Complex impedance load amp FR influence - WiiM Amp review by Erin

[grogi.giant]

/.../ I still stress that we’re talking about relatively small* differences, which in any case only occur when this extremely low cost device is pushed.

The inaccuracies of frequency response are effectively the same, regardless of how hard the device is pushed.

 

[Yorkshire Mouth]

The inaccuracies of frequency response are effectively the same, regardless of how hard the device is pushed.

Apologies, cross wires (no pun intended).

The point is, the deviations are small, and whilst they could be picked up in an immediate A/B comparison, I don't think you'd notice them in isolation.

Meanwhile, deviations in your speakers are likely to be greater.

Subsequently, as we're talking about a device with an amp section costing <£100, it's hard to complain, and churlish to paint it as some sort of unmitigated disaster.

As I say, I look forward to the upcoming WiiM Ultra, and any future pre amp from them which render this discussion irrelevant.

Best wishes.

 

[grogi.giant]

Subsequently, as we're talking about a device with an amp section costing <£100, it's hard to complain, and churlish to paint it as some sort of unmitigated disaster.

But WiiM AMP doesn't cost <100 monies... It is in the region of $€£300-350 - and at that price point I expect certain level of performance...

The load dependency of TPA325x is not something discovered yesterday. WiiM certainly invested a lot of money to design and tightly integrate the AMP, but did not go forward with another $3 of elements to enable PFFB...

 

[Yorkshire Mouth]

But WiiM AMP doesn't cost <100 monies... It is in the region of $€£300-350 - and at that price point I expect certain level of performance...

The load dependency of TPA325x is not something discovered yesterday. WiiM certainly invested a lot of money to design and tightly integrate the AMP, but did not go forward with another $3 of elements to enable PFFB...

Which is why I said the amp stage.

This is a £220 Pro Plus with an amp stage tagged on.

 

[Miker 1102]

Your first sentence says this product is unacceptable. Many would beg to differ.

Both Amir and Erin’s reviews suggest the issues discussed are only really a problem in large rooms/longer seating distances/tough to drive speakers.

Most of the issues are at over 10 kHz. I’m not sure more than half us (the older half) can hear a 1 dB deviation between 10 kHz and 20 kHz.

I agree. My room and current speakers in combination with this amp and a small subwoofer produced an wonderful listening experience. I am 53 years old and used to listen to a transistor radio. My wife and kids enjoyed the way this looks and sounds. I cannot even remember the last time that happened.

 

[Deleted member 48726]

Then you've not been looking. I think it is common in Amir's reviews.

Marantz Cinema 40 AVR Review

This is a review and detailed measurements of the Marantz Cinema 40 9.4 channel AV Receiver (AVR). It is on kind loan from a member and costs US $3,500. As is typical in this class, you get a very large box. At least the look is unique and to my liking. The port hole display while limiting...

www.audiosciencereview.com

Or:

Anthem AVM70 Review (AV Processor)

This is a review and detailed measurements of the Anthem AVM70. It was sent to me by the company for review and costs US $3799. The AVM70 has a very modern and attractive front panel and display: Company takes full use of the large display as you see with nice icons. Importantly, the...

www.audiosciencereview.com

Just the fact there is a separate Sinad chart for AVR's shows they are judged to their own standard.

So, if AVR's have their own chart because they are judged differently, then chip amps should too? Or all in ones? Or chip amp with tone controls? Is it "fair" then that cheap chip amps without any features get to be above in chart than say an integrated Yamaha?

You see where this is going?
Either all amplifiers should be judged on the same scale or there should be more definitions and groups. Right now review recommendations are purely subjective it seems.

 

[Deleted member 48726]

Which is why I said the amp stage.

This is a £220 Pro Plus with an amp stage tagged on.

So we are down to BOM breakdown to defend the thing..

Where are your source of cost information for the different parts? Do you know the markup on the different parts of the amp?

If not, this way of reasoning is nonsense.

 

[antcollinet]

Is it "fair" then that cheap chip amps without any features get to be above in chart than say an integrated Yamaha?

Well, in that the chart is a purely objective chart based on SINAD, then yes.

And separating charts based on amp technology doesn't make sense. With AVR's they are a different class in that someone looking for a stereo amp will normally NOT be after an AVR, and vice versa. I'd guess the separation of the charts is primarily for this purpose rather than that the two classes tend to have two ranges of SINAD. Certainly if I were looking for an AVR I'd not want to go searching through an Amp/AVR chart to find the few AVRs dotted around through it.

