lindemann musicbook:55
ON TEST
lindemann musicbook:55
power amplifier
M
any electronics
designers say
that the newest
amplifier design
on the block,
Class-D, is still ‘a
work in progress’, such that each new Class-D design
is an improvement on earlier versions and
implementations. If it were true—and I’m not
necessarily saying it is—it would certainly not
be surprising, because although the concept
of Class-D has been around for more than
half a century, practical implementations of
the Class-D circuit have only been around
since the turn of the century. (†)
There are many completely different types
of Class-D amplifier, because there are many
different ways of getting transistors to switch
on and off rapidly enough to be able to derive an analogue audio signal, including pulse
width modulation (PWM), pulse density
modulation (PDM), delta-sigma modulation
(DSM) and self-oscillating modulation (SOM).
It is worth noting that all these systems are
actually analogue—it’s a misnomer that the
‘D’ in Class-D stands for digital… it’s actually
an analogue switching amplifier.
18
Australian Hi-Fi
The Equipment
Readers familiar with Lindemann’s range will
immediately recognise the musicbook:55
as being a Lindemann product, because it’s
presented in exactly the same livery as all
Lindemann’s other products: amplifiers,
DACs, Network Players, etc. This means that
not only is it gorgeous, its size and cosmetic
appearance are also a perfect match for those
products, which will be of immense importance to the fashion-conscious member of
your household.
As you’d expect of any power amplifier,
there aren’t many controls on the musicbook:55: not to put too fine a point on it,
there are none at all. There’s a 240-volt mains
power switch on the rear panel, and that’s it.
(Sure, there’s a small rotary switch to adjust
the amplifier between the available stereo,
mono-biamp and mono-bridge modes, plus
a really teensy toggle to switch between the
XLR and RCA inputs, but I don’t really count
these as true ‘controls’.) Although volume
controls are increasingly becoming a rarity
on audio power amplifiers, so I cannot take
Lindemann to task for not providing one (or
two) on the musicbook:55, I can’t help but
bemoan the disappearance of the volume
control in general… but back to the small
rotary switch used to switch modes, it’s rather
like a tiny trim-pot and must be turned via a
tiny screwdriver (Philips or flat-blade).
The problem is that there’s no indication
of which setting is which… or at least I didn’t
think there was until I mentioned this to my
son, who said it would be stupid not to have
some indication. When he looked, he told
me that one of the four ‘slots’ into which you
place the screwdriver was arrowed at one end,
whereas the other three had squared-off ends.
So it will be simple if you have better eyesight
than me! The switch will come pointed at ‘1’
(stereo) from the factory, and if you want to
bi-amp you’d turn it a quarter-of-a-turn to
† One of the earliest Class-D patents was granted to none other than Amar G Bose,
founder of Bose Corporation, whose research for his PhD thesis later resulted in him
being granted a patent [Signal Translation Employing Two-State Techniques, Patent No.
3,294,981] for a non-linear Class-D power amplifier ‘way back in 1966.
www.avhub.com.au
Lindemann musicbook:55 Power Amplifier
the right (‘2’) and to bridge the two channels
to mono, another quarter-of-a-turn to the
right (‘3’). The last position is labelled ‘0’ and
described as ‘Reserved’, but I think this is just
another way of saying ‘Not Used’.
As for that lack of a power switch on the
front panel, Lindemann has that covered
by way of providing the musicbook:55 with
automatic power switching. Lindemann’s
could damage them. Although practical,
this type of terminal does make it difficult
to source the necessary connectors to which
you’d attach your speaker cables. All hi-fi
dealers who sell Lindemann will be able
to sell you the necessary connectors, but it
would have made sense for Lindemann to
include some with the musicbook:55.
Popping the top of the Lindemann
musicbook:25 to have
a butcher’s inside, I
wasn’t surprised to discover that Lindemann
has used PCBs designed
and made by two other
specialist manufacturers. The musicbook:55’s
switch-mode power
supply (SMPS) is made
in China to a design
originally developed
by Philips but whose
copyright is now owned
by Bobitrans Power Solutions of Spain.
