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Equalizers mounted on threegang switch. See Fig. 4.
NEW DEVELOPMENTS
IN PHONO EQUALIZERS
By CHARLES P. BOEGLI
Cincinnati Research Company
FOR the past several years a series
of articles dealing with the design
of equalizers for high -fidelity phonographs
has appeared in RADIO & TELE-
VISION NEWS (References 1, 2, 3, 4). At
the beginning, this work was complicated
by the lack of detailed knowledge
regarding the recording characteristics
employed by the various companies,
and much reliance had to be
placed upon secondary sources of information
and upon "ear" tests. While
the writer has always recognized that
the latter forms a very flimsy basis
for equalizer design (or, for that matter,
for any audio equipment design)
the assumption was always made that
secondary sources of information
could be considered fairly reliable. It
has been quite distressing to learn
that this is not always the case, and
that in spite of considerable effort,
errors have appeared in previous
tables.
Since the appearance of the last
table, therefore, a great deal of work
has been done in approaching the
various record manufacturers directly
for data on recording characteristics
used not only at present, but also in
the past. The cooperation extended
by some concerns (notably RCA Victor)
has been gratifying; others have
been quite slow in replying to inquiries,
and a third class has unfortunately
chosen to ignore the letters addressetlto
them. The attitude of this
last group makes the task of equalizer
design particularly thankless.
The present table is based upon the
best data now available to the writer.
With the exception of the "flat"
equalizers, the circuits shown in the
table actually comprise two sections;
54
Fig. 1. Design and performance of a flat.
500 cycle -per second equalizer circuit.
DIFFERENCE BETWEEN ACTUAL
DESIRED RESPONSES
/AND
ASYMPTOTES OF DESIRED RESPONSE
o
ó
FREQUENCY )CPS)
8
New and revised data covering equalizers designed to be
used with all of the commercially- available phono discs.
NEW ORTHOPHONIC
NEW ORTHOPHONIC
OLD CNARACTERISTIC
BASS ASYMPTOTE
I
FOR OLD CHARACTERISTIC ...
OLD CHARACTERISTIC
$ g
o
FREOUENCT(CP51
Fig. 2. Characteristics used by RCA
Victor for records at all three speeds.
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the first compensates for the bass
turnover and the second for the treble
pre- emphasis. Now, the bass characteristic
in most recordings takes the
form of a "turnover" at 500, 400, or
250 cps below which the velocities are
attenuated at the rate of 6 db -peroctave.
This bass drop may continue
to the lowest frequencies recorded or
there may be another turnover at 50
or 100 cps below which the attenua-
tion ceases. A single -stage equalizer
to correct fully for a 6 db- per -octave
bass slope can only be made at the
cost of almost infinite attenuation of
high frequencies.- It is consequently
necessary to accept a certain amount
of undercorrection; with a high -frequency
attenuation of 33 db, which
is probably as high as desirable, a
slope of 5.75 db -per- octave can be attained.
If a circuit providing 5.75 db -peroctave
of bass emphasis has its turnover
at 500 cps, there will of course
be a residual bass drop amounting to
about 1 db at 30 cps when this equalizer
is being used with the disc for
which it was designed. A number of
persons consider it desirable to avoid
bass drop whenever possible, and this
objective can be attained by designing
the equalizer for a turnover of approximately
550 cps instead of 500 cps; the
greater part of the difference between
the actual and ideal performance
then occurs around 500 cps where it is
probably less objectionable. Fig. 1,
the curves for a "flat" 500 -cps equalizer,
shows the response obtained with
the circuit modified in this manner.
With an 0.1 pfd. input condenser and
an output shunt resistance of 1.0
megohm, the circuit shown will have
the correct response ±
1 db down
to less than 10 cps. Of course, a full
6 db- per -octave correction can be obtained
with a two- section bass equalizer
but such circuits have certain
disadvantages of their own (Reference
2).
High- impedance circuits have an
unfortunate tendency to pick up hum
and noise from stray fields; it has
been quite noticeable with previous
designs which have used quite large
(approximately 1.5- megohm) input resistors.
This is the reason for lowering
the impedance of all the equalizers,
which is evidenced by a reduction
in resistor values and a corresponding
increase in condenser sizes.
The modification has been carried out
so that condensers of commercially -
available sizes can be used; in many
c: ses the employment of resistors
that do not appear in the RTMA 10%
tolerance series is thus necessitated.
Since most enthusiasts have been constructing
their equalizers with 5%
resistors, however, this is not considered
a serious objection. There is a
further advantage in the reduction
of impedance: the input capacitance
of the tube following the equalizer is
now quite neglegible in its effect on
the treble response.
Beside the changes just noted, individual
features of certain equalizers
should be stressed:
Flat 250 cps: This equalizer is to be
used with HMV and English Columbia
78 -rpm discs, which are both
manufactured by Electrical and Musical
Industries, Ltd. Although the literature
supplied by this company indicates
that the characteristic is flat
above 250 cps, discs played with such
an equalizer generally sound somewhat
shrill. This probably can be attributed
to the type of microphone
used in recording, which introduces a
noticeable resonant peak in the upper
frequency ranges. It has been found
that the introduction of a shunt condenser
across the equalizer, causing
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