running 160 melcor 1731's. power/current draw?

What would you say would be the current draw for running around 160 X 1731's Yes 160 of them. Working on a console that has a total of 160 of those?    I am thinking  25 milliamps each on average. so minimum 4 amps...

I don't have the 1731 specs, but attached are the API 2520 specs.  Looks like it depends on PSU voltage and output load.  25mA is probably a good average bet though, just be sure to oversize the supply ;-)

the current one attached to the console that was there when I got there outputs +/-15VDC at 3 amps which seems awfully small for so many discrete circuits.  The owner got it that and it lasted about a month where it developed a hum.  I am fairly certain this all has to do with the  fact that the PSU is being pushed to the max and beyond  because 3 amps seems like not enough current to power all of that efficiently

Sounds logical.  I would go with 5 amps at least.  Can you post photos of the board?

Would be best if you could isolate a channel strip to test current draw for the unit since each opamp is performing a different task. 

How big is it? how many channels/groups/etc?

If the PSU is running at max, it should be VERY hot.  Not saying it's not the PSU, but if you can run it off a large bench supply you'd know right away. It could also be a shitty PSU.   

obvious question,  do you have schematics you could share?    Some pics would be good too, I have very little melcor stuff.

FWIW, Niels once told me of a recording truck with a (IIRC 32 channel?) API console running off two 1-Amp power one supplies.  Seemed very low to me, but if you're going into high impedances and the console has minimal other circuitry (groups/eqs/comps) your current draw could be very low. 

mjrippe said:

Sounds logical.  I would go with 5 amps at least.  Can you post photos of the board?

Click to expand...

I will have photos and some schematics as it is unique fo sure

For starters the console is very 70's API design style but using melcor.

O.k. here is why i think it's related to the power supply's. For starters  the console went in without a problem and then suddenly three nights ago it started and fist thing to happen was it popped a fuse on the bipolar 28VDC. The guy who sold the console TO the current owner said just put another fuse in there and up the size a little. So the owner did.  Has  not blown a fuse since but now the hum/noise. I got called  in and am trying to sort all this and get the noise/hum out.  The console runs of +/-15VDC(1731's), +/-28VDC(discrete section on each card after the 1731's) and 24VDC(logic) The noise/hum is everywhere. This had me thinking there was a problem in the master section.

The master section is fed from the same cards that make up the booster amps, summing amps, mic pre. The entire console is full of these cards that are all housed in card cages in  the bottom of the console at the front.  We first swapped master section cards around no change.  We then shut off the 28VDC which feeds the circuit on the cards past the discrete op amps. You'll see when i post info later on that. It went away.  o.k. that makes sense as we now cut power to part of the audio circuit on these cards.  o.k.  turned it back on and the PSU does not get real warm.  But the noise returns.  So we then  switched off the +/-15VDC but left on the +/-28VDC again noise went away.  so the fact we left the parts of the circuit that cover the output having me thinking that 15VDC  has a shit supply. Yeah it should be warm when touching it but  was in a spot where it was hard to get a body part on it.

The fact it's everywhere has me thinking it's got to be the power supply which was added by the seller as the console didn't have one. The  +/-15VDC supply does not get real hot to touch

The console and PSU  grounds seems to be intact and all voltages test out correctly.  I would think that being a house it was the house electrical but if it was, it would have been that way from day one. 

Honestly,  a pair of modest ammeters wired into some number of modules will be a far better answer than debating the issue. Good lab bench supplies (like the HP/Agilent E3631) sometimes have current readouts built in, so you just power it up and see what's happening.  If you are short on cash, two $5 multimeters properly wired in series with the supply will tell you at least two digits of accuracy, which is far better than what we can accomplish here.

Also, don't forget to measure supply current at idle as well as driving some signal to a load. The latter case has to be higher than idle, and is important to put a boundary on how much supply current you need in a realistic situation.

Best of luck,

all well and good I assure you but even if the 3 amps is enough, looking at the datasheets I have been able to procure lead me to thinking at minimum the 3 amps supply output is being pushed to max or close to it.  More importantly the heart of the issue is noise/junk/hum on every bit of the console. At first I was thinking it was the monitor section .  still working it all outs.  Since I was not there from ground zero, I can only go by what has been said to me which was it was fine until a couple of nights ago when it started to do all the noise and so forth. Started on one side only then went to both. monitors Swapped monitor cards as the entire console is made up of the same amp cards.  It did not fix anything which had me thinking it's further up the chain.  It's on everything so either the mains power in the building developed a hum at the drop of a hat a few nights ago or there is a power issue.

The console in question is this. No this is not the exact one but the same model. Apparently there were 3 built.  The one I just started to look after has 6 compressors which are not in the photo. They are Melcor and would go on the left hand side where the 6 black panels are. Then across the rest of the console is melcor eq's(eq 20's)

Interesting one....
Regarding the first question I´d put up a jig, feed one of these Melcor DOAs with max signal before clipping into worst case load and measure the current draw. Quiescent current doesn´t say enough about the overall current draw of the desk.
Then I´d look at the ripple on the rails with a scope and pull all modules one after another to see if there is one single strip causing the load or if it´s the sheer amount of DOAs causing this problem.
After these two manouvers you´ll know if the stock PSU is underrated.

> 25mA is probably a good average bet

Note that at +/-15V supply to 75 Ohm load each can suck _60_mA.

And this will be generally true for any amp driving 75 Ohms.

60mA times 160 modules is near 10 Amps.

Yes, odds are that few modules face 75 Ohm loads.

