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Old 22nd December 2011, 02:43 PM   #1
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Default classdaudio vs hypex

Has anyone heard these amps in the same system? If so how do they compare. I have narrowed down my choices to these two. I would like some real world comparisons of the two. Thanks
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Old 19th January 2012, 01:21 PM   #2
Eric is offline Eric  United States
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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:
Click the image to open in full size.

1kHz squarewave input, 10vRMS output into 4-ohm resistive load:
Click the image to open in full size.

1kHz squarewave input, 10vRMS output into 8-ohm resistive load:
Click the image to open in full size.

10kHz squarewave input, 10vRMS output into 4-ohm resistive load:
Click the image to open in full size.

10kHz squarewave input, 10vRMS output into 8-ohm resistive load:
Click the image to open in full size.

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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Last edited by Eric; 19th January 2012 at 01:31 PM. Reason: adding clarification
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Old 19th January 2012, 04:00 PM   #3
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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....
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Old 19th January 2012, 04:41 PM   #4
Eric is offline Eric  United States
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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
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Old 19th January 2012, 04:50 PM   #5
rdf is offline rdf  Canada
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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.
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Old 19th January 2012, 05:20 PM   #6
Eric is offline Eric  United States
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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.
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Old 19th January 2012, 08:40 PM   #7
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...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.
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Old 19th January 2012, 09:50 PM   #8
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It seems that the output filter of that ClassDaudio amp is tuned to an 8 ohm load.
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Old 19th January 2012, 10:32 PM   #9
wwenze is offline wwenze  Singapore
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160kHz @ -20dB is normal...?

Last edited by wwenze; 19th January 2012 at 10:37 PM.
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Old 20th January 2012, 12:29 AM   #10
Eric is offline Eric  United States
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Quote:
Originally Posted by pieter t View Post
It seems that the output filter of that ClassDaudio amp is tuned to an 8 ohm load.
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
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