mathematically and electrically possible? I can see how a differential preamp to output twice the voltage, but we are talking about class AB power amp when you referred to rail voltage right?

If so, can you kindly elaborate on how you can get twice the output (again, I assume you meant the power amplifier output) using half the rail voltage, simply by using differential drive?
 
Matthew J Poes

Matthew J Poes

Audioholic Chief
Staff member
Interesting, how is it mathematically and electrically possible? I can see how a differential preamp to output twice the voltage, but we are talking about class AB power amp when you referred to rail voltage right?

If so, can you kindly elaborate on how you can get twice the output (again, I assume you meant the power amplifier output) using half the rail voltage, simply by using differential drive?
Because the total swing is what matters. So here is how a differential output amplifier using this bridge tied load approach works. Take two identical push pull amps (as an example). Each has 75 volt rails. For the sake of argument, you have 10 amps of current from the power supply. That is 1500 watts based on the swing between +/-75 volts (150 volts), but we need to divide by 1.41 to get to RMS. Class AB would only be 65% efficient. Class D would be 90% efficient. Let's go with Class AB. Now we are down to 975 watts peak and 691.5 watts RMS and that would be pushing things pretty hard. Now we go to a bridge tied load version, the second amp is inverted (phase is reversed. You tied the positive terminal of the second amp to the negative terminal of first amp. That is now referenced to ground. It doesn't touch the speaker. The positive terminal of the first amp goes to the positive terminal of the speaker. The negative terminal of the second amp goes to the negative terminal of the speaker. Again, no ground connection here. The two swing between these amps is now going to be much higher, 300 volts, like having one amp with +/-150 volts. Now we have basically 1300 watts of RMS power output ability. Or...you could go down to 35 volt rails, now the caps are very small and way cheaper/way more reliable.

Here is a schematic of one of the older SDA speakers:
1617981694060.png


Basically it is pulling the sound information from just the positive terminal, which on a single ended output amplifier, is fine, all of the signal is present on the positive terminal of a single ended output amplifier.

1617981942412.png

Here is a PP amplifier. You can see there is a positive output, load, ground. All the signal is on the output here.

But a differential amplifier is different.
1617982015501.png

Half the signal is on the + output, half is on the - output. Go back to the rail voltage comment. To obtain the total swing, the sinewave isn't swinging up and down the full amount on just the positive output. Instead, half the voltage level is on the positive terminal and half is on the negative terminal, with inverted phase. That is what gives the total voltage swing. So for a given wattage of output, the stereo array speakers would only see half the voltage swing that the main drivers see, thus 6dB quieter.

Does it make sense now?
 

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