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Frequently asked questions about AMR equipment

 
Both the AM-77 and CD-77 are available in dark titanium or champagne gold. The former is a more modern and technical presentation. The latter offers a more classic and reto look. The RC-77 remote commander will be included in the same corresponding colour.
Thermionic Electron valves when used in audio circuits offer a far more musically satisfying and engaging sound than transistors (please refer to Appendix A
for a more detailed explanation).
The Valves used in AMR products
are qualified by the manufacturer
for operation under maximum
operating conditions of 10,000
hours. AMR operates the signal
valves used in the CD-77 processor
and the drive-stage of the AM-77
very conservatively.

U.S. military data from the 1950s
suggests an average lifespan of
100,000 operational hours under
such conditions. If the average
listening time is 10 Hours per day
this equates to around 27 Years. Individual valves may fail earlier
or last longer than such average
predicted lifespan.

From auditions and electrical tests,
AMR found most modern production valves were inconsistent in sonic terms
and quality control/reliability was poor.
In the AM-77 and CD-77, the majority
of Valves are NOS.

These Valves were originally manufactured and selected from production for military and
communication use. Large
stocks accumulated throughout
the Cold War remain to secure
supplies for years to come.

AMR products come factory-fitted with
New Old Stock Valves that have been selected to offer the highest sound
quality. However, we appreciate that changing valves may change the
sound.

Please be aware that doing so may
void your warranty and may very well change the sound quality of your
AMR product for the worse
(please refer to the
Warranty section
for more details).

In the case of AMR products, when
a part such as capacitor or resistor
has been selected for a certain
section, it had to pass very stringent
trials and is most likely the best
available choice disregarding cost.

Therefore, replacement of parts may
very well change the sound quality
of your AMR product for the worse.
Please also be aware that doing so
may void your warranty (please refer
to the
Warranty section for
more details).

AMR use copper and aluminium for
the chassis for three reasons. First,
these materials are non-magnetic. Second, they allow a relatively 'light' structure to provide the required
structural integrity and freedom
from resonance AMR require. Third, aluminium and copper offer excellent shielding properties. .

At AMR we machine the front panel
from a 25mm thick Aluminium billet.
The extruded structures for the sides
are on average 10mm thick and are additionally reinforced for increased
rigidity. The top plate for the CD
opening is made from 5mm Aluminium. The bottom of the chassis is formed
from a 7mm thick sandwich of 2mm
pure solid copper and 5mm solid
aluminium with the rear panel formed
from a 5mm thick sandwich of 2mm
pure solid copper and 3mm solid aluminium.

Hence the strength of the main structure
of AMR's chassis varies between 5mm to 10mm with all main structural parts being 7mm or thicker. The resultant empty case weighs in at around 18Kg and is supremely resistant to vibrations of any kind.

OptiIsolate® supports were drawn from
the exacting aerospace industry to
provide maximum mechanical isolation
of the solid aluminium chassis from unwanted vibrations and resonances.

Therefore, your AMR component
already offers superior resonance dissipation compared to peer products. However, different supports may slightly change the sound, feel free to experiment.

AMR strongly believes that its components should offer a superior sonic performance when only fed common mains electricity without the need for expensive after-
market power conditioning products.
Hence, AMR has integrated several unique technologies in its products to minimse the impact of mains quality.

AMR components incorporate critical electrical management designs - OptiMains®; OptiPower®; OptiReg®; OptiSeal® and OptiTrans® (please
refer to the
Advanced Design Features section for more details). The effect is
that AMR components are minimally influenced by the constant variations
in the quality of standard electrical
power supply. Exotic power re-generation equipment may be of an added sonic benefit, but the increment is likely to be
very limited.

After assembly, every AMR component is placed on a test bench where it is run continuously for 7 days or 168 hours. Measurements are taken all the while to ensure that it is operating within the intended parameters. However, AMR is aware that equipment tends to change in a more gradual fashion over the first
1,000 hours of operation.


AMR take measurements according
to many different principles during
design and quality and control. The measurements quoted in the Specifications section however,
were taken according to standards established by a variety of bodies.

These bodies include the Fair Trade Commission of the U.S., the IEC and
the Institute of High Fidelity. The
standard applied (where applicable)
is quoted with the measurement in
the Specifications section.

At this stage, AMR chose CD over the competing new formats because most modern recordings regardless of their
being issued as DVD-Audio, Super
Audio CD® or CD have been manipulated so that they do not even start to approach the limits available from the CD format.

Many earlier recordings that have a
wider dynamic range nevertheless,
were engineered to fit within the
capabilities of CD. Hence, at this stage,
very little music material exists that takes advantage of the improvements offered
by the new media standards. Therefore, AMR elected to provide a player that
offers the recreation of music from the existing catalogue of recordings to the highest degree of realism.

 
 

Frequently asked questions about the CD-77 Reference Class Compact Disk Processor

 

AMR auditioned many of the recent and historical contenders for the title "the
King of the Digital to Analogue Converters" and awarded the crown to the Philips TDA1541A. The rationale is that the
Philips TDA1541A chipset offers a superior sonic performance which to this day,
remains unsurpassed.

The Philips TDA1541A was designed
in the late eighties: a time when performance not profitability was the objective. As a result, when properly
and fully maximised, not even the so-called "latest generation" of DAC chipsets come remotely close to reproducing music as faithfully and involving as the venerable Philips TDA1541A chipset.

AMR's research and development team's
prior experience with the Philips
TDA1541A in other compact disc players
as well as designs of their own told them
that the full performance of this chipset
had yet to be extracted. Some
manufacturers advertise the use of Philips Double Crown TDA1541A chips, which
were selected at the time of
manufacturing for specific performance parameters by Philips. These can sound slightly better than the standard
TDA1541A's when used in conventional circuits. However, Philips Double Crown TDA1541A chips are not longer widely available and the differences have
often been overplayed. AMR went to the drawing board and analysed the Philips TDA1541A at the silicon-die level in
extreme detail. AMR discovered a number
of areas where correctly designed external circuitry would significantly bridge the performance gap between the Double
Crown and standard TDA1541A chips.
From the power supply to the digital input signal conditioning, the CD-77 circuit has been implemented to a degree of optimisation extending beyond anything
we are aware of. The implementation is
far beyond the simple manufacturer's datasheet application schematic
commonly found in CD-Players and
Dac's employing the TDA1541A. The resulting core technologies are the OptiSignal® conditioning circuit and OptiClockLock®. Under testing, these
circuits considerably improved the performance of standard TDA1541As to
a level that was largely on par with Double Crown chips in measured and subjective performance evaluation. (Note AMR used verified genuine Double Crown chips for these tests, as the market has recently seen many standard TDA1541A which were surreptitiously remarked as Double Crown
to increase their sales value.) We at AMR believe that the result of all this surpasses most if not all other TDA1541A designs,
even those fitted with a genuine Double Crown chip.(please refer to the Advanced Design Features section for more details).

"SuperClock" is simply a marketing term.
The CD-77 uses a military-grade crystal oscillator equal or superior in specification
to any of the industrial-grade so-called "Super Clocks" offered as an aftermarket modification. AMR has combined this
master clock with a dedicated power supply whose noise performance is at the limits of what is physically possible.

Going beyond even this, all other clocks within the CD-77 are synchronised to the superior precision timing signal of the master clock module to avoid increased jitter due to multiple oscillators creating "beat frequencies."

CD clock crystals are often promoted using
a measure that specifies the difference of
the actual frequency from the nominal frequency, usually specified as the deviation in ppm (parts per million). It should be noted that this specification has no bearing whatsoever on the jitter in CD players as
it is a purely static measurement.

Phase noise in the clock is the primary
cause of jitter. No reliable and standardised protocol exists to measure and specify
jitter in a form that makes measurements comparable. AMR as a result, has chosen
not to publish a specific number that would not be reliably comparable to the numbers published by others. Rest assured that the
CD-77 offers one of the lowest levels of
phase noise of any CD player produced.

In recent years, a technology known
as "Upsampling" as well as one called
"Zero-Oversampling®" or "
Non-Oversampling" have gained
popularity with CD player and DAC manufacturers. Having compared these approaches to traditional "Oversampling"
we at AMR feel that in most cases, the omission of Oversampling or Upsampling
and the use of a simple analogue circuit,
is the most musically satisfying.

However, in a significant minority of recordings, we found that either traditional Oversampling or Upsampling was preferable. Therefore, we have implemented all of
them and made them user-selectable
(please refer to OptiSample® in the Advanced Design Features section for
more details).

Philips Electronics ceased production of
its own 'industrial standard' compact disc transports in 2001. Presently, new so-called Philips CD transports are far-eastern clones constructed to much lesser standards and
are of questionable reliability and are the only remaining off the self dedicated CD transports. Furthermore the exceptional
Victor (JVC) transport the XL-Z900 was
based upon has been discontinued recently and several other suppliers of high quality transport solutions have cancelled their CD Lines in favour of Multiformat DVD designs. At AMR we found that if CD replay is the desired application neither computer CD Drives nor the current "universal" DVD
based solutions perform equal to the
highest quality dedicated transport mechanisms. Hence AMR set out to
create its CD transport system that would provide the same or better performance
than the legendary transport solutions of yesteryear, be they the original Philips
swing arm type transport or Victor's XL-Z900. AMR combined genuine parts from a
number of major manufacturers and
mounted these to our platform machined from a solid aluminium billet over 42mm thick. This platform is then suspended on dampers to isolate the Transport further
from any vibrations. The resultant
OptiDrive ® transport maximises the
accuracy of the data retrieval. (please
refer to OptiDrive® in the Advanced Design Features section for more details).

Yes, the CD-77 has a 16-frame buffer (or 512K Bit of Memory). That said, nearly all CD players have a memory buffer of one form or other. This buffer is used among other things to allow for error correction if the data is read incorrectly. The error correction, IF implemented correctly recovers the original data through the use of redundant data written to the CD in almost all cases. This process does NOT "interpolate" but actually recovers the exact data originally written on the CD. The necessary calculation may however take a short time, without a large enough buffer, the timing of the Data would be effected negatively. Only in case of gross problems does the error correction fail and then ALL CD-Players, even computer-based ones or software such as Exact Audio Copy, will first attempt to interpolate samples that cannot be read correctly (in many cases the software will first attempt to re-read the samples several times) from samples before and after the affected area. The Data transmission from the drive to the DAC is timed by the clock supplied to the CD-Transport or generated in the transport itself. The spindle motor servo keeps the buffer normally 1/2 full and therefore is linked to that master clock indirectly. If the master clock driving the transport is accurate and the buffer large enough (8 frames or more) the output from the memory buffer is "jitter free". Some Chipsets designed for portable CD Players have a larger amount of memory, to provide "jog proofing". This means that even if the player is violently shaken and hence mistracks, several seconds of music are in the buffer and the player normally can re-aquire tracking before the buffer runs out. Other than this (which is of no relevance to a CD Player used at home on a solid surface) there is no further advantage from excessively large memory buffers and such buffers usually slow down operation, by delaying the time from pressing play to hearing music by the time needed to fill the buffer, track skipping etc is equally delayed. The situation changes dramatically if we use a seperate Transport and DAC linked via S/P-DIF or AES/EBU cables. The problem when using a DAC with an S/P-DIF or AES/EBU input, which combine the Data and Clock is that the DAC itself must recover the Clock from the datastream, which is not a very accurate process and made more problematic by the specific way the Clock is modulated to carry the data. To address this significant problem, additional memory buffers have been occasionally used in the highest "high end" DAC's, both pro-audio and consumer audio in order to allow a local clock which is NOT PLL syncronised (PLL = Phase Locked Loop - a process analogus to Negative Feedback, except in the time domain). The clocks between Transport and DAC may now differ to a small but significant degree. The CD Specification allows +/- 1000ppm or 0.1% deviation, meaning that up to 1 in thousand samples would be "too much" or "missing" per second, so the memory buffer must hold many more frames than the one in the CD Player. In fact, it needs to be able hold around 44 frames per second playtime of CD. Thus the minimum memory buffer needs to handle around 210,000 32-Bit frames of under or overrun and needs to at least 2MB in size to accommodate this. Further, the buffer would need filling with 210,000 frames and would introduce around 5 second delay between data being read and received on the audio output. As no "off the shelf" solution exists to provide such a memory buffer it would need to be designed from ground-up. We hope this brief explanation has shed some light as to why DAC's very rarely have memory buffers (no simple and readily available solutions) and CD-Players always have them (< 1KB required to work properly and built into most commonly available chipsets). The key to high sound quality is not in the size of the memory buffer, but first in the implementation of the mechanical system to maximise the correct reading of data followed by the error correction routines, which requires extended routines beyond the original "red book" specification to correctly recover the original data. This correct and original data then needs to clocked into the DAC with a precise, jitter-free clock, so following the mechanical implementation of the drive, equally important is the master clock for the Player. In summary, yes the CD-77 as a one-box processor, has a more than ample 16-frame memory buffer but so does almost every other compact disc player. However, DACs need much larger memory buffers but due to a lack of a readily available solution, very few actually have a buffer.

