Amplifier Comparison SpeakerPower SP2-12000 and Powersoft K20-DSP-Aesop

[Ricci]

Last year at about this time Brian Oppegaard at Speakerpower sent me one of his 240V, dual mono, SP2-12000 rack amplifiers so I could try it out and do a comparison against one of my Powersoft K10's. This was after discussing back and forth for a while. Of course a few weeks after that I became very busy and made a move to sell my K10's for K20's with all the bells and whistles which put the project off for a while. So here we are a year later and I've finally gotten around to what I intended to do from the get go. (My apologies for the extreme delay Brian.)

 

I'm more of an objective data driven person rather than subjective so to that end the plan was to actually gather some form of rough comparison data on the amplifiers. Also I do not have an amplifier test bench and need another test regimen and equipment setup like I need a hole in the head, so the plan was to test the amplifiers while powering subwoofers. This should be more representative of a complex real world load on the amplifiers but also has a number of challenges. First is that these amplifiers are among the most powerful on the market and it will take a number of drivers to be able to handle their full output without being the limiting factor. #2 is that it is going to be loud. I already had 8 sealed,  dual voice coil, RE Audio XXX 18's in my HT space so I figured that should take care of the speaker load. I never planned to do any testing or listening to either amplifier above the bass range. Really who buys 10kW rated amplifiers and uses them for anything other than bass? I wanted to use a sealed system in order not bandwidth limit the amplifiers and to require them to supply power down deep. I originally planned to do the measurements outdoors to remove room effects and to keep from subjecting my house to the absurd level of bass that would be produced, but after looking at the logistics of transporting the four 260lb dual opposed cabinets, the amplifiers, the wiring change overs, setup time and worrying about a good window in the weather I gave up on that. Indoors was so much easier I decided to take the lazy route.

 

The test regime was to wire up as many XXX drivers in groups of two that would meet nominal loads of 8, 4 and 2 ohms. The amplifiers would be connected to each nominal load and then be run up to maximum output with 6.5 cycle shaped CEA-2010 type bursts in order to look at dynamic output and also with the same 23 second long sine sweeps that I use for testing speakers outdoors in order to look at something with an extremely high duty cycle, much worse than any music or movie content I'm aware of. this would give a good glimpse of long term output and short term. The amplifiers were both run from the same 240V AC line, the output was recorded by an Earthwork's M30 microphone placed at the headrest of the main listening position. The signals used were all software based and the microphone preamp providing the signal to the amplifiers was a Presonus Firebox. The gain for both amplifiers was set to the highest available setting. No DSP or other filtering was used for either amplifier. Originally I only planned to do a set of single channel driven tests since the SP2-12000 is a dual mono design with each internal channel already bridged. However after discussions with Brian and encountering issues with bus pumping with the K20 (Which is not a full bridge design)  when running a single channel a set of tests running both channels was also done. For the dual channel tests the SP2-12000 was run in stereo since each channel is already a full bridge design and the K20 was operated in bridged mode. An effort was made to use wiring that was as close to each other as possible for each amplifier. It could not be exact but in general was very, very close. I used my existing 4 runs of 4 pole Speakon cables in the room and put together a wood panel with a set of Speakon jacks that would allow me to use a variety of jumper schemes to switch the load presented to each amp. In general I would always use the same drivers for both amplifiers if all 8 were not used and maintained parallel / series wiring of voice coils and drivers for both amplifiers with any changes needed in the wiring scheme accomplished at the amplifier or at the Speakon switch plate so that the loads were as close as possible. Obviously by using speakers as the load and amplifiers with this much juice thermal and dynamic compression in the drivers is a concern. However the tests using the full compliment of 8 drivers seemed to suffer from this very little and those using 4 drivers much less than I expected. Still it will be noted which amplifier was tested first at each nominal resistance as the second one to be tested may have been dealing with a bit of heat in the drivers.  

 

More to come...

 

 

[Ricci]

Attached here is an example of the wiring plate used and also an example impedance of one of the drivers in a sealed cab of about the same airspace with the coils in series.

