The Model 535 Dual Graphic Equalizer is a studio quality product which has wide application in recording, sound reinforcement, radio and TV production, and high fidelity music systems. It uses Harris HA 4741-5 Op Amps. It can be modded, if you wish, by switching out these op-amp ICs. Some people online have done this with good results. However this item is 100% vintage and original. The system is designed to operate at any nominal level from as low as -20dB Ref. 0.775 V allowing its use with low level mixers, high or low impedances, and balanced or single-ended circuits. The Model 535 provides 12 dB of boost or cut at each of its 10 frequencies which are centred at ISO l-octave increments from 31.5 to 16 kHz. The positions of the control arms of the vertical, stepless controls provide a graphic representation of the response curve selected. The filters are active, minimum phase, 2-pole synthesized LC networks, whose skirts properly combine for minimum ripple and phase shift when used in combination. A front panel control adjusts the gain from -10 dB to +20 dB. Bypass switches permit switching the equalization out for A-B test. An optional (SC-1) security cover may be installed to protect all operating controls against inadvertent disturbance or tampering in fixed installations. If you are using the amp with a modern high impedance system, you need to put a 600 (or 620) ohm resistor across the output terminals in parallel with the load. A printed copy of the manual is included. The item is in overall good condition, but as with all vintage audio equipment, the controls may need application of some Deoxit to function at their best after being sat unused for so long.