The Class-D amplifier module is made in
Batam, in Indonesia, for a Dutch company
called Heliox that specialises in building
SMPSs and Class-D amplifiers. The particu-
Visually gorgeous: a feast
for the eyes. It’s so beautiful
I would have liked it if
Lindemann had made it twice
or even four times as large
auto-switching circuitry is smarter than most,
such that the musicbook:55 will switch on
automatically whenever it detects an audio
signal at its input. It will do so within two
seconds if it’s in ‘idle’ mode, which is when
the amplifier is actually switched off, but
the power supply is switched on. If both
the amplifier and power supply boards are
switched off (leaving only a standby sensor
powered-up), switch-on takes around eight
seconds. When no audio signal is detected
at the musicbook:55’s input, the amplifier
will wait for around 30 minutes, then switch
to ‘idle’ mode, leaving the power supply
switched on. If no audio signal is detected
for a further 30 minutes, the power supply is
switched off, leaving only the sensor itself active, waiting for an audio signal to bring the
amplifier back to life. Power consumption in
idle mode is around one-watt, and in standby
mode, around half that again.
The rear panel has both balanced (via XLR)
and unbalanced (via RCA) line-level inputs.
The speaker terminals are those odd ‘safety’
shrouded banana terminals that were first
used by Naim but are now, since Europe has
tightened up on its electrical safety regulations, becoming more common on European
amplifiers (Lindemann is headquartered
in Germany). It’s particularly useful to use
this type of terminal with Class-D amplifiers, because on many of them the negative
speaker terminal is not at ground potential,
as it mostly (but not always!) is with ClassA/B amplifiers, so accidentally grounding the
negative terminal of some Class-D amplifiers
At left of the chassis is an SMPS module designed by
Philips and now made in China for Bobitrans Power
Solutions of Spain. Bottom right is the Class-D
amplifier module made in Batam, Indonesia, for Dutch
company Heliox. Top right is the analogue input PCB
made by AVM in Germany.
lar Heliox module used in the Lindemann
musicbook:55 is a two-channel half-bridge
design using the SOM technique, where the
oscillation conditions are created by a phase
shift in the feedback loop between the output
terminals to the input terminals, apparently
patented by Heliox and called ‘UCD’. The
main advantages of this particular technique
is that because the output filter is included in
the feedback loop it has no affect on performance, plus there’s no need for a Zobel network, which means output impedance is very
low and the amplifier can deliver full power
into loads as low as 2Ω. The input circuitry
is designed and manufactured in Germany
by Lindemann itself. I was not surprised that
Lindemann is using OEM SMPS and Class-D
modules, because both circuits are fiendishly
difficult to design and build. Because of the
multinational nature of the build, there really
should be a label affixed to the underside of
the musicbook:55 that says ‘Assembled in
Germany.’
In Use and Operation
For the purpose of this review I paired the
musicbook:55 with Lindemann’s musicbook:25 Network Music Player, which was
a match made in heaven… well, Germany
really. I know the musicbook:25 was given
a rave review in this very magazine by my
fellow reviewer, Hugh Douglas [Australian
Hi-Fi Magazine Volume 47 No 2], and you
Australian Hi-Fi
19
ON TEST
should read that review for a full report and
his conclusion, but I just have to add my
half-pence worth, because I found Lindemann’s musicbook:25 to be an absolutely
fabulous component… I think I liked it even
better than Hugh appeared to. It’s also one
of the first Network Music Players I’ve ever
used that connected instantly and perfectly
to my home computer network without any
fussing around, and once connected, it stayed
connected: no need for re-boots. I also loved
the interface, which worked seamlessly and
satisfyingly with both with my little iPod and
with my iPad. [Editor’s Note: The Lindemann musicbook:25 review is now online at:
www.tinyurl.com/ahf-mb25-review]
But if I loved the musicbook:25, I also loved
the musicbook:55. First up, like the musicbook:25, it’s visually gorgeous: a feast for the
eyes. It’s so beautiful I would have liked it if
Lindemann had made it twice as large as it is,
or even four times as large. Space isn’t an issue
at my place, and I do like ‘big’ components,
just as I’d take an eight litre V8 over a fuel-efficient three-lunger any day of the week. But
enough of my personal philosophy and onto
the nitty-gritty of the musicbook:55’s sound
quality.