 

Obviously the impedance of the cabs shifted somewhat with heat and high displacement of the drivers but this is the baseline. The use of so many drivers should have kept the amount of heat buildup in the coils from being excessive and causing notable compression or parameter shifting. Regardless both amplifiers were driven into the same loading so should have experienced the same things or close enough for the rough general nature of this type of testing. By looking at the impedance we can get a rough guideline of the areas that required the highest current from the amps and which areas were near the peak and required more voltage.

 

 

[Ricci]

Let's start with the single channel measurements as they were done first.

 

8 OHM Single Channel Results

The SP2-12000 SpeakerPower was up first at 8ohms. The burst tests were done first followed by the long term sine wave test. Nothing notable to report. The SP2-12000 had no issues even when driven several dB into clipping. The fans didn't even turn on. The K20 followed also at 8ohms again nothing to report here. The amp chugged along.

 

The single channel 8 ohm burst data is represented in the chart below. The K20 is set as the baseline measurement in the blue column and it's results are set at "0". The SP2-12000 result is shown in dBW relative to that recorded by the K20. A positive number indicates higher output a negative number equals less. Again this is the recorded acoustic output of a group of heavy duty speakers when driven with both amplifiers. We can see that during this test the K20 recorded about 1.6dB higher output on average. No long term comparison data. The reason will be explained below.

 

 

 

 

4 OHM Single Channel Results

The K20 went first at 4 ohms. The burst tests were done first followed by the long term sine wave test. The burst tests went as expected. No issues. However when running the sine wave test which starts way down near 2Hz, the K20 muted the outputs. The output level had to be dropped way down by 7-9dB to get it to complete the sweep without muting the output during the beginning of the sweep in the extreme low frequencies. I had never encountered this behavior before so I tried the other channel with the same result. I even tried the other K20 I have and it repeated the same. I started moving the start frequency of the sweep up to see if this would help and once the sweep started closer to 20Hz the K20 would complete the sweep at full output without protecting. The SP2-12000 however, after running through its burst testing, had no issues with the full sine wave sweep even when driven several dB into clipping as before. Since the K20 wouldn't complete the full sweep in this setup unless the bandwidth was restricted or the level was reduced some 6dB or more below the SP2-12000 capability I saw no point in presenting the long term data comparison for this test. The SP2-12000 clearly won that portion of the test. The burst testing is presented below in the same manner as previously outlined. The K20 is the baseline "0" result and the SP2-12000 output is shown referenced to that baseline. Similar to the 8ohm single channel results the K20 has about 1.9dB greater output on average with the 4ohm burst tests.

 

 

 

 

 

2 OHM Single Channel Results

The SP2-12000 went first at 2 ohms. The burst tests were done first followed by the long term sine wave test. The burst tests went as expected. The only thing that cropped up was that I was able to get the SP2-12000 to mute briefly with the 10Hz burst with it driven WAY into clipping while trying to get every last 0.1dB. Otherwise it was uneventful and backing off a little bit removed that issue. The long term test was also not an issue for the SP2-12000 after backing off the signal a bit from the burst testing. The clip lights were still illuminated for nearly the entire sweep. I knew that the K20 would likely repeat the issue from the 4 ohm long term sweep test at 2 ohms, so the SP2-12000 would win the long term single channel portion with ease. This was the case. The K20 did not like being asked for full power in the extreme low frequencies with one channel driven. It muted the outputs unless the level was greatly reduced or unless the bandwidth below 20Hz was skipped. The issue even started to show up in the 2 ohm burst testing.  The K20 would mute at the 10 through 16Hz bands if pushed too hard. At 10 and 12.5Hz the output prior to muting was down some 8 to 10dB from the SP2-12000's recorded results. At 16Hz the K20 kind of got its act together and was around 3dB low from what it would have been. The SP2-12000 produced notably more output at that band as well. At 20Hz and above the K20 finally started doing its thing and averaged about 1.3dB higher output.

