Dumb question about SSL master buss amps

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MaxDM

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I was looking at schematics of an 80’s SSL.

I am not sure about the operating levels but I saw what at first glance seems to be a diode clipper, probably to protect a the discrete differential front end from overload.

am I correct?

If so, does that mean that when the buss clips it’s primarily diode clipping?
 

abbey road d enfer

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I was looking at schematics of an 80’s SSL.
Can you post a picture or a link? My crystal ball is at the garage for repair now.
I am not sure about the operating levels but I saw what at first glance seems to be a diode clipper, probably to protect a the discrete differential front end from overload.

am I correct?

If so, does that mean that when the buss clips it’s primarily diode clipping?
The only way a virtual earth "bus" can clip is when the active stage runs out of juice. In a VE summing amp, the bus voltage is nil, as long as the linear operation is not exceeded.
In case of clipping, it is possible that some significant voltage happens on the bus.
 

gyraf

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Aah well (there's one hell of a lot more in a ssl4000 than this schematic, this is the quad bus mix amp section)

If you mean D8/D9, then they're there to protect the LM394 transistor array against excessive differential voltages, that would possibly mess with its precisely matched spacs..

/Jakob E.
 

MaxDM

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Aah well (there's one hell of a lot more in a ssl4000 than this schematic, this is the quad bus mix amp section)

If you mean D8/D9, then they're there to protect the LM394 transistor array against excessive differential voltages, that would possibly mess with its precisely matched spacs..

/Jakob E.

Figured as much, but do you think that it would be possible to clip the quad summing amp on peaks, as is, or is that something that would rarely happen?

Does the main L-R buss use a different type of circuit?

I am curious about the overload characteristics of the board, in use, so I'd like to know more about the summing and line amps on the master buss
 

JohnRoberts

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The clamp diodes across the low noise LM394s prevent them from zenering during turn on transients or whatever... If a low noise base emitