So Completely Confused, ARGH!~

Ugh, I have a problem.

My console has a point to the bay off the mix buss, I suspect that the point is likely taken from the mix buss summing amps, the return goes to the stereo master fader which drives I think another opamp and a 600:600 transformer. The opamps are discrete little things throughout. The more I think of it, the mix insert point is almost definitely transformerless and its definitely pre master fader.

I plug this into most of my limiters and all hell breaks loose. Into an api 2500, Ive got NOOO bottom end at all, Im talking like hipass filter at 800 HZ. It wont feed the gyraf ssl at all, same thing happens. Ive got a compex with a 10K input transformer thats just total distortion. It sort of sounds better with the daking limiters which have a 10K jensen, but it still sounds like shit. Also, the taper on the master fader is ALL fucked up, its off point moves from the bottom of the fader to the center, very odd.

Oddly enough, a dbx 162 works. I always thought that box had a balanced front end, maybe not... Anyhow, that works great, but I'll be damned if Im gonna put a 162 on my mix buss...

Anyway, the bottom end thing I know form other stuff is likely an impedance mismatch. What do I need to do to make this work? Like an api, my console has all the same opamps that do all the amplifying. Im nearly certain that the buss outputs use two opamps and a transformer to get out of the console. If I put two transformers in a box and patch that between the bay and the limiter, if I used a 1:1 600r transformer, would that do the trick, or am I stuck with having to build an active box with a single gain stage, argh!

I guess what I really need to know is if something wont drive a box with a 10K input transformer, will it drive a 600r transformer?

Only way is to try, but thought Id pose this one to the gurus to see what kind of info I can gather for a fix it day tomorrow. I wasnt expecting this... GRRRR.

thanks for any thoughts-

dave

Hi Dave,

Can you sketch out exactly what's going on- have you got a manual page you can scan, or a hand-drawn diagram? Are the outputs on TRS bantams/GPOs/jacks?

It sounds like you may be feeding a balanced input with an unbalanced signal. I've done this by accident before- I fed a balanced input with a signal between chassis ground and the +ve side of a balanced input. There was an audio signal, but it sounded exactly as you mentioned- flat and weedy with no bottom end. The unbalanced signal was across one side of the input transformer and audio/chassis ground. So what I was hearing was the signal coming through the leakage capacitance of the input transformer! Hence the slightly odd sound.

It sounds like the signal at this point in the console will be unbalanced- if the output of the mix amp is an op-amp which then drives a fader, I should imagine it'll be unbalanced. So the insert-point will be unbalanced also. I'll draw a diagram of something to try. I'll post it soon.

Mark

not knowing your setup....
if the pre-fader insert and fader comes between the mixer and the post fader buffer and driver.
Could well be an unbalanced insert, and with potentially duff signal caps?

Dave,

Here's a sketch of what you've told me...and I've presumed that the inserts are wired unbalanced:



...and then I presumed that you inserted a balanced patchcord which would connect the outboard like this:



But, in reality, with the Send jack being unbalanced, this is what you have electrically:



Try wiring the send jack like this:

And the return jack like this: [...his. Hope this makes sense! :thumb: Mark

I think you're correct, Mark. In my experience when something loses gain and, in particular bottom, it has lost one leg of the balanced signal somewhere. If the inserts are brought out to the bays the problem could be there.

Good luck, Dave

Just to cap it off, I've had this problem as well, and it's always been a balanced to unbalanced thing.

I had an identical problem (i.e zilch below about 6-800hz) recently with some '62 vari-mu compressors. Initially I thought that the impedance of the outputs driving the vari-mu was at fault, and posted a query here whereupon Jakob and friends also pointed the finger at an impedance mismatch...

Having tried all kinds of powerful discrete o/p stages to drive it with no joy I established that a jumper was loose on the rear panel and I was only tapping into one side of the transformer, just as Mark has described... Reconnected the jumper and had no problem whatsoever, I can't figure out why the jumper would've been disconnected in the first place which is why I didn't check it, doh!

Note to self: check the obvious before hitting the post button

I reckon Mark has hit the nail on the head here.

Cheers,
Justin

> a sketch of what you've told me...and I've presumed that the inserts are wired unbalanced:

I hope that is wrong.... but I fear you are right.

When you have an unbalanced signal on 1/4" jacks, and hope to drive true balanced gear, and do not know if it is true floating or what, it probably should be wired more like this:

Yes,

Which makes me think that maybe this insert point was an add-on at some time? I've had more than enough trouble with unbalanced insert-points that have been "tacked on" to a desk hoping it would interface with the real world without any tears.

Dave, what discrete op-amps are used in the console? Are the main buss-out stages driving the transformers the same type of amp that drives the fader? If not, you may have trouble when pushing level into a true 600R input from the insert send.

Hope you sort it!

Mark

damn, you guys are awesome. All the opamps arre the same... The point on the bay wasnt tacked on, it came from the original install this way. I happen to have some extra transformers for the console, I think what Im gonna do first is make a cable into a transformer and see if that solves the problem, which I hope it will. I dont want to mess around with the point at the bay, as its a normalled connection and dont want to get into stirring the behive just yet, Ive been to that place before with this thing...

mark- Im nearly certain that your sketch is whats going on, the console is unbalanced until it goes out the busses, so there'd be no reason to have an interstage transformer just for this one point. There's lots of wacky design stuff with this thing, it was built in 1978 and there are cave drawings on the inside of the desk depicting how an inline console should work... Total stone age on some stuff...

Jon- all the caps on the summing and driver opamps are new, so I can assume with some degree of confidence thats not the problem.

PRR- what does the 300R to ground on the return do in this scenario?

My other thought was the problem may have been adding a balanced signal to the prefader point, but thats not really what it sounds like. I need to wake up (love working nights) clear my head and build a transformer into a tt cable and see what happens, I'll let you guys know.

Thanks for everyones help, this place totally rules.

dave

ok, spent some time with a sledgehammer today...

First off, here's the block diagram, please follow the link, I kept it big so its readable-

P M S K

If you look, the insert point is after the L/R summing amp, before the L/R master fader which then feeds the output amps. All the opamps are identical, similar somewhat to 2520's.

I took a spare 600:600 output tx that I had and hooked it up and got the same thing, no change. The WIERDEST thing about this is with the output fader at full throw, its totally distorted with no bass, as you bring the fader to unity, there is no top end and it gets all muddy and the level drops then as you bring it to minus five, the bottom falls out and is all distorted and the level increases, VERY strange. I would love to understand why that happens...

Ive been looking at the Echo Send/ Return. The two transformers there are both round cans, one is a jensen JE-11P-6 which looks to be the return and a smaller can that is stamped "76113". I cant turn up any info on the web for either (outside the fact that the 11p-6 is a line input), anyone familiar with these?

Id like to set this up so I can use the point to feed a balanced input and the return with a balanced output. Speaking outloud, I wonder if the problem is not feeding the balanced input but actually receiving a balanced output... This works fine with a unbalanced gear, or so it seems. I tried feeding it to a mic pre with a 150r input transformer and got repeated behavior, switched that same transformer to 10K and the same thing. Short of bypassing the input transformer on a box, now that we know what we are dealing with, are any of the suggestions earlier in this thread now more or less appropriate than before seeing the block diagram? I would love to meet the designer someday, the freakin echo send is set up for balanced operation, but the mix insert isnt... Were they expecting everyone to just use an unbalanced patch there? That doesnt seem to make much sense for a 1978 design.

Im open for suggestion, my whole mix is stalled around this. Whats worse is that beacuse of the monitoring on the board, I cant even patch the limiter between the console and the 2 track (as Id prefer to do anyway) because I cant monitor the 2 track the way the routing works on the console. ARGH.... I wanted an old console...

help!

dave

Dave,

Could the fact that there is no physical break shown for the master bus inserts the way there is for the echo send mean that there wasn't an insert point originally? The arrows would seem to indicate an insert point, but maybe they weren't actually there? If the arrows do indicate an insert, it looks like there's another after the meters, before the output. It wouldn't be the first time documentation is wrong. Maybe your console has been modded to add the insert? If so, maybe the mod was incorrectly done? Or maybe the unbalanced insert was a custom feature the original buyer wanted for some other reason than balanced gear. Maybe those arrows indicate where there are edge connector terminals on cards rather then inserts. I'm just thinking.


....hmmm...what's that smell?....


:guinness:

the arrows on the drawing represent points on the patchbay. Its not a modern "insert" where there is a send and return on a TRS from one point in the bay, its just a point on the bay... Ive got all the original documentation for the console, I redid the bays so had to go over them with a fine tooth comb and the wiring matches the blueprints. I have no idea what that point is supposed to be for, its quite possible that its just there for the normalling, there is other wiring that goes to the bay for no good reason other than the normal connection, perhaps Im hoping to use it for something it cant be used for. grrrr...

dave

Hi Dave,

You got me hooked on this one now :grin:

Its not a modern "insert" where there is a send and return on a TRS from one point in the bay, its just a point on the bay..

Click to expand...

Just verify this for me...the schematic show a single "listen" jack on the bay, whereas the echo send has a two-point "insert" point (not normalled). Is the master buss "insert" point you're trying just a single point? Or has a second point been added for return? If this is just a "listen" jack with no break, then really weird stuff will happen if you try to use it as an insert.

Just got some stuff to do...I'll be back with something to try...

Mark

Okay,

Your bay points must look something like this:



Figure A would indicate either a "listen" point- which allows you to listen in to the signal without breaking its path, or an unbalanced insert- the tip is usually the "send" and the ring the "return". The tip-to-ring path is normalled, so when a jack is inserted it breaks the path.

Figure B is what I call a "proper" insert- the signal is sent from the top jack, and is returned via the bottom jack. This connection is the most versatile as it can be fully normalled (inserting a jack in either point breaks the connection), or half-normalled- usually the send jack is wired this way to be a Listen jack, with the signal only being broken if a jack is inserted in the Return point.

To find out what's happening, you need to sniff out your signals. Here's the basic plan:



You'll need a TT lead which can have one of its ends cut off, and prepare the end so that the screen, tip and ring wires are stripped and can be easily connected. Check with a DMM which wire is which- the screen will be obvious, but the colours for the tip and ring don't always make sense. You'll then have the setup in the diagram above.

What we want to do is find out where the signal is coming from in the bay point, and then where it goes back into the console. Have you got a small battery operated amp lying about? One of those mini guitar practice amps would be good. Even high-impedance headphones will work. Get a cable from the amp (or phones) and prepare the ends so that it can be easily connected to the TT test lead. Put program through the console, and route it to the L and R outputs. To start with, twist the Tip wire of the TT test lead to the "hot" terminal of the test amp. Then twist the shield wire of the TT lead to shield of the shield of the test-amp wire. The reason you need a battery powered amp here is that there is no danger of shorting the outputs of the console. The console will have its 0V point tied to mains ground, so a fully floating test amp allows you to connect across a balanced or unbalanced connection in any way.

Then, insert the TT lead to the bay point. What happens when you insert it? Does it break the output to the Master L+R Outs? Make sure the connections you've made aren't touching. Have a listen. If you're getting a full signal, then it's an unbalanced output from screen-to-tip (or a centre-tapped balanced output, listening to one leg- it'd be 6dB down...) Then remove the TT and connect the test amp leads hot terminal to the ring wire of the TT lead. Keep the shields connected. Plug the TT lead back in and have a listen. Signal? If yes, then the output is centre-tapped balanced (which I'm sure won't be the case here- no transformer sending to the point from the schematic!), or else the tip and ring in the point are commoned. If no signal, then we can presume that a. the ring connection is not connected, or b. the ring is actually a return.

If you got no signal at all, or a weakish signal with both wirings, then connect the amp across the Tip and Ring terminals of the TT lead. It doesn't matter which lead goes to which terminal of the amp lead because the monitor amp/phones are floating. BTW, don't wear the headphones- they're purely being used as a go/no go tester. The levels could be a bit hot which could jeopardise any mixing youw want to do :shock:

Give this a go and tell us what you're getting.

:thumb:

Mark