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GENERAL
What kind of on/off switches do you recommend? As far as I can tell
from my reading, no one is particularly fussy about the power switch,
but it would seem just as critical as the line cord. Or, will any 3~4
Amp DPST switch do?
The on/off switch is important since it must cope with the inrush current
of two beefy toroidal transformers, which can be considerable. We suggest
using a minimum 15A for 110/120V mains, and 10A for 220/240V mains –
for both the 55W and the 100W. Any less a rating and you can expect the
switch to fail within two years.
A snubber network of 0.22uF in series with 100R 1W should be placed across
the active and neutral terminations ON THE SWITCHED SIDE OF THE PRIMARY.
This greatly improves mains noise rejection of the amplifier and slightly
reduces the surge on switch-on. The capacitor is supplied in later kitsets
commencing around May 2002, and is rated for any mains grid across the
world to 250VAC.
What's the best way to do a power on/off light?
We select a high efficiency blue LED, because they look cool!
Light intensity from a LED depends on the efficiency of the LED and the
supply voltage. Most LEDs run at around 2V from anode to cathode and a
current from 0.15mA to 10mA; red down to 1V6, and blue to 2V2. Thus setting
up an indicator LED must be done empirically; you need to judge the intensity
for yourself and dimension the dropper resistor to taste. Tip: don’t
set it too bright. LEDs have a tendency to bore holes in the head if run
at too high a current, and since, like most audiophiles, you will stare
at the amp and speakers while playing, too bright an LED is 1 BAD THING.
Here’s the math. For the 36V rail of the 55W AKSA, we need to drop
around 34 volts across the dropper resistor. At 0.5mA, for example, a
reasonable level for a blue LED, this means the dropper resistor, by Ohms
Law, is 34/0.0005, which is 68,000 ohms, or 68K nearest preferred value
(NPV). For 2mA, the dropper resistor would be 34/0.002, which is 17,000
ohms, or 18K NPV. At 5mA however, this resistor should be 34/0.005 = 6,800
ohms, or 6K8 NPV. For the 49V rail of the 100W AKSA, the NPVs respectively
are 100K, 22K, and 10K. All of these resistors should be rated at no less
than half a watt; in the extreme case, 5mA from a 49V supply, the resistor
dissipation is a mere 220mW, which is a long way shy of the 500mW rating
of a half watt resistor. Nonetheless, keep it well ventilated!
Are there any special or student discounts?
Aspen Amplifiers is a small company with one full time principal and two
part time employees. As such we are building up our infrastructure and
constantly struggling with cash flow and time constraints.
The AKSA products are already discounted. They are inexpensive. This is
a competitive market place, and these products sell for little more than
the US dollar cost of the constituent parts. Accordingly, please do not
ask for a discount, as refusal may offend. This policy cannot and will
not change for the foreseeable future, and like everyone else we may even
be obliged to raise our prices in the months ahead as economic conditions
in Australia and abroad change.
That said, we do offer quantity discounts. Orders for three AKSAs attract
a 5% discount on total price. Five AKSAs attract a 10% discount, while
an order for ten AKSAs will receive a 15% discount.
What pot do you recommend for volume control?
This is, in fact, a very important topic, and one that has been written
about interminably since hifi began. Conventional potentiometers damage
the sonics considerably, and we recommend a 100 kOhm linear, double ganged
cermet, law-faked with a 100 kOhm 1W 1% resistor between output and ground.
This solution takes full account of the AKSA input impedance. This
is why:
*
Potentiometers are generally of poor sonic quality, with profound and
deleterious effects on the sound quality.
*
The human ear requires a logarithmic curve on the track of the pot to
give any kind of even progression as you increase the level. Fabricating
these log tracks istricky and achieving a match better than 1.5dB between
gangs is almost impossible, even with sophisticated laser trimming.
The best tracking pots (the Alp Black Beauty) use conductive plastic,
which is trimmed to great accuracy using programmed lasers, but they are
markedly below the sonic standards of a properly executed stepped attenuator.
*
The best sonics come from stepped attenuators, which typically use a long
series of linked and switched high quality resistors from input to ground.
The best attenuators use just two resistors to form individual voltage
division, selectable with a high quality switch from a large array of
resistor pairs. The resistors are often surface mounted components.
By use of a switch, usually with 24 positions and up to 60, the voltage
division is moved up or down with respect to ground, achieving a switched
voltage divider function. This system is simple, but expensive to
implement - but it avoids the sonic horrors of carbon potentiometers.
Stepped attenuators are made by DACT, Goldpoint, CoherentTechnologies
and others, but are very expensive because of the switch, which must be
of very high quality to deliver negligible signal interaction and yet
withstand constant, unrelenting operation. We know of one which
costs around $US1200 for a stereo! This is serious, high end territory!
*
A myriad of potentiometer ideas have come and gone over the years, in
attempts to improve the sonics whilst keeping the cost down. The
Alps Blue and the Black Beauty series (both conductive plastic and widely
used in recording studios) are highly regarded and a lot cheaper than
the stepped attenuator.
*
Generally, it is far too expensive to manufacture a compact logarithmic,
wirewound pot. Furthermore, the wire used on a typical 50K or 100K
pot is very fine, with many turns, and this creates high inductance.
Sadly this makes such pots unsuited for audio, since at 20K the response
is at least 2.5dB down and this rolloff is particularly noticeable, particularly
on percussion.
*
The pot to buy is the cermet type. In our opinion the best cost/benefit
choice are the cermet pot’s, which really work well if law-faked
and have the advantage of wonderful durability and infinite adjustment,
unlike a stepped attenuator. Try to find a dual gang version.
Thecermet pots are only available with linear curve, and are not made
for audio, although they work very well. They were originally designed
for the precision industrial and instrumentation markets. Cermet
pots are sonically superior to and a little cheaper than the conductive
plastic types, far less than the stepped attenuators and arguably just
as good sonically. Bourns make dual gang cermet pots; try
to purchase from the 81 or 82 series, but be sure the resistive element
is cermet (ceramic/metal).
*
You can law fake a linear 100 kOhm pot with a simple 15 kOhm resistor
from middle terminal (wiper, or centre; output) to ground.
The 47K input impedance of the AKSA combines with this resistor to give
around 11K4 from wiper to ground. Signal input is taken to the top
terminal, bottom terminal is earthed, and the middle (wiper) terminal
is then taken to the AKSA input. This gives you an excellent and
progressive volume control, with a minimum system input impedance (with
the 47kOhm Zinof the AKSA) of 10K2 kOhm (at full volume), a maximum of
100 kOhm (at lowest volume), and a full range with additional sensitivity
in the earlier stages for easy adjustment during quiet listening.
You have infinite adjustment, and a double ganged cermet Bourns should
cost no more than $US18. This is far cheaper than the starting price
of the DACT, which is around $US190 plus postage, and superior because
the DACT offering is only 24 position, which is a tad coarse for most
audiophiles. No change is required to the AKSA; no mod’s
to the board, and the wiper of the pot is simply hooked to the input terminal
on the PCB. Leads should be keep short for reasons of stray capacitance,
and all earthing should be referred to the signal earth terminal on the
amp, slightly to the right and above the star earth spade connector.
Are there guarantees?
Guarantees are very difficult. Since we do not assemble and quality test
the amplifier, it would be very foolish for us to guarantee your final
product. If insured during transit, we can guarantee that the kitset arrives
in good order and condition. We also guarantee the operational integrity
of our pre-assembled and tested AKSA modules where sold. If we’ve
made some, albeit highly unlikely, error in putting together your kit
we will quickly correct it for you at no charge if you alert us within
three days of receiving the package. Of course, it cannot be overstated
that we offer comprehensive technical backup via email; the AKSA may only
be a kitset, but it is extremely well supported and this, we feel, is
important to its commercial success.
We also guarantee that the design is well engineered, and delivers the
sonic performance we say it will deliver. We even ask that when you have
completed and auditioned the amplifier, you consider writing a testimonial
for our website. We are really that confident; this is an exceptional
amplifier by any standard.
If the Aspen Amps products are so good and so simple to build, why don't
you yourself manufacture them , even on a small scale?
In modern consumer societies, the economists describe business enterprises
in terms of ‘barriers to entry’. In essence, this phrase describes
the cost of infrastructure and production equipment necessary to create
the product. Today we live in a highly regulated and litigious environment,
and all electrical appliances, including amplifiers, must pass a number
of rigorous safety and electromagnetic emission tests. No manufacturer
can sell his product without passing these tests and winning proper certification.
In a world market, each and every country has its own tests (but of course!!)
and all consumer goods must be passed locally for sale within that market.
A moment’s thought reveals that this is the major barrier to entry
for the AKSA on world markets. Aside from the technical aspects, which
are straightforward and easy to arrange, there is the matter of negotiating
the various government agencies and bureaucracies. And then there is often
a maze of complex trading and marketing controls, which make it difficult
to mass market any complex consumer product – taken as a whole, these
are substantial issues and at this point in time we are not able to negotiate
them. Not yet, at least.
That said, a commercial agreement has been reached with a Californian
company to produce a retail product based entirely upon the AKSA design.
A similar discussion is in process with a manufacturer in Turkey. Of course,
it is too early to say when fully complete AKSAs will be available and
at what cost, but they will certainly be marketed across the Internet,
at first in North America and Korea, and then throughout Europe.
Is it possible to buy the PCB's alone, without components and heatsinks,
since I have ready access to most of the parts locally to me?
No. Parts are not sold separately. The AKSA is sold only as a kitset.
The reason is that the AKSA is highly sensitive to component choice, and
to guarantee the sonics, we must specify all components precisely. The
best, most convenient and most cost-effective way to do this is to sell
a kitset.
A second, important consideration is intellectual property, which in this
design is considerable and worth thousands if incorporated into a fully
assembled, retail product. The value of a circuit board takes no account
of this.
Thirdly, the electronic components market in Australia is relatively cheaper
than North America for the same quality. This is a happy circumstance
for our North American and European buyers, and clearly is a win-win.
How difficult will I find the actual construction process?
The instructions for the 55W AKSA run to 12,200 words, with 22 photographs
and diagrams. For the 100W AKSA these figures are respectively 12,700
and 28.
The AKSA boards are easy to assemble and solder because the tracks are
thick and the pads large. This is quite deliberate, to maximize copper
area and reduce any chance of destroying the PCB. Someone who knows how
to use a soldering iron, has moderate patience, and recognizes different
components, and can follow instructions will have no problems successfully
assembling the AKSA.
As well, there is an extensive diagnostics section which covers most faultfinding
issues. If you are a patient and careful person, and you can follow instructions,
you should have no difficulty reading and digesting these instructions.
However, the actual construction does require some skill at soldering,
and you are advised to practise this art to a reasonable standard so that
you can construct a compact, single sided printed circuit board with track
sizes down to 47 thou (1.2mm). If you have no experience in wiring or
electronics at all, you are advised NOT to tackle this project unless
you have a suitably experienced mentor looking over your shoulder…..!
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