in audio amps

48 Pages·2011·6.33 MB·English

A CALL FOR INTELLIGENT SPEAKER COMPARISON
J a n u a r y 2 0 1 0
US $7.00/Canada $10.00
Tube, Solid State,
Loudspeaker Technology
O R T
P
W R E
O
H
Y S
D I
Phono Preamp
UPDATE
Finding the Sweet Spot AC Power Line
IMPROVEMENTS
IN AUDIO AMPS
The CableCaddy:
A Hatrack for Wire
Tube Amp Power FOR LESS
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VOLUME 41 NUMBER 1 JANUARY 2010
Editor and Publisher
Edward T. Dell, Jr.
FEATURES DEPARTMENTS
Vice President
Karen Hebert
THE SWEET SPOT EDITORIAL Dennis Brisson ....................Assistant Publisher
This master circuit designer reveals some secrets A New Year Richard Surrette .................Editorial Assistant
apto sthseib lceo smopuonnde innt yleovuer l atuod aioch aiemvpinsg. the best By Edward T. Dell, Jr. .......................................6 JDasaovnid H Haannaafoforrdd .. ..................................GGrraapphhiiccs D Deisriegcnteorr
Laurel Humphrey ..............Marketing Director
By Nelson Pass .....................................................................8 TOOLS & TECHNIQUES Sharon LeClair ...................Customer Service
The CableCaddy Mike Biron ..........................Shipping Manager
15 WATTS PER CHANNEL FOR By Jan Didden ................................................................37 Regular Contributors
LESS THAN $150 Eric Barbour Gary Galo
Erno Borbely Chuck Hansen
Our frugal friend presents a tube amp design for
Richard Campbell G.R. Koonce
first-time builders.
Dennis Colin Tom Lyle
By Bruce Brown ................................................................22 Joseph D’Appolito James Moriyasu
Vance Dickason Nelson Pass
Jan Didden Richard Pierce
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Editorial
A New Year
O
ur heartfelt thanks to all of For those who choose it, the print
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6 audioXpress 1/10 www.audioXpress.com

circuit design
By Nelson Pass
The Sweet Spot
What you can do to ensure the best possible sound from your system.
Every audiophile understands the For every gain device, there is a number illustration shows simplified examples of
concept of the “sweet spot,” that which characterizes the amount of gain. the two most common gain stages using
happy balance of conditions that When you talk about distortion, you are any kind of gain device, and the device
elicits the best possible sound. It talking about the alteration of the amount pins are the Grid, Cathode, and Plate. In
can be a particular listening spot, the of gain in the device as the conditions both cases the input signal appears at the
best positioning of loudspeakers, or the change. The gain of a tube or transistor Grid and variation of voltage between
fortuitous combination of components is altered primarily by changes in volt- the Grid and Cathode causes a change
which complement each other perfectly. age across the device, current through the in the current flow from the Plate to the
It should be no surprise that the desire device, and the temperature of the de- Cathode. (All the examples you will see
for the best performance takes the search vice. Changes in any of these conditions are simplified, and omit DC bias that may
for the sweet spot into the interiors of the change the gain, and all gain changes pro- be present on the pins except for the V+
components themselves. duce distortion. More precisely, they are supply).
This article will concern itself with the distortion. In Common-Cathode mode, the tube
finding the sweet spot for each gain device Of course, the concept of a sweet spot develops both current and voltage gain,
in audio amplifiers. It is a commonly held depends on having an idea of what con- and the output is taken off the plate, in-
belief in audio that the best amplifiers stitutes the best performance. It could be verted in phase, and usually increased in
are composed of one or more active gain that you want the lowest measured distor- amplitude. In Common-Plate mode, the
stages, each made as intrinsically linear as tion, a particular mix or phase of harmon- tube seeks merely to have the Cathode
possible before negative feedback is ap- ics in the distortion waveform, the great- voltage follow the Grid voltage, so the
plied to further improve performance. est efficiency, greatest power, or simply output voltage is ideally the same as the
Taken to the extreme, linear stage = the best subjective experience when you input, but with some current gain. That is
good/negative feedback = bad. The lat- listen to it. The sweet spot is whatever you why it’s called a follower.
ter half of that statement is the subject want—after all, you are the designer. For both modes you would normally
of controversy, but there is no argument look for the sweet spot—the conditions
I BRAKE FOR TUTORIALS
about linear stages being good. giving the best performance—by varying
Achieving intrinsically low distortion If you don’t mind a few basics, let’s look the values of the supply V+, the value of
in active devices (tubes or transistors) is at examples of gain devices in generic cir- the load resistance, and the bias current
usually not easy. You can use good parts, cuits, starting with a triode (Fig. 1). This through the device. Inevitably there will
see to it that they have adequate voltage
and current available, and use them in to-
pologies that play to their strengths. These
are the basics, and this is what you gener-
ally see when you examine schematics of
popular amplifiers.
However, each individual device has a
sweet spot where its performance is the
best—often dramatically better—and
finding that sweet spot is a powerful tech-
nique for maximizing the performance. I
am writing about this because after years
of communicating with DIYers and many
professionals, I have discovered that this is
a poorly understood concept, and I have
seen many circuits which have failed to
FIGURE 1: Triode single-ended Class A.
take advantage of it.
8 audioXpress 1/10 www.audioXpress.com

be some combination of these which is the best. Sometimes one
of the circuit conditions is fixed; often it is the load value, in
www.hs-devices.com [email protected]
which case you vary the other conditions—the supply voltage
ENGINEERED and the bias.
In all of the examples later, I have chosen to simply look at
Common-Plate (or Common-Drain/Common-Collector) per-
TO PERFORM...
formance, but the same ideas apply to Common-Cathode (or
Source or Emitter) operation. Actually, Common-Drain circuits
AMT would seem to offer the least opportunity for improvement—as
AIR MOTION TRANSFORMER followers enjoying 100% degenerative feedback, they already
by MUNDORF
have a much lower distortion character than Common-Source
“There are many dome circuits. But you will see that even they can be improved, often
tweeters out there [...] But dramatically, if you find the sweet spot.
none of them sound as Figure 2 shows the example of the JFET counterparts to the
dynamic as the AMT.”
tube circuits seen above. Here the pins are Gate, Source, and
Drain. In the Common-Drain follower, the input voltage pretty
Dick Olsher in
The Absolute Sound® 11/2008, much follows the output voltage, and in the Common-Source
reviewing a prominent speaker amplifier, the output is inverted and has both voltage and cur-
featuring our AMT®2440
rent gain available. Figure 3 shows the MOSFET example,
HOLO RAPHIC which has the same pin designations as the JFET.
And last, Fig. 4 is the example of a bipolar transistor, where
SOUND DE ICES
by MUNDORF the pins are Base, Emitter, and Collector. The choice of a par-
ticular type of gain device—tube or transistor—is often arbi-
...THE BEAUTY
trary, but ideally it plays to the particular strength of the device
for a given application. In principle you can substitute one type
OF MUSIC. Handcrafted in for another, but as a practical matter, there is usually a good rea-
the Homeland of
Ludwig v. Beethoven son for the selection of one over the others.
It doesn’t matter in terms of the idea of finding the
sweet spot—the terrain changes a bit, but the game
remains the same.
GETTING TO THE SWEET SPOT
For any of the above circuit types and devices, if you start with
HHoollooggrraapphhiicc SSoouunndd DDeevviicceess®®®®bbyy MMUUNNDDOORRFF •• AAMMTT ®®®TTwweeeetteerr SSeerriieess reasonable textbook values for the circuit, you will get reason-
able performance. If you start to play with the values a little bit,
you will find that the performance changes for better or worse.
Often you will find a combination of voltage and resistor values
which give a lot better than the generic performance. That’s the
sweet spot.
If you have a distortion analyzer, you could simply run
Superior components for more than 20 years. Such as the
through the range of combinations of values and select the re-
MCap® Supreme Silver/Gold/Oil sult you like best. If you simply want good measurements, you
might be able to stop there. If you are looking for better subjec-
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FIGURE 2: JFET single-ended Class A.
10 audioXpress 1/10 www.audioXpress.com