• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required. There are many reviews of audio hardware and expert members to help answer your questions. Click here to have your audio equipment measured for free!

AIYIMA A07 TPA3255 Review (Amplifier)

One area where I often find myself with a bit of confusion with low-cost class D amps is with the output impedance. I recall that in some cases Amir has measured the output impedance and mentioned it in the text overlaying the graphs. Perhaps I missed it, but I didn't see it with this one. I wonder if there is a reason why it isn't relevant and whether I'll end up feeling stupid when someone explains it. As Amir and others have variously pointed out, if the output impedance is not sufficiently lower than the nominal speaker impedance, the frequency response of the speaker will be altered, due to frequency-dependent variation in the speaker's proportional share of the voltage output by the amplifier output device. (The "damping factor" is the usual way of expressing this, however the damping factor can be misleading since it is calculated using an assumed nominal speaker impedance rather than the true nominal impedance of actual speaker.)

The output impedance is of course primarily a consequence of the low-pass filter customarily used to block the ultrasonic frequencies that are inherent with class D. This is why the output impedance of a class D amp is characteristically much greater at 20 kHz than at 1 kHz. Eight or ten months back I spent a little time one evening looking over TI's spec sheets and literature for several of their class D chips. I recall that TI advocated "filterless Class D", omitting the low-pass filter altogether. This begs the question of why this filter has customarily been used if it isn't needed.

In any case, I struggle with the understanding of why the output impedance wouldn't be a fundamentally important consideration for class D amplifiers in general, and especially with low-cost class D amplifiers, since the way in which this fundamental concern is addressed by the amplifier designer seems to be a major differentiating factor for low-cost class D amps vs. the expensive ones. Someone here may be willing to shed some light on this. I suppose that it probably has to do with the feedback topology. While any explanation that is half-decent will be welcome, I hope that it will not draw attention away from the salient question of why it would not be fundamentally important, with class D amps generally and particularly, to pay close attention to the output impedance.
 
It is mostly the output LC filter to define output impedance of the Class D amp.
 
I bought several Chinese TPA3255 boards 2 years ago. They sound great, but 2 died, most probably they can't take 48V. I read somewhere to drive them below 40V.
For the money I buy now Behringer A800's. 150 euro, great sounding, much more powerful and power supply included.

I've been powering two A07s with 48VDC for the better part of a year with nary a hint of impending failure -- and spending $50-65 every few years sounds like a reasonable expense for decent performance without a big one-time expenditure. In fact, I'm on the verge of buying two more of these little wonders -- one to replace a relatively elderly TAS5630 2.1 (I don't use or need the subwoofer channel) "chip amp," the other as a spare -- for less than $100 USD with free shipping. I have nothing against Behringer gear and use one of their "U-Phoria" audio interfaces as my DAC -- but my first one of those failed just as the warranty expired. I hope you have better luck with your A800 purchase(s)!
 
Happy new year Amir. Hey, this looks like a nice little amplifier, with great bang for the buck. Thanks for the review!!
 
Happy N.Y. all....

Just wanted to put this new class d implementation of the "Axign" design on everyone's radar displays...

Apologies if this has been mentioned b4....

Was initially only available in EU and UK, but now B+H has it on their site....for $600 might be interesting????

Love to see this get Amir's attention on the AP.....


https://audioxpress.com/news/harman...ring-axign-ax5689-class-d-controller-solution
 
I still like the NE5532 ( used in the module) because of its well balanced parameters. I prefer it to LME49720, because, according to my measurements, it is more resistent to EMI, even if it has slightly higher distortion - which remains below audibility threshold.
Do the EMI levels hit audible levels on the LME49720? Thinking of getting one of these now :p
 
Although I have been viewing this site with great fascination for more than a year, I must say I don't understand a lot. Regarding measurements, is Intermodulation Distortion (IMD) not also of high importance in evaluating and comparing amplifiers? Can the AP device measure this? Might IMD be poor in some amplifiers for which the provided measurements are good? If IMD can be measured and shown to be good, perhaps that can give further confidence or insight in comparing different amplifiers to each other (e.g. class D to class A/B etc). I'm pondering which direction to take in upgrading my amp...
 
View attachment 102787
They both feel kind of cheap as they would be at these prices ($21 for the 48 volt one). Personally I would not leave these plugged in permanently if I am not around. The included power supply has a two-pronged AC cable that was not polarized. Rotating it around made a small difference in mains leakage.
So, if the power supply is a weak link, can someone suggest a good replacement that offers more confidence in safety (and maybe a little more performance as well)?
It would make sense for it to be a high-value option (ie, not twice the cost of the amp unit).
Is there a trusted company that makes good power supplies without breaking the bank?

