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I haven't done any direct comparisons of these two amps, but I do have a pair of CDA-254 amps from ClassDAudio that I built for my surround sound amps in my theater.
I used my scope and signal generator to do some measurements of this amp and I'm not really sure what to make of what I saw on the scope. Since I haven't measured any other Class-D amps, I don't know if this type of performance is consistent for Class-D topology or not, but it's not nearly as clean as other Class-A and Class-AB amps I've used. Overall, I'd say this amp was optimized to drive 4-ohm loads rather than 8-ohms.
Power bandwidth is 10Hz to about 35kHz. Output power of the kit is 90w into 8 ohms and 150w into 4 ohms. I suspect power output would be greater with greater PSU voltage as indicated in the writeups provided by ClassDAudio.
10kHz input sinewave, 10vRMS output into 4- and 8-ohm resistive load:
1kHz squarewave input, 10vRMS output into 4-ohm resistive load:
1kHz squarewave input, 10vRMS output into 8-ohm resistive load:
10kHz squarewave input, 10vRMS output into 4-ohm resistive load:
10kHz squarewave input, 10vRMS output into 8-ohm resistive load:
My full write up of the construction of this project can be found here: ClassDAudio CDA-254 Class D Surround Amplifier
I used my scope and signal generator to do some measurements of this amp and I'm not really sure what to make of what I saw on the scope. Since I haven't measured any other Class-D amps, I don't know if this type of performance is consistent for Class-D topology or not, but it's not nearly as clean as other Class-A and Class-AB amps I've used. Overall, I'd say this amp was optimized to drive 4-ohm loads rather than 8-ohms.
Power bandwidth is 10Hz to about 35kHz. Output power of the kit is 90w into 8 ohms and 150w into 4 ohms. I suspect power output would be greater with greater PSU voltage as indicated in the writeups provided by ClassDAudio.
10kHz input sinewave, 10vRMS output into 4- and 8-ohm resistive load:
An externally hosted image should be here but it was not working when we last tested it.
1kHz squarewave input, 10vRMS output into 4-ohm resistive load:
An externally hosted image should be here but it was not working when we last tested it.
1kHz squarewave input, 10vRMS output into 8-ohm resistive load:
An externally hosted image should be here but it was not working when we last tested it.
10kHz squarewave input, 10vRMS output into 4-ohm resistive load:
An externally hosted image should be here but it was not working when we last tested it.
10kHz squarewave input, 10vRMS output into 8-ohm resistive load:
An externally hosted image should be here but it was not working when we last tested it.
My full write up of the construction of this project can be found here: ClassDAudio CDA-254 Class D Surround Amplifier
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Hi Eric,
the superimposed HF ripple is normal.
The rectangular response appears to me acceptable, you will find lots of worse amps and some better ones.
P.S.
Are you sure that your 10kHz are really 10kHz?
...doesn't fit to normal scope time scales...
Looks more like 8kHz.
P.P.S.
In any case it is difficult to extrapolate from scope findings to sonic quality.
I am saying this, well knowing my personal scope friendly mind setting....
the superimposed HF ripple is normal.
The rectangular response appears to me acceptable, you will find lots of worse amps and some better ones.
P.S.
Are you sure that your 10kHz are really 10kHz?
...doesn't fit to normal scope time scales...
Looks more like 8kHz.
P.P.S.
In any case it is difficult to extrapolate from scope findings to sonic quality.
I am saying this, well knowing my personal scope friendly mind setting....
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Hi ChocoHolic,
Thanks for your reply. I wasn't sure what to make of the output, but my guess was that the HF ripple was fairly normal for Class-D stuff, especially since it showed up in both the sine and square waves. I appreciate your confirmation of my hunch - this is the first amp of this type I've put together, so I was eager to do some measurements.
The 10kHz may be off a bit. I'm using old, second-hand equipment that hasn't been adjusted in a long time. The input signal is from an analog signal generator that was set to 10kHz, my suspicion is that the scope needs a small adjustment to the display parameters - something I haven't taken the time to do. I was just looking to see the waveforms.