Just like the unit looks, the musicbook:55’s
bass is also gorgeous. It goes deeper than you’ll
ever need, lower than the deepest synth I’ve
ever heard played and even when it’s reproducing low notes, there’s no ‘waffle’ there at
all, just clean bass that’s always perfect for the
instrument it’s reproducing: silky-smooth with
bowed double-bass or cello, and hugely impactful with slapped bass or percussion. There’s no
overhang, no ringing, exceptional tonality… I
can’t enthuse enough about the pellucidity of
its bass. I absolutely loved using the musicbook:55 to listen to Club Cheval’s new album,
Discipline, with my favourite track to demo the
musicbook:55’s bass to visitors being the first
30 seconds of the track Legends (Intro), though
the bass in the track from which the disc takes
its name is just awesome!
20
Australian Hi-Fi
Lindemann musicbook:55 Power Amplifier
(or any of the other tracks on this fabulous
album. If you can’t find the LP, stream it!).
Your only real decision
will be whether to buy
the musicbook:55 or
its seemingly identical
but rather lowerpowered twin, the
musicbook:50
The midrange surprised me, not because it
was so great-sounding (which it was), but because the tonal quality reminded me of some
of the better valve amplifiers I’ve heard…
and that’s something I don’t think I’ve ever
said before about a Class-D design! Although
tonally accurate and well-balanced against the
bass, it has that soft, silky tone that seemed
to wrap a gossamer veil around every vocalist,
though most attractively with female vocalists,
as was comprehensively proved to me when
listening to Lapsley’s Love is Blind, from her debut album Long Way Home. Although much
of the backing on this album is over-produced, as is her voice when it’s multi-tracked,
when she’s singing solo I could—thanks to
the musicbook:55—hear perfectly clearly her
unique vocal attributes. But the clarity of the
musicbook:55’s midrange also let me hear the
heavy-handed engineering, so it clearly didn’t
pull a gossamer veil over this.
Also surprising was the treble, this time because I auditioned it with pub rock band Bad
Dreems’s debut album, Dogs At Bay. ‘Judging
treble using a garage rock band?’ I hear readers
decry. Yep, guilty, but first I was using vinyl
and second, there’s just so much high-frequency energy in this album that if the amp
doesn’t get it right, the result is just ear-glazing
sizzle. The musicbook:55 got the treble exactly
right—pure high-frequency energy, but excitingly real, as Ghost Gums is my witness
Conclusion
As you can see, I have had nothing but good
things to say about Lindemann’s musicbook:55 power amplifier, and it’s on that
note that I will conclude this review: It’s a
great little amp. If you’re thinking about a
musicbook:25 Network Music Player—and,
even from my brief exposure, you should be
if you’re looking for a Network Music Player
(and don’t forget it’s a CD Player and DAC as
well)—your only real decision will be whether
to buy the musicbook:55 or its seemingly
identical (but rather lower-powered) twin, the
musicbook:50. And even that’s an easy decision: small room—musicbook:50; big room
Leslie Dudman
musicbook:55. Go for it!
Readers interested in a full technical appraisal of the
performance of the Lindemann musicbook:55 Power
Amplifier should continue on and read the LABORATORY TEST REPORT published on page 22.
Contact details
Brand: Lindemann
Model: musicbook:55
Category: Power Amplifier
RRP: $4,460
Warranty: Five Years
Distributor: Audio Magic
Address: 23/22 French Avenue
Northcote VIC 3070
T: (03) 9489 5122
E: info@audiomagic.com.au
W: www.audiomagic.com.au
•• High power
•• Size and appearance
•• Superb sound
•• Speaker terminals
•• Volume control
see Lab Report on page 22
www.avhub.com.au
LAB REPORT
Lindemann musicbook:55 Power Amplifier
Figure 1. Total
harmonic distortion
(THD) at 1kHz at an
output of 1-watt
into an 8-ohm
non-inductive
load, referenced to
0dB. [Lindemann
MusicBook:55 Power
Amplifier]
dBFS
0.00
Newport Test Labs
-20.00
-40.00
-60.00
-80.00
-100.00
-120.00
Laboratory
Test
Results
The Owner’s Manual provided with the musicbook:55
says the amplifier is rated to deliver a power output of
240-watts into 4Ω loads for durations of less than one
minute. Newport Test Labs found it delivered around
0.6dB less than this, albeit over a little longer period of
time: 205-watts per channel (single or both channels
driven into 4Ω). Lindemann does not provide a power
output specification into 8Ω loads, but Newport Test Labs
measured 110-watts per channel into 8Ω. (The musicbook:55’s protection circuit triggered automatically
when the laboratory attempted to measure power output
into 2Ω, hence the lack of results for this test load.) The
protection circuit auto-resets very quickly, so once the 2Ω
load was replaced with a higher resistance, the protection
instantly reset itself—no user intervention was required.
The frequency response of the musicbook:55 was measured as extending from less than 1Hz to 45kHz –1dB,
and from less than 1Hz to 63dB –3dB, as shown in the
accompanying results table. Channel separation was measured as 67dB at 20Hz, 70dB at 1kHz and 61dB at 20kHz.
Although these are all satisfactory results that will enable
excellent sonic performance, I am used to seeing higher
separation figures returned by power amplifiers. Channel
phase errors were low at the same frequencies, as you can
see from the tabulated figures but these results are fairly
typical of a power amplifier and in any case would not be
audible.
Overall distortion (THD+N) was measured by Newport
Test Labs at 0.023% for an output of one watt, and 0.18%
at an output of 100-watts. These are good results, but
you have to remember that they’re band-limited as a
result of the lab having to use a 20kHz low-pass filter to
prevent switching noise from the amplifier affecting the
test instruments. [Editor’s Note: I am advised that as a
result of the number of Class-D amplifiers now being tested, in
future the lab will be using an 80kHz low-pass filter to remove
switching noise.] The harmonic spectra of the musicbook:55’s output is shown in Graphs 1 through 4.
Graph 1 shows performance at one watt into an 8Ω
load, and there’s a second harmonic at –70dB (0.0316%),
a third at –90dB (0.0031%) a fourth at –110dB (0.0003%),
a sixth at –112dB (0.0002%) and a seventh at –107dB
(0.0004%). You can see that the noise floor is over 100dB
down at low frequencies, and 120dB down across the
rest of the audio band. Results into a 4Ω load at one watt
(Graph 2) show an increase in distortion, with the second
harmonic sitting at –63dB (0.0707%), the third at –80dB
(0.01%), a fourth at –86dB (0.005%), and a fifth at –88dB
(0.0039%).
22
Australian Hi-Fi
-140.00 0.00 Hz
Figure 2. Total
harmonic distortion
(THD) at 1kHz at an
output of 1-watt into
a 4-ohm non-inductive load, referenced
to 0dB. [Lindemann
MusicBook:55 Power
Amplifier]
dBFS
0.00
4000.00
8000.00
12000.00
16000.00
20000.00
Graph 1: Total harmonic distortion (THD) at 1kHz at an output of 1-watt into an 8-ohm
non-inductive load, referenced to 0dB. [Lindemann MusicBook:55 Power Amplifier]
Newport Test Labs
-20.00
-40.00
-60.00
-80.00
-100.00
-120.00
-140.00
0.00 Hz
4000.00
8000.00
12000.00
16000.00
20000.00
Graph 2: Total harmonic distortion (THD) at 1kHz at an output of 1-watt into a 4-ohm
Figure 3. Total
harmonic distortion (THD) at 1kHz
at an output of
100-watts into an
8-ohm non-inductive
load, referenced to
0dB. [Lindemann
MusicBook:55 Power
Amplifier]
dBFS non-inductive load, referenced to 0dB. [Lindemann MusicBook:55 Power Amplifier]
0.00
Newport Test Labs
-20.00
-40.00
-60.00
-80.00
-100.00
-120.00
-140.00
Figure 4. Total
harmonic distortion (THD) at 1kHz
at an output of
200-watts into a
4-ohm non-inductive
load, referenced to
0dB. [Lindemann
MusicBook:55 Power
Amplifier]
0.00 Hz
4000.00
8000.00
12000.00
16000.00
20000.00
Graph 3: Total harmonic distortion (THD) at 1kHz at an output of 100-watts into an 8-ohm
dBFS non-inductive load, referenced to 0dB. [Lindemann MusicBook:55 Power Amplifier]
0.00
Newport Test Labs
-20.00
-40.00
-60.00
-80.00
-100.00
-120.00
-140.00 0.00 Hz
Figure 5. Intermodulation distortion
(CCIF-IMD) using test
signals at 19kHz and
20kHz, at an output
of 1-watt into an
8-ohm non-inductive
load, referenced to
0dB. [Lindemann
MB:55]
dBFS
0.00
4000.00
8000.00
12000.00
16000.00
20000.00
Graph 4: Total harmonic distortion (THD) at 1kHz at an output of 200-watts into a 4-ohm
non-inductive load, referenced to 0dB. [Lindemann MusicBook:55 Power Amplifier]
Newport Test Labs
-20.00
-40.00
-60.00
-80.00
-100.00
-120.00
-140.00
Figure 6. Frequency
response of line
input at an output of
1-watt into an 8-ohm
non-inductive load
(black trace), 4-ohm
load (green) and
into a combination
resistive/inductive/
capacitive load representative of a typical
two-way loudspeaker
system (red trace).
[Lindemann MB:55]
dBr
3.00
0Hz
6000.00
12000.00
18000.00
24000.00
30000
Graph 5: Intermodulation distortion (CCIF-IMD) using test signals at 19kHz and 20kHz, at an
output of 1-watt into an 8-ohm non-inductive load, referenced to 0dB. [Lindemann MB:55]
Newport Test Labs
2.50
2.00
1.50
1.00
0.50
0.00
-0.50
-1.00
-1.50
-2.00
-2.50
-3.00
10.00 Hz
100.00
1000.00
10000.00
www.avhub.com.au
Lindemann musicbook:55 Power Amplifier
LAB REPORT
Newport Test Labs
measured the signal
to noise ratio at
101dB A-weighted
referenced to an
output of 100-watts
per channel
harmonic component now sitting up at –50dB
(0.3162%).
Intermodulation distortion is shown in
Graph 5, and you can see the unwanted 1kHz
difference signal generated by the 19kHz
and 20kHz test signals is sitting at –71dB
(0.0281%). The sidebands at 18kHz and 21kHz
are sitting at around –76dB (0.0158%), while
the pair at 17kHz and 22kHz are at –93dB
(0.0022%) and –90dB (0.0031%) respectively.
Newport Test Labs measured the signalto-noise ratio of the musicbook:55 at 81dB
A-weighted referenced to an output of one
watt, and 101dB A-weighted referenced to
100-watts.
Square wave performance was good, though
interpretation is made a little difficult by the
presence of the high-frequency switching of
the Class-D design (at around 355kHz), which
has not been filtered out either by the amplifier itself or by Newport Test Labs.
The 100Hz square wave shows excellent frequency response, extending down to d.c. and
a complete lack of phase shift (also excellent).
The slight overshoot visible on the leading
edge is the result of a rise in the frequency
response above 20kHz.
The 1kHz square wave shows this same
overshoot, as well as the high-frequency
switching noise. The 10kHz wave begins to
resolve the noise, but the waveform shows
There are seven higher-order harmonic distortion components visible above the noise floor,
but all are more than 100dB (0.001%) down.
Graph 3 shows distortion when the musicbook:55 is driving an 8Ω load at an output
of 100-watts per channel. You can see that
odd-order harmonics dominate, and that with
the exception of the fourth harmonic the first
six harmonics all lie between –68dB (0.0398%)
and –82dB (0.0079%). When driven into 4Ω
loads, at 200-watts output (Graph 4), distortion increases further, again with odd-order
harmonics dominating, and with the third
Lindemann MusicBook:55 — Power Output Test Results
Channel
Load (Ω)
20Hz
(watts)
20Hz
(dBW)
1kHz
(watts)
1kHz
(dBW)
10kHz
(watts)
10kHz
(dBW)
1
8Ω
111
20.4
110
20.4
110
20.4
2
8Ω
110
20.4
110
20.4
110
20.4
1
4Ω
205
23.1
205
23.1
205
23.1
2
4Ω
205
23.1
205
23.1
205
23.1
Lindemann MusicBook:55 — Laboratory Test Results
Test
Measured Result
Units/Comment
Frequency Response @ 1 watt o/p
<1Hz – 45kHz
–1dB
Frequency Response @ 1 watt o/p
<1Hz – 63kHz
–3dB
Channel Separation (dB)
Channel Balance
Interchannel Phase
THD+N
67dB / 70dB / 61dB
0.045
0.11 / 0.07 / 1.22
0.023% / 0.181%
(20Hz / 1kHz / 20kHz)
dB @ 1kHz
degrees ( 20Hz / 1kHz / 20kHz)
@ 1-watt / @ rated output
Signal-to-Noise (unwghted/wghted)
79dB / 81dB
dB referred to 1-watt output
Signal-to-Noise (unwghted/wghted)
100dB / 101dB
dB referred to 100-watts output
Input Sensitivity (Balanced)
118mV / 1.15V
(1-watt / 100-watts output)
Power Consumption
0.48 / 19.47
watts (Standby / On)
Power Consumption
21.70 / 234
watts at 1-watt / at rated output
Mains Voltage Variation during Test
24
Australian Hi-Fi
237 – 243
Minimum – Maximum
significant rounding, reflecting the amplifier’s
–3dB down-point of 63kHz.
The fourth oscillogram reveals that the
musicbook:55 will be completely stable into
highly reactive loads.
Overall, the Lindemann’s musicbook:55
is an excellent Class-D design, though given
the increase in distortion with decreasing
impedance combined with the fact that its
protection circuit triggered when driving 2Ω
loads, I’d recommend using speakers with a
nominal impedance of 6Ω or higher.
Steve Holding
Readers should note that the results mentioned in
the report, tabulated in performance charts and/or
displayed using graphs and/or photographs should
be construed as applying only to the specific sample
tested.
www.avhub.com.au
SUBSCRIBE
ANDSAVE!
AND
SAVE!
REVIEWED
Harbeth Super HL5plus
Loudspeakers
Pioneer SC-LX89
AV Receiver
Gold Note CD-1000
CD Player & DAC
40
Q Acoustics Conceptkers
Loudspea
Jan/Feb 2016 | $9.95
625 S2
Jeff Rowland Design Amp
lifier
Stereo Power
| www.avhub.com.au
ATOMIC CLOCKS
yer!
Italy’s Gold Note CD pla
Improve the sound of
Yes, you can read some of our reviews in full on
AVHub.com.au. You can now swipe your way through
every page of Australian Hi-Fi on your iPad or Android
tablet, (or computer) using our snazzy Digital Issues.
But there’s nothing like the proper print edition, landing
in your letterbox sealed in shiny cellophane, ready for
you to read at your leisure, at full size, in
the order we intended, in the bath, if you like.
Get every issue of Australian Hi-Fi the way it should be.
YES! PLEASE SEND ME A
SUBSCRIPTION TO AUSTRALIAN HI-FI
ETS:
New Subscription
Subscription
or
RE CABIN
L- COExtend
GEor
RK?
DO THEY RE ALLY WO
Mr
Mrs
Miss
Ms
Gift Subscription
* See inside for details.
6
Competition Ends on 14/02/201
12/9/2015 3:42:28 PM
14 ISSUES (2 YEARS) $109 SAVE OVER $30
7 ISSUES (1 YEAR) $59 SAVE OVER 15%
INCLUDES HOME CINEMA & HI-FI LIVING SPECIAL EACH YEAR
W
d 1
AHF Jan16_001 Cover.ind
Your Name
MYMAGAZINES.COM.AU
Your Address
Postcode
Daytime Phone (
)
SUBSCRIPTIONS
PO BOX 3355
ST LEONARDS NSW 1590
PAYMENT DETAILS
I enclose a cheque/money order for $
E-mail
Please provide phone or email in case of delivery issues
IF A GIFT SUBSCRIPTION
SYDNEY (02) 9901 6111
TOLLFREE 1300 361 146
Mr
Mrs
Miss
Ms
payable to Next Media Pty Ltd OR Charge my credit card:
Mastercard
Visa
American Express
Recipient’s Name
Recipient’s Address
Daytime Phone (
E-mail
)
Name on Card
Expiry Date
/
CVV:
Cardholder’s Signature
Terms and conditions: Price offer available to Australian and NZ residents. Expires 1/5/16. Includes GST. Savings based on total cover price. Overseas Airmail 14 issues
A$195 or 7 issues A$99. Australian Hi-Fi subscription comprises of 7 issues PA – 6 regular issues and 1 special issue. This form may be used as a Tax Invoice. nextmedia
Pty Ltd ABN 84 128 805 970. Please tick if you do not wish to receive special offers or information from nextmedia or its partners via email mail. Please refer to
www.nextmedia.com.au for the full Privacy Notice.
MA/HF
Postcode
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertising