MEP-250 Bridged-T equalizer
Posted: Thu Sep 06, 2018 6:51 pm
The recent thread regarding "negative impedance converter" interestingly wandered into a subject I've been pondering recently, namely the ITI/Sontec/and perhaps current Massenburg EQ designs.
Many years ago when I was a very young studio recordist, I recall being blown away by the original ITI parametric. Later dabblings and designs of mine moved into the SVF universe, but the bridged-T design has always stuck in my mind.
Here's a link to George's old AES paper which is quite thin on details:
http://www.aes.org/e-lib/browse.cfm?elib=16171
Pages 5 and 6 of the PDF contain a very brief discussion of the circuits shown on pages 8 and 9 of the PDF.
This DIY kit pages has the schemo linked in the negative impedance thread, but the first one on this page was actually easier for me to read:
http://www.thedonclassics.com/250eq-diy_schematic.html
In the center of the lower part of the schematic is a diagram of the basic midband building blocks.
I've been puzzling over it for awhile. The bridged-T is wrapped around IC-7, while IC8 seems to have a dual purpose. As a unity gain inverter at audio frequencies, it would appear to be offering negative feedback around IC7 by virtue of connecting from IC7 output to non inverting input. In addition, it appears to be operating as a DC servo.
For the bandwidth/"Q" control there is an interesting R network which involves the "raw" signal and feedback at the input of IC7, probably to somehow keep the gain through the filter constant as the bandwidth control is adjusted. Here I become confused as to how the "negative" feedback impacts the bandwidth, and why did they do it that way (lol)?
Perhaps someone smarter than me can elaborate!
Bri
Many years ago when I was a very young studio recordist, I recall being blown away by the original ITI parametric. Later dabblings and designs of mine moved into the SVF universe, but the bridged-T design has always stuck in my mind.
Here's a link to George's old AES paper which is quite thin on details:
http://www.aes.org/e-lib/browse.cfm?elib=16171
Pages 5 and 6 of the PDF contain a very brief discussion of the circuits shown on pages 8 and 9 of the PDF.
This DIY kit pages has the schemo linked in the negative impedance thread, but the first one on this page was actually easier for me to read:
http://www.thedonclassics.com/250eq-diy_schematic.html
In the center of the lower part of the schematic is a diagram of the basic midband building blocks.
I've been puzzling over it for awhile. The bridged-T is wrapped around IC-7, while IC8 seems to have a dual purpose. As a unity gain inverter at audio frequencies, it would appear to be offering negative feedback around IC7 by virtue of connecting from IC7 output to non inverting input. In addition, it appears to be operating as a DC servo.
For the bandwidth/"Q" control there is an interesting R network which involves the "raw" signal and feedback at the input of IC7, probably to somehow keep the gain through the filter constant as the bandwidth control is adjusted. Here I become confused as to how the "negative" feedback impacts the bandwidth, and why did they do it that way (lol)?
Perhaps someone smarter than me can elaborate!
Bri