Because the TI needs to install a high-quality modulator, high-quality low-noise operational amplifiers and high-current high-speed keys at the output in a cheap, small-sized product. This is very difficult, if not impossible. In integrated amplifiers of class D, the output transistors always have worse parameters than discrete MOSFETs in the TO220 package. Simply by definition. One high-quality MOSFET will cost more than the entire TPA3255 microcircuit. TPA3255 is a budget one and is tailored for the function "satisfactory for little money". And not for the function "excellent for little money".
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2025: Does somebody needs Hypex etc...?
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I guess that depends on what you consider "excellent" vs "satisfactory".
With realistic audio signals, and into common loads, it seems well-designed TPA3255-based amps are capable of extremely clean audio reproduction, as shown here.
But there are indeed other designs that can provide equally clean reproduction even with unrealistic and dificult signals, which can produce more power and drive more difficult loads, and which can dissipate more heat effectively.
Some use-cases may require such capabilities, in which case TPA3255-based designs are simply unsuitable.
In other cases even a good TPA3255 design would be massive overkill.
With realistic audio signals, and into common loads, it seems well-designed TPA3255-based amps are capable of extremely clean audio reproduction, as shown here.
But there are indeed other designs that can provide equally clean reproduction even with unrealistic and dificult signals, which can produce more power and drive more difficult loads, and which can dissipate more heat effectively.
Some use-cases may require such capabilities, in which case TPA3255-based designs are simply unsuitable.
In other cases even a good TPA3255 design would be massive overkill.
I take it from your Avatar and Setup that you're a bit of a NAD Master Series fan.
Me too, although I'm cooling a bit as they seem to be dropping useful features with each new Masters product! For example:
My earlier M12 and M32 allowed me to set a different (or no) crossover when I connected subs. Thus, I could run my main speakers full range while the subs acted to resolve room modes, etc. The M33, M66 (and probably M10) don't allow this. As soon as you mention via Settings there's a sub present you have to specify a single XO and this applies to both the main and all sub speakers. No good if you have paid good money for full-range main speakers only for NAD to cripple them!
The M33 and M50.2 allowed the album artwork or radio station notes to be output to an HDMI-connected TV. This useful feature (especially when you have friends around) is denied us on the M66.
The HTRM-2 remote that was supplied with most earlier Master units offered far more control than the SRM-1 that's supplied with the M66 and I believe the new M33 V2. For example, you can't scroll through the Inputs via the Up and Down buttons as you can with the earlier remote
Volume could be very accurately controlled by 0.5 dB steps with the earlier kit, but the M66 jumps 2 or 3 dB for each touch of the remote or the app volume bar. There are no 0.5 dB steps available.
I've raised these points with NAD following my recent M66 purchase and am anxiously anticipating a fobbing off reply from their Customer Service expert fobbing off team! But perhaps they'll surprise me by saying these missing or reduced features will be restored with the next firmware update - I'm not holding my breath!
I must say I am most disappointed that the M66 falls short of my expectations, specially after being generally very pleased with the M33 and earlier (M12, M32 and M50.2) units.
Mike_Trueman
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Yeah, i am using the NAD every day for nearfield listening and i am enjoying the music it plays.
My KEF's need the crossover (HPF).
What do you mean by cripple them? Here are my NAD subwoofer settings.
Do you mean that you can not disable the crossover?
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Guddu
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I am late to this party but is this very graph not very similar for any class D? I mean TPA3255 / Hypex / Purify, specially above 5 KHz*.
*irrespective of the fact if it does not matter much from auditability point of view.
I started a thread sometime back to understand how to read this graph properly for meaningful conclusion and all I could get to is no point in looing beyond 5/10 KHz.
You can think probably 10/15/20 KHz should be excluded to avoid any confusion but probably its there for some purpose.
Yeah, i am using the NAD every day for nearfield listening and i am enjoying the music it plays.
My KEF's need the crossover (HPF).
What do you mean by cripple them? Here are my NAD subwoofer settings.
Do you mean that you can not disable the crossover?
View attachment 467849
Thanks for your reply. The screenshot you show is similar to those for the M33 and M66 regarding the Subwoofer settings.
What I mean by "crippling" my main speakers is that NAD will cut off the legs of their performance! The main speakers won't receive and therefore can't deliver anything below the chosen XO frequency. Good news for most small main speakers, but if you own ones capable of delivering the full frequency range (as mine can from their twin 12" drivers) you are deprived of the main purpose and justification of your big expenditure in buying these full-range speakers.
By contract, the M12 and M32 (predecessors of M66 and M33) allowed me to choose any XO between 40 - 200 Hz, or No XO at all. Thus I could keep the full range going to my main speakers, while the subs receive bass only. Here's the screenshot from the User Manual. Furthermore, you'll notice that these older units allow the ubs to receive a stereo signal - M33 and M66 are Mono only. Also the relative volumes of Main and Subs can be adjusted. Why has NAD deprived us of these useful (arguably essential) features?
Mike_Trueman
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Prolly to keep it simple for people like me.
You could always use speaker cable splitters and the RCA adapters for your sub but yeah it is more work.
TurtlePaul
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I think that one of the biggest differences is that Hypex has 40-50 dB of open loop gain across the audio spectrum while TI cites 21.5 dB. The TI PFFB sample implementation shows 6-7 dB of PFFB to keep gain in the high teens. With its huge open loop bandwidth, the Hypex is able to use a ton of negative feedback. If the TIs gain more s falling at higher frequencies, it ends up with little feedback to suppress distortions. I think the low gain on the TI may be because the chip has an internal feedback loop to decrease noise (and that feedback is therefore not available to use post-filter).
Also, Purifi uses a 10% higher self oscillation frequency and has only 20 dB of attenuation at the 500 khz frequency, so they push the LC filter and the associated distortion from the inductor farther out of the audible band.
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Prolly to keep it simple for people like me.
You could always use speaker cable splitters and the RCA adapters for your sub but yeah it is more work.
View attachment 467861View attachment 467859
Yes, of course (in fact the M33 has 2 sets of speaker terminals so no need for splitters) but by doing this, you don't benefit from Dirac or DLBC with the M66
In fact, I'm about to rewire the subs so they get a high level signal direct from the amps
PS - You can't connect subs using amp + and - speaker terminals if the amp is Class D. You have to connect to + and Earth
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