New master section for Peavey AMR

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KMB-AUDIO

Well-known member
Joined
Dec 9, 2007
Messages
161
Location
San Diego, CA
Hello all:

After a long hiatus from music/diy...I am back in it and re-energized to get some projects done.  With that said, I would like to build a new master section for my console. I've thought a lot about what I need in a master section, and this is it.
I'm hoping some knowledgable people wouldn't mind taking a look at the attached schematic and let me know if you see any errors. 

The board I woule like to build this for is a Peavey AMR 2400.  It has 32 inputs from the left side of the board via 22k resistors and another 48 or so from the bus side of the board via (from what JR has said here) synthesized current sources.


My questions right off the bat are:
1. Do I need the 22k resistors from the 2 track inputs to the summing amp? I put them there because the L/R bus feeds are 22k. 

2. Will there be interaction between the master output and the control room output volume controls? my gut feeling is that there should be some sort of buffer between the two, but I don't know for sure. 

Any help is appreciated and I will be sure to take pictures along th way (people like pictures, right?  ;))

 

Attachments

  • Master Section.jpg
    Master Section.jpg
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> 1. Do I need the 22k resistors from the 2 track inputs to the summing amp?
Yes. You don't want the That1200s dead shorted to reference voltage.

> 2. Will there be interaction between the master output and the control room output volume controls?
The connected load to the JH990 will vary down to ~1.1K with both rotary switches in their CW position, but IMHO this won't cause interaction.

Your rotary switches might need a 2nd look. A 24 pos. rotary, other than a 23 pos., most often will be missing an end-stop and an accidently dial thru from silence to full level might wake up the neighbours. Maybe intentional, but for a log behaviour some R values at your attenuator series strings (2.49K at virtual ground node in parallel to the shunt arm resistor value) seem off. Your 12 pos.rotary might be missing a 13th position for silence as well.
The 10Ks at the 990 summer and 918 inverters non-inverting input most likely will be a typo. (will be the paralleled value of all connected input resistors and feedback resistor. I'd prefer a direct connection to reference voltage at least for these varying conditions. YMMV)
 
Hi KMB... nice project... I'm making a discrete mixer, for your Q...
1 you need those 22k resistor to get the right gain for the aux... I would said use 10k resistor to get 0dB gain.
2 shouldnt be interaction like that, if you are too purist you could use 2 separate interstage caps instead of 1 (.22µF+470µF). As Harpo said 1k1 load wont be nothing to a 990, it can drive 100Ω without harm.

I dont know the rotary switches very well, so, no opinion at that.

The only thing I would cange is the resistor at the non inverting input of the 990, it shouldnt be the same value as the feedback, it should be the value of parallel between all summing resistors and feedback, so, for 32 inputs is about 680Ω, with the 2TK around 5k (if you use 10k resistors instead of 22k). In all cases, this resistor is to reduce the offset voltage, if you use caps you dont need to use this resistor, Im using a servo instead of resistors and caps. If you want the resistor, you could use a one more switch at Sw1 that change it or use a 680Ω resistor that would work find with the most critical situation (32 channels) and let the offset of the 2TK to the caps.

Cheers

JS

 
Thanks for the answers guys!

The 24 pos switch that I am using has a stop to prevent "dial-through"...so no problem there. The 12 position does not have a silence position by design.  I use that output to my master recorder and have no need to ever turn it completely off.

So if I connect the non-inverting input to ground, do I need to be worried about offset? Since I'm using caps on the output is this a big concern? Perhaps I should input compensation current to minimize DC offset at the output?

I'm going to experiment either way to see what sounds the best to me.

Thanks again!
 
Hi, I don't have any advice here, but it seems that JohnRoberts, who is a moderator here and has a lot to say about console design in general, actually designed the AMR 2400 (or partly). may be he could be one of the very best personnes to contact for help in this project.

Just my two cents...
Laurent.
 
Hi Laurent:

Thanks for the advice!

Indeed I did know that and was hoping that he would chime in...but I did not want to bother the man :)
 
You shouldnt have problems with the non inverting input to ground, but you could try to use a cap in series with the OLI at the non-inverting input, and use a resistor from non-inverting to ground the same value of feedback, so you'll never have offset and you could take out the output caps... going like this never have dc so always will have the best headroom and dont need to add a cap, just move it backwards.

In my project Im using servos because I cant get good caps, dont seems you have that issue, so dont think you have to use servos, but if you want i tell my experience, something is on my topic (discrete mixer) http://www.groupdiy.com/index.php?topic=48042.0 but if you like I can send you the schems Im working on... anything finished yet, that's why i didnt upload them.

JS
 

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