Saturday, December 22, 2012

My Newest Akai Roberts Mod Creation: The Pre+EQ

Akai / Roberts Pre+EQ Mod:

About a year ago I started messing with some EQ circuits for a new design that incorporated a microphone preamplifier and an EQ in one Akai donor module. I knew that for the preamp part of the device I wanted something totally different than my Brute Force Preamp, so I settled on the mic preamp circuit from the old Tube RCA Broadcast Consoles. It uses two 12AY7/6072 tubes and the circuit I prototyped years ago sounded great. It really is a completely different tone than the Brute Force, so it fit well with my plans. SO, for the EQ I wanted something a bit different: not the same old boring Pultec style for me. Instead I wanted to find the best PA-Style, 'James', 2-band EQ. I also wanted to incorporate some tone switching and a variable low cut filter. What I settled on was a hodge-podge of circuit bits. I used most of the 2-band circuit from the Bogen MXM mixer, a simple variable low cut circuit found on the net, a boost switch that moves the frequency band of the treble control down into the mid range a bit, and finally a 'Beef' switch that removes the negative feedback from the circuit and adds bypass caps to two cathode resistors simultaneously. I will post more details and a final schematic soon.