But the person looking for a stereo amp doesn't want to look through two charts separated simply based on how the amp is made. They are quite able to use the chart to select a range of devices suitable from the audio performance perspective, then judge between those based on other aspects.

I think the only way to improve on this would be to have filterable charts where you could "build your own chart" by having filter options to include/exclude devices based on a list of common features. This would be a hell of a lot of work to implement, and even if implemented, the database doesn't currently hold the necessary information, so it'd not work for anything reviewed to date.

Having said all that - yes, the recommendation or not does have a subjective element to it. Such as the lack of reccomendation for the Wiim mini based on it's lack of support for roon. Amir will also sometimes take price into account if the price/performance is particularly off base. Again we are all capable of judging whether that matters to us.

 

[morillon]

Which is why I said the amp stage.

This is a £220 Pro Plus with an amp stage tagged on.

on the streaming functions side yes..
but on the audio side the "plus" has a dac stage on akm which was the big effort compared to the pro, but the "amp" is on ess...

 

[Yorkshire Mouth]

on the streaming functions side yes..
but on the audio side the "plus" has a dac stage on akm which was the big effort compared to the pro, but the "amp" is on ess...

Ah! I'll cover that in a post I'm just going to make in reply to another.

Best wishes.

 

[Yorkshire Mouth]

So we are down to BOM breakdown to defend the thing..

Where are your source of cost information for the different parts? Do you know the markup on the different parts of the amp?

If not, this way of reasoning is nonsense.

Let's try to keep this polite discussion just a little less strident.

Okay, firstly I agree that, unless we were to do a complete strip down, assessing the cost of each individual component, we're never going to be 100% spot on. But let's not make the perfect the enemy of the good. We really shouldn't be just saying "We can't know to the penny how much the streamer-DAC stage cost, so we're just going to compare this to other £300 amps".

It is more than reasonable to point out that this isn't just an amp, it has a streaming-DAC stage, so we can't compare this to other amps of a similar price which have no streaming-DAC stage. That would be poor reasoning.

Okay, we don't know everything. But what do we know?

Okay, firstly we can compare the Mini, Pro and Pro Plus.

Mini = basic DAC (TI PCM 5121), and limited processing power (Dualcore A7 CPU, 128MB DRAM, 128MB Flash).
Pro = basic DAC (TI PCM 5121), and better processing power (Quadcore A53 CPU, 128MB DRAM, 128MB Flash).
Pro Plus = better DAC (AKM 4493SEQ), and better processing power (Quadcore A53 CPU, 128MB DRAM, 128MB Flash).

So how does the Amp compare.

Well, we know the SNR of the TI PCM 5121 is 106 dB(both the chip manufacturer's specs, and WiiM's specs, though end performance may not be as good.

We know the claimed specs for the AKM is 125 dB (manufacturer) and 121 dB (WiiM's claim).

We know the ESS Sabre DAC's claimed manufacturer's spec is 130 dB.

Now, whether that's completely accurate or not, it's not unfair to say that this isn't a poor DAC, and probably better than the WiiM Mini and Pro, and possibly as good or better than the Pro Plus.

Processing? We know that the Amp almost certainly has better processing power than the Mini, and is in line with the Pro and Pro Plus. Why? Well, WiiM have been working on room correction and increasing the 4-band PEQ to 8-band on all three streamers. But they've now dropped this on the Mini's route map, whilst keeping it on the Pro, Pro Plus, and Amp. Indeed the routemap for all 3 higher-tier WiiM products is near-identical.

So we can safely say that the DAC-streamer stage of the Amp is definitely better than the Mini, definitely better than the Pro, and possibly as good, if not better, than the Pro Plus. At the very worst, it sits between the Pro and Pro Plus.

What else? We know the Amp has a very nice, and not inexpensive, aluminium body, whilst the Mini, Pro, and Pro Plus are plastic, so that's going to eat into the cost.

Now if the DAC-streamer stage of the Amp PLUS the cost of the aluminium case, are as much as the Pro Plus, that leaves £80 for the amplification stage.

Okay, where would that put in the market? Well, the WiiM Amp uses a TI TPA 3255 amp chip. Just like the Fosi Audio V3.

The WiiM Amp was measured here by Amir as having SINAD of c.88 dB The V3 was also measured at 88 dB.