I also completely agree with the correlation (...and lack thereof...) of measurements to what you hear from your speakers. Sometime over the next few days, I plan to move this amp and use it to drive my Avro mains (large, floor standing di-poles) to see how things really sound. It was just that with my Aleph-X amps, I was able to tweak the shape of the square wave output by swapping cap values.
Thanks for your input! I always welcome the opportunity to learn something new!
Eric
Thanks for your reply. I wasn't sure what to make of the output, but my guess was that the HF ripple was fairly normal for Class-D stuff, especially since it showed up in both the sine and square waves. I appreciate your confirmation of my hunch - this is the first amp of this type I've put together, so I was eager to do some measurements.
The 10kHz may be off a bit. I'm using old, second-hand equipment that hasn't been adjusted in a long time. The input signal is from an analog signal generator that was set to 10kHz, my suspicion is that the scope needs a small adjustment to the display parameters - something I haven't taken the time to do. I was just looking to see the waveforms.
I also completely agree with the correlation (...and lack thereof...) of measurements to what you hear from your speakers. Sometime over the next few days, I plan to move this amp and use it to drive my Avro mains (large, floor standing di-poles) to see how things really sound. It was just that with my Aleph-X amps, I was able to tweak the shape of the square wave output by swapping cap values.
Thanks for your input! I always welcome the opportunity to learn something new!
Eric
Eric, is that 10 Vrms or 10 Vp-p? At Vrms that ultrasonic calcs out to roughly 1/10 watt into 4 ohms. At normal speaker efficiencies, in the low 80 dB range. The tweeters will break in nicely. Does that overlaying sine change in level with drive? Still there with no drive? I'm interested in the SDS-224 for a spare transformer project and yours are the only measurements I've seen on classdaudio's amps. Thanks.
BTW, at least one of classd's amps (TI-600) appears to 'suggest' an incorrect max 8 ohm RMS output for max rated power supply voltage, off by a factor of ~two. It might account for your commented disappointment in measured output.
BTW, at least one of classd's amps (TI-600) appears to 'suggest' an incorrect max 8 ohm RMS output for max rated power supply voltage, off by a factor of ~two. It might account for your commented disappointment in measured output.
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rdf,
Output is 10vRMS, so more like ~14vp-p output. My signal generator, while continuously adjustable, isn't able to output a signal that results in less than about 7vRMS after amplification. Perhaps I need to disassemble it and see if the adjustment pot can be cleaned a little bit. I'm not sure that I'll be able to reach it...
While I didn't look extensively across all frequencies and output voltages, the oscillations seemed to be present rather consistently - it seems like this should be expected based on what ChocoHolic indicated. I poked around mostly in the range of 1kHz to 10kHz. While I was exploring power bandwidth I was just looking at the peak values and not really paying attention to zoom into a single sine wave at different frequencies. I also never looked at the output with no input signal (input RCA shorted). Output at the speaker, though, is very quiet - you have to have your ear within 2-3 inches of the tweeter to hear any hiss at all. Beyond that distance, there is nothing to hear when no input is presentl.
As for maximum power output, I suspect this is directly related to the 400VA transformer. It looks like they offer two "standard" transformers with their kits: 400VA and 500VA. This kit shipped with the 400VA transformer that has dual ~35Vac secondaries that provide +/- 49Vdc to the amp boards. Their literature indicates that the amps work with power supply voltages of +/-45Vdc to +/-60Vdc. I suspect an additional 20Vdc input to the amp boards would provide the "missing" output power that I noted. I could easily pick up a different transformer, but I'm not convinced that it would be worth it to pick up the additional 30w (or about 1 to 1.5dB at the speaker) of output power.
Output is 10vRMS, so more like ~14vp-p output. My signal generator, while continuously adjustable, isn't able to output a signal that results in less than about 7vRMS after amplification. Perhaps I need to disassemble it and see if the adjustment pot can be cleaned a little bit. I'm not sure that I'll be able to reach it...
While I didn't look extensively across all frequencies and output voltages, the oscillations seemed to be present rather consistently - it seems like this should be expected based on what ChocoHolic indicated. I poked around mostly in the range of 1kHz to 10kHz. While I was exploring power bandwidth I was just looking at the peak values and not really paying attention to zoom into a single sine wave at different frequencies. I also never looked at the output with no input signal (input RCA shorted). Output at the speaker, though, is very quiet - you have to have your ear within 2-3 inches of the tweeter to hear any hiss at all. Beyond that distance, there is nothing to hear when no input is presentl.
As for maximum power output, I suspect this is directly related to the 400VA transformer. It looks like they offer two "standard" transformers with their kits: 400VA and 500VA. This kit shipped with the 400VA transformer that has dual ~35Vac secondaries that provide +/- 49Vdc to the amp boards. Their literature indicates that the amps work with power supply voltages of +/-45Vdc to +/-60Vdc. I suspect an additional 20Vdc input to the amp boards would provide the "missing" output power that I noted. I could easily pick up a different transformer, but I'm not convinced that it would be worth it to pick up the additional 30w (or about 1 to 1.5dB at the speaker) of output power.
...yupp, there is definitely no clue in pushing an amp to its supply limits, just for gaining the last 1-2db.
One more information for the HF ripple.
The power transistors of class D amps are usually switching between positive and negative rail. Most designs do this with frequencies between 200kHz-500kHz. The signal information is most commonly coded in a simple pulse width modulation. The advantage of this are low losses, because most of the time the switches are ON or OFF. Just during transition from HIGH to LOW ( some tens of ns) they are doing something pretty lossy...
Between this PWM power signal (i.e. +/-50V rectangular with 300kHz) and the speaker - there is just a simple filter. Usually a LC filter. This filter reduces the high frequency content massively and allows to pass the LF audio signal.
One more information for the HF ripple.
The power transistors of class D amps are usually switching between positive and negative rail. Most designs do this with frequencies between 200kHz-500kHz. The signal information is most commonly coded in a simple pulse width modulation. The advantage of this are low losses, because most of the time the switches are ON or OFF. Just during transition from HIGH to LOW ( some tens of ns) they are doing something pretty lossy...
Between this PWM power signal (i.e. +/-50V rectangular with 300kHz) and the speaker - there is just a simple filter. Usually a LC filter. This filter reduces the high frequency content massively and allows to pass the LF audio signal.
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What makes you think this is the case? My impression (though I'm likely incorrect) is that the output is tuned to a 4ohm load since the 4ohm square waves are much cleaner than the 8ohm square waves. Is there something that I am missing?
Eric
The 4 ohm square waves show overshoot, which can be caused by less effective filtering.
I am not sure how the ClassDaudio amps work in this regard (Hypex UCD is not sensitive to loudspeaker impedance) but normally the output filter in class d amplifiers must be tuned to the load (loudspeaker impedance).
I am not sure how the ClassDaudio amps work in this regard (Hypex UCD is not sensitive to loudspeaker impedance) but normally the output filter in class d amplifiers must be tuned to the load (loudspeaker impedance).
It is the other way around actually - it shows overshoot with the 8 Ohm load. This is quite logical because a higher load impedance means less damping at the filter pole frequency.
The load-independancy of UcD etc is due to the fact that the filter is included in the NFB loop.
Regards
Charles
The load-independancy of UcD etc is due to the fact that the filter is included in the NFB loop.
Regards
Charles
@pieter t:
Please note Eric has written the picture information above the picture, not below.
The overshoot happens with 8 Ohms.
EDIT: Time overlap during posting....
@ wwenze:
...if we want to filter out the 300kHz switching frequency - yes then there will be also pretty much attentuation at 160kHz.
Most typical filters start to roll off second order above 25..-40kHz.
Not really great, on the other hand most high quality dome tweeters start to roll off second order between 17kHz...22kHz. More often class D amps are not suffering from missing treble, but in fact the observed filter peaking at high load impedances can lead to increased treble (i.e. multiple db at 16kHz), which is giving some class D amps a bright sound coloration.
Funny thing that this effect is typical for designs with slow filters.
Last but not least, class D amps with post filter feedback can achieve larger bandwidth than the filter bandwidth and allow reduced influence of the load impedance.
Please note Eric has written the picture information above the picture, not below.
The overshoot happens with 8 Ohms.
EDIT: Time overlap during posting....
@ wwenze:
...if we want to filter out the 300kHz switching frequency - yes then there will be also pretty much attentuation at 160kHz.
Most typical filters start to roll off second order above 25..-40kHz.
Not really great, on the other hand most high quality dome tweeters start to roll off second order between 17kHz...22kHz. More often class D amps are not suffering from missing treble, but in fact the observed filter peaking at high load impedances can lead to increased treble (i.e. multiple db at 16kHz), which is giving some class D amps a bright sound coloration.
Funny thing that this effect is typical for designs with slow filters.
Last but not least, class D amps with post filter feedback can achieve larger bandwidth than the filter bandwidth and allow reduced influence of the load impedance.
First question, of course, is what is the actual load impedance you are driving (at 10kHz or 20kHz). It's unlikely to be the nominal impedance of the speaker, or resistive . . .
Second question derives from that . . . what is the actual application. I use classDaudio amps on my woofers and mids . . . and once past stability considerations really don't care about the effect of filter impedance at ten or more times the crossover frequency, since there's no signal there anyway. If I decide to use them on the tweeters I'll match/mod the filter as necessary, and/or include frequency response compensation (which is there in the crossover anyway, "tuned" for the drivers).
The residual switching frequency component is all but meaningless with an inductive (and mechanical) load . . . it is not easy to push current at a couple hundred kHz through an inductive voice coil (the driver is itself a filter), and what you do push doesn't end up moving any air. Even if it did I wouldn't be able to hear it. The absense of crossover distortion I can hear, though, and I like not hearing it . . .
Second question derives from that . . . what is the actual application. I use classDaudio amps on my woofers and mids . . . and once past stability considerations really don't care about the effect of filter impedance at ten or more times the crossover frequency, since there's no signal there anyway. If I decide to use them on the tweeters I'll match/mod the filter as necessary, and/or include frequency response compensation (which is there in the crossover anyway, "tuned" for the drivers).
The residual switching frequency component is all but meaningless with an inductive (and mechanical) load . . . it is not easy to push current at a couple hundred kHz through an inductive voice coil (the driver is itself a filter), and what you do push doesn't end up moving any air. Even if it did I wouldn't be able to hear it. The absense of crossover distortion I can hear, though, and I like not hearing it . . .
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Sorry for the confusion, guys. Yes, the labels are ABOVE each image. I should have found a better way to make it more clear in my first post.
I used the CDA-1000 kit from ClassDAudio which is basically two CDA-254 stereo amplifier modules, a power supply board (rectifier and caps) and a transformer. I use this 4-channel amp to run my surround sound speakers for my dedicated basement theater (my avatar). Since this was my first Class-D "project," I wanted to see how it compared measurement-wise to my Class-A amps.
I figured the small amount of overshoot into 8 ohms vs. that of 4 ohms is simply a matter of a few pF in an output filter somewhere on the board. I was also thinking that the overshoot into 8 ohms would probably compensate to some degree for the effects of the 35kHz upper -3dB point that I measured and keep the tweeter relatively "flat" until it reached its upper frequency limit. The tweeters that I use in these speakers are Focal TC120TD5's.
Over the weekend, I'll see if I can find some time to hook it to my main speakers and give it a more critical listen. I'm still working on repairing my preamp. I had disassembled it last week to clean some of the wire jumper connections that were causing intermittent connections on my surround speakers. Now that its back together again, the subwoofer output doesn't work when its decoding DD and DTS soundtracks - though it does work with 2-channel PCM such as my CD player and satellite dish and it can generate internal test tones to the sub for setting SPL levels. I figure something didn't get seated properly during the reassembly process - its just a matter of finding it... The fun part is that I have to remove 5 large PCBs just to get to the problem area - what fun!
I used the CDA-1000 kit from ClassDAudio which is basically two CDA-254 stereo amplifier modules, a power supply board (rectifier and caps) and a transformer. I use this 4-channel amp to run my surround sound speakers for my dedicated basement theater (my avatar). Since this was my first Class-D "project," I wanted to see how it compared measurement-wise to my Class-A amps.
