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The Elliptic Equalizer is Offered as A Bare Circuit Board, PC Board with ICs, or Assembled and Tested Shown Above.
The KA Electronics EEQ-12 Elliptic
Equalizer for vinyl mastering is a unique 12 dB per octave design based on work published in the
Pro Audio Design Forum.
What is an Elliptic Equalizer?
An "elliptic equalizer" is used during vinyl mastering to prevent playback
skipping on music having wide stereo bass. The bass-to-mono elliptic EQ gets its name
from the width of ellipses in the recorded groove or
Lissajous patterns on an
oscilloscope. If the width of the recorded groove becomes too narrow from stereo
bass, which produces large "vertical modulation" of the cutter head, the
playback stylus can be pinched out of the groove.
EQ high-pass filters the difference, or "Side" information, to convert
low frequency vertical
modulation of the cutter head to lateral. Elliptic equalizers (EEQ) historically had 6 dB per
octave filter slopes. When Side is high-pass filtered to provide EEQ, a low-pass
response, also at 6 dB per octave, is created in the stereo crosstalk curve.
When elliptic equalization is used to make low frequencies mono,
midrange separation and stereo imaging are impaired at a 6 dB per octave rate.
Due to their sonic footprint, EEQs are only used when necessary. The EEQ-12's
Steeper high-pass filters don't improve crosstalk.
high-pass filter with a slope greater than 6 dB per octave steepens the
EEQ response, but the crosstalk curve, which is subtracted or "derived," will
have a 6 dB per octave slope regardless of the high-pass filter's steepness. The
Left and Right outputs also produce undesirable
response peaking. The steeper
the high-pass filter the greater the peaking.
The limitation of
asymmetric slopes and response peaking in an EEQ with slopes greater than 6 dB
per octave is similar to derived loudspeaker crossovers. The work of crossover
designers Linkwitz, Self and others in solving these limitations in speaker
systems have been applied to the EEQ-12.
Elliptic Equalization based on crossover
The KA Electronics EEQ-12 has 12 dB per octave
slopes in both its
By borrowing techniques from crossover design, a
phase-correcting network in the EEQ-12's Mid signal path compensates for the
phase response of the Side filter. The result is a
symmetrical, 12 dB per octave
response in both the vertical high-pass and low-pass crosstalk curves. Midrange
imaging is maintained with a low sonic footprint.
The EEQ-12's steeper
high-pass response allows the EEQ frequency to often be set an octave lower and, when combined
with a crosstalk curve an octave lower and falling twice as fast, midrange
separation and imaging are doubly improved.
Compare the difference.
The following curves show the EEQ-12 switched between 6 and 12 dB per octave at
300 Hz. At 1 kHz the EEQ-12 has about 35 dB separation versus only 20 dB for a
conventional 6 dB per octave EEQ. A phono cartridge may have up to 30-35 dB of midband separation. The improvement in stereo imaging
on playback is significant.
Hear the difference.
The following sound file compares
the EEQ-12's crosstalk at 12 dB per octave (0-0:12 seconds), 6 dB per octave
(0:13-0:28) and 12 dB per octave (0:29-0:37). The EEQ-12 frequency is 150 Hz.
One channel is driven. The opposite channel's output, containing desirable bass
cross-feed and undesirable crosstalk, was recorded as a mono file. The midrange
leakage heard in the 6 dB per octave example is the reduction in separation that
occurs with conventional EEQ.
12 dB versus 6 dB per octave crosstalk comparison (Mono, mp3.)
Other applications for the EEQ-12 Elliptic Equalizer.
Vinyl playback: Elliptic equalization is ideal
for warp reduction during vinyl playback or transfer. Warp is primarily vertical. Due
to playback and cutting limitations described earlier, vinyl recordings do not
typically contain recorded vertical content at low frequencies. The EEQ-12 filters low
frequency vertical warp without filtering the lateral mono bass. When compared
to conventional Left/Right infrasonic high-pass filtering, the EEQ's cutoff
frequency works octaves higher without audibility.
Headphones: The "bass-to-mono" cross-feed of the EEQ-12 creates
an "in head" low frequency focus for headphone listening. No amount of
EQ can correct a wide, flabby-sounding, bottom end. Slightly "woolly" or
"tubby" bass in recordings with too wide a low end for 'phones tighten up
to develop "point" and impact.
You don't want stereo bass that's panned to the opposite end of the room.
Bass-to-mono processing during mastering with the EEQ-12 limits ultra-wide hyped
stereo bass detrimental to dance, club and other large venues. Bass-to-mono also
assists small speakers by distributing low frequency content more evenly between
drivers. The EEQ-12's low sonic footprint and increased separation are
particularly important for formats not destined for vinyl release.
The EEQ-32 Elliptic Equalizer internal
Mid/Side "MS" filters and can be used with conventional Left and Right inputs or
MS encoded material.
For Left/Right sources,
the EEQ-12 provides on-board Mid Side encoding.
Fully-balanced THAT1246 ICs receive the Left and Right inputs. MS duties are
performed by THAT1240 precision sum and difference amplifiers.
can accept external unbalanced MS inputs to add EEQ functionality to the
Precision MS Matrix. When the EEQ-12 is
inserted at point "F" the MS matrix's balanced inserts remain available for
upstream Mid/Side processing. External MS requires only four ICs to be installed
in the EEQ-12.
The EEQ-12's internal MS signals are decoded on-board to provide balanced
Left/Right outputs, using THAT1646 OutSmarts®
line drivers, or jumper-configured to provide direct unbalanced MS outputs
for external decoding.
The EEQ-12 can be configured for either relay or
switch-based tuning. On-board relays select Off, 75, 150 and 300 Hz with a
single-pole rotary switch or contact closure. (Other frequencies are available
by changing resistors.) A four pole multi-position rotary switch with external resistors
may also be used to directly tune the EEQ-12. Relays provide a very
cost-effective solution for modulation plus preview applications which require
a costly eight pole switch.
The EEQ-12 can also be relay-switched from
symmetrical 12/12 to 6/6 dB response to emulate conventional 6 dB per octave EEQ
identical to the Neumann EE-70 or EE-77.
An optional 12/6 "uncorrected" mode is also available for experimentation.
The EEQ-12 provides hard-wire relay bypass for external MS-encoded sources. When the
internal MS matrix is used, relays provide comparative "soft" bypass. Full
hard-wire bypass using Left/Right connections require an
Current-limited LED tally outputs are provided for
switch illumination of bypass and filter slope. The relay and LED supply is
electrically-isolated from audio ground. An optional on-board relay voltage
regulator permits the EEQ to be used with an unregulated or higher-voltage supply.
The EEQ-12 Elliptic Equalizer is sold as a bare PC board, a "kit" including
semiconductors, or an assembled and fully-tested module.
A Mouser Electronics Project Manager
is available for DIY builds of the EEQ-12 Elliptic EQ.
Other items from KA-Electronics RockmoreLabs:
Bare PC Board
Bare PC Board Including ICs
(4 THAT1240, 2 1246, 2 1646, 4 NJM2114.)
Assembled and Tested EEQ-12 Board
Buy it on
KA-Electronics EEQ-12 early adopters include Misjah
van der Heiden 24 Mastering, Zander
Macharis Studio Margueritte, Barry
Wolifson Sterling Sound, James Sillitoe
Technology, Simon Riat, Tony Eichler
Masterworks, Wim Nel Knop Recordings, Simon Wood, Peter Montgomery
CHR Mastering, Kevin Park
Lacquer Channel, Skander Mensi, Leon
Lothspeich, Danny Collart, Christophe Chapelle, Sylvain Dubois, Chris Hesse...