Simple active summing mixer.

[radiance]

If you want sends than..yes.

 

[macwanj]

Nice job radiance! Just one question though... where did you get the PCB made? Must have been expensive...did you do it on 2 or 4 layers? I'm building some thing similar right now using 990s with a DC servo for 48 channels...Any tips on pcb layout would be appreciated.

 

[radiance]

Nice job radiance! Just one question though... where did you get the PCB made? Must have been expensive...did you do it on 2 or 4 layers? I'm building some thing similar right now using 990s with a DC servo for 48 channels...Any tips on pcb layout would be appreciated.

Thanks for the kind words. My pcb's where made by Euro circuits and yes, it was expensive, something like 200 euro for both pcb's.
It's a 2 layer pcb. This was my first pcb design attempt and it literally took me a year of free time to design this.
As for the pcb layout tips, just read everything you can find on this subject, than read everything about grounding schemes and everything about opamps and psu decoupling caps. Beside the usual 100nF decoupling caps it's good to also use some 220 uf ones, if there's pcb space make room for a lot of them, you can always leave some out.
Don't try to make everything fit on a small space like I did    Make your design so you can choose between multipple options, say D|C servo or caps. This way you can experiment a little.
It's best if you can make the whole project on 1 big pcb. I had  problems with internal audio connections between both pcb's since they both had a slightly defferent ground potential. This was easely solved with shorter & thicker ground wires but that's not a elegant option.

 

[guitarguy12387]

Hey guys,

Bit of a noob here with some noob questions. I have been studying this thread and other similar resources for the past few weeks and i'm trying to understand the circuit better. So please bear with me.

Ok. Looking back at the first page (the summing schem). Whats the purpose of the 510ohm R from the non-inverting input of the 990 to ground? Is that compensating for input offset current of the amp? If so... why not one on the opa604/opa134?

Secondly, whats the function of the opa604/opa134 even? I thought the DOA was providing the makeup gain?

Also, any specific reason why .22uf is in parallel with 100uf coupling caps instead of just one cap? Or is it just hard to find the right value...

Finally, whats the purpose of the caps in the feedback network of the amps? Just for stability?

Thanks in advance. Again, i apologize if i made any noob assumptions/conclusions haha.

 

[macwanj]

Hey guitarguy,

Read the section on summing amps in this document; it will makes things a litle more clear
http://www.johnhardyco.com/pdf/990-2007.pdf

Hope this helps...

 

[guitarguy12387]

Wow this seems to help ALOT for a few of my questions. I will read through this alot. Thanks a ton!

 

[radiance]

Yes, do read the John Hardy article first....

Ok. Looking back at the first page (the summing schem). Whats the purpose of the 510ohm R from the non-inverting input of the 990 to ground? Is that compensating for input offset current of the amp? If so... why not one on the opa604/opa134?

Answer here...

http://www.groupdiy.com/index.php?topic=27351.0

Secondly, whats the function of the opa604/opa134 even? I thought the DOA was providing the makeup gain?

The opa604/opa134 is there to flip the polarity back to normal (was inverted by the 990) AND it also acts as a buffer for the THAT1646/DRV134 line driver.

 

[macwanj]

Hey radiance,

I'm still trying to figureout the best way to do my pcb layout and I was hoping you could help me out by answering a few questions. First off, what did you have on your 2 pcbs? I'm assuming one had all the differential inputs and outputs and amplifiers and the other bigger pcb had the summing amp, vu buffers pots etc. My next question is how did you do your power and grounds? Did you use traces or segmented planes?

Right now I have a ground plane and power traces running on one layer and the top layer is the signal layer. The other approach is using 4 layers with 2 power planes and one segmented ground plane. I have two pcbs as well with one for the inputs and outputs and the other for the summing circuit. Each pcb has its own star point which connects to the main star on the chassis through a 14awg wire. Another difference with my layout is I have the summing buss on seperate solid core wires that connect to the summing amp pcb on one end and supported by a terminal strip on the other.

Thanks in advance and by the way any pics of the finished product with the front panel and all? The thing looks massively cool...

 

[radiance]

Hey radiance,

I'm still trying to figureout the best way to do my pcb layout and I was hoping you could help me out by answering a few questions. First off, what did you have on your 2 pcbs? I'm assuming one had all the differential inputs and outputs and amplifiers and the other bigger pcb had the summing amp, vu buffers pots etc.

The front pcb with the lorlins has all channel inputs, group inputs, master out buffers + line drivers , VU buffers, monitor buffers + line drivers + mono stereo switching, headhone amp. Bigger one that lays horizontal contains all summing amps for groups and master which all have a separate mono summing opapm (990) as well. That's a total of 9 (mono) summing busses, so for example the master has left , mono and right summing. Furthermore this pcb contains the line drivers for the 2 inserts per bus + insert 1 on both groups has a dry/wet control to allow parallel compresion . Both the inserts on the master can be switched to behave in m/s mode so there's m/s encoder + decoder on the pcb as well.

how did you do your power and grounds? Did you use traces or segmented planes?