 

 

 

 

 

Ok so at this point a couple of things are clear. On a single channel versus single channel basis the K20 can burst higher numbers short term but the SP2-12000 kicks its ass with longer term or higher duty cycle signals at very low frequencies and lower impedance loads. The limited K20 output and aversion to extreme low frequency information is why the long term data is not compared for the single channel tests. It's a clear cut result. At this point I sent some of this information to Brian and asked his opinion on some things since he is a certified amplifier guru and I'm more like the guy who stayed at the Holiday Inn Express last night when it comes to amplifiers. I know a little about thery and design but I don't kid myself when comparing my limited understanding to someone who does it for a living. This will be covered in the next post.

[Ricci]

After talking with Brian about the results for the single channel tests and the issues with the K20 and low frequencies he mentioned that it was likely due to either DC protection or bus pumping. My thoughts were that it was one of these or perhaps current limiting also. I'd heard of bus pumping in Class-D amplifiers before and was familiar with the idea but had never really experienced it happening with an amplifier before in such a conclusive way. Brian recommended doing a dual channel test with both channels run and the Powersoft bridged. The SP2-12000 is a dual mono design with two separate amplifier modules each with their own power supply which are already in a full bridged configuration. The K20 and other amps in the Digam series have each channel sharing a power supply and are half bridged when run in stereo but can be placed in a full bridged configuration. In a nutshell what happens when a Class-D amplifier is in a half bridged configuration is that there is an induced current back from the load to the amplifier which can cause fluctuation and instability in the amplifier. The issue is worst at very low frequencies. There are a number of technical papers on Class-D amplifier design that explain the issue in depth with a quick internet search. By placing the amplifier in a full bridged configuration the bus pumping issue is resolved. This sounded like the issue being experienced with the K20 during the single channel testing and indeed after thinking about it I virtually always run the K series amps I've owned in a bridged config. The few times that I hadn't were during testing of a single speaker where it could not begin to handle the full power from even a single channel of the amplifier so the limits were never approached. That would explain why the issue didn't appear in those couple of tests using just a single channel. 

 

Armed with this bit of information the dual channel tests would be with the SP2-12000 run in stereo since it is already a pair of bridged amplifiers. The K20 would be run in a mono bridged configuration. In bridged mode the amplifier will see half of the impedance on each channel so the loading works out the same for say an amplifier run into two 4 ohm loads in stereo and the same 4 ohm loads presented as 8 ohms mono to a bridged amp.

 

 

 

8 Ohm Stereo SP2-12000 / 16 Ohm Mono K20 / Both Channels Driven

For this test the SP2-12000 was run first. The burst testing was without issue. The long term sine wave test was also without issue. The K20 also ran through both tests without issue. Good so far.

 

The burst results are presented below. As with the single channel 8ohm results the K20 produces a bit higher output with an average about 1.9dB higher. The long term sine wave results show much the same thing with the K20 maintaining a similar lead over the entire range of the test. By the way the response shape shown is the raw response in my room so it isn't pretty but it works for the purpose of illustrating the comparison.

 

 

 

 

 

4 Ohm Stereo SP2-12000 / 8 Ohm Mono K20 / Both Channels Driven

For this test the K20 was run first. The burst testing was without issue. The long term sine wave test was also without issue. WOOHOO! I was relieved that bridging the amp fixed the issue to say the least. The SP2-12000 did its thing as usual without complaint.

 

The burst results are similar to the other tests so far with the K20 able to push the speakers about 1.7dB louder on average. The two channel driven long term testing follows a similar pattern as at 8 ohms and the K20 is able to push the speakers to slightly higher output levels for the duration of the measurement signal.

 

 

 

 

 

2 Ohm Stereo SP2-12000 / 4 Ohm Mono K20 / Both Channels Driven

For this test the SP2-12000 was run first. The burst testing was completed first. I did manage to get the SP2-12000 to briefly mute or protect with the 10Hz burst while trying to get everything possible out of it. As before this was pushing the amp deep into clipping and reducing the level just a hair took care of it. The long term testing was then done and the SP2-12000 could be caused to mute or protect briefly below <10Hz if the level was too high but again this was with a signal that was a good 4 or 5dB past the point that the clip lights would be lit up. Reducing the input signal level a dB or two allowed the SP2-12000 to complete the sweep at full power without any problems. The K20 completed the burst testing without issue even at 10Hz and I could not get it to protect or mute even running the input signal up 6dB or more past what would cause clipping. The 2ohm long term test also was completed without any muting or protection triggering. Looks like bridging the amp did indeed entirely remove the issue seen in the single channel unbridged testing.

