surround sound low-cost amp

65 Pages·2011·9.03 MB·English

B OX D ES I G N MAD E FAST AN D EASY
O c t o b e r 2 0 1 0
US $7.00/Canada $10.00
Tube, Solid State,
Loudspeaker Technology
LOW-COST
AM P
FOR
SURROUND
SOUND
A Tonearm Fix
From Jelco
Digital Filters:
A Cure-All for Speaker Reflections?
Building a Vintage
BCB Receiver Clone
www.audioXpress.com

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VOLUME 41 NUMBER 10 OCTOBER 2010 THE STAFF
FEATURES DEPARTMENTS Editor and Publisher
Edward T. Dell, Jr.
THE BRICK XPRESS MAIL ..............................................32 Vice President
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Enhance your digital home theater equipment CLASSIFIEDS ...............................................................44 Dennis Brisson ....................Assistant Publisher
with this low-cost add-on amp. Richard Surrette .................Editorial Assistant
By Raj K. Gorkhali .................................................................6 AD INDEX ......................................................................44 Jason Hanaford ...................Graphics Director
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Re-discover how pleasant audio can sound with
this simple radio receiver.
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ELIMINATING SPEAKER Richard Campbell G.R. Koonce
REFLECTIONS WITH DIGITAL FILTERS Dennis Colin Tom Lyle
Joseph D’Appolito James Moriyasu
Using digital filters to remove reflections from Vance Dickason Nelson Pass
speaker cabinets. Jan Didden Richard Pierce
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4 audioXpress 10/10 www.audioXpress.com
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Stanford Research Systems
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solid state
By Raj.K.Gorkhali
The Brick
This compact, low-cost, add-on amplifier will deliver a cool 15W/channel in stereo mode or about 29W into
a single speaker in bridged mono mode—all from an unassuming little box no larger than a house brick!
Also, with Dolby Digital, as with
MPEG and DTS encoding, there are
two true and separate rear surrounding
channel signals rather than a single de-
rived “difference” signal. So if you want
to hear all of the directional surround
information, you really need two sepa-
rate amplifier channels and speakers.
Finally, with Dolby Digital encoding,
the subwoofer track (actually called the
“LFE,” or “low frequency effects,” track)
again seems to be quite discrete. In fact,
on many action blockbuster DVDs it’s
used to deliver most of the “grunt” for
explosions and other effects. So without
a subwoofer, you can again miss most of
the impact of these effects.
All of which meant that in order
to really enjoy Dolby Digital movies
(cid:2) from DVD, I was going to need at least
ike many popular audio projects, surround-sound effect either, because two—possibly three—more speak-
this one arose from a clear per- there’s really only one “rear” signal with ers and matching amplifiers. I suspect
sonal need. After getting a new this type of decoding anyway, derived many readers are going to be in a similar
DVD player, I soon realized that from the “L-R” or “difference” signal. position when they upgrade to digital
I would need to upgrade my modest Nor did we miss a subwoofer, because surround sound as part of DVD-based
“home theater” amplifier and speaker my main speaker has pretty good bass home theater.
setup in order to take full advantage response. The extra speakers aren’t likely to be
of the Dolby Digital (AC-3) surround too much of a problem, of course; many
UPGRADE NEEDS
sound on most DVDs. of us have a few lying around from dis-
Up until now I’d been “making do” But when I tried playing Dolby Digi- carded stereo systems. But I soon found
with a simple analog-type surround- tal signals through this “economy” myself looking for a low-cost add-on
sound setup, using the el-cheapo matrix- surround-sound setup, it soon became amplifier, for either stereo or bridged
type decoder. I hadn’t even been using all clear that I would need to make some mono use.
of the outputs from this. I’d been getting changes. For starters, all of the Dolby What I really needed was a kind of
quite satisfying sound from most video Digital “5.1 channels” are discrete and “bare essentials” version of the Shoe-
movies using just the front left and right separate, and the center–front channel string Amp. Just the basic power chan-
speaker (driven by my main stereo amp) signal is quite different from a mere sum nels with all of the frills removed,
and a single rear-channel speaker, driven of the front L and R signals. But as it housed in a compact box, and designed
(in bridged mono mode) by one of the happens, many DVDs actually have the so you can build it for a rock-bottom
stereo amps. main screen dialog on the center-front price. The concept of the Brick was
My family and I didn’t really miss track, so you really do need a center- born.
the center-front speaker, which, after front amp and speaker if you want to Why the name? Well, it’s roughly the
all, handles only a “sum” of L+R signal hear most of the dialog! (Unless you use same size and shape as a house brick,
with this type of analog surround decod- the two-channel mix down outputs, an and almost as unattractive. Functional
er. We didn’t miss out on much of the analog decoder.) rather than pretty, if you like.
