BALANCED SUMMING MIXER: Best Gain Stage?

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kpeek

Active member
Joined
Sep 1, 2004
Messages
35
Hello:

I want to build a 24 channel discrete balanced summing mixer with gain, mute and pan controls on each channel while having four AUX SENDS with gain and pan controls per channel and just two main outputs. So that would mean I want:

  Channel Strips:
            24 Input Transformers (line level)
            24 Output Transformers (line output or interstage)
            24 Discrete gain stages (set for highest headroom)
            24 (2) Deck rotary switches (gain controls)
            24 (4) Deck rotary switches (pan controls)
            96 (2) Deck rotary switches (gain controls)-AUX SENDS
            96 (4) Deck rotary switches (pan controls) -AUX SENDS
            24 Relays for Mute functions
 
    Each channel would have to have an Input line level transformer followed by a gain stage
    with gain and pan controls followed by an Output or Interstage transformer. I assume this
    would be enough current to feed the signal to four AUX SEND gain stages and the Left and Right
    MAIN OUTPUT stages. Is this correct?

    Then each channel would have control over level and pan on the main channels while having level
    and pan control on the AUX SENDS.

    The AUX RETURNS would be treated the same as four additional channels with an Input line level
    transformer into a gain stage with level control and then routed to the MAIN OUTPUT summing
    stages.
    This would add the following:
           
  AUX RETURNS:
            4 Input Transformers (line level)
            4 Discrete gain stages (set at line level)
            4 Relays for Mute functions
            4 (4) Deck rotary switches (gain controls)-AUX RETURNS

    For the output section there would be LEFT and RIGHT Discrete gain SUMMING output stages for
    the summing of the 24 channels and the 4 AUX RETURN channels followed by two Output
    Transformers. A headphone amplifier would be included. There would be balanced outputs from
    the transformers as well as two sets of unbalanced transformers to drive "stereo Hi-fi amplifiers."
    There will be analog VU meters on at least the MASTER OUTPUTS if not every channel.

  MASTER SUMMING:
            2 (2) Deck rotary switches (master gain controls)-MASTER SUMMING
            2 Output Transformers (line level) MASTER OUTPUTS
            2 to 30 Analog VU Meters (at minimum 2 on MASTER OUTPUTS)


I have drawn preliminary schematics for this by looking at Fred Forssell's and other mixer schematics.

Does this seem possible? I know it will be expensive and hard to complete, but the rewards would be great.

I could cheapen the project but not using transformers, using unbalanced signals, using cheap switches, and of course using monolithic op-amps. But this is going to be the "CENTERPIECE" of my studio like an old small Neve 8016 or a API 1608.
   

Any suggestions?

Thanks 
Kevin Peek
Modern Recording
Tampa, FL






 
Yes...it's possible. Technicaly speaking that is.
However, practically it's a very big and expensive undertaking and that's mainly because of the rotary switches.
I presume you want the high quality Elma 24 position switches right?
I count a total of 720 decks you would need to solder  :eek:  (that's only the channel strips) I've recently soldered some and I can do 4 decks a day without getting totally bonkers. That includes resistor matching and attaching cables.
In your case that would be 180 days of full time soldering rotary switches. That's mental slavery man  ;) No can't do...

Not to mention the switches alone are going to cost you at least 7200 euro. This is a very low estimate since I think you'll get some serious discount ordering this much. Normally this switches will cost you at least double that price if not more.

The ones with resistors pre assembled are even more expensive.

Why not go for high quality conductive plastc pots or so? Leave the rotary's for master and maybe gain setting?
 
Radiance,

I saw your mixer which was beautiful and the closest things anyone some has done to my idea.

I agree with you. This is more of an "high-quality" summing box. So the levels I can set and automate in the computer so I was thinking 12 step switches for levels and 12 steps for the pan. I was hoping to hit a surplus shop here in the US.

What kind of pots would you suggest, Alps? Bourns? They have to be four decked if they are to be blanced pan-pots.

What about gain stages- something not an op-amp like quad eight CA-227
http://cms.orphanaudio.com/index.php?id=134,0,0,1,0,0    - buy one and copy into my design?
Or op-amps like Hardy, Forssell, DOA, any other gain designs?
I might try working on my own. But that's difficult.

What do you think?

 
OHHHHHH.....

Cinemag makes great transformers and give good discounts. I was likely going to use those and maybe make a copy of the CA-227 card but with out all the "mic pre" extras-DI, 48V, etc. just the level and that is it.
 
kpeek said:
What kind of pots would you suggest, Alps? Bourns? They have to be four decked if they are to be blanced pan-pots.

