frying summing amp/bus soundcraft 1624 ! help

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Rob Flinn

Well-known member
Joined
Jun 3, 2004
Messages
5,207
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Between Sussex, UK & Aude, France.
I've been trying to solve a problem in a Soundcraft 1624 which has a frying right st bus .

The bus feeds a basic 5534 summing amp with 3 resistors an output cap & a 12pf cap. From there it goes to an insert point then the fader, & subsequently the fader buffer. 

The problem is around the summing bus or amp since the frying goes when I deck the fader the frying goes.  All the channels & tape returns are switched off, which disconnects them from the bus.

What I've done so far, swapped out all the summing amp parts & tarted up all the soldering.    exactly the same frying sound.  I disconnected the input from the bus  to the summing amp & it disappears. 

What I'm notunderstanding that the bus is just a piece of wire that is disconnected from everything else, so why do a still get the frying ?  Or is it possible that one of the F2 (like the 1176 push switches) switches that switch the channels on & off the bus is puuting some crap on the bus ?/

Any ideas ?
 
If there is a short to ground on the bus of a typical virtual earth cum amp, you will get the frying sound and that is calling for very high noise gain. Also if there is excessive capacitance on the bus that could cause instability and noise, while a short to ground is the far more common fault than extra capacitance.

You ohmmeter is your friend.

JR
 
The frying sound, could be either the opamp oscillating at a frequency above audio... or a shunt from the - input to ground causing high noise gain.
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If it was a dead short, the opamp would drift to one rail or the other, but if instead it is just a very low ohm short or there is some DCR in the path (like wire), the opamp could still work but at very high noise gain. Also if the short is cap coupled in, the DC operating point would be fine, but you would still have big noise.

The 5534 IIRC is not unity gain stable so to guarantee stability needs to be compensated somehow, in a marginal stability situation, the bus may be adding enough capacitance to ground to cause some oscillation. But I wouldn't expect something like this to just happen years later.

For debugging you can add a larger than needed compensation cap to insure the 5534 is unconditionally stable and not oscillating. 
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I'm not sure I understand your comment  "since the frying goes when I deck the fader the frying goes."  Does deck the fader, mean pull down the master fader? and does "goes" mean goes a away, or starts?  If the noise is in the bus amp then it would drop as the master fader is pulled down.

Note the short to the bus, could be on an input module or anything sending to the bus, and it could be on the other side of a blocking capacitor.

JR
 
If there's say 50 ohms bus to ground, normal gain is hardly affected, the summer compensates. But the noise-gain is very high.

Power off, try to read the ohms of the summing bus. If the un-assign goes open, it would go to infinity. If there's some aux feed that never disconnects, maybe 10K. If un-assign switches the 10K channel resistor to ground, then you might have ~~600 ohms. Anything much less is dubious.

Did you change the opamp? It seems unlikely that a bus amp would get some external zap which would degrade the input transistors, but stuff happens.
 
Thanks for your response PRR.

I did try several replacement op amps, but it made no difference.    I'm not sure what reading I should get to ground on the bus, but since the left channel is fine I can easily make a comparison tomorow.

On other thing with this desk, which may be part of the problem is that all the aux outs have a spikey (i.e non pure sine wave) sounding mains hum. 

If there is some sort of short on the bus, I guess yanking the channels a few at a time is the way to go ?
 
John

Sorry it's been a long day & my typing is not brilliant.  By "since the frying goes when I deck the fader the frying goes."  I actually mean when I pull the fader down in other words the wiper goes to ground (or the deck - English expression) then the frying disappears.  This is why I believe it's something about the summing area.   I also managed to disconnect the bus from the summing amp input  & the frying also disappeared, so I also believe that the problem is either on the bus or something feeding it.
 
OK my answer holds... that's what I guessed.

BTW if the short is on the other side of a cap you may not be able to measure with DC ohmmeter.

Yes, yanking channels one at a time could identify where the short is...

JR
 
I started to pull the channels & it was the 1st one I tried, which is 8 extra channels on a single strip.  My mate got the channel from ebay & doesn't use this much so I'm going to recap it & give it a good check over.

Thanks for your advice again.
 
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