Tubes versus Transistors - Is There An Audible Difference?

SEAB-liATTBEWS

AUDIO SYSTEMS

20 WEST 84ih STREET

NEW YORK, NEW YORK 10024

(212) 874-0137

Tubes Versus Transistors-
Is There an Audible Difference?*

RUSSELL O. HAMM

Sear Sound Studios, New York, N. Y.

Engineers and musicians have long debated the question of tube sound versus
transistor sound. Previous attempts to measure this difference have always assumed
linear operation' of the test amplifier. This conventional method of frequency re-
sponse, distortion, and noise measurement has shown that no significant difference
exists. This paper, however, points out that amplifiers are often severely overloaded
by signal transients (THD 30%). Under this condition there is a major difference
in the harmonic distortion components of the amplified signal, with tubes, transistors,
and operational amplifiers separating into distinct groups.

INTRODUCTION: As a recording engineer we became
directly involved with the tube sound versus transistor
sound controversy as it related to pop recording. The dif-
ference became markedly noticeable as more solid-state
consoles made their appearance. Of course there are so
many sound problems related to studio acoustics that
electronic problems are generally considered the least
of one's worries. After acoustically rebuilding several
studios, however, we began to question just how much of
a role acoustics played.

During one session in a studio notorious for bad sound
we plugged the microphones into Ampex portable
mixers instead of the regular console. The change in
sound quality was nothing short of incredible. All the
acoustic changes we had made in that studio never had
brought about the vast improvement in the sound that a
single change in electronics had. Over a period of sev-
eral years we continued this rather informal investiga-

* Presented September 14, 1972, at the 43 rd Conven-
tion of the Audio Engineering Society, New York.

MAY 1973, VOLUME 21, NUMBER 4

tion of the electronic sound problem. In the past, we have
heard many widely varied theories that explain the prob-
lem, but no one, however, could actually measure it
in meaningful terms.

PSYCHOACOUSTICS

Anyone who listens to phonograph records closely can
tell that tubes sound different from transistors. Defin-
ing what this difference is, however, is a complex psy-
choacoustical problem. Any investigation of this admit-
tedly subtle phenomenon must really begin with a few
human observations. Some people try to point out and
describe valid differences. Others just object to the en-
tire thesis and resort to spouting opinions. It is the lis-
tener's job to sort out the facts from the fiction.

Electrical engineers, especially the ones who design
recording equipment, can prove that there is no differ-
ence in tube or transistor sound. They do this by show-
ing the latest specification sheets and quoting electronic
figures which are visually quite impressive. It is true, ac-
cording to the parameters being measured, that there is

267

RUSSELL O. HAMM

Fig. 1. Simplified bipolar logarithmic amplifier schematic.

only a marginal difference in the signal quality. But are
there some important parameters which are not being
measured? One engineer who admits that there might
be some marginal difference in the sound, says, "You just
have to get used to the nice clean sound of transistors.
What you've been listening to on tubes is a lot of dis-
tortion." Of course the question which comes to mind is,
What is this distortion and how is it measured?

Psychoacoustically, musicians make more objective sub-
jects than engineers. While their terms may not be ex-
pressed in standard units, the musician's "by ear" measur-
ing technique seems quite valid. Consider the possibili-
ty that the ear's response may be quite different than an
oscilloscope's.

"Tube records have more bass. . . . The bass actually
sounds an octave lower," says one rock guitarist. A
couple of professional studio players have pointed out
on numerous occasions that the middle range of tube re-
cordings is very clear, each instrument has presence, even
at very low playback levels. Transistor recordings tend
to emphasize the sibilants and cymbals, especially at low
levels. 'Transistor recordings are very clean but they lack
the 'air' of a good tube recording." "With tubes there
is a space between the instruments even when they play
loud . . . transistors make a lot of buzzing." Two people
commented that transistors added a lot of musically un-
related harmonics or white noise, especially on attack
transients. This same phenomenon was expressed by an-
other person as a "shattered glass" sound that restricted
the dynamics. It was generally agreed that tubes did not
have this problem because they overload gently. Final-
ly, according to one record producer, 'Transistor records
sound restricted like they're under a blanket. Tube rec-
ords jump out of the speaker at you. . . . Transistors
have highs and lows but there is no punch to the sound."