However this is clearly not a DuKane with all 100K loads.

> a pair of modest ammeters

+1 , except $5 DMMs don't do 10 Amps and cheap analog meters have become hard to find (and Pucho probably needs it fixed yesterday).

Look through the dead power-amps for a pair of 0.1 Ohm resistors.

Thanks Prr, when ever you comment I think two things, one I said a gaff and your correcting it and or your dropping some knowledge that is always appreciated.  The main  amp cards that make up the desk are known as The UOAB which is short for The universal operational amplifier card. It is used everywhere from the mic pre's to boosters, to summing cards, etc. I have it's schematic along side notes on how to hook it all up to run things. It's all utc Iron, CJ would have a field day. I have sent documents to Ethan so he can make them fit and be available in the tech documents.  8)

If the desk is humming at idle then you can presume that the idle/max current use and therefore the power supply size is not the primary issue, no?
Sometimes customer information can be partial or misleading. maybe you are looking after a fault that that took the fuse, but is still present now with the slightly lager fuse. or these are two separate issues.... just to get you to loose some time over this project.you are sure no other changes have been made to the system in the same time frame?

- Michael

Well I didn't do the install I came in after the fact. I have been in contact with the guys who have. we are doing the run around due to them not having any techs they can call in L.A.,do not know my work and they are somewhere out of state. It does purpose an interesting idea that at Idol(just turned on) noise is there but would one component effect the entire system so that it causes noise at every possible input and or output? as with any console fault finding, it's a matter of taking big chunks and making them more manageable.  so here is a list of what is known so far humming at idol could be related to dead filter caps or other things in the power supply.

1. noise started a month and some change after the console was installed. It had run well up until then...
2. a fuse blew on the power supply that runs the emitter follower after the melcor's on the UOAB's it is +/-28VDC
3. fuse was replaced and suggested by the sale's guys tech to up it from 1.5 to 1.75
4.  +/-28VDC does not blow fuses now or blows fuses when the original value is back in
5. the +/-28VDC PSU  is not racked and is open frame attached to a face plate. it does not run very hot.

Then I came into the picture when the owner called me...
I found the following that I can turn of the +/-28vdc and the noise goes away, I can also do that with the +/-15VDC and the noise goes away. I can leave the +/-28VDC on and shut the +/-15VDC off and noise disappears.  all that tells me at this point is I am shutting down  a section on all the UOAB cards which make up the console.  the UOAB's are comprised of 2 basic stages the melcor at +/-15VDC and the emitter follower running at +/-28VDC
I can't see one part causing noise for every card but you never know. I am back there for a little time this evening. I will remove all the cards in the card cage's and plug them in one by one and see if and when the noise starts.  Start with the masters and work backwardsThe board was supposedly recapped before sold.  more over if I could remove the eq's and the compressor i would really be styling but doing so requires a little but of undoing as the eq's were mounted into the frame before the one large faceplate piece with all the control knobs.  ???

Fault finding is fun, this is a good challenge for sure.

Set your Fluke DVM on the AC Volts scale and measure the noise on each PSU rail??


Bri

You have two different bi-polar power supplies...

Not necessarily a bad thing....But what's the ground like between them?

Since you know that the emitter-follower stage is "clean" with the UOABs powered off, you can get to the outputs of the UOAB's with a scope/squawk box with the emitter-follower stage powered off and watch/listen to what's going on there.

If the output stage is clean with it's +-28, and in the UOABs are clean on their +-15, but everything hums when it's together...You've probably got a ground reference issue between the two PSUs.

I got beaten to to it I guess but..

pucho812 said:

I found the following that I can turn of the +/-28vdc and the noise goes away, I can also do that with the +/-15VDC and the noise goes away. I can leave the +/-28VDC on and shut the +/-15VDC off and noise disappears.  all that tells me at this point is I am shutting down  a section on all the UOAB cards which make up the console.  the UOAB's are comprised of 2 basic stages the melcor at +/-15VDC and the emitter follower running at +/-28VDC

Click to expand...

This logic does leave your option but it also leaves the option that the 2 supplies are interacting in some way.

So basically…

TheJames said:

You have two different bi-polar power supplies...

Not necessarily a bad thing....But what's the ground like between them?

Click to expand...

What he said

ground between then... hmmmmm good call except the sellers had a tech team out to install the board.  worked sans issue for a month before noise started. Doesn't mean a crimp didn't go bad or whatever.  but definitely worth confirming. To be clear the UOAB cards run off both +/-15VDC and +/-28VDC.

Tonights adventure pulled all the UOAB cards and went by banks of putting them back in.  no one card behaved any differently or caused noise to change in volume or pitch.

Oh and the PSU for 28VDC gets warm but not to hot to touch...

The UAOB documents are up in the technical documents section. It includes schematics along side app notes and design notes explaining the circuit.  8) Thanks Ethan as usual you rock..

> AC Volts scale and measure the noise on each PSU rail??

+1

> You have two different bi-polar power supplies...

That's not clear. The docs (posted under "RCA") show only +/-28V to each card. These "emitter followers" are actually local voltage drop/filter sections to get +/-15V very-clean.

If there IS a board-wide +/-15V supply, we need more info about that. From this glimpse, I don't think there should be. The approach is per-board filtering from 28V to 15V, so supply can be "a considerable distance from the amplifiers", and to eliminate cross-talk.

The Main Outs will be 50 Ohm loads (see 1:3.5 transformer 8469300-1) and at clipping these cards will be sucking 0.1 Amps each.