Well, Alps, Bourns...it's all good. Conductive plastic is best but does not have a solid feel at all. Knobs will feel very "loose".
Alps blue velvet do have a very good feel.

I don't know if I understand this right but...why do you want te keep the signal balanced inside the mixer as well? You have input trannies, so from there on the signal can be unbalanced untill it leaves your mixer at the output transformer. This means you'll "only" need single deck and dual deck pots.
Keeping a balanced signal "inside" the mixer will double part count and cost. Finding the right 4 deck pots will be difficult.

Designing a mixer means you'll have to do research on several things at once. That is: making sure the circuit works, are the parts available, are the parts going to fit, how hot do things get (in case of using 990's for example),  part costs, can things be simplefied. You have to go back and forth between these steps a lot of times before you have something that actually CAN be build. I fount that the metal work part, the "are the parts going to fit" part ....was by far the most difficult aspect of builing a mixer.


Another thing...what are you going to connect to this mixer? Does it really needs to be ultra high quality?
 
Well these are the key points:

1) The inputs will be connected to balanced ADA converters (ECHO Audiofire 12- the same D/A
    converters as Pro Tools HD). Relays would be used to mute the channels.

2) You are right about making the signal unbalanced once inside the mixer, it makes sense.

3) I would use a summing box but most are 16 channel, don't have pan controls or AUX sends and
    returns.

4) In its simpliest form I need:
    a) 24 channel mixer with balanced inputs (could be unbalanced from there) and pan controls
    b) four AUX sends per channel with pan controls--this is so they could be two mono and one stereo,
        two stereo, or four mono, basically the pan controls would be due to my having several outboard
        Lexicon (PCM80 etc.) FX that I use
    c) an output section that has a high enough headroom to accommodate 24 channels with pan, 4 AUX
        returns with pan
    d) have balanced outputs and two sets of unbalanced outputs for stereo HiFi amplifiers
    e) a headphone amplifier

Is this more reasonable?

I have access to a full metal shop so I can make my own case. Drill presses, metal brakes, and other metal working equipment.

Lastly I would need a racked power supply. +/- 24 VDC for discrete op-amps and OPA604 and 12VDC or 15VDC for Relays.

Anything else?



 
kpeek said:
1) The inputs will be connected to balanced ADA converters (ECHO Audiofire 12- the same D/A
   converters as Pro Tools HD). Relays would be used to mute the channels.

Have a look inside your ECHO Audiofire 12 to see what opamps are used. It is THOSE opamps which will have the most significant impact on your sound (as Tubemooley said). Maybe expensive input transformers are a bit overkill. They are not going to make your signal better, they'll just ad some color.

kpeek said:
4) In its simpliest form I need:
    a) 24 channel mixer with balanced inputs (could be unbalanced from there) and pan controls
    b) four AUX sends per channel with pan controls--this is so they could be two mono and one stereo,
        two stereo, or four mono, basically the pan controls would be due to my having several outboard
        Lexicon (PCM80 etc.) FX that I use
    c) an output section that has a high enough headroom to accommodate 24 channels with pan, 4 AUX
        returns with pan
    d) have balanced outputs and two sets of unbalanced outputs for stereo HiFi amplifiers
    e) a headphone amplifier

Is this more reasonable?

Hey hey, you're the designer/builder here...you don't have to negotiate  ;)
No, but seriously, to me...the above looks a lot more do-able than your first idea.

Have you considdered:
- inserts on the master?
- meters?

I guess you're going to use the opa604 as buffer opamp (in various places) because it can run on 24+/- V right?

Also, I've a lot (really an awefull lot) relay's left from my mixer project so if you need some for cheap pm me. They are 24v and surface mount though...but very easy to solder and they don't take much space on a PCB.
Are you going to design a pcb for this project? If so, it's best to make one big pcb, not two like I did. Keep the wireing to a minimum (unlike what I did ), maybe try to go for PCB mount pots. It saves so much time and wireing trouble.
 
Why don't you buy a console that has a bunch of iron in it, like an old MCI? That will probably be a lot cheaper. There is a lot of information on improving the 400 series consoles, and Sage Electronics designed discrete op-amp replacements
 
For two reasons:

1) Most are over $10K USD , more than my budget

2) I want to DIY

Remember, I need a 24 channel summing mixer with 4 aux sends  and pans on the channels and sends - that's it- no EQ or other "stuff".
I think I can do it for less than $5K USD.
 
kpeek said:
    c) an output section that has a high enough headroom to accommodate 24 channels with pan, 4 AUX
        returns with pan

Best gain stage would be a discrete opamp like the John Hardy 990 or for example the discrete opamp from JLM audio which uses a opa2604 as building block

http://www.jlmaudio.com/shop/index.php?_a=viewProd&productId=32

This one has on board DC Servo...meaning less caps in the audio path.

You can practically make all parts of your mixer with it, line receivers/drivers headphone amp, summing amp (according to the jlm website that is...)

 
Hey there,

You might want to consider Jeff's Active Combining Amplifier ACA as a block of your summing stage.  It is loosely based around an API approach but is really an inverting amplifier opamp based scheme.

You could use some of these for your aux busses and main buss too.  He has a couple different models.  They invert the polarity so you'll need to flip it with an inverting opamp running at unity gain or a transformer (which could be nice for balancing to).

He has some affordable but good quality transformers too on there.  Jeff posts here often but his website is www.classicapi.com

I was looking to figure out a distribution/aux scheme.  PPR came up with a decent one that includes some gain in the aux.  Look towards the bottom of the thread at his diagram.

http://www.groupdiy.com/index.php?topic=36913.0

CC
 
radiance said:
Have a look inside your ECHO Audiofire 12 to see what opamps are used.
Totally!
Got this Audiofire too, I just soldered a 4562 instead of the cheap opamp (don't remember the model, 20XX something?) in there, big difference!
Interestingly enough, it uses only one opamp on the output and one np cap after it, so I got an impression that it's not even balanced output...
Anyhow, the sound cleared up quite a bit, still sounds somewhat harsh to my taste, next to try is a transformer output, then maybe transformer+tube buffer...
It surely isn't a very high end card, and a high end mixer would be a waste, IMO...
 
The outputs may be impedance balanced.  Is there a resistor and cap in parallel grounded to pin 3 (or the ring)?  If so, then it would have decent CMMR but it would not be symmetrical output.  The symmetrical output means nothing in terms of noise rejection but does affect how much output can be had (I believe)....

CC
 
It's smd and multilayer pcb, so I couldn't trace it well, particularly the ground.. I could see on the scope that the signal goes via opamp and capacitor to the positive (tip), and there was no signal on the ring, however it wasn't short to the ground (sleeve), and sleeve in it's turn isn't connected to power ground...
Motu 828 that I was modding before uses 3 opamps for the output, and no capacitors, and produces balanced out. Subjectively, I like it's sound more, it's smoother in mids/highs and has more bass. Could be the codecs though.
 
I suspect that the Audiofire is impedance balanced then.  The ring would connect to ground via a resistor and capacitor.  I don't have access to draw a schematic but look at the output of JLMaudio's DI here: http://www.jlmaudio.com/JLM%20DI%20Schematic.pdf

It would be the same idea likely for the Audiofire.

There are some advantages in that you don't have to be careful about how you run it unbalanced.  The CMMR would be good to.  The fellow at Jensen Transformers advocates this same approach in his paper connecting unbalanced to balanced configurations.  So if it's good enough for him - it's good enough for me.

Other than that - I have no idea what the Audiofire sounds like - I've never used one.

CC
 
kpeek said:
For two reasons:

1) Most are over $10K USD , more than my budget

2) I want to DIY

Remember, I need a 24 channel summing mixer with 4 aux sends  and pans on the channels and sends - that's it- no EQ or other "stuff".
I think I can do it for less than $5K USD.

Hi,

This sounds like a lot of work...

So, if you think about it in a channel-strip approach, you have 24 input channels, four aux-bus / output-channel-masters, and two output-master-bus channels. That is thirty channelstrips. With a budget of 5k you can spend about 167.- per channel and with a budget of 10k you can spend 333.- channel...

If you approach as a modular mixer with a backplane/motherboard you can make it fairly minimal at first and have a functioning unit for less initial cash and time outlay and design/upgrade your modules as time/budget permits.

For instance. Maybe design each input channel-strip to have a module for Input-Section, aux-bus module, fader-module, and routing-pan-module.
The master/output modules could also have an input-module, routing-module, fader-module, and output-driver.. Why not design in inserts, and space to make your input section mic/line with direct-outs

That way, you just need to initially design/build/commission two master/output modules, the backplane, 24 fader-modules (buffered?), and the routing/pan module (LCR assign to keep it simple initialy)... And the chassis.

I guess the input strip-aux-modules would be similar to the fader module except with a rotary and maybe a pre/post switch to take the signal from either the input section or output of the fader section...

I don't know. Just throwing some ideas out there for you.

Best,
j
 

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