When we heard an unusually loud and clear pop-

ular-music studio recording, we tried to trace its origin.
In almost every case we found that the recording console
had vacuum-tube preamplifiers. We are specific in men-
tioning preamplifiers because in many cases we found
hybrid systems. Typically this is a three- or four-track
console that is modified with solid-state line amplifiers
to feed a solid-state eight- or sixteen-track tape machine.
Our extensive checking has indicated only two areas
where vacuum-tube circuitry makes a definite audible
difference in the sound quality: microphone preampli-
fiers and power amplifiers driving speakers or disc cut-
ters. Both are applications where there is a mechanical-
electrical interface.

As the preliminary basis for our further investiga-
tion we decided to look into microphone and preampli-
fier signal levels under actual studio operating conditions.
Hoping to find some clues here we would then try to
carry this work further and relate electrical operating
conditions to acoustically subjective sound colorations.
Our search through published literature showed that little
work has been undertaken in this area. Most microphone
manufacturers publish extensive data on output levels
under standard test conditions [1], but this is rather hard
to convert to terms of microphone distances and play-
ing volumes. Preamplifier circuit design is well covered
for noise considerations [2], but not from the standpoint
of actual microphone operating levels. Distortion has been
treated in numerous ways [3-5], but with very few ref-
erences to musical sound quality [10].

MICROPHONE OUTPUT LEVELS

To get a rough idea of the voltage output from dif-
ferent types of microphones, an oscilloscope was paral-
leled across inputs of a console. During the normal pop-
ular-music type sessions, peak readings of 1 volt or more
were common, especially from closeup microphones on
voice and drums. Due to the linear voltage scale, oscil-
loscope measurements over more than a 10-dB range
are difficult. By building a simple bipolar logarithmic
amplifier, the useful measuring range was extended to
about four decades (Fig. 1). Considerable studio ob-
servation finally led to the construction of a peak hold-
ing type decibel meter. This circuit retained transient
peaks of more than 50 microseconds within 2-dB ac-
curacy for about 10 seconds; long enough to write them
down. Using the logarithmic oscilloscope display and
the peak meter together proved very useful in gathering
a wealth of data about real-life microphone signals.

Table I shows the normal peak outputs from several

Instrument

Bass drum

(single head)
Large torn torn
Small torn torn
Piano (single note)
Piano (chord)
Orchestra bells
Cow bell
Loud yell

Table 1. Peak microphone output levels for percussive sounds

Distance
(inches)

12

12

6

6

18

12

4

Microphone Voltage. Open Circuit. dB Ref. 0.775 V

U-87

U-47

77-DX

— 1

— 1
-25
-23
-16
-10

—6

-6

-5
—29
-27
-25
-12
— 11

-9

—9

—7
-38
-36
-33
-29

* U-87 and U-47 by Neumann, 77DX by RCA, C-28 by AKG, 666 by Electro- Voice.

C-28

— 15

-10
-9

-35
-33
-33
-19
-10

666

— 1

—5

— 1
-32
-33
-30
— 15
-10

268

JOURNAL OF THE AUDIO ENGINEERING SOCIETY

TUBES VERSUS TRANSISTORS-IS THERE AN AUDIBLE DIFFERENCE?

popular types of studio microphones. All the readings
are taken with the microphone operating into the pri-
mary of an unloaded transformer. Pickup distances are
indicated for each instrument and were determined by
normal studio practice. Table II is an abridgement of a

Table 2. Peak output for a U-47 microphone for various
sounds.

Peak ■

Pressure

Microphone

Distance

Frequency

Voltage (dB

Instrument

(feet)

(Hz)

Ref. 0.775 V)

75-piece orchestra

15

350

-10

15-piece orchestra

10

350

— 12

Trumpet

3

600

-16

Trombone

3

600

— 15

French horn

3

300

-13

Flute

3.5

800

-26

Piccolo

3.5

2500

— 18

Clarinet

3.5

350

-22

Bass sax

3.5

350

—8

Bass viol

5

150

— 13

similar study done by Fine Recording, Inc., several years
ago. Details of this test setup are not available but the
readings are probably taken without the 6-dB pad com-
monly used on the U-47 microphone today. Some cal-
culations based on the manufacturer's published sensi-
tivity for these microphones indicates that acoustic
sound-pressure levels in excess of 130 dB are common.
While the latest console preamplifiers have less noise,
less distortion, and more knobs than ever before, they
are not designed to handle this kind of input level. In
most commercially available preamplifiers, head room
runs on the order of +20 dBm, 1 and gain is common-
ly set at 40 dB. With these basic parameters it is clear
from the data shown in Tables I and II that severe
overloads can occur on peaks from almost all instru-
ments. For example, a U-87 microphone gives a peak
output of —1 dBm from a large floor torn. Amplifica-
tion by 40 dB in the microphone preamplifier results in
an output swing of + 39 dBm, or almost 20 dB above the
overload point. Logically a peak of this magnitude should
be severely distorted.

Most recording consoles today have variable resistive
pads on the microphone inputs to attenuate signal levels
which are beyond the capabilities of the preamplifier.
The common use of these input pads supposedly came
about with the advent of loud rock music; however, this
is not true in fact For some 20 years it has been com-
mon to use a Neumann U-47 microphone for close mi-
crophone recording of brass and voice. Table II shows
output levels requiring 10-20 dB of padding under these
conditions, and this does agree with recording practice
today where solid-state amplifiers are used. But most
tube consoles did not have input pads and yet the same
microphone performed with little noticeable distortion.
Certainly brass players and singers are not that much
louder today than they were yesterday. The microphone
distance is about the same. The preamplifier specifications
have not changed that much. Yet transistors require pads
and tubes do not.

Here then is the hypothesis for further investigation.
In the usual evaluation of audio preamplifiers it is as-

1 dBm is 1 mW into 600 ohms.

sumed that they are operated in their linear range, i.e.,
harmonic distortion less than 10%. In this range tubes
and transistors do have very similar performance char-
acteristics. But the preceding section points out that am-
plifiers nre often operated far out of their linear range
at signal levels which would cause severe distortion.
Under these conditions, tubes and transistors appear to
behave quite differently from a sound viewpoint.

DISTORTION CHARACTERISTICS OF
PREAMPLIFIERS

Three commercially available microphone preampli-
fiers of different designs were set up in the recording
studio. Each amplifier was adjusted for a gain of 40 dB
and an overload point of 3% total harmonic distortion
(THD) at +18 dBm. Preamplifier 1 was a transistor
design, preamplifier 2 was a hybrid operational amplifier,
and preamplifier 3 was a vacuum-tube triode design. The
amplifier outputs were terminated in 600-ohm loads and
bridged by the monitoring system. The test signal, U-87
microphone, and large floor torn were switchable to each
preamplifier input.

An informal group of studio personnel listened to the
outputs of the three amplifiers on the normal control
room monitor speakers. As the test signal was switched
from one amplifier to another, the listeners were asked
to judge the sound quality. The output of amplifiers 1
and 2 was unanimously judged to be severely distorted.
Amplifier 3, however, sounded clean. The test was re-
peated several times inserting attenuating pads in the
microphone line until each amplifier sounded undis-
torted. Amplifier 1 could stand overloads of 5-10 dB
without noticeable distortion. Amplifier 2 showed notice-
able distortion at about 5-dB overload. Further listening
revealed that it was only in the range of early overload
where the amplifiers differed appreciably in sound quali-
ty. Once the amplifiers were well into the distortion re-
gion, they all sounded alike — distorted. In their normal
nonoverload range all three amplifiers sounded very clean.

The listening tests clearly indicate that the overload
margin varies widely between different types of ampli-
fiers. Engineering studies show that any amplifier adds
distortion as soon as the overload point is reached. The
tests show that all amplifiers could be overloaded to a
certain degree without this distortion becoming notice-
able. It may be concluded that these inaudible harmonics
in the early overload condition might very well be caus-
ing the difference in sound coloration between tubes and
transistors.

To get a general representation of the character of
harmonic distortion in audio amplifiers, overload curves
were plotted for about fifty different circuits. The tube
circuits used the popular 12AY7 and 12AX7 triodes, the
8628 and 7586 triode nuvistors, and the 5879 pentode.
These tubes have all been extensively used in recording
console preamplifiers. The 2N3391A, 2N5089, and
2N3117 silicon NPN transistors were also chosen be-
cause of their extensive use in console and tape recorder
circuitry. For comparison purposes tests were also run on
the 2N5087 which is the PNP sister of the 2N5089. Op-
erational amplifiers included the popular 709 and LM301
monolithic units and two commercially available hybrid
designs used in recording consoles.

The curves shown in Fig. 2 are representative of the
general distortion characteristics of single-stage class A

MAY 1973, VOLUME 21, NUMBER 4

269

RUSSELL O. HAMM

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Fig. 2. Single-stage amplifier comparison of total har-
monic distortion (THD).

Fig. 3. Multistage amplifier comparison of total harmonic
distortion (THD).

audio amplifiers. The devices are all operating open loop
(no feedback) with a bias point which allows for maxi-
mum undistortcd output swing. The curves are referenced
to a common point of 3% (THD), regardless of actual
input or output levels. Since the objective of these com-
parisons is to detect variations in the slopes of the dis-
tortion characteristics, the x axis is a scale of relative
level independent of circuit impedance considerations.
These particular curves were chosen from the many
plotted as representative of different families: silicon
transistors, triodes, and pentodes. A quick look shows that
the often versed opinion that tubes overload more gently
than transistors is obviously a myth.

single-stage amplifiers, a review of the many different
amplifiers tested shows that the slopes of all THD curves
run about the same. The lack of a wide variation be-
tween the curves indicates that THD plots are not very
relevant to what the ear hears in the listening tests.

Another series of tests were made on the same group of
preamplifiers using a spectrum analyzer to measure the
amplitude of individual harmonics. Each amplifier was
driven 12 dB into overload, starting from a reference
point of 1% third harmonic distortion. Every harmonic
to the seventh was plotted. Since it is not possible to mea-
sure the relative phase of the harmonics on the spectrum
analyzer, the overload waveforms were recorded for

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Fig. 6. Distortion components for two-stage pentode am-
plifier.

Fig. 3 shows the distortion characteristics for four
different commercially available preamplifiers, using two
or more stages of amplification. All the circuits use feed-
back, a couple are push— pull. Each ampifier is operating
into 600 ohms at a gain of 40 dB. As in the previous
curves, there is a common reference point of 1% THD.
While these curves show a marked difference from the

Fourier analysis on the digital computer. The resulting
plots divided amplifiers into three distinct categories.

1) Tube Characteristics

Fig. 4 shows the distortion components for a typical
two-stage 12AY7 amplifier. This particular design is quite
representative of several single-ended, multistage triode
tube amplifiers tested. The outstanding characteristic is

Fig. 5. Waveform of triode amplifier of Fig. 4 at 12-dB
overload. 1000-Hz tone.

Fig. 7. Waveform of pentode amplifier of Fig. 6 at 12-
dB overload, 1000-Hz tone.

270

JOURNAL OF THE AUDIO ENGINEERING SOCIETY

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Fig. 8. Distortion components for multistage capacitor-
coupled transistor amplifier.

the dominance of the second harmonic followed closely
by the third. The fourth harmonic rises 3-4 dB later,
running parallel to the third. The fifth, sixth, and seventh
remain below 5% out to the 12-dB overload point. These
curves seem to be a general characteristic of all the triode
amplifiers tested, whether octal, miniature, nuvistor,
single-ended, or pusfa-i>ull. Fig. 5 is the waveform at 12
dB of overload. The clipping is unsymmetrical with a
shifted duty cycle. Again this is characteristic of all the
triode amplifiers tested. Fig. 6 shows the distortion com-
ponents for a two-stage single-ended pentode amplifier.
Here the third harmonic is dominant and the second
rises about 3 dB later with the same slope. Both the
fourth and the fifth are prominent while the sixth and
seventh remain under 5%. The waveform at 12-dB over-
load (Fig. 7), is similar to the triode, but its duty cycle
is not shifted as much. It is not reasonable to assume
that virtually all tube amplifiers can be represented by
these two examples. However, the major characteristic
of the tube amplifier is the presence of strong second and
third harmonics, sometimes in concert with the fourth
and fifth, but always much greater in amplitude. Har-
monics higher than the fifth are not significant until the
overload is beyond 12 dB. These characteristics seem
to hold true for wide variations in circuit design param-
eters. The extreme difference in the tube amplifiers is the
interchanging of the position of the second and third
harmonics. This effect is not just a characteristic of the
pentode, it is common to triodes too.

Fig. 9. Waveform for transistor amplifier of Fig. 8 at
12-dB overload, 1000-Hz tone.

2) Transistor Characteristics

Figs. 8 and 10 show the characteristics of two transistor
amplifiers. Like the previous figures the curves are repre-
sentative of all the transistor amplifiers tested. The dis-
tinguishing feature is the strong third harmonic com-
ponent All other harmonics are present, but at a much
lower amplitude than the third. When the overload

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coupled transistor amplifier.

reaches a break point, all the higher harmonics begin to
rise simultaneously. This point is generally within 3-6
dB of the 1% third harmonic point. The waveforms of
these amplifiers (Figs. 9 and 11) are distinctly square
wave in form with symmetrical clipping and an almost
perfect duty cycle. Both amplifiers shown have single-
ended inputs and push-pull outputs. However, the cir-
cuit designs are radically different.

Fig. 11. Waveform for transistor amplifier of Fig. 10 at
12-dB overload, 1000-Hz tone.

3) OperationaLAmplifier Characteristics

Fig. 12 is a hybrid operational amplifier. The third
harmonic rises steeply as the dominant distortion com-
ponent in a characteristic similar to the transistor. Also
rising very strongly from the same point are the fifth and
seventh harmonics. All even harmonics are suppressed
completely. The waveform of Fig. 13 is a perfect square
wave. As a classification group, operational amplifiers
have the most uniform characteristics with almost no de-
viation from the curves shown in this example.

In view of the transient nature of audio signals, steady-

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MAY 1973, VOLUME 21, NUMBER 4

271

RUSSELL O. HAMM

Fig. 13. Waveform for operational amplifier of Fig. 12 at
12-dB overload, 1000-Hz tone.

state single-frequency distortion analysis could yield ques-
tionable results. Indeed, the arguments for and against
sine-wave and pulse test signals for audio system testing
have been the subject for a number of engineering papers
[4], [7]. For our purposes, however, a few minutes toy-
ing with an electronic synthesizer quickly proved that
musical instruments do not produce fast pulses. For ex-
ample, a good simulation of the large floor torn used in
the amplifier listening tests is a 100-Hz tone modulated
with an envelope rise time of 5 ms and a decay time of
300 ms. Also an extensive study of trumpet tones [6]
measured the rise time of the fastest staccato notes at 12
ms. Certainly, rise times of these orders can not be con-
sidered pulses for audio amplifiers with passbands ex-
tending to 20 kHz or better. Just to further prove the
correctness of the preceding steady-state results, the
synthesized floor torn signal was used to test the same
amplifiers at the same level as the microphone signal.
Careful observation of the amplified signal showed
that envelope clipping was identical to the steady-state
clipping level (Fig. 14). There were no glitches or other
fast transient phenomena in the output signal.

SIGNIFICANCE OF MUSICAL HARMONICS

Having divided amplifiers into three groups of dis-
tortion characteristics, the next step is to determine how
the harmonics relate to hearing. There is a close parallel
here between electronic distortion and musical tone col-
oration that is the real key to why tubes and transis-
tors sound different. Perhaps the most knowledgeable
authorities in this area are the craftsmen who build or-
gans and musical instruments [8], [9], Through many
years of careful experimentation these artisans have de-

Fig. 14. a. Envelope of Moog-generated floor torn test
signal, b. Envelope clipping of transient signals by amplifier
is identical to single-frequency clipping levels.

termined how various harmonics relate to the colora-
tion of an instrument's tonal quality.

The primary color characteristic of an instrument is
determined by the strength of the first few harmonics.
Each of the lower harmonics produces its own charac-
teristic effect when it is dominant or it can modify the
effect of another dominant harmonic if it is prominent.
In the simplest classification, the lower harmonics are
divided into two tonal groups. The odd harmonics (third
and fifth) produce a "stopped" or "covered" sound. The
even harmonics (second, fourth, and sixth) produce
"choral" or "singing" sounds.

The second and third harmonics are the most impor-
tant from the viewpoint of the electronic distortion
graphs in the previous section. Musically the second is an
octave above the fundamental and is almost inaudible;
yet it adds body to the sound, making it fuller. The third
is termed a quint or musical twelfth. It produces a sound
many musicians refer to as "blanketed." Instead of mak-
ing the tone fuller, a strong third actually makes the
tone softer. Adding a fifth to a strong third gives the
sound a metallic quality that gets annoying in character
as its amplitude increases. A strong second with a strong
third tends to open the "covered" effect. Adding the
fourth and the fifth to this changes the sound to an "open
horn" like character.

The higher harmonics, above the seventh, give the
tone "edge" or "bite." Provided the edge is balanced to
the basic musical tone, it tends to reinforce the funda-
mental, giving the sound a sharp attack quality. Many
of the edge harmonics are musically unrelated pitches
such as the seventh, ninth, and eleventh. Therefore, too
much edge can produce a raspy dissonant quality. Since
the ear seems very sensitive to the edge harmonics, con-
trolling their amplitude is of paramount importance. The
previously mentioned study of the trumpet tone [6] shows
that the edge effect is directly related to the loudness of
the tone. Playing the same trumpet note loud or soft
makes little difference in the amplitude of the funda-
mental and the lower harmonics. However, harmonics
above the sixth increase and decrease in amplitude in al-
most direct proportion to the loudness. This edge bal-
ance is a critically important loudness signal for the
human ear.

RELATIONSHIP OF FACTORS AND FINDINGS

The basic cause of the difference in tube and transistor
sound is the weighting of harmonic distortion com-
ponents in the amplifier's overload region. Transistor am-
plifiers exhibit a strong component of third harmonic
distortion when driven into overload. This harmonic pro-
duces a "covered" sound, giving the recording a re-
stricted quality. Alternatively a tube amplifier when over-
loaded generates a whole spectrum of harmonics. Par-
ticularly strong are the second, third, fourth, and fifth
overtones which give a full-bodied "brassy" quality to
the sound. The further any amplifier is driven into satura-
tion, the greater the amplitude of the higher harmonics
like the seventh, eighth, ninth, etc. These add edge to the
sound which the ear translates to loudness information.
Overloading an operational amplifier produces such steep-
ly rising edge harmonics that they become objectionable
within a 5-dB range. Transistors extend this overload
range to about 10 dB and tubes widen it 20 dB or more.
Using this basic analysis, the psychoacoustic character-

272

JOURNAL OF THE AUDIO ENGINEERING SOCIETY

TUBES VERSUS TRANSISTORS-IS THERE AN AUDIBLE DIFFERENCE?

istics stated in the beginning of this paper can be related
to the electrical harmonic properties of each type of
amplifier.

It was not part of the original intent of this paper to
analyze operational amplifiers. However, the tests show
that they fall into a distinct class of their own. Basical-
ly, operational amplifiers produce strong third, fifth, and
seventh harmonics when driven only a few dB into over-
load. The resultant sound is metallic with a very harsh
edge which the ear hears as strong distortion. Since this
sound is so objectionable, it acts as a clearly audible
overload warning signal. Consequently, operational am-
plifiers are rarely operated in their saturated region. This
results in a very cleanly amplified sound with little colora-
tion and true dynamic range within the limitations of
the amplifier. True dynamic range is not necessarily the
determinant of good sound reproduction, however, since
it is much greater than any disc or tape system presently
available. Because of their characteristics, operational am-
plifiers produce only the top end of the dynamic range
which contains all the transients but lacks the solid pitch
information which the ear hears as music. When records
of true dynamic range are played on a limited-range
system, they sound very thin. This relates directly to the
originally cited listener's comment that transistor rec-
ords were very clean but sounded sibilant and cymbally.

The transistor characteristics which our subjects noted
were the buzzing or white-noise sound and the lack of
'punch." The buzz is of course directly related to the
edge produced by overloading on transients. The guess
that this is white noise is due to the fact that many of
the edge harmonics like the seventh and ninth are not
musically related to the fundamental. The ear hears these
dissonant tones as a kind of noise accompanying every
attack. The lack of punch is due to the strong third har-
monic which is inaudibly "blanketing" the sound. This is
correctable by using a large enough pad to prevent all
peaks from reaching the amplifier's saturated region. But
from a practical standpoint, there is no way of determin-
ing this on most consoles. Adding auxiliary peak indi-
cators on the input preamplifiers could alleviate both
these problems, and the sound would be very close to
that of the operational amplifier in its linear region.

