Pultec inductor based equalizers are legendary. Most audio engineers find that the sound of an LC EQ is smoother than other types, mostly due to the minimal phase shift throughout the audio bandwidth. The EQ-1 passive filter was designed by the engineers at Western Electric's Bell Labs. Pultec licensed the design from WE and added a tube gain makeup amp, later to be replaced with a discrete solid state API 2520 Op-amp. The early EQP-1's used two toroid coils, one 100 mH and one 50mH which were switched in and out of the circuit in various combinations to make the required values. Most Pultec EQP-1A, EQP-1A3 models used a single tapped inductor to get the values needed for the hi-frequency section of the EQ. The VintageWindings EQ-1A Toroid Filter Coil is a direct replacement for the original tapped single coil.

This is a Very Important Note:

The math used on the modern version equivalent of the
original EQP-1A filter is WRONG!

The modern EQP-1A Filter tap values that almost 100% of
manufacturers of gear and coils are using:

27mH - .0033uF
33mH - .0056uF / .0082uF
47mH - .0082uF
68mH - .015uF
82mH - .018uF
150mH - .018uF

These values are mathematically INCORRECT!!

I don't know who the person was that re-engineered the original Pultec
EQP-1A tapped toroid and gave us the values most commonly used
today, but I can say that no one checked the work. Here is a chart that
I assembled that shows the conventional modern circuit math compared
to the corrected math that we are currently winding our Revised EQ-1A
Coils to:




Here are the values as they should be to obtain frequency values that
are as close to the original manufactures advertised frequencies as possible:

16 KHz. - Inductor - 29.9 mH    Capacitor - 3.3nF (.0033 uF)
12 KHz. - Inductor - 31.4 mH    Capacitor - 5.6nF (.0056 uF)
10 KHz. - Inductor - 31.4 mH    Capacitor - 8.2 nF (.0082 uF)
8 KHz. - Inductor - 48.2 mH    Capacitor - 8.2 nF (.0082 uF)
5 KHz. - Inductor - 67.5 mH    Capacitor - 15 nF (.015 uF)
4 KHz. - Inductor - 87.9 mH    Capacitor - 18 nF (.018 uF)
3 KHz. - Inductor - 156 mH    Capacitor - 18 nF (.018 uF)

I used a nifty program that I made for my trusty TI-83plus but if you
don't believe my work you can easily check it right here:

Calctool.org

Bandwidth:

The bandwidth in an LCR circuit is largely determined by the amount
of total resistance of the LC(R) combination. In the EQP circuit the total
resistance (Rt) of each LCR leg (tap/cap) includes the resistance of the
coil winding(s) and any other resistors in that leg of the circuit. In the
EQP circuit the bandwidth pot adds extra resistance to widen out the
response curves. In some LCR circuits, like the Quad Eight/Sphere EQ's,
extra resistors are added in series with the inductors and capacitors.
These are carefully chosen to keep the bandwidths consistent. In the EQP
the dc resistance of the coil itself determines the narrowest bandwidth of
the curve. The lower the coil's dc resistance is, the tighter the curve will
be. The actual ORIGINAL Pultec tapped toroid had only 5 taps (see
info below). The resistances ranged from about 14 ohms at the first tap
to about 34 ohms on the final tap. VintageWindings tapped toroids
have six taps (for the modern cap values) and the resistances are in the
same 14-34 ohm range as the originals to keep the response bandwidths
as close to the originals as possible.


Here are the response curves for the new inductor as sent from a
wonderful VintageWindings customer:



Revised VintageWindings EQ-1A Data Sheet



The Vintage Windings EQ-1A is a Molybdenum Permalloy Powder
toroid core inductor coil suitable for use in the passive equalizer sec-
tion of the Pultec EQP-1A, EQP-1A3 tube and solid state equalizers.
The core material formulation was chosen to compliment this appli-
cation specifically. The coils are potted in a vacuum oven.
More details on VintageWindings inductors Here.


This unit can also be wound on a core with a higher permeability
which results in a coil with less resistance and a higher Q.

Also available in Matched Stereo Pairs !!

Some customers who don't mind searching for old school capacitors
want a coil wound to the actual Pultec 5 tap single toroid specifications.
Here they are:

32.0 mH
51.1 mH
84.3 mH
100.2 mH
150.5 mH

Note:
There is no magic to these inductance values. If you use good
caps there will be no audible or measurable sound difference
no matter which tapped coil/cap combo that you use. However,
for those who need an actual replacement coil for their original
coil to be used with original caps, VintageWindings sells a direct
replacement for the same price as the new 6 tap version.

 E-mail for availability and details.





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