Part I: Topping PA5 Mk II Plus (PA5II+) Class D amplifier review. (And RME ADI-2 Remote, "cultural pressure" for subjectivist audiophile reviews to include measurements?)

Over the last few months, I've been exploring the performance of the TI TPA3255 Class D chip amps including the AIYIMA A08 Pro, Fosi Audio TB10D, and AOSHIDA A7. Overall, we've seen very good results from these little amplifiers and I think they represent a level of low-distortion performance at a price point that is very attractive to many hi-fi hobbyists.

Depending on the power supply mated to the amplifier, measurements have been showing low output impedance up to 10kHz typically (<0.2Ω up to 10kHz), something like 80+W into 4Ω with less than 0.1% THD+N, and a -80dB Triple-Tone TD+N result at 1W into 4Ω which is excellent considering the prices of these little amps typically at less than US$150 depending on the features you want like Bluetooth input, VU meter, etc.!

Given the track record of these TI devices so far, this time, let's look at another chip amp which is based on an undisclosed part, not the TPA3255, although I can confirm it is a TI TPA chip. In my discussions with John Yang at Topping, the intent is to allow audiophiles to judge the product based on sound quality merits rather than focus too much on exactly which Class D part was used since the design around the chip is very important. Fair enough, and that's exactly what we aim to do here anyways since even with the previous TPA3255 amps reviewed, there were objective differences to find even if subjectively, differences might not be as noticeable.

I bought this Topping PA5 Mk II Plus (which I'll abbreviate as PA5II+ for simplicity) myself from Amazon, currently around US$325 with the intent to use in my own system. Since I'm interested in "high-fidelity" (not merely "euphonia"), I was interested to see how well this would compete with the DIY Hypex nCore NC252MP we measured a few years back. I like the feature set that it offers. On paper, it should provide the power I need and if the hi-fidelity performance is excellent, it could certainly be a listening "reference" here even though to be honest, I still love the Hypex's capabilities and would have no issue with enjoying the music from the inexpensive TPA3255 devices either.

So, let's start with the PA5II+ specs and appearance. Here's a picture of the open box and parts that it came with:

As usual, nothing fancy; I prefer companies put the money into the device rather than box-opening experience.😉 It came well-packaged with foam surrounding the amplifier and the power supply, cables tucked away so no scratches during transportation. Here's the power supply and the locking "aviation" connector which is more robust than less expensive amps although it would be more of a hassle to find a replacement power supply if needed.

When plugged in, there's a green LED on the other side to indicate that it's active.

It's a switching power supply rated as 38V/4A for this "Plus" model, an increase over the 38V/3A for the less expensive non-plus Topping PA5 Mk II (US$250). On paper, using a high threshold of 10% THD, the difference in power into 4Ω is advertised as 140W for the Plus, and 100W for the non-Plus. Of course we'll see using measurements the power output for ourselves as I typically rate peak power at 0.1%.

Other than the power supply, there's a basic leaflet manual and information card. Warranty is 1 year.

Up front there are the volume knob and switches:

Size comparison of the Topping PA5II+ with the AOSHIDA A7 below which is wider but not as deep. Notice that the PA5II+ is turned on, small white LED lit. I didn't peel off the plastic protectors yet...

The large volume gain knob to the right feels smooth with decent resistance. And on the left side are the power switch (On, Off, or 12V Trigger) and TRS balanced or RCA single-ended input switch. This device is a balanced amplifier which implies benefits with lower noise and distortion when fed a clean signal to the TRS input. Notice that the power LED is a small white point; I don't think anyone would complain that it's "too bright"! If anything, given the small size, if used in a home theater or media room, it could be hard to tell if the light is on sitting a few feet away. Compared to the amber UV meters of something like the AIYIMA or AOSHIDA amps listed above, this device is on the very unobtrusive side.

This amp does not have a remote control but this is the intent of the convenient 12V trigger. You can pair the device with a pre-amp with trigger mechanism to turn on/off. I very much appreciate this feature and I prefer it over the less elegant Iot Relay which still works but abruptly cuts off power which creates more of a "click" to the speakers that the gracious silence using the 12V trigger.

Here's a look behind the box:

We can see the TRS and RCA (under the rubber covers) inputs. A note to folks with unusually high output impedance DACs and preamps, the PA5II+'s TRS input impedance is 10kΩ and the RCA is around 2kΩ (you could use a RCA to TRS adaptor for higher unbalanced input impedance). In principle this should not be a problem with modern DACs which typically have <200Ω balanced (TRS/XLR) and <100Ω RCA output values. Some device specs will publish the output impedance values such as the Topping D10s with 20Ω RCA out, Topping D10 Balanced 88Ω TRS, SMSL DO100 100Ω XLR, Sabaj A20d 2022 100Ω XLR, etc.

It's nice to have wider ratio of output to input impedance to reduce voltage drops but with line-level input, we're typically dealing with resistive loads within the audio spectrum and not the complex reactive impedance with potential frequency fluctuations of speakers or even headphones with their crossovers and voice coils. Something like 1:10 should be OK, better at 1:20, and no need to get hung up on even bigger ratios like 1:100.

To the right of the input connectors are the 12V trigger, a pair of speaker binding posts, and the locking 38V 2-pin power supply connector. Not the most sturdy speaker binding posts but they work just fine and my banana plug speaker cables were held tightly. Notice that there's ventilation out back as well as the sides and at the bottom of the metal enclosure which feels sturdy in the hand:

As with other Class D amplifiers, the technology is highly efficient, up to 90+% at higher output levels, thus running relatively cool. There's some mild warmth if you push the output level over a few minutes but the box never gets uncomfortably warm. Class D amps are also less heavy and can be smaller - this box weighs 700g without power supply, about 13cm wide x 16cm deep x4.5cm tall. Obviously also less expensive.

I did my usual procedure of putting the PA5II+ in my sound system for 2 weeks to listen with music and movies to form some subjective opinions prior to measurement-testing. Since these days life is more busy than usual, for this Part I article, let me start with some standard measurements like output impedance and frequency response. In Part II, we'll look at power, distortions, and subjective opinions.

Quickly, let me highlight the test system for these measurements. For signal source, I will use the dual-ES9038Q2M SMSL DO100 DAC which has both RCA and XLR outputs. As per my previous review, performance of the DAC is excellent with better than 120dB dynamic range, and THD+N of around -119dB RCA and -122dB XLR output; this level of precision would be better than current amplifiers. Depending on what I'm measuring, the RME ADI-2 Pro FS as ADC is already excellent. For more precise distortion and power measurements (Part II), I will use the E1DA Cosmos Scaler prototype + Cosmos ADC (MONO mode).

As usual, with power amplifiers, I'll use Linear Audio's AutoRanger and the typical 4Ω and 8Ω non-inductive resistor loads as indicated. Unless specified, all digital signals at 24/96.

Volume gain, switching noise

To start, I double checked the voltage gain provided by this amp with the volume knob turned to maximum:

        +24.9dB RCA input

        +18.9dB TRS input

Very close to the specs sheet from Topping with 25dB RCA and 19.2dB balanced listed. Note that this is not the lowest gain amp I've measured over the years; that distinction would go to the Pass ACA v1.1 with +16dB gain, RCA input.

Let's have a peek at what a 1kHz 1Vrms sine wave looks like using the Topping PA5II+. No filtering applied to the signal so we can intentionally see the Class D switching noise:

Indeed, the switching noise is present. However, this is different compared to other TI TPA amps I've examined. The switching frequency looks a bit higher and more attenuated compared to something like the AIYIMA A08 PRO.

Using a wide bandwidth, but lower resolution FFT on the oscilloscope, we can better characterize the nature of that switching frequency:

That looks pretty clean. The switching frequency is up at 600kHz and we see that the level is attenuated by about -30dB compared to the 5kHz tone at 1Vrms. Otherwise, there's no obvious noise all the way to 1.5MHz. Notice that the 93kHz signal has been attenuated as well which is a reflection of ultrasonic roll-off presumably due to filtering applied.

Output impedance

Impressive low output impedance across the whole audible spectrum. This is significantly better than what we saw with the TPA3255 amplifiers previously.

Translated to damping factor using a 4Ω load:

Nice. With damping factor >100 across the audio spectrum, we should be looking at quite a "load invariant" amplifier.

Frequency response

Let's see if we can prove this "load invariance" characteristic with some frequency response measurements:

Overall, looking good. There's a little bit of high frequency roll-off into 20kHz of about -0.3dB for both the 4 and 8Ω loads. A little more roll-off of -0.5dB into 20kHz with the actual Sony bookshelf load but that 0.2dB difference between actual speaker and resistive loads at such high frequencies would be of no audible consequence. There is less than -1dB roll-off out to 40kHz which is great.

Looking at an even wider bandwidth frequency response measurement (not shown), roll-off is about -2.5dB with the 4Ω load by 50kHz whereas the 8Ω and Sony speaker loads rise a little by 45-50kHz to -0.25dB (using the SMSL DO100 DAC which like most DACs has its own roll-off), then dip to -3dB by 60kHz. Given that some speakers can show tweeter "ringing" (cone break-up) around 25-40kHz, it's good that the frequency response is not increasing into this low-ultrasonic range.

On the bass end, it's about -0.4dB at 20Hz, -1dB at 10Hz. Again, nothing to be concerned of at all. This amp will have no difficulty feeding well-controlled bass to your speakers.

Above, we see the 4Ω load frequency response and phase relationship. No problem at all. And here it is with the Sony bookshelf speaker load. For reference, I've included the Sony SS-H1600's impedance and phase graph.

Again, no concerns. Overall the PA5II+ does not show as much variability in frequency response as we've seen with TPA3255 amplifiers thanks to the much lower output impedance (higher damping factor) basically across the audible spectrum. Comparatively, with TPA3255 amps, an 8Ω load can typically cause a frequency rise into 20kHz; whether that is an issue depends on speakers and the person listening, but should still be relatively subtle.

Stereo crosstalk

Since this is a stereo amp, let's have a quick look at crosstalk. Note that crosstalk doesn't really need to be very high especially when we're talking about speaker amps since there's inherent cross-contamination between the ears in listening rooms (hence the idea of employing crosstalk cancellation to improve soundstage). 

Using a 4kHz tone in the left channel, and a 300Hz signal in the right, playing at 0dBFS independently, let's see how many dBs seep through between the channels, first with RCA input:

And now here's with the balanced TRS input:

We can see the superiority of the balanced input with the significantly lower stereo crosstalk (>20dB better). Other factors such as the DAC itself and RCA cable used will play into the result. In any event, as I mentioned above, it would be very unusual for stereo crosstalk to be an issue using speakers and it would also be very odd to have music with very low noise level plus strong "hard pan" between left and right channels (maybe some of the early stereo mixes).

While I'm not trying to demonstrate distortion levels yet (and am not employing anything special like USB isolators), we can see in the FFTs above early hints as to the low noise floor already.

Summary, Part I...

So far what we're seeing objectively is very encouraging.

The Topping PA5 Mk II Plus is a relatively inexpensive (currently around US$325) Class D amplifier with well-attenuated 600kHz switching noise. While the maximum amplifier gain of +25dB RCA and +19dB TRS is lower compared to many other amps, this should not be a problem with 2Vrms RCA outputs or 4Vrms balanced (1.4Vrms RCA and 2.7Vrms balanced enough to achieve full rated power). If you're integrating this amp into a multichannel system or mixed with other amps, just make sure you have a way such as level settings in an AV receiver to match output levels.

It's great to see the higher damping factor (lower output impedance). This is reflected in the frequency response graphs showing significantly less load variance compared to most other Class D devices I've examined. The frequency response shows less than 1dB roll-off out to 40kHz which is great.

Crosstalk is very low and we can see the benefit of using the balanced input with easily better than 100dB channel separation.

One reason I had actually not tried the original Topping PA5 "Mk I" was because of concerns around a high failure rate (as per this poll thread, there's repair module information for those with issues). It is good to hear that the warranty on those was extended to 2 years and refunds offered by the company. While the Mk II here is a new product, time will tell if there are any issues. It's probably reasonable to assume that with Topping aware of the concerns from the first time around, they'll be much more vigilant of potential issues!

Next time, in Part II, we'll examine the power and distortion characteristics of this little amplifier. Maybe we'll even talk about subjective impressions... ;-)

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For those using the RME ADI-2 line of devices (like their DAC or latest ADI-2/4 Pro SE ADC/DAC), you'll know that these boxes can be quite complicated with all kinds of features to turn on/off as well as DSP settings. This is clearly not a bad thing and is reflective of the power of these devices! However, it could get complex when you're deep in the multi-level submenus trying to figure out which knobs and buttons to push.

Check out the new RME ADI-2 Remote Software available for Mac, iPad, and Windows! I've been using it with my Windows 11 test system for the measurements above and it's really very convenient to adjust settings quickly or confirm values. Also very easy to set EQ. Make sure to update your firmware and OS drivers to support the app.

Excellent stuff and amazing example of customer support from RME over the years. Other companies need to take note. This is how it's done right.

One last observation. Over the last few years, I think it's great that audiophile discussions overall seem to be getting more technically oriented and measurements are becoming more important. Whether with headphone measurements, or talking about DACs and amps, the performance figures will help discerning consumers to at least make sure that what they're buying is well-engineered. However, I think "subjectivist" audiophiles who aren't particularly versed in technical matters should really refrain from publishing measurements unless they've done a bit of research and have a clear idea of what they're trying to show.

For example, I see The Absolute Sound has this video review of the Schiit Freya+ and LISST preamp posted with some measurements at the end starting around here:

What the heck is that!? Clearly, it's more than a little weird to be tacking on some REW graphs into the end of the review with no meaningful context.

Anyhow, maybe TAS can get some more objective folks over there to fix things up and put the measurements for viewers in a way that is somewhat useful if they're going to be serious about it. Measurements and the presentation of the data require a level of understanding in order to interpret the significance. You can't just throw up a bunch of REW acoustic measurements and call it a day! Objective testing requires a much more demanding level of discipline than just opening one's mouth or putting finger to keyboard and writing creative stories about what one supposedly heard (or did not hear).

I'm guessing that within the audiophile community, there's now some cultural "pressure" for pure subjective reviewers to take steps to introduce some objectivity/measurements in order to at least appear like they know what they're talking about. Or at least not look so embarrassing with their claims. If this is the case, that's excellent, and this ultimately will be for the betterment of the hobby.

I see that Darko has been doing REW room measurements. Others, like Hans Beekhuyzen has been adding measurements as well even though I think he still has strange beliefs in his hearing abilities and interprets things in ways he wants to believe especially around unnecessary products he seems to be trying to sell. :-|

Alright audiophiles, off to get some other stuff done and I will be back with more on the PA5II+ later in Part II. Going to check out the Rolling Stones' new Hackney Diamonds this weekend (can be streamed multichannel/Atmos as well). First original album from the boys in 18 years (since 2005's A Bigger Bang although they've had new music like "Living In A Ghost Town" released during the pandemic). 18 years is like a generation ago!

Hope you're enjoying the music!