 

This test series was the only one that got the SP2-12000's fans to engage at all and produced any heat in the amp. It got barely warm. The K20 did spool the fans up a bit on this one as well and also became a bit warm. Warmer than the SP2-12000. The K20 seems to run with the fans set to cool the amp much more aggressively and the case does run warm to the touch even at idle where the SP2-12000 won't warm up significantly unless you are really beating on it for awhile.

 

The 2 ohm, both channels driven, burst tests show much the same story as we've seen so far. The K20 is able to coerce the XXX's into producing about 1.6dB higher output on average. The long term sine wave testing at this lower impedance load is a little more interesting. The K20 exhibited some limiting during this and was held about 3-4dB below clipping for large sections of the sweep. The general trend is that the K20 would produce near full power for a section of the sweep then reduce output for a while and then ramp back up and then limit a bit for awhile. We can see that the K20 starts off with a bit higher output down in the resonance of the earth frequencies of 2-5Hz and then the SP2-12000 takes over from 5-15Hz. The K20 rises back up and takes back over from 16-34Hz. Then the SP2-12000 takes over or runs even with the K20 over the 35-70Hz octave. The K20 comes back on from 70-120Hz and produces a bit higher output again. To me this looks like long term current limiting going on in the K20. The Digam series of amplifiers has a number of settings for limiting the amplifiers output or matching it to the circuit that it is on. The maximum current setting is 33A which is a long term average which is there to prevent tripping breakers. With typical content I doubt that this is ever an issue but the sine wave measurement used here is 100% duty cycle for 23 seconds or so and with the relatively low impedance minimums and the amplifier being run at full output I suspect this may actually be coming into effect and the software may be limiting the current a bit. The most notable area of the K20's measurement is between 30-40Hz where clearly the output is limited abruptly for a bit.  If this set of sweeps is compared with the impedance curve of the driver provided earlier we can see that the output squishing happens in the areas of high current demand. I will say that there is no way that I could hear anything happening or tell anything different during this measurement by ear. The entire house was being shook violently and a reduction of a couple dB in output simply doesn't audibly register to me when my home and body is being punished like that. Perhaps outside...I don't know. Anyway I'd call the 2 ohm long term sine wave test a draw or a slight win for the SP2-12000.

 

 

 

[Ricci]

The wrap up.

 

Looking at the data as a whole the K20 does have a higher short term output capability and also long term into higher impedance loads. The SP2-12000 is certainly no slouch either and closes the gap at low impedance loads and with high duty cycle signals into extreme low frequencies. Based on the power ratings the results are pretty much as expected. Both of these amplifiers are monsters and offer among the highest potential output on the market with the capability to drive multiple of the heaviest duty drivers or bass cabinets to their limits. The K20-DSP-Aesop certainly offers more bells and whistles with a huge variety of settings, on board DSP, remote control and monitoring, networking, AES digital inputs, cable compensation, universal power supply, etc. Most of which would not be used in most cases unless you are running them as part of a huge networked system in a stadium or something. The SP2-12000 is more straight forward and offers comparable amounts of power. The SP2-12000 feels a bit lighter and is physically a little smaller. The K20 is a single rack space but it is a very deep amplifier requiring a deep rack. The SP2-12000 is 2 rack spaces but is a much shorter case so the overall effect is that it feels smaller despite being 2 rack spaces. The SP2-12000 is also much quieter. The K20's fans always run and while they are not as loud as some other amplifiers they are audible in a quiet home setting. The SP2-12000's fans don't even seem to run most of the time and didn't even turn on until asked to produce very large amounts of power for an extended time. That being the case it is virtually silent which is awesome for a quieter home or studio setting. It also ran cooler. It produces barely any heat until really loaded down for an extended period at which point it got barely warm. The K20 idles with a bit of warmth after being on for a while and will warm up a bit after being used heavily. It has never gotten what I'd call hot by any means but it does run a little warmer than the SP2-12000. The SP2-12000 also has a little less roll off below 10Hz for the ultra low frequency fanatics. The Powersoft is designed and manufactured in the EU while the SP2-12000 is a North American design and manufacture. The SP2-12000 is also much cheaper than the K20. Even if you get an extremely good deal on the K20 or K10 series like I did and you eschew all of the extra DSP or AESOP options they are still notably more expensive than the SP2-12000 on average. Both of these amps tolerated all of the abuse with ease and are obviously well protected and designed such that you will really have to do something stupid to blow one up. I mean if running them way into clipping below 10Hz multiple times with sine waves won't do it it's going to take something really extreme. Both are absolute mules and I would proudly own either.