Note: just for the record, I do not intend to buy one of these (don't have the need), but figure there are others wondering what to substitute for the power supply given Amir's comment above. I am also a bit interested because every now and then I will see a used product which does not include the power supply and finding a good brand to trust for replacement PS would be a good thing!
 
So, if the power supply is a weak link, can someone suggest a good replacement that offers more confidence in safety (and maybe a little more performance as well)?
It would make sense for it to be a high-value option (ie, not twice the cost of the amp unit).
Is there a trusted company that makes good power supplies without breaking the bank?

Note: just for the record, I do not intend to buy one of these (don't have the need), but figure there are others wondering what to substitute for the power supply given Amir's comment above. I am also a bit interested because every now and then I will see a used product which does not include the power supply and finding a good brand to trust for replacement PS would be a good thing!

I find this product to be interesting. Credit to @DosThou .
https://www.ebay.com/itm/Douk-Audio...er-Transformer-US-EU-UK-AU-Plug-/283456878291
There is a 32v 5A version.

It cost double the price of a generic one but claims to be better built. Anyone had experience ?
 
Last edited:
I find this product to be interesting. Credit to @DosThou .
https://www.ebay.com/itm/Douk-Audio...er-Transformer-US-EU-UK-AU-Plug-/283456878291
There is a 32v 5A version.

It cost double the price of a generic one but claims to be better built. Anyone had experience ?

Just going by the specs -- and its likely decent quality notwithstanding -- this thing will not unlock the A07's potential, not even close. Aiyima's recommendation of a 48VDC 7.5A SMPS is at the lower edge of what it takes to unleash that potential.

This is what powers the A07 that drives my main L+R speakers -- I got mine for $25 USD via eBay. I like its voltage adjustment knob, LED voltage display, and thermostatically-controlled fan -- but those are "bells & whistles" and virtually any unit with similar specs will do. You'll need to do a little DIY and buy a few $ worth of parts -- a three-prong AC mains cord, a suitable coaxial DC plug, and a length of 2-conductor cable (ordinary 18 AWG "zipcord" will do) to carry the DC to the amp -- in order to get it connected up, but IMO it's much more than worth that modest cost and effort.

HJS-480-0-48 SMPS

s-l500[1].jpg
 
One area where I often find myself with a bit of confusion with low-cost class D amps is with the output impedance. I recall that in some cases Amir has measured the output impedance and mentioned it in the text overlaying the graphs. Perhaps I missed it, but I didn't see it with this one. I wonder if there is a reason why it isn't relevant and whether I'll end up feeling stupid when someone explains it. As Amir and others have variously pointed out, if the output impedance is not sufficiently lower than the nominal speaker impedance, the frequency response of the speaker will be altered, due to frequency-dependent variation in the speaker's proportional share of the voltage output by the amplifier output device. (The "damping factor" is the usual way of expressing this, however the damping factor can be misleading since it is calculated using an assumed nominal speaker impedance rather than the true nominal impedance of actual speaker.)

The output impedance is of course primarily a consequence of the low-pass filter customarily used to block the ultrasonic frequencies that are inherent with class D. This is why the output impedance of a class D amp is characteristically much greater at 20 kHz than at 1 kHz. Eight or ten months back I spent a little time one evening looking over TI's spec sheets and literature for several of their class D chips. I recall that TI advocated "filterless Class D", omitting the low-pass filter altogether. This begs the question of why this filter has customarily been used if it isn't needed.

In any case, I struggle with the understanding of why the output impedance wouldn't be a fundamentally important consideration for class D amplifiers in general, and especially with low-cost class D amplifiers, since the way in which this fundamental concern is addressed by the amplifier designer seems to be a major differentiating factor for low-cost class D amps vs. the expensive ones. Someone here may be willing to shed some light on this. I suppose that it probably has to do with the feedback topology. While any explanation that is half-decent will be welcome, I hope that it will not draw attention away from the salient question of why it would not be fundamentally important, with class D amps generally and particularly, to pay close attention to the output impedance.

Because if the effect of output impedance is the frequency response, then we already have that measured. Output Z from the filter is minimal at 1kHz, while at 20kHz it is high enough to cause the FR to dip a bit. Or even ring.

"This begs the question of why this filter has customarily been used if it isn't needed."
- Because it is customarily been needed. It still is needed today. Filterless class D have their own set of limitation, like generally not allowed to have long cables / being limited to the same enclosure as the driver.

Or maybe we think of it another way. The class D amp itself, without the filter, creates a ton of extra frequencies. We don't want that. Hence we add the filter, which has high impedance (seen from amplifier direction), to alter the frequency response to -9999dB. But at the same time a properly designed filter will also have 0dB insertion loss in the passband.
 
Last edited:
Based on many complaints on short life time of the amp, I guess there are thermal issues if 48V PSU is used and low impedance load. Local overheating and death of the chip.
 
  • Like
Reactions: KEW
Back
Top Bottom