macwanj
Active member
Hi folks,
I'm new to the forums and I know that there has been a lot of discussion about summing bus circuits. However, this being my first pcb project I am a little bit nervous about going forward and ordering them before I got some feedback from most of you knowledgable folks out there. So here goes
I'm trying to build 24 channels ito a 2U rack space unit. I'm using THAT 1240 chips for the differential input stange, with a 990 for the summing amp and a JLM audio hybrid opamp as a buffer. I also have a pre fader expand out to daisy chain multiple units (the expand ins connect directly to the summing buss with a THAT1246 -6dB chip to balance out the +6dB for the expand out balanced line driver . The 25 THAT chips fit on two 11" BY 3" PCBs (12 channels and one mono expand in per pcb) and the summing amp and buffers fit on two 5" by 3" pcb (one for left and the other for right).The plan was to stack them up using nylon standoffs. Custom pcb offers a 11" by 8" 4 layer pcb for $120 so I'm going to go for that for all the pcbs. Using the 4 layer board for the summing amp pcb makes things a whole lot easier and since I have so much extra space I'm going to do the differential input pcb on 4 layers as well. For the diff input pcb I have a + and - 24V power plane and a ground plane. The THAT 1240 chip reference as well as all the decoupling caps connect to this ground plane. The summing circuit pcb has a segmented ground plane that meets at the local star point on the board. Both pcb's ground connects to the star point on the 2U chassis which ultimately goes to the star point in the external PSU through a 5 pin XLR. The pin 1's on the XLR are terminated directly at the chassis with the option to conect it to the ground plane of the diff input pcb as well for unbalanced connections. The summing buss is a solid copper 10 gauge wire.
I was looking at the forssell summing buss circuit and I wanted to make room for a ground buss. However I am a bit confused because it requires each channels ground to conect to the ground buss via a 1k resistor which then ties to the non-inverting input of the 990. Since I have one ground plane can I connect this plane to the non-inverting ground of the 990 with just one 1k resistor? In the schematic attached right now, I have a 415ohm resistor to ground on the non-inverting input of the 990 which is the equvalent to the resistance as seen by the inverting input.
I'm posting the schematic and tomorrow I'll be able to post a jpeg of the pcb's too. (I'm using Diptrace). Any comments, suggestions etc would be most welcome. I'm excited about this project as its my first pro audio one; built a lot of tube amps and horns on the past. Once I have he pcb's figured out I can hook them up to the audio precision test setup at work.
I'm new to the forums and I know that there has been a lot of discussion about summing bus circuits. However, this being my first pcb project I am a little bit nervous about going forward and ordering them before I got some feedback from most of you knowledgable folks out there. So here goes
I'm trying to build 24 channels ito a 2U rack space unit. I'm using THAT 1240 chips for the differential input stange, with a 990 for the summing amp and a JLM audio hybrid opamp as a buffer. I also have a pre fader expand out to daisy chain multiple units (the expand ins connect directly to the summing buss with a THAT1246 -6dB chip to balance out the +6dB for the expand out balanced line driver . The 25 THAT chips fit on two 11" BY 3" PCBs (12 channels and one mono expand in per pcb) and the summing amp and buffers fit on two 5" by 3" pcb (one for left and the other for right).The plan was to stack them up using nylon standoffs. Custom pcb offers a 11" by 8" 4 layer pcb for $120 so I'm going to go for that for all the pcbs. Using the 4 layer board for the summing amp pcb makes things a whole lot easier and since I have so much extra space I'm going to do the differential input pcb on 4 layers as well. For the diff input pcb I have a + and - 24V power plane and a ground plane. The THAT 1240 chip reference as well as all the decoupling caps connect to this ground plane. The summing circuit pcb has a segmented ground plane that meets at the local star point on the board. Both pcb's ground connects to the star point on the 2U chassis which ultimately goes to the star point in the external PSU through a 5 pin XLR. The pin 1's on the XLR are terminated directly at the chassis with the option to conect it to the ground plane of the diff input pcb as well for unbalanced connections. The summing buss is a solid copper 10 gauge wire.
I was looking at the forssell summing buss circuit and I wanted to make room for a ground buss. However I am a bit confused because it requires each channels ground to conect to the ground buss via a 1k resistor which then ties to the non-inverting input of the 990. Since I have one ground plane can I connect this plane to the non-inverting ground of the 990 with just one 1k resistor? In the schematic attached right now, I have a 415ohm resistor to ground on the non-inverting input of the 990 which is the equvalent to the resistance as seen by the inverting input.
I'm posting the schematic and tomorrow I'll be able to post a jpeg of the pcb's too. (I'm using Diptrace). Any comments, suggestions etc would be most welcome. I'm excited about this project as its my first pro audio one; built a lot of tube amps and horns on the past. Once I have he pcb's figured out I can hook them up to the audio precision test setup at work.