Vacuum-tube amplifiers differ from transistor and op-
erational amplifiers because they can be operated in the
overload region without adding objectionable distortion.
The combination of the slow rising edge and the open
harmonic structure of the overload characteristics form
an almost ideal sound-recording compressor. Within the
15-20-dB "safe" overload range, the electrical output of
the tube amplifier increases by only 2-4 dB, acting like
a limiter. However, since the edge is increasing within
this range, the subjective loudness remains uncompressed

to the ear. This effect causes tube-amplified signals to have
a high apparent level which is not indicated on a volume
indicator (VU meter). Tubes sound louder and have a
better signal-to-noise ratio because of this extra subjec-
tive head room that transistor amplifiers do not have.
Tubes get punch from their naturally brassy overload
characteristics. Since the loud signals can be recorded at
higher levels, the softer signals are also louder, so they
are not lost in tape hiss and they effectively give the
tube sound greater clarity. The feeling of more bass re-
sponse is directly related to the strong second and third
harmonic components which reinforce the "natural" bass
with "synthetic" bass [5]. In the context of a limited
dynamic range system like the phonograph, recordings
made with vacuum-tube preamplifiers will have more ap-
parent level and a greater signal to system noise ratio
than recordings made with transistors or operational am-
plifiers.

ACKNOWLEDGMENT

The author wishes to thank Walter Sear and Peter
Scheiber for innumerable helpful discussions on the musi-
cian's viewpoint of sound. He also wishes to thank John
Olson of RCA and Steve Temmer of Gotham Audio for
the loaning of amplifiers.

REFERENCES

[1] "Neumann Transistor Condensor Microphones,"
Gotham Audio Corp., Sales Bull. 1971.

[2] A. D. Smith and P. H. Wittman, "Design Con-
siderations of Low-Noise Audio Input Circuitry for a
Professional Microphone Mixer," /. Audio Eng. Soc, vol.
13. pp. 140-156 (Apr. 1970).

[3] A. Schaumberger. "The Application of Impulse
Measurement Techniques to the Detection of Linear Dis-
tortion," /. Audio Eng. Soc, vol. 19, pp. 664-668 (Sept.
1971).

[4] M. Otala, "Circuit Design Modifications for Mini-
mizing Transient Intermodulation Distortion in Audio
Amplifiers." /. Audio Eng. Soc. vol. 20, pp. 396-399
(June 1972).

[5] F. Langford-Smith, Radiotron Designer's Hand-
book (RCA. 1953). chap. 14.

(61 J. C. Risset, "Computer Study of Trumpet Tones,"
Bell Telephone Laboratories, File MM-66-1222-2.

[7] J. R. Ashley. T. A. Saponas, and R. C. Matson,
"Test Signals for Music Reproduction Svstems," IEEE
Spectrum, vol. 8, pp. 53-61 (July 1971).

[81 A. H. Benade, Horns, Strings and Harmony
(Doublcday, New York, 1960).

[9] R. A. Schaefer, "New Techniques for Organ Tone
Generation," /. Audio Eng. Soc, vol. 19, pp. 570-575
(July/ Aug. 1971).

[10] P.. Langevin, "Intermodulation Distortion in Tape
Recording," /. Audio Eng. Soc, vol. 11, pp. 270-278
(July 1963).

THE AUTHOR

Russell O. Hamm received his engineering training
at the University of New Hampshire. He worked for
Vidcom Electronics and later the Fine Recording divi-
sion of that Company, designing and supervising the
installation of their extensive sixteen-track recording
facilities. While with Fine Recordings, Mr. Hamm did
a great deal of experimentation in stereophonic and
quadraphonic sound for records and motion pictures

which, in conjunction with Peter Scheiber, formed the
basis for the development of the present matrix-quad
record. Mr. Hamm's record-producing and engineering
credits include albums, commercials, and motion-pic-
ture sound tracks by many well-known artists. Mr.
Hamm presently serves as a Consultant to Sear Sound
Studios in New York and is constructing Ditanfra Stu-
dios in the Virgin Islands.

MAY 1973, VOLUME 21, NUMBER 4

273