First of all, these are my first pcb's so I don't considder myself an expert. My pcb's are double sided. First I wanted to keep the power on one side and the audio signal on the other. I did not menage to do this cause space was tight but I always kept a comfortable distance between power and audio signal traces and most of the time each where on one side.
As for grounds: the signal side has a star ground made of thick traces (no plane). All traces come together at the star point from which a thick coper wire connects to a so called ground buss bar. The power side has a groundplane which connects to the audio ground at the star point. This way the psu dirt can't get in the audio ground.
Another thing to consider is how to connect the power decoupling caps from the opamps. I connected the power direct to the pin of the opamp. From that same pin a separate (very short) trace goes to one pin of the (100nf) decoupling cap. The other pin connects to the ground plane. So I did not put the decoupling caps in the power trace.

 

[macwanj]

Thanks radiance, that was a most helpful reply indeed. This is my first pcb project and I need to stop thinking point to point but its hard if thats what you did for years. I'm going to use a similar layout as you yours wih the addition of power and audio ground terminals on the back of the chassis. Did you use a thick copper bar for the ground buss bar and thick traces for the summing bus and ground bus traces?  This weekend I'm going to have a lot of work to do. Its going to be hard routing all that stuff on a 5" by 3" board; guess its going to be a lot of vias! Thanks and have a good weekend!

 

[Davo]

Radiance, Fantastic job, I'm jealous!!!

I have a quick noob question for you.... I'm not familiar with line receivers, but I think that it's a great idea to sum the balanced input by way of the IC. 
My question has to do with impedance and the line receiver.  The output impedance of my mixer is 50 ohms or thereabouts per channel.... is the THAT 12xx designed to handle "line" inputs, or will it do just fine with lower impedances?  (I know, it's called a "line" receiver, but I just don't know hehe)
I could always use an opamp (THAT 1510) to sum the input instead.

Thanks!!!
Dave

 

[radiance]

Did you use a thick copper bar for the ground buss bar

Yes

and thick traces for the summing bus and ground bus traces? 

no....maybe I should have but everything seems to work OK.

 

[radiance]

is the THAT 12xx designed to handle "line" inputs, or will it do just fine with lower impedances?  (I know, it's called a "line" receiver, but I just don't know hehe)

I guess it's designed for line imputs hence the name  but you can check the data sheet here 
http://www.thatcorp.com/datashts/1240data.pdf

What exactly are you trying to achieve with your mixer outputs? I'm not sure I understoot it correctly...

 

[Davo]

I guess it's designed for line imputs hence the name  but you can check the data sheet here 
http://www.thatcorp.com/datashts/1240data.pdf

What exactly are you trying to achieve with your mixer outputs? I'm not sure I understoot it correctly...

Hi Radiance,

Sorry bout the stupid questions... I'm learning all about opamps, impedances etc. 

I'm just thinking in terms of matching impedances as I would with an input transformer.  It looks like the 1243 has 21k input impedance (10.5k per rail) and I've got a 50 ohm output impedance per rail....... of course, I don't know if that matters in the least, as the 1243 provides -3 dbu output.  I've probably already answered my own question (told you it was noobish )  In any case, I'll shoot THAT an email and see what they recommend.

Thanks!!!

 

[synthetic]

Hello, I'm another beginner looking at active summing mixers. Mine will be based on the NYDave 16-channel passive mixer into the Forssel summing amp 2, though possibly using OP275 instead of 990-based op amp at least at first. (The NYDave schematic seems to be hard to find, so let me know if I should upload it anywhere.)

I have a general question about the resistor tolerances. I've been thinking about using 0.1% resistors at least on the inputs so that imaging, etc. doesn't get messed up going through the mixer. Is this necessary or am I crazy?

 

[radiance]

Hello, I'm another beginner looking at active summing mixers. Mine will be based on the NYDave 16-channel passive mixer into the Forssel summing amp 2, though possibly using OP275 instead of 990-based op amp at least at first. (The NYDave schematic seems to be hard to find, so let me know if I should upload it anywhere.)

I have a general question about the resistor tolerances. I've been thinking about using 0.1% resistors at least on the inputs so that imaging, etc. doesn't get messed up going through the mixer. Is this necessary or am I crazy?

I don't have NYD's scem here at the moment and I think you should ask Dave first before uploading anything..
....but to answer your question...I don't know exactly where those 0,1% resistors are going to be but to give you an idea let say the feedback resistor from the summing opamp is not 10K as supposed, but 9900R instead (1% off). This will give you a gain of minus 0,0873 dB instead of unity gain when you've had a resistor of 10K.

That's good enough for me 

Just buy a hundred pack of your choice 1% resistors and match them yourself. If they're from the same batch they usually are not to far off each other

 

[synthetic]

My ohm meter isn't too great but I'll try it with 1% resistors. I'll draw up a schematic for what I want to build and y'all can punch holes in it.

Basically, each input has a 4.4kk resistor, then feed a 4P3T switch. The switch positions are open (mute) mono (a pair of 2k resistors to L and R for summing to mono) and stereo (odd channels L even channels R). That then goes to Forssel's "summing buss 2" makeup stage. On;y I'm thinking of using an OP275 instead of a 990 here to save cost since I'm building it fully balanced. More later. Should I post to another thread? I didn't mean to hijack, but didn't want two separate active summing threads either.

 

[radiance]

Should I post to another thread? I didn't mean to hijack, but didn't want two separate active summing threads either.

Do what you like best....

You probabely get more response having your own thread.

 

[radiance]

Here pics of the finished unit...

http://www.groupdiy.com/index.php?topic=33104.0

 

[3nity]

I see on the finished unit a Wet and Dry function.
But i dont see it on any schematic!!
Whats its function? and do you have any schem?

Thanks.