The only methods to transmit data between transport and DAC that are standardised are S/P-DIF and AES/EBU. As these combine clock and data signals, they produce higher levels of jitter than found in well-engineered single unit players. While AMR could have implemented a non-standard, proprietary connection, such as an implementation of the I2S bus, none are suited to long transmission distances and no common standards exist. By keeping all digital circuits within a very compact 4 inch square area, we negated the introduction of jitter arising from long connections. We kept the analogue stages very close to the digital to analogue converter, to avoid introducing noise and distortion picked up from the outside. We also include the power supply in the same case to ensure that clean power is generated where it is consumed rather than being sent over long cables as with a two-box setup, which would again, pickup noise and distortion.

The Digital to Analogue Converter
section selected by AMR together with
the thermionic electron valve-based analogues stages provide a performance far superior to that of most stand-alone DACs. In addition, as no standard exists that allows the connection of a transport
to a DAC without introducing substantial levels of jitter, AMR felt that providing such an output would serve no purpose and would compromise the integrity of
our own highly-optimised digital circuitry.

In recent times, although many music formats (including WAV, AAC, WMA,
MP3 etc.) have become computer-based, the sound system of most computers however, leaves much to be desired.
The USB input of the CD-77 allows its superior digital-to-analogue conversion and analogue stage to be used in the replay of computer-based music formats
to the same level of quality provided by the CD-77 for compact discs. (please refer to OptiBus® in the Advanced Design Features section for more details)

CD is an optical format. The presence of varying external light conditions such as that admitted by a transparent lid was found detrimental to the sonic performance.

Virtually all CD Recordings are made for speaker replay. Thus reproducing said recordings via headphones is neither satisfying nor realistic. In order to improve the situation somewhat, a significant amount of analogue or digital processing is essential.

AMR feel that the inclusion of these would have exceeded the design remit for a CD source and is better placed in a dedicated headphone amplifier which includes the required processing. Such properly executed headphone amplifiers are readily available from specialist manufacturers.

Virtually all CD Recordings are made for speaker replay. Thus reproducing said recordings via headphones is neither satisfying nor realistic. In order to improve the situation somewhat, a significant amount of analogue or digital processing is essential. AMR feel that the inclusion of these would have exceeded the design remit for a CD source and is better placed in a dedicated headphone amplifier which includes the required processing. Such properly executed headphone amplifiers are readily available from specialist manufacturers.


The CD-77 uses a completely signal-ended signal path as we have found this to give a more realistic reproduction of music. However, in terms of sound quality, we found the XLR connector preferable to the RCA connector, even when used with un-balanced signals.

Consequently, we have offered the XLR output in a manner that is fully compatible with balanced equipment: takes full advantage of balanced connections yet retains the desirable single ended signal circuit.


