Altronics K 5171 Handmatig

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This high-quality, low-distortion and low-noise
stereo preamplifier can be used with just
about any amplifier modules to form a stereo
amplifier. It can also be used as a standalone
preamp. A low-cost infrared remote control
is used to switch between 3 to 6 separate
inputs, adjust the volume or temporarily mute
the output. It also includes manual volume,
bass and treble controls and pushbuttons to
select between the three stereo inputs. LED
indicators in the pushbuttons show which
input is active. It also has power, acknowledge
and mute status LEDs. All in all, it offers con-
siderable advantages over previous models.
You could build it into an amplifier based on
our Ultra-LD series of amplifier modules, such
as the Ultra-LD Mk.4 (August-October 2015;
www.siliconchip.com.au/Series/289). Or
you could use easy-to-build, low-cost SC200
amplifier modules (January-March 2017;
conchip.com.au/Series/308; Altronics kit Cat
K5157). Or build it in a case and use it with
an existing power amp. It's up to you.
Since it has a motorised potentiometer for
volume control, you can adjust the volume di-
rectly with a knob if you don't want to use the
remote. It has an effectively-infinite number of
possible volume settings, unlike most digital
volume controls, which can have quite large
steps.
This preamp has much better performance
than most. While we have published a couple
of very low noise and distortion preamps
designs over the last decade or so, none of
them had tone controls. This one provides
wide-range bass and treble adjustment knobs
to allow you to overcome deficiencies in your
loudspeakers, compensate for the room
response or just adjust the sound to be the
way you like it.
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K 5171
While the performance is excellent when the
tone controls are active, we have provided the
option to bypass them using a push on, push
off switch. Its integrated LED indicator shows
when the tone controls are switched in or out.
This switch has three benefits. One, it's
difficult to centre the tone controls precisely
when you want the response to be flat, so the
switch provides an easy way to achieve that.
Two, it provides slightly better performance
with the tone controls switched out. And
three, it gives you an easy way to hear exactly
what effect the tone controls are having, by
toggling them on and off. A PIC microcon-
troller is used to provide the remote control,
muting and input selection functions.
Input selection is by way of a separate PCB
unit (Altronics Kit - K 5172) interconnected
to the main preamplifier using 10-way ribbon
sili-
cable. If you don't need the input selector, you
can build the project without it.
Performance
This preamplifier has excellent performance.
It uses low-distortion, low-noise op amps
throughout, plus we have taken great care to
specify very linear types of capacitor and to
keep resistor values low, where their Johnson
(thermal) noise contribution is likely to affect
the signal.
Inevitably, the tone control circuitry adds
some noise when it is switched in. But perfor-
mance is still very good with the tone controls
in, giving a THD+N figure of just 0.00054%
at 1kHz and 0.0007% at 10kHz. By compari-
son, with the tone controls out, those figures
become 0.00044% and 0.00048% respective-
ly – see Fig.1.
Those measurements were made with a
bandwidth of 20Hz-80kHz, which is necessary
to measure distortion at higher frequencies
174 Roe Street, PERTH. Western Australia. 6000
Phone: (08) 9428 2188
Ultra Low Distortion
Preamplifier with
Tone Controls
Introducing our state-of-the-art stereo preamplifier. Along
with almost unmeasurable noise and distortion (typically
0.0003% THD+N!) it sports remote volume control, input
selection and muting plus bass and treble adjustment
knobs on the front panel.
Reprinted with kind permission of Silicon Chip Magazine
accurately. But such a measurement includes
a significant amount of ultrasonic noise (ie,
in the 20-80kHz range). And Fig.1 shows
that the distortion performance is dominated
by noise. So we also made measurements
with a 20Hz-22kHz bandwidth, shown in
blue on Fig.1, and this reveals that the true
audible distortion and noise level is closer to
0.00025% – an astonishingly low figure.
Fig.3 shows the frequency response with the
tone control at either extreme, and switched
out (the blue curve). This demonstrates that
when you're not using the tone controls, the
frequency response is very flat. You can barely
see the deviation on this plot; zooming in, we
can see that the response is down only 0.2dB
at 20Hz and less than 0.1dB at 20kHz.
Fig.4 shows the coupling between channels,
which is typically less than -80dB, and the
coupling between adjacent inputs, typically
around -100dB. So isolation between channels
and inputs is very good. The signal-to-noise ra-
tio figure is especially good; over 120dB with
a 2.2V RMS input signal (typical for CD/DVD/
Blu-ray players), the tone controls switched
out and the volume pot at unity gain.
In summary, you can be confident when using
this preamp that it will not negatively affect the
audio signals passing through it, regardless of
whether you are using the tone controls.
Capacitor and potentiometer selection
We mentioned earlier that we're using linear
capacitor types where that's important, and
also keeping resistance values low to mini-
mise thermal noise.
For capacitors between 10nF and 100nF, we
have specified MKT polyester (plastic dielec-
tric) types. While polyester is not quite as line-
Fax: (08) 9428 2187