 

A final word for owners of Class-D amplifiers.

I always tried to run my amps bridged due to the advantages I've read about but after the clear data and first hand behavioral differences I noted with the K20 when running full bridged versus not, I am truly a believer. If you own a Class D based amplifier that has the ability to be bridged that pretty much means that it is half bridged when not running in bridged mode and you could be severely limiting the amplifiers performance for bass duties. If you want the best performance possible from the amplifier for driving subwoofers you need to bridge your amplifier. This is especially true if you are driving a sealed or IB bass system and running it unfiltered on the bottom for the most extension possible. The extreme low frequencies combined with high current demand seem to be especially problematic. The K20 seemed to be alright at 20Hz and above with the loadings used here even when unbridged, but who knows how other amplifiers may behave, so I'd advise against taking the chance with diminished head room and instability in the low bass range and just go straight to bridged mode.

[lukeamdman]

Awesome!  Really looking forward to the results. 

[chrapladm]

AAwwww YA. Looking forward to this.

[Ricci]

Crap...Maybe should've reserved one more.

[chrapladm]

Ricci I can delete my comments if you want to add more reserve spots?

 

Let me know.

[Ricci]

No worries man. Almost done!

[Spacebug]

Interesting read about these amps, waay more power than I need but still interesting

 

But, from what i've heard bus pumping is not a problem assosiated with SMPS itself but of class-d amplifiers, the amplifier stage itself that is.

If class-d modules don't operate in full bridge mode bus pumping will occur.

The powersupply is not the cause but rather the component that takes the beating of fluctuating rail voltages.

Not sure whether SMPS is more susceptible than "linear" power supplies or not but the main problem comes from class-d amps not operating in full bridge...

In full bridge mode the rail fluctuation caused by one amp "channel" will be consumed by the other, inverted amp channel causing an equal but inverted fluctuation.

In non bridge mode the rail fluctuation is not canceled out by the other channel so the fluctuation reach the powersupply and it is here the problem starts.

 

That is atleast what i've heard...

[Ricci]

You are correct Spacebug. Thanks for catching that. I always assume SMPS is synonymous with Class-D but it is not.

[Spacebug]

An understandable assumption though.

I guess there isn't too many amps employing switching output stage but does not have a switch mode power supply...

 

But for example the LG FP series uses SMPS but usual, linear output stages, hence not susceptible to bus pumping, if i'm correct that is...

 

 

EDIT: nevermind, brainfart on my behalf, misinterpreted your last post

[SME]

Wow, big props to the SP amp!  I'm amazed you couldn't get the fans to spin without running a continuous clipped signal into a 2 ohm load.  I also like that the amp can drive 2-ohm loads in stereo without issue.

[Bossobass Dave]

Thanks for this. I like Brian and his products but have never liked his comparison data because I believe it is heavily biased toward his product.

 

To be more specific, his amplifiers are all full bridge configuration. The SP 2 channel amplifiers are the equivalent to 2 full bridge amplifiers and thus cannot be bridged. Comparing a full bridge amp to a single channel of a half bridge configuration is like comparing 2 dual VC drivers with one of the drivers using only one of it's VCs. Although interesting, it's not relevant data.

 

The SP amps weren't running 2 ohms in stereo.

