I spent the past week building this custom stereo passive inductor EQ for a customer named, Dave Grundy. I think its obvious how the name, Grundy Q came to be. Dave wanted an EQ for his home hi-fi stereo, but he also wanted to be able to use it with his home recording setup. I designed the EQ to have useful frequencies for home room acoustics with a stereo hi-fi system in mind. Balancing that with usability in the studio world required some careful thought into what frequencies would be best. Budget and ease of use were both important factors in the design. Dave will be the first to tell you he doesn't have a lot of experience with equalizers, so I did my best to make it a no-brainer when it comes to practical use. Set the knobs at 12:00 and the EQ has a flat response from 20Hz-25kHz. The bass, mid, and treble knobs can boost or cut. Bass has selection of 20Hz, 40Hz, and 60Hz. I chose those frequencies because I know many speakers lack those deep tones. With boosting 20Hz on the EQ its like adding a subwoofer to your room. In mixing, I tend to gravitate to 60Hz for boosting bass guitar or kick drum. The mid section has choices of 300Hz, 500Hz, 700Hz. I feel these frequencies are general areas of mud in both mixes and in room acoustics. I would probably cut somewhere around 700Hz for my own living room, but if I were using the EQ to mix bass or kick I might boost 300Hz a tad. The treble is 10kHz or 20kHz. I felt 10k is a pretty standard high frequency control to have. Boosting 20kHz on this EQ adds a nice bit of air to anything. Each channel has a bypass switch on the top (near the center vacuum tube). There is a front panel switch (center) to choose unbalanced RCA ins/outs or balanced XLR ins/outs.
The Grundy Q tube compliment is a pair of 6922/6DJ8 dual triodes, a pair of EF806S pentodes, and a 12AU7 dual triode. 295v B+ for loads of sexy headroom. Only the highest quality parts throughout and of course, its wired point to point.
Overall, I'm super pleased with the results of this EQ. It sounds incredible and delivers useful control over frequencies in an easy to use package. Its flexible in operation, offers super low noise (which I'm all about), and sounds fantastic! I'm seriously considering offering something based on this as a product in the future. Unfortunately, that will probably be a long way off. My to-do list is long. As with anything, it comes down to time and money. Hopefully I can afford to bring something like this to market in the coming years.
Noise is -110dB A weighted. The EQ can boost your signal by about 6dB over unity. THD is 0.125%
About the Author
Mike Congilosi II, Inventor/Engineer