Auditronics 110A phantom/grounding issue

[dukeofearl]

I am restoring a 110a console and am going through the modules to make sure they all work. I have recapped the two big electrolytic caps and the 6 tantalum power caps. I put together some edge connectors and have started testing. I have the +15v jumpered to the 24v. I do not have the phantom power hooked up right now.

The first few have been working nicely but there is a weird issue. They only work when the phantom power button is depressed. When the phantom button is off, then they don't work. When they aren't hooked up and the phantom power button is off, the program out is shorted to ground. P1-7 is somehow connected to P1-1 when phantom is off. Anyone know what might be causing this? Is this something obvious I'm missing? Because it seems to be the case on the five modules I've tested.

 

[dukeofearl]

Another question concerns the jumpering of the +15v to the +24v. I did it because I've seen others do it here when racking these. Is it okay to do on the entire console? My original power supply has issues but I got a good acopian 4.5 amp +15/-15v supply to replace it. I can get a +24v supply but if this would work then why not. But there must be some reason for the 24.

 

[Whoops]

Another question concerns the jumpering of the +15v to the +24v.

I will not jumper +15v to the +24v without knowing why people do it, or without knowing if you could it.

If there was no need for 24V then Audiotronics would just have used +15v for everything, it would be simpler and less expensive to do so. There must be a good reason for it

 

[dukeofearl]

i saw brian roth do it in another thread so figured it was legit. but there must be a reason for 24v for the full console.

confused on that phantom thingy...

 

[Brian Roth]

In my situations, I was just racking up a few modules.  The 24V line only fed a couple of LEDs on each module, so I "cheated" and used the 15V rail for that function.

I don't remember what other items in a full desk were fed from the 24 rail.

I never had a phantom issue like you describe.  But, I did had to change out the phantom resistors to the correct 6k8 value. 

Bri

 

[Brian Roth]

Also, what phantom power switch are you referring to?  I just looked at a 110B schemo I have on the computer and the "50 Volts" isn't switched.

Bri

 

[dukeofearl]

Wait sorry I'm a compete idiot! I thought the red button on the bottom was to turn the 50v on or off on each channel - but it's just to turn the channel on and off. Duh.

 

[Brian Roth]

LOL!  Yep, that switch shorts the output line to the connector.

Still, double check the values of the phantom resistors.  Stock was 20K (!!!)...what were they thinking?

Bri

 

[dukeofearl]

Yes I should change those resistors. Thanks for the tip.

So is 50v always going to the channels?

 

[Brian Roth]

48/50 DCV  ...not a worry.

Needs to be clean...no ripple, yada  yada.

20K.....WRONG!!!!!!!!!  lol

Oh...duke..also look at the "pile of resistors" at the incoming side of the A/B switch and the "mic" transformer.

Auditronics wanted to sell to anybody  (which was GOOD!)...so they had various resistor configs...


"A/B" input switch.  Mic into "A" , line into "B"???  Mic into both?  Line level into both?  Hence the "stuffing pattern" thru that single input path.

It's perhaps a "broadcast" concept  (look back thru all of the ancients...Gates, RCA,  yada yada.)

Bri

 

[dukeofearl]

i think mine is one of the early ones since it has the lm709 opamps. my understanding of these things is still pretty basic - but basically the mic and line are both going into the same transformer and the line is just attenuated?

 

[Brian Roth]

That's correct.  In the most "standard" configuration, "A" is mic and "B" is a line input which adds a pad ahead of the transformer.  But, it can be strapped in various other combinations.  The schematic shows the strapping PC board pads and resistor values.  I have seen all sorts of combinations, often done in the field by the end user.

Bri

 

[kramerb1]

I've racked up a few channels of Auditronics 501 strips. I've found quite a bit of info along the way including upgrades to the LM709 and LM301. The part numbers apply to 501 but can easily be deciphered to apply to the 110.

"Auditronics is currently using (Signetics) NE5534 in place of 709 and 301 in model 501 console.  The 5534 gives improved performance in terms of distortion and signal-to-noise. 
301 to 5534 8 pin:
301 Stage
Remove compensation capacitor between pins 1 and 8
Install 150 pf capacitor in parallel with R47 5.1 k feedback resistor (noted in schematic)

709 to 5534 (14pin to 8 pin)
Output Stage
Remove 2 compensating networks consisting of C25 and R41 and C29.
Add 150 pf capacitor in parallel with R29 - the feedback resistor

709 Mic stage
Remove C2, R7 and C5
Add 150 pf capacitor in parallel with feedback resistor consisting of R6 and P1 in series."

 

[kramerb1]

If you look at the bottom of the A501draft_section_6_short_list.pdf it shows the proper orientation of fitting the 5534 in the 14 pin socket.

 

[kramerb1]

501 schematic

 

[dukeofearl]

Thanks for that info, kramerb1. I had heard about that mod and was wondering about the specifics. I'm going to start by getting the console working as is and decide if I want to do that and/or other upgrades. There's always a chance I'll like the sound of the less clean opamps.

So far, I've got 16 of 17 working channels. I found a shorted tantalum cap on the summing module so recapped that. So I think I'll see if I can get the 16 channels working with the summing amp and main output cards, leaving the problematic monitor section out for now. I won't be using that anyways. Fingers crossed!

Another mod I had considered was replacing the summing section with a Classic Api ACA-BO. 

And figuring out some way of balanced, post-eq direct outs. Seems like there are several ways of doing this - tapping into program out or solo out (how I would actually do that I would need help with) - and using output cards or ideally just a transformer.

 

[dukeofearl]

just wanted to check that these are a good choice for phantom before ordering...

http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=218453021&uq=636229302449282153

wasn't sure with 6.8 vs 6.81 K, 1/2 vs 1/4 W, 1 vs 5 %

these are 6.81k, 1/2w, 1%

thanks!

 

[Brian Roth]

That should work.

Bri

 

[mrbobbailey]

Hi All,

I realize this is an old post, but it hit right on the topic I've been wondering about.  I also would like to add phantom power to my 110a.  I came across Radio Design Labs (RDL) that made me think I could in a discrete way on the console back or at least in a rack plate on the I/O patchbay.  Are you familiar with their products, particularly the ST-MPA48?  Or is there another recommendation for adding switchable phantom power to these vintage consoles?

Much thanks,

Bob

 

[abbey road d enfer]

Hi All,

I realize this is an old post, but it hit right on the topic I've been wondering about.  I also would like to add phantom power to my 110a.  I came across Radio Design Labs (RDL) that made me think I could in a discrete way on the console back or at least in a rack plate on the I/O patchbay.  Are you familiar with their products, particularly the ST-MPA48?  Or is there another recommendation for adding switchable phantom power to these vintage consoles?

Much thanks,

Bob

This product is just 4 off 6.81k resistors in a box with screw-type connectors, with a DC/DC converter that allows using the standard 12/24V RDL PSU's. That makes it a quite expensive unit.
Once you have a 48V PSU of sufficient capacity (14mA per channel), the phantom feed circuit is actually very pedestrian.
For a 24 ch mixer, you need not more than 16W. Beware that smps (Switch Mode Power Supply) do not work well when not enough current is drawn, so you may want to favour a linear PSU, which are becoming increasingly rare...