I figured the small amount of overshoot into 8 ohms vs. that of 4 ohms is simply a matter of a few pF in an output filter somewhere on the board. I was also thinking that the overshoot into 8 ohms would probably compensate to some degree for the effects of the 35kHz upper -3dB point that I measured and keep the tweeter relatively "flat" until it reached its upper frequency limit. The tweeters that I use in these speakers are Focal TC120TD5's.
Over the weekend, I'll see if I can find some time to hook it to my main speakers and give it a more critical listen. I'm still working on repairing my preamp. I had disassembled it last week to clean some of the wire jumper connections that were causing intermittent connections on my surround speakers. Now that its back together again, the subwoofer output doesn't work when its decoding DD and DTS soundtracks - though it does work with 2-channel PCM such as my CD player and satellite dish and it can generate internal test tones to the sub for setting SPL levels. I figure something didn't get seated properly during the reassembly process - its just a matter of finding it... The fun part is that I have to remove 5 large PCBs just to get to the problem area - what fun!
Last edited:
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I finally had some time to do some extended listening to my new ClassDAudio amp and to compare it to my Class-A DIY Aleph-X amp (if such an act is not considered heresy here in the Class-D forum...). I've had the Aleph-X amps up and running for about a year and a half at this point, so I am very familiar with their sound.
My first impressions of the CDA-254 from ClassDAudio were using an MP3 player as input to this amp to drive my surround sound speakers (these are 8 ohm speaker). I started here because this was the easiest physical setup to complete, though it's far from you'd call a critical listening experience. The shortcomings of an MP3 source were pretty clearly evident, but it sounded very good despite this obvious handicap. Plenty of solid bass, nice highs - not too shrill, plenty of midrange presence to provide some weight and body to the music- nothing seemed out of proportion. So far, so good. This amp sounds like a very nice overall performer!
A few days later, I started moving things around so I could drive my power-hungry Avro open baffle speakers with this amp to see how it compared with my heavyweight Aleph-X Class-A monoblock amps. I was especially interested in this comparison since I've read so many positive comments about this Class-D amp - many people here and on other forums have commented that this little over-achiever has replaced much more expensive amps in their setups. Others have commented on its "tube-like" sound characteristic. As many have also indicated better sonic results with the amp warmed up a little as opposed to listening as soon as it is powered up, I gave it about an hour before I sat down to really listen.
First things first. Turn off the subwoofer (makes it hard to judge bass performance of the individual amps with a 1000w sub running on top of them)... Then I ran through a few CDs including Mark Knopfler's "Sailing to Philadelphia" disk, some Paul Simon, Tracy Chapman, and a few others. The first thing I noticed about this amp was a brighter top end to the treble than what I am used to. This was a bit of a welcome finding as I've sometimes thought the Aleph-Xs were are little light on top for my taste. There was plenty of bass (which is a bit of a trick to get from open baffle speakers), so I was pleased here as well. After listening more critically, I found that if I closed my eyes I was more able to point directly to the speakers when using this amp. The Aleph-X amps are rather adept at making these enormous speakers "disappear" into the room. That is, the sound is clearly there, but it becomes difficult to pinpoint the physical location of the speaker unless your eyes are open. With the Class-D amp, I could point directly to the speakers each and every time. I wonder if this had to do with the higher treble level that this amp produced?
Overall, music sounded great! My Avros didn't quite produce the same imaging and soundstage with the Class-D amp as they do with my Class-A amps. The Aleph-Xs have the ability to create a "layered" presentation, especially with the vocals on Track 5 (The Last Laugh) of Mark Knopfler's "Sailing" disk. Mark's voice is very clearly more diffused and in the background compared to when Van Morrison's vocals enter the song. Van's voice is strong and firmly anchored front and center ahead of Mark's - almost like he is suspended between and out in front of my speakers. It's a startling and eerie experience if you haven't heard it before. The Class-D amp presented things cleanly, but didn't quite have the same imaging and soundstage that I was accustomed to. These differences are noticeable, but not what I'd call "huge."
As I got more and more into the music, I found myself turning the volume level higher and higher. This is where things always get fun! The bass becomes more palpable. Your body begins to resonate with the music... (it's time to turn it down, though, when your vision blurs in unison with the attack of the bass lines...) The last push of the "Vol+" button on the remote, however, engaged the protection mode of the Class-D amps. I'm not sure whether it was the amplitude of the input signal that shut it down, or that I was asking the amp to dump even more current into the dual 12" woofers that dip down below 4 ohms impedance at about 100Hz that caused the problem. Either way, the music was restored as soon as I backed the volume level down a bit. Wondering if it was just a fluke, I pushed the volume level higher again with exactly the same results. Curious to understand where this was happening, I grabbed my SPL meter and began measuring. At the listening position of about 12 feet from the speakers, I was unable to get the speakers to produce more than 95dB (this is very loud in my 14x24 foot room!) before the protection kicked in. Fair enough - I'd rather have an amp tell me "please, no more..." (rather politely, too!) than for it to damage itself or my speakers and leave me on my own to discover the damage later on. At higher volume levels, this amp never seemed congested or distorted - it seemed that the protection circuit engaged before this happened. The top end, though, was a bit more edgy that I was used to and was almost enough for me to want to back the volume down a bit... But remember, we're talking very high volume levels here. Perhaps there is some sweet spot (for me) between the Aleph-X and the CDA-254 in terms of treble brightness. As rdf pointed out just above, this might play out differently with different cables, or the simple addition of a post-amp Zobel network...
OK, swap the wires back to the Aleph-X amps. Wow- more authority in the bass (to what was already a clear presence with the previous amp). Velvety smooth midrange with a bloom that enveloped the entire room. A smooth and sweet top end. Ahhh - that was better. The image was more focused and the soundstage more diffused. The Aleph-X amps can easily push the Avros to the 110dB mark at the listening position, though this is just too much power for my listening room.
All in all, the Class-D amp is a very capable amp! Bass response is strong, though I found the top end a bit bright for my taste. Depending on individual preferences, though, this may be more to your liking than mine. Midrange is clearly balanced with the bass & treble and doesn't sound like a hole between the bass and treble, nor does it sound bloated. The sound stage is diffuse, but the treble calls attention to the speakers making localization relatively easy. I can imagine this literally causing chills when screening movies like "The Matrix" with bullets whizzing by in all different directions! While these amps had a bit of trouble with my power hungry Avro speakers (not really a surprise there), they perform quite well especially with my surround speakers (the intended purpose for this amp). I was able to drive my surrounds to very high volume levels without the protection circuitry cutting in at all.
Overall, I'm very impressed with the price/performance ratio of this amp!
My first impressions of the CDA-254 from ClassDAudio were using an MP3 player as input to this amp to drive my surround sound speakers (these are 8 ohm speaker). I started here because this was the easiest physical setup to complete, though it's far from you'd call a critical listening experience. The shortcomings of an MP3 source were pretty clearly evident, but it sounded very good despite this obvious handicap. Plenty of solid bass, nice highs - not too shrill, plenty of midrange presence to provide some weight and body to the music- nothing seemed out of proportion. So far, so good. This amp sounds like a very nice overall performer!
A few days later, I started moving things around so I could drive my power-hungry Avro open baffle speakers with this amp to see how it compared with my heavyweight Aleph-X Class-A monoblock amps. I was especially interested in this comparison since I've read so many positive comments about this Class-D amp - many people here and on other forums have commented that this little over-achiever has replaced much more expensive amps in their setups. Others have commented on its "tube-like" sound characteristic. As many have also indicated better sonic results with the amp warmed up a little as opposed to listening as soon as it is powered up, I gave it about an hour before I sat down to really listen.
First things first. Turn off the subwoofer (makes it hard to judge bass performance of the individual amps with a 1000w sub running on top of them)... Then I ran through a few CDs including Mark Knopfler's "Sailing to Philadelphia" disk, some Paul Simon, Tracy Chapman, and a few others. The first thing I noticed about this amp was a brighter top end to the treble than what I am used to. This was a bit of a welcome finding as I've sometimes thought the Aleph-Xs were are little light on top for my taste. There was plenty of bass (which is a bit of a trick to get from open baffle speakers), so I was pleased here as well. After listening more critically, I found that if I closed my eyes I was more able to point directly to the speakers when using this amp. The Aleph-X amps are rather adept at making these enormous speakers "disappear" into the room. That is, the sound is clearly there, but it becomes difficult to pinpoint the physical location of the speaker unless your eyes are open. With the Class-D amp, I could point directly to the speakers each and every time. I wonder if this had to do with the higher treble level that this amp produced?