The Wiim Amp was measured here by Amir as having an SNR of 93 dB at 5W. The Fosi was 91 dB.

At full power, Wiim Amp 105 dB, V3 103 dB.

If you take the WiiM Amp's price of £300 and deduct £220 for everything before the amplification stage, that leaves £80.

The V3 costs £80.

Now this isn't absolute proof, I agree. But it's a hell of a good ballpark indicator of the standard of the amplification we can reasonably expect from the WiiM Amp.

Given all of the above, comparing the Wiim to the Fosi, I'd say they both walk like Chuck Berry playing guitar, say "Quack!" and go well with a Morello Cherry Sauce.

And I think everything I've said here is 'reasonable',

 

[morillon]

the ES 9018K2M is a bit old chip and not expensive now...that it is "better" than the t-i probably...but that the Akm is recent..(the figures have to be put into perspective)...the effort on the card side dac of the "plus" (even if wiim did not take advantage of it to produce sym outputs which would have been quite welcome) does not seem to be comparable in the all in one amp..but it should also be put into perspective of course..

but this ess is certainly a very reasonable choice... not like the TI...

;- )

(wiim had to spend a lot of energy trying the mqa for a long time just, I believe, for not having taken the version of the dac t-i mqa... was a very very long episode of "quest" for wiim which seemed really care about this tidal-mqa...)

 

[Inertiaman]

After reviewing some of these posts, Erin's test, VintageFlanker's FR plots, TI app notes, etc I have some observations/questions. Feel free to correct or enlighten as appropriate:

1) we know little or nothing about Erin's reactive loads. I applaud his effort to find a "better" load for testing and he did post the impedance plot of them. But if I understand correctly, this is the first time he has used these reactive loads. We have no comparison or baseline to the same loads tested with a "flat" amplifier? This is also Erin's first test of an amplifier (previously only speakers), and while he seems both well-intentioned and competent, I don't think it is unreasonable to have a little skepticism for his initial data on his first-ever amp test. Given all this, it seems like there is at least the possibility that his reactive loads may not behave as we think? Could the way they are built be causing oscillations with the output filtering of the Wiim amp, for example? I'd feel better about Erin's data if there were some corroborating examples, but I've not seen any (maybe due to laziness on my reading??).

2) @VintageFlanker's FR plot with an actual speaker as the load showed not the slightest hint of the 2k-10k behavior shown in Erin's plot. Is this because the aging Boston Acoustics speaker may have an uncomplicated impedance curve (I don't know if it does or not) that doesn't "trigger" the Wiim amps load dependency? An anomaly? Or is Flanker's speaker perhaps a more representative load of "a typical speaker" than Erin's contrived reactive loads, so what we should be looking for is the explanation of how/why Erin got those odd curves and Flanker didn't?

3) many posts express concern about Erin's loads not going high enough in impedance, and show examples of speakers with 50+ ohm at some points, the implication clearly being that these FR deviations would be even worse if a "better' reactive dummy load were used, with higher impedances. But as far as I can tell, the dB peaks in Erin's FR plots correspond well with his load's impedance dips, not the load peaks. And those load dips are sub-3ohm, which, although not unheard of in speakers, is atypical (at least anecdotally to me). My point being that it seems to be that the low impedances of any speaker are what should, possibly, concern us w.r.t. amplifier (un)transparency more so than the high impedances.

Its easy for me to sit on my sofa and toss out homework assignments for others to labor over, but it sure would be nice to see a handful of additional FR plots with actual speaker loads to bring more data to this topic. At ~ 5 watts I don't think the tests represent a genuine risk to damage a speaker, do they? Hearing protection + blankets over speaker (and/or in another room, etc) would minimize the irritation for the tester.

Finding even one speaker that exhibits anything remotely like Erin's plots would add credibility to the argument that there is problematic behavior in the 2k-10k range. Alternatively, seeing several speakers with plots as flat as Flanker's current example would support the argument that the Wiim/TPA3255 load dependency is a negligible concern.

Certainly such testing is time better spent than arguing the audibility of 1dB ad infinitum.

 

[Yorkshire Mouth]

1) we know little or nothing about Erin's reactive loads. I applaud his effort to find a "better" load for testing and he did post the impedance plot of them. But if I understand correctly, this is the first time he has used these reactive loads. We have no comparison or baseline to the same loads tested with a "flat" amplifier?

I mean THIS ^^^ quite obviously.

I mean we are ASR, aren't we?