 
 

Frequently asked questions about the AM-77 Reference Class Dual Mono Amplifier

 

Many of the most desirable speakers in the world require high levels of power to work well. AMR designed the AM-77 reference class dual-mono amplifier based on OptiGain® which is an innovative implementation of Fusion Technology. AMR's OptiGain® is a unique circuit design approach and ingenious implementation of the valve voltage amplification circuit coupled with a near zero solid-state amplification stage using the patent-pending Feed-Forward Error Correction circuit. With an extremely low-level of distortion achieved, the AM-77 delivers enough power to comfortably drive modern speakers with the desirable characteristics of a single-ended valve amplifier.

Traditionally, amplifiers use feedback by which a proportion of the output signal is fed-back to the input of the amplifier which is then compared to the input signal. If differences are present, the amplifier will attempt to correct this error after it has already happened. Contrastingly, AMR's patent-pending Feed-Forward Error Correction circuit uses two identical amplifiers operating as a symmetrical output stage. The Feed-Forward Error Correction circuit extracts only the error of each amplifier and feeds it into the opposite polarity amplifier. As a result, both the original error and its fed-forward replica are available at the exact same amplitude on both terminals of the speaker. As the speaker reacts only to the difference in the signal and none is present, the error has in effect, disappeared. Similar error-correction schemes have been used for many years in military control applications. To our knowledge this is the first consequential application in audio (please refer to OptiGain® in the Advanced Design Features section for further details).
It has been long acknowledged that switched attenuators of various types and designs offer the highest sound quality. However, they are usually limited in range, number of steps and are not easily remote-controlled. AMR has adopted a similar approach of using switched resistors. However, the switched resistor arrays of the OptiLevel® precision volume control allow the use of far fewer contacts and resistors to achieve a much wider range of control with finer steps. Sonically, unlike other methods of volume control, dynamics remain, even at low-listening levels. The AM-77 volume control is in addition, microprocessor-controlled allowing for remote control as matter of course while still offering a superior sonic performance allied with convenience
AMR's AM-77 Dual Mono Amplifier can be used with all kinds of dynamic, electrostatic and planar speakers, provided they are efficient enough to provide the desired sound pressure levels in your room and are of a suitable impedance. In normal-sized modern living rooms, we recommend that speakers should have a sensitivity of at least 82dB/2.83v/1m. We also recommend that speaker impedance should not drop significantly below 3 Ohm and on average, be at least 4 Ohm. Speakers with higher sensitivity and higher impedances can be used without any problems. Indeed, as the room size increases, it is desirable to have more efficient speakers. Please note however, that the AM-77's very high power output can, when combined with exceptionally efficient speakers, produce sound pressure levels that may impair hearing. AMR assumes no responsibility for such results.
We feel that our customers should not be constrained in their use of our products by omitting functionality that can be provided without a loss in sound quality. We equally do not feel it fair our customers pay extra for such functionality in the form of a paid-for option.

AMR felt that the kind of phono stage we would have been able to integrate inside the amplifier or to provide as an option board would not have been in keeping with the high standards set by AMR products. This is mainly a question of size and space as a true no compromise phono stage would require considerable effort, circuit and power supply resources. For casual, occasional listening to LPs, simple and inexpensive "phono boxes" are available from a range of specialist manufacturers. For enthusiasts whose music listening is based mainly around listening to LPs, AMR is currently developing the PH-77 standalone phono stage whose implementation is in keeping with AMR's extraordinarily high standards.

The professional Speakon Speaker connector offers touch proof and airtight silver contacts of superlative contact integrity that provides significantly improved sound quality over conventional binding posts. While sadly uncommon in high fidelity applications, AMR felt that this connector is highly desirable on grounds of sonic integrity, safety and long-term integrity.

The ubiquity of portable digital media should not be ignored. AMR therefore designed the AM-77 to offer a 3.5mm jack for the connection of portable digital media such as an iPod®.

Many portable digital media devices offer charging from a USB connector. The USB connector on side of the front panel provides this charging voltage to such devices. It has no other function or digital connection facility.

The mechanical structure of a heat sink resembles many tuning forks and usually resonates strongly as such. As these resonances invariably find their way into the signal passing through the amplifier, AMR felt it wise to omit them. Instead, AMR has taken great pains to design their chassis so it can effectively reject the excess heat produced by the circuit through radiation and convection without adding sonic artifacts.



   
   


     
 
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