 

With Class D the efficiency is increased by feeding the back EMF to the PS to recycle the power that is otherwise lost to heat. That's why the 2nd channel cancellation is the cure for bus pumping at <100 Hz amplification, but the FR is pertinent. Class D amps that offer half bridge channels usually have a relatively high FR roll off and aren't intended for use to reproduce content to 3 Hz at maximum output. Also, combined with a higher, steeper roll off, a large enough cap reservoir could also prevent bus pumping in a half bridge configuration.

 

This ^^^ is why I also would love to see the FRs of the amps. It's a simple procedure and roll off characteristic is imperative data in any sub amp comparison for obvious reasons. Amplifiers enter a quantum dimension proportional to frequency as frequency decreases.

[Ricci]

I did post the FR of the PS amps and also the SP amp in comparison in the electronics FR thread I believe. You can use the comparison with the PS to derive the SP FR. It is pretty good below 10Hz.

[SME]

Dave, what are you trying to say here?  You talk about the lack of bridge capability like it's a flaw, when in fact the amps are already bridged internally.  And in fact, they bridge into 2 ohm loads, unlike most amps that need 4 ohms to be operated bridged.  Or, is there something special about delivering your power into 4 ohm as opposed to 2 ohm?

 

The power supply capabilities are especially impressive.  That long duration sweep likely drains all the reserves, so it really measures what the power supply (and the AC supply line) can provide up to the point at which current limiters kick in for thermal protection or to prevent circuit breakers from flipping.

 

As for efficiency, I am fairly certain that the efficiency advantages of class D do not have to do with recycling of back EMF energy, and in fact, the presence of back EMF is a nuisance that appears in their design that is most adequately quelled by bridging the outputs.  To have not one but two fully-bridged outputs is an advantage, not a disadvantage.

 

It looks like the SP2 and K10 probably have very similar roll-off characteristics.  The signal chain measurements thread shows the K10 down about 3.5 dB down at 5 Hz and 7.5 dB at 3 Hz.  That doesn't look too bad at all, of course, assuming the K20 and K10 have identical roll-off.

[lukeamdman]

Awesome information Josh!

 

I don't think Dave believed me when I was telling him how powerful the sp2-12000 was, but these tests shouldn't leave any question.

 

As you know, I'm running mine to loads very close to 1ohm per channel, and it never complains. I've seen the fans skin up maybe twice in the last year I've had it.

 

I've said it before and I'll say it again, unless you plan to use the SP amps for strictly 2ohm use you're wasting their potential and what they were really designed for.

[chrapladm]

That is awesome. Quite impressive specs. And a great value for the price.

[Ricci]

One quick update. I am putting my system back together this morning and was reconnecting everything and double checking all of my settings between the 2 K20's and the K20 used for the comparison's current setting was actually set to 32A Fast instead of 32A Slow. That may have affected the 4 ohm bridged long term test by having it limit current draw a bit quicker. I don't know for sure and I'm not setting everything back up to find out. Maybe down the road some time.

 

Dave by stereo I meant that the amplifier was running 2 separate channels. It is 2 bridged amps of course but it is run with 2 channels. The k20 was run into a mono load. The SP could not be so it was 2 loads with a mono signal.

[Kvalsvoll]

Great test, @Ricci.

 

Mind that I have not read through everything here, but I noticed the results for the single channel Powersoft and your comments on bus pumping.

So my comments here may be redundant.

 

For bass duty it is necessary to either run a bridged output stage, or you need a power supply with regenerative capacity.

Either will work fine.

This is not an option, it is necessary.

 

A different approach sometimes seen suggested is to run two channels on the same rails with opposing phased signal.

If the load and the signal is the same for both channels, this will work like bridge mode.

In a practical situation this is not a good approach; say you choose to turn off one sub, leaving the amp running on one channel only..

 

A pcm/"digital" amplifier with single output stage and no regenerative power supply will pump up the rail voltages when operated at low frequencies and high load.

This will eventually blow the output stage if the amplifier does not shut down due to some kind of protection circuit.

 

This is why it is necessary to test all bass-duty amplifiers at low frequencies.

It may be good at say 40Hz, but when you put a 14Hz signal on, it can fail, even blow the output stage.

