From the Sound On Sound site:

The 511 is a two-band equalizer with very typical features for an EQ from the '60s. Back then, although many manufacturers strived to expand the simple tone controls of the '50s, the fully parametric EQ remained beyond the horizon, and this is probably a reason why so many manufacturers came up with various unique yet very useful EQ layouts in this 'start-up' period.

As we'll see in the course of this review, the Electrodyne 511 certainly fits this description. Each of its two filter bands utilises a dual-concentric rotary switch to control frequency and amplitude, allowing for boosts and cuts of ± 2, 4, 6, 9 and 12 dB. The high band offers shelving curves at 1.5, 3, 5 and 10 kHz, and the low band at 40, 100, 250 and 500 Hz. Additionally, each band can be switched to a peaking characteristic at some frequencies: 1.5, 3 and 5 kHz on the high band, 250 and 500 Hz on the low band. The controls on the 1U 500-series front-panel are completed by a bypass button with a status LED.

The electronic circuitry inside the module is equally characteristic of the era in which this EQ's predecessor was designed. The 511 is based on the filter network of the original Electrodyne 711 channel strip (see the photo of the 'Sinatra' console). The filters are embedded in a very short and elegant discrete signal path consisting of only three stages. First comes the active balanced input buffer amp, and second in line is the actual EQ circuitry. The 511 features two inductor-based filter bands wrapped around an op-amp stage, which then directly drives the output transformer. Thus, in contrast to the venerable Pultec EQ, the 511 is an active inductor-based EQ, and this has several effects on the sound and behaviour of the filters, which will be discussed below. According to designer Ken Hirsch, the discrete A5000 op-amp used in the 511 is running in class A/B mode, and it is a development of the last 'historical' Electrodyne op-amp, ironing out some of the shortcomings of its predecessors. It is based on design notes from the original Electrodyne archives, and has received approval from the original design staff. The output transformer covers almost a third of the available space inside the module, which means that it is probably the biggest transformer inside any 500-series module (it's certainly the biggest I've seen to date). Like the inductors, the transformer is manufactured by CineMag to original specifications. (Remember that CineMag are the successors of Reichenbach, supplier of the originals to Electrodyne.)

All these technical details are evidence of how meticulously the 511 has been crafted, and how faithful to its vintage predecessors this module is. Even the transistors, which, fortunately, are still in production today, are the same types used in vintage units. The rotary switches are manufactured especially for the 511, and even the knobs and levers are NOS (new old stock) parts, to which Orphan Audio gained access when taking over all assets of the vintage Electrodyne company. According to Peter Montessi, many beta testers have approved the new Electrodynes as being sonically totally on par with their vintage counterparts.

Speaking of the sound, I should explain some points that are vital to the sonic character of the 511's filters. Both are extremely wide, with peaking-mode Q-factors of 0.35 for the high band, and a slightly steeper (but still extraordinarily gentle) 0.7 for the bass band. In contrast to passive inductor-based EQs, the 511 is a reciprocal constant-Q design. In other, less technical words: boosts have the same curves as cuts, and the Q factor does not vary with amplitude.