I fully appreciate Erin's excellent efforts here. But to extrapolate conclusions from a data set of one, without any reservation or qualification whatsoever, as some have tried here, is not even remotely scientific.

 

[morillon]

not to mention reactive loads etc.. I would be curious about measurements even on a simple load at 10 or 16 ohms , easy to do... ?

(ps :as a precaution, I concluded for myself to avoid the use, however tempting, of this type of small amp for speaker development.. etc.)

 

[NTK]

The frequency response of the Wiim Amp can be almost entirely predicted as a response of the LC output filter. So there isn't any big mystery as to how the Wiim Amp responds to "complex loads" (i.e. the response is highly predictable when the speaker impedance is known).

The L and C values from TI's Class D Amplifier LC Filter Design application note example works perfectly.

https://www.ti.com/lit/an/slaa701a/slaa701a.pdf

The filter L is: 11.25 μH
The filter C is: 2 * C_btl = 2 * 0.68 μF = 1.38 μF
R_load = R_btl/2 = R_speaker/2

The complex impedance of an inductor: Z_l = 1j * ω * L
The complex impedance of a capacitor: Z_c = -1j / (ω * C)

Using the voltage divider formula, the frequency response is given as:


The magnitude and phase of Z_spkr ( | Z_spkr | and angle( Z_spk r) ) are read from the impedance curve with help from WebPlotDigitizer and interpolated for the calculations.

WebPlotDigitizer - Extract data from plots, images, and maps

automeris.io

Below are the plots of the digitized impedance magnitude and phase of Erin's "complex load":


Here is the calculated frequency (magnitude) response, and it well matched Erin's measurement.

 

[Inertiaman]

The frequency response of the Wiim Amp can be almost entirely predicted as a response of the LC output filter. So there isn't any big mystery as to how the Wiim Amp responds to "complex loads" (i.e. the response is highly predictable when the speaker impedance is known).

How do you know the component values of the Wiim Amp output filter?

 

[antcollinet]

How do you know the component values of the Wiim Amp output filter?

He doesn't. For the calculation he used the values from TI's application note.

However, the fact that the result of the calculation closely matches the experimental result with a Wiim amp - suggests that the Wiim amp design also conforms reasonably closely with the application note.

Here is the calculated frequency (magnitude) response, and it well matched Erin's measurement.

Great work. (Are you using a maths package for the calculations? if so, which one? - Thanks)

 

[dominikz]

The frequency response of the Wiim Amp can be almost entirely predicted as a response of the LC output filter. So there isn't any big mystery as to how the Wiim Amp responds to "complex loads" (i.e. the response is highly predictable when the speaker impedance is known).

The L and C values from TI's Class D Amplifier LC Filter Design application note example works perfectly.

https://www.ti.com/lit/an/slaa701a/slaa701a.pdf

View attachment 352542

The filter L is: 11.25 μH
The filter C is: 2 * C_btl = 2 * 0.68 μF = 1.38 μF
R_load = R_btl/2 = R_speaker/2

The complex impedance of an inductor: Z_l = 1j * ω * L
The complex impedance of a capacitor: Z_c = -1j / (ω * C)

Using the voltage divider formula, the frequency response is given as:
View attachment 352558

The magnitude and phase of Z_spkr ( | Z_spkr | and angle( Z_spk r) ) are read from the impedance curve with help from WebPlotDigitizer and interpolated for the calculations.

WebPlotDigitizer - Extract data from plots, images, and maps

automeris.io

Below are the plots of the digitized impedance magnitude and phase of Erin's "complex load":
View attachment 352553View attachment 352554

Here is the calculated frequency (magnitude) response, and it well matched Erin's measurement.
View attachment 352555

Just wanted to applaud you for the effort - great work, concise explanation (with references) and nice diagrams! It would be really difficult to hit the nail on the head more than you did in this post!

 

[NTK]

He doesn't. For the calculation he used the values from TI's application note.

However, the fact that the result of the calculation closely matches the experimental result with a Wiim amp - suggests that the Wiim amp design also conforms reasonably closely with the application note.



Great work. (Are you using a maths package for the calculations? if so, which one? - Thanks)

Thanks for your kind words. I use Mathematica a lot for quick analyses. It has a learning curve but I have gotten comfortable with it. But the home license costs USD$195/yr.

Attached is PDF of my Mathematica notebook, and the WebPlotDigitizer digitized CSV files for the impedance numbers of Erin's "complex" load.