The amplifier may be good for normal PA-use, >30-40hz, but it will fail when Tom Cruise is "Coming in hot".

 

Another issue is that smps power supplies can have significantly lower output for low frequency loads.

This is why many class-D amplifiers have reduced output at lower frequencies.

This is, however, not as critical, nothing will blow, it is just a performance issue.

[Bossobass Dave]

I did post the FR of the PS amps and also the SP amp in comparison in the electronics FR thread I believe. You can use the comparison with the PS to derive the SP FR. It is pretty good below 10Hz.

 

Nope. I see the K-10 but no FR for the K-20 and no FR I can find for any SP amp. Mic'd sweeps are not reliable FR measurements of amp rolloff.   

 

Dave, what are you trying to say here?  You talk about the lack of bridge capability like it's a flaw, when in fact the amps are already bridged internally.  And in fact, they bridge into 2 ohm loads, unlike most amps that need 4 ohms to be operated bridged.  Or, is there something special about delivering your power into 4 ohm as opposed to 2 ohm?

 

The power supply capabilities are especially impressive.  That long duration sweep likely drains all the reserves, so it really measures what the power supply (and the AC supply line) can provide up to the point at which current limiters kick in for thermal protection or to prevent circuit breakers from flipping.

 

As for efficiency, I am fairly certain that the efficiency advantages of class D do not have to do with recycling of back EMF energy, and in fact, the presence of back EMF is a nuisance that appears in their design that is most adequately quelled by bridging the outputs.  To have not one but two fully-bridged outputs is an advantage, not a disadvantage.

 

It looks like the SP2 and K10 probably have very similar roll-off characteristics.  The signal chain measurements thread shows the K10 down about 3.5 dB down at 5 Hz and 7.5 dB at 3 Hz.  That doesn't look too bad at all, of course, assuming the K20 and K10 have identical roll-off.

 

What I'm saying is that it is not a proper comparison of amplifiers to compare a single channel that is a full bridge configuration amplifier of amplifier A to a single channel that is a half bridge configuration channel of amplifier B. The 'flaw' is in the test methodology and, therefore, the results. I would have thought that was obvious in looking at the comparisons page of the SP site.

 

The roll off characteristics of either of the amplifiers is not apparent from any data presented and in that respect I assume nothing.

 

How many bridged channels an amplifier may have is not relevant to my comment.

 

I don't think Dave believed me when I was telling him how powerful the sp2-12000 was, but these tests shouldn't leave any question.

 

As you know, I'm running mine to loads very close to 1ohm per channel, and it never complains. I've seen the fans skin up maybe twice in the last year I've had it.

 

I haven't doubted the raw output data, but I do question the 1 ohm operation claim. Again, I don't doubt certain situations permit that sort of stupid-high current draw, but I don't think your system presents such a situation. I don't believe you operate anywhere near 1 ohm.

 

If you have any data to the contrary, please post it.

[Ricci]

The K20 roll off is the same as the K10 FWTW.

[Bossobass Dave]

The K20 roll off is the same as the K10 FWTW.

 

 

Gotcha. I have the K-10 FR graph, thanks.

 

Anyone have the SP FR?

[lukeamdman]

I haven't doubted the raw output data, but I do question the 1 ohm operation claim. Again, I don't doubt certain situations permit that sort of stupid-high current draw, but I don't think your system presents such a situation. I don't believe you operate anywhere near 1 ohm.

 

If you have any data to the contrary, please post it.

 

Here are the drivers I'm using:

 

http://www.bcspeakers.com/products/lf-driver/21-0/21ipal

 

They have a DC resistance of 0.7ohm, and I also measured 0.7ohm with my meter.  

 

 

 

 

In Josh's measurements of the Othorn cabinet with the 21sw152-4 (DC resistance of 3.3ohm), the impedance minimums are at about ~28hz and ~59hz

 

 

 

 

 

28hz: 3.8ohm

59hz: 3.9ohm

 

 

 

21sw152-4 specs:

 

 

 

Josh also tested the 21-IPAL in the Othorn cabinet and stated the impedance between 25-30hz to be ~1.1ohm.