6 audioXpress 10/10 www.audioXpress.com
Introducing the
DEQX HDP-Express
There’s never been anything like DEQX’s Active linear-phase crossover features:
• Steeper than 6dB/octave crossover filters required to reduce distortion
HDP-3DSPprocessor.Its-140dBTHDdigital
don’t maintain linear-phase using traditional crossovers. Now they do.
transparencyprovidesDEQX’suniquespeaker
• Traditional active Butterworth and Linkwitz Riley, mild to steep crossovers
androomcorrectionandactivelinear-phase and four ‘Profile’ presets lets you directly compare linear-phase advantages.
crossovers. • Using 48dB/octave or steeper linear-phase crossovers distortion vanishes,
resolution increases, while volume can double or quadruple, 3 to 6dB.
It also provides Preamp and DAC features
• When used with speaker correction, stereo 3D imaging and dispersion
toavoidunnecessaryconversions.And yet, also improves to such a degree that a centre speaker is unnecessary.
despitebeingmodestlypricedfromjust Speaker correction features:
US$3950,it’s been out of reach for many • Anechoically measure and correct your passive or active speakers using
a simple measurement technique, even in your listening room.
serious music lover DIYs.
• Corrects anechoic (native) speaker frequency-response from typically
EntertheHDP-Express™. plus/minus 3dB to plus/minus 0.3dB*: about a 6dB improvement!
• Corrects Group-delay (phase/timing errors at different frequencies) about
DesignedforDIYbudgetsfromUS$1950
tenfold* e.g. 1ms reduces to 0.1ms —especially noticeable in midrange.
(pluscalmicandfreight)ifpurchasedfactory-
Room Correction features and preference EQ:
direct,theHDP-Express™retainsthekey • One or more room measurements displayed graphically, allow manual,
performancefeaturesoftheHDP-3,using real-time, 7-band parametric EQ settings
thesameDEQX-calsetupsoftwarefor • Time-domain correction of subwoofers/bass speakers measured in-room.
• Adjust delay between main speakers and subwoofers in real-time.
Windows(orMacwithWindows).
Media correction—forensic tone control:
• Remote controlled Low-shelf, Midband-fully parametric and High-shelf
• Low, mid and Hi frequencies adjustable in octaves and semitones
• Adjustable Q from one semitone to four-octave wide
• 99-memory storage for instant recall
Pre-amp and processing features
• Four profiles for instant selection of crossovers, correction and EQ
For maximum convenience,
• Four inputs: S/PDIF, AES3, analogue unbalanced and balanced.
all preamp functions are
controllable by the remote. • Integrate one or two subwoofers
• Remote controlled Input and profile selection.
• Six unbalanced outs: Stereo low, mid, Hi (mid used for passive speakers)
• Optional balanced outs (6 x XLRs) transformer or active.
• Dual 32-bit SHARC DSPs provide minus 140dB THD digital transparency
Mention Audio Express
for a special offer:
HIGH DEFINITION AUDIO BY DEQX
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*Anechoic correction may have limited resolution at bass frequencies (20Hz - 200Hz), where room measurement and correction can be used.
All the same, it provides two 15W quite effective low-cost analog surround familiar and low-cost LM1875 power
RMS power amplifier channels, with sound system. Of course, the Brick op amp.
low distortion and noise, and good fre- would also be quite suitable for enhanc- In stereo mode, each half of the
quency response. You can use the two ing a multimedia computer setup, allow- TL072 operates as a non-inverting
channels for two separate signals, if you ing you to drive a pair of decent external input amplifier with a gain of about 2.8
wish, or combine them with the flick speakers. times—as determined by feedback re-
of a switch into a single bridge-mode sistors R7 and R5, for example. At the
OPERATION
mono channel, with an output of about front of each input amp there’s a low-
29W RMS. So it’s equally suitable for There’s nothing very novel about the pass filter for RF suppression (R1/C1
driving a pair of rear–channel speakers, Brick’s circuitry; it’s basically just the and R2/C2, followed by the volume/
a single center-front channel, or even a power supply and power amps with a gain control RV1a/b) and blocking ca-
reasonably sensitive subwoofer. “front end” added. The only real twist pacitors C3/C4.
The input sensitivity is pretty good, is a simple scheme that allows the two Switch SW1 is used to configure the
too. You need only around 200mV input channels to be driven from one of the circuit for bridge-mode mono operation.
to produce full output, so it’s very suit- inputs with opposite polarity, for hassle- When it’s switched to the mono posi-
able for use with “line” output on DVD free bridge-mode mono operation. The tion, the non-inverting input of IC1a is
players with built-in Dolby Digital de- schematic (Fig. 1) shows how it’s done. connected to ground, so the “left” input
coders. In fact, you could use one or All of the front-end circuitry is based on of the Brick becomes ineffective. How-
two of the Bricks and the new surround a TL072 dual FET input op amp (IC1), ever, the “cold” end of resistor R6, used
decoder to produce a complete and while each channel’s power stage uses a as that stage’s lower feedback resistor in
FIGURE 1: The Brick amp circuit.
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stereo mode, is now disconnected from front-end circuit earth to maximize AC low-cost brick-shaped aluminum utility
ground and connected instead to R9, stability. There’s also a zobel impedence box, measuring only 125 (cid:114) 100 (cid:114) 75mm
running to the output of IC1b, the right stabilizing circuit across each power (Fig. 2). This wasn’t easy, especially as
channel’s input stage. As a result, IC1a is amp’s output (R16/C19 and R17/C20) the 30V/1A power transformer takes up
now converted into a unity-gain invert- to again ensure maximum AC stabil- over half of the internal volume.
ing stage, taking the signal from IC1b ity. In stereo mode, each of the output The power supply and input stag-
and producing an opposite-polarity rep- stages drives its own speaker, which is es are mounted on a small PCB just
lica at its own output. This converts the connected between the “+” output termi- behind the front panel—in this case,
front end so that it now produces dual- nals to receive the combined output of mounted vertically—while the power
phase drive signals from the right chan- both stages. amp stages are mounted on a small
nel input. The output stages operate from DC narrow PCB mounted just behind the
Each input stage drives one of the rails of (cid:111)21V, which is provided from rear panel and heatsink. Because of the
LM1875 power stages (IC2 and IC3), a simple power supply using a 30V/1A smaller box and the need to use a low-
with very traditional circuitry. Coupling center-tapped transformer driving a cost finned heatsink for the output de-
caps C7 and C8 provide DC blocking, bridge rectifier (or, if you prefer, two vices, even the power amp PCB had
with the power stage voltage gains set full-wave rectifiers), with two 2200(cid:77)F to be smaller than that in the amp. I
to around 18 by the feedback network reservoir capacitors for each output rail. achieved this by moving the input cou-
R14/R12 and R15/R13. Resistors R10 The input stages operate from (cid:111)21V pling capacitors to the front end board,
and R11 provide biasing for the non- rails via simple dropping circuits using and also by using separate power supply
inverting inputs, while C9/C10 provide resistors R18/19, zener diodes ZD1/2, connections for each channel. As with
AC grounding for the bottom of the and decoupling capacitors C25/26. the corresponding board, this one is sup-
feedback networks, yet allow the power ported purely by the two power amp
CONSTRUCTION
stages to operate as DC voltage follow- chips. This causes no problems as the
ers for maximum thermal stability. The electronics side of the Brick is board assembly is very light.
The earth returns for the output pretty conventional. The only real trick The front end/power supply PCB is
amp input circuits are taken back to the is to squeeze everything into a small, automatically much smaller than in the
FIGURE 2: Box dimensions (all in metric). Note: All unmarked holes are 3mm diameter.
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