Overall, music sounded great! My Avros didn't quite produce the same imaging and soundstage with the Class-D amp as they do with my Class-A amps. The Aleph-Xs have the ability to create a "layered" presentation, especially with the vocals on Track 5 (The Last Laugh) of Mark Knopfler's "Sailing" disk. Mark's voice is very clearly more diffused and in the background compared to when Van Morrison's vocals enter the song. Van's voice is strong and firmly anchored front and center ahead of Mark's - almost like he is suspended between and out in front of my speakers. It's a startling and eerie experience if you haven't heard it before. The Class-D amp presented things cleanly, but didn't quite have the same imaging and soundstage that I was accustomed to. These differences are noticeable, but not what I'd call "huge."
As I got more and more into the music, I found myself turning the volume level higher and higher. This is where things always get fun! The bass becomes more palpable. Your body begins to resonate with the music... (it's time to turn it down, though, when your vision blurs in unison with the attack of the bass lines...) The last push of the "Vol+" button on the remote, however, engaged the protection mode of the Class-D amps. I'm not sure whether it was the amplitude of the input signal that shut it down, or that I was asking the amp to dump even more current into the dual 12" woofers that dip down below 4 ohms impedance at about 100Hz that caused the problem. Either way, the music was restored as soon as I backed the volume level down a bit. Wondering if it was just a fluke, I pushed the volume level higher again with exactly the same results. Curious to understand where this was happening, I grabbed my SPL meter and began measuring. At the listening position of about 12 feet from the speakers, I was unable to get the speakers to produce more than 95dB (this is very loud in my 14x24 foot room!) before the protection kicked in. Fair enough - I'd rather have an amp tell me "please, no more..." (rather politely, too!) than for it to damage itself or my speakers and leave me on my own to discover the damage later on. At higher volume levels, this amp never seemed congested or distorted - it seemed that the protection circuit engaged before this happened. The top end, though, was a bit more edgy that I was used to and was almost enough for me to want to back the volume down a bit... But remember, we're talking very high volume levels here. Perhaps there is some sweet spot (for me) between the Aleph-X and the CDA-254 in terms of treble brightness. As rdf pointed out just above, this might play out differently with different cables, or the simple addition of a post-amp Zobel network...
OK, swap the wires back to the Aleph-X amps. Wow- more authority in the bass (to what was already a clear presence with the previous amp). Velvety smooth midrange with a bloom that enveloped the entire room. A smooth and sweet top end. Ahhh - that was better. The image was more focused and the soundstage more diffused. The Aleph-X amps can easily push the Avros to the 110dB mark at the listening position, though this is just too much power for my listening room.
All in all, the Class-D amp is a very capable amp! Bass response is strong, though I found the top end a bit bright for my taste. Depending on individual preferences, though, this may be more to your liking than mine. Midrange is clearly balanced with the bass & treble and doesn't sound like a hole between the bass and treble, nor does it sound bloated. The sound stage is diffuse, but the treble calls attention to the speakers making localization relatively easy. I can imagine this literally causing chills when screening movies like "The Matrix" with bullets whizzing by in all different directions! While these amps had a bit of trouble with my power hungry Avro speakers (not really a surprise there), they perform quite well especially with my surround speakers (the intended purpose for this amp). I was able to drive my surrounds to very high volume levels without the protection circuitry cutting in at all.
Overall, I'm very impressed with the price/performance ratio of this amp!
Eric, thanks for the extended review. From your description, I doubt that the Classd amps are a match for my modded ASL Monsoons. I think I will pass. Now if we could just have a similar review of the new Hypex amps, I could rest in peace. My current amps driving Paradigm Sig1v3s allows them to completley disappear leaving only a full orchestra in all its panoply. I was hoping that some class d amp could do the same. Unless the Hypex does it, it seems that we are not there yet. Thanks again for the time and trouble. Kindest regards John Dozier
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