Summing amp choice

[shot]

Hi!

I'm being driven by an idea to butcher my lovely Allen&Heath MOD3 console to make it more usefull in this modern era.

It's a modular 16 channel input and 8 ch. output desk from circa 1979. It has fantastic sounding mic preamps, but not so versatile eq. The "problem" is that it has 8 outputs, with only first two output channels being passed to the central section and availiable for monitoring. And this monitor output on the central section is unbalanced. The entire console is discreete, it has nice warm sound and fine mic input transformers (unknown brand to me - called Electronic Technologies Anglia Limited (ETAL) 276-2). Headroom is not it's strong spot, as far as I've heard when working on it few years ago (shortly after I got new little Trident so Allen&Heath went to storage). It is +-24V operated. I have almost all the schematics so if anyone's interested I can upload them...

So, my idea is to completely cut the frame and ditch all eight output channels (they are unbalanced, with no eq so I don't consider it a waste) and to build a new better suming amp for the console. Aside from that, I'll be adding unbalanced direct outs but that's an easy one.

I was googling and browsing through old threads here trying to find out what would be the appropriate choice. Went through threads on Neumann 475 summing amp and maybe it's the best choice. But... huh...

What do you guys (and gals of course!) think I should go for?

Since this console is old school all discrete, should I blindly stick to the idea of building all discrete summing amp? I'm not against using chip opamps but if there's a reason to avoid them I'll take the advice.

My only guidelines would be to have enough headroom and some vibe in it's sound. Don't want to ruin the vibe of the console, so it should be something to go along with it. Probably output transformers would give me that (I guess??).

Just point me in some direction. What summing amp designs I should look for and dig into reading?
What would be best described as warm and creamy (I hate this type of descriptions but...) summ?



thnx

Luka

 

[boji]

Grab a six pack, or some coffee and get comfortable:

http://www.groupdiy.com/index.php?topic=2779

 

[shot]

Damn! I never went into META thread, but instead I used search function. So I stumbled accross some of the threads but never dig into META!

big big thanks boji!!

But question is still here - based on it's sound (kinda vintage) what would you reccomend?

 

[tv]

http://www.freepatentsonline.com/4268798.html

here's an idea.. two in fact.
but you would have to sim. the component values yourself.

 

[sodderboy]

Butcher my lovely console?  Why not fix whats wrong?
Since you like the console, why not use the amps from the output modules?  Keep like with like.  Add some output iron and you are in business.
As far as the headroom is concerned, you can make measurements with a scope to determine where it gets squarshed.  It should have beaucoup headroom with bipolar 24.  Neve 80 series does it with half that.  Add your direct outs and measure the channel headroom with a scope- find where you hit the ceiling.  If it is the mix buses that get slammed, all you would need to do is reduce the level post channel fader.  If it is post mix bus, then there you can change levels in the monitor section.  You can measure the gain structure at points from mic in to monitor out and find where it clips, and address that specifically.  +4 to line in, panned to one side, fader at 0, should yield 0Vu out the mix bus.  What does your console do?
As far as different bus amps are concerned, Neve and API circuits with their in and out iron would be on the short list, with many PCB and parts sources available here.
Mike

 

[shot]

In past few days, especcially after Sodderboy's post, I'm more and more thinking about using existing output summing scheme but maybe recapping it or possibly upgrading it. It could be a mess if I try to fit some other summing amp into this, and it's impossible to know how it will sound before I actually build it and put it in the console. I was hoping to find some "sound info" on other summing amps but that's not avaliable anywhere. Most of the thereads here and on other forums talk only electronic-wise, and noone mentions how it sounds. I don't want to go blind and butcher the console just to see if I feel it's sound or not.

So anyway. First of all I need to find a way how to test for bad components. I don't have hardware scope, but I hope to manage it through computer input/output passing signal to the console and recording it back.
As far as I remember, the few mixes I did way back on this console, when recorded into my comp had waveforms slightly cut on one side. For instance, positive peaks were full but their negative counterparts were a bit shorter. Not like cut off tottaly but they were like 70 percent strong. As far as I understand that could be caused by outdated capacitors. It's not strange since this console is 30+ years old. Seems to me some input channel capacitors were recapped by previous owner(s), but not all. I haven't opened output channels so I don't know what is in them. I just poped up few inputs to see the insides.

The entire console is built around ZTX109C transistors. I don't know anything about them, but I found out that 2SC732 could be it's replacement. Maybe I'll try to swap a few transistors to see if the sound would change (to better hopefully). What do you think guys, should I try to do that??

Here's the output channel's block diagram. I'd use only the top part - signal from input channels flowing to xlr output. I have no need for tape monitor inputs. It was built for 8ch tape recorder use.
I have couple of pikatron 1:1 transformers and I was thinking of using them to balance the output (since it's not balanced here)



Luka

p.s. more schems will follow!
Maybe I should have put this in the Lab section of the forum... sorry guys...

 

[shot]

Here is the schematic of the output mix amp and line amp
Mix amp's output is connected to fader and fader's output goes to line amp's input. They didn't draw it here but you can see it in the output channel's block diagram I've posted earlyer...

Is there anything you see as a weak link in this schematic that would probably need to be replaced?

 

[shot]

And also, I need to take direct outs from input channels.
I have no need to have them balanced so I was thinking that the easy way would be to steal the signal from the pan pot. Any other suggestion?
I would first try a few points to see what would be the level of direct out signal on different spots. Maybe the most elegant way would be if I could take it from solo switch, but I'm affraid it would be insufficient level. I don't have a clue on what level to expect. I need to try it.

Anyway, here's the block diagram of input channel:

 

[sodderboy]

Well you can do frequency sweeps and measure on a DMM to see if your frequency response is good.  Bad electrolytic coupling caps will cause lower response in the low end before that drop overall level.  A good software oscillator is the discontinued Virtual MR1. 

Snooping around measuring headroom is tougher without a scope, because it is difficult to "hear" the same amount of harmonic distortion of a 1K signal at different places- how do you quantify what you hear, how can you listen to 1K for very long before it all sounds like distortion, and what is really distorting? your measurement point or your monitors.  A re-cap is a good place to start, and see if you can borrow a scope from someone who might have one not in constant use.  Checking repair shops will get you out and meeting other electronics people.  Also look for people with huge antennas on their roofs.  Ham radio dudes are the most encouraging and giving people you can meet, and they are easy to find.  It was ham dudes that brought me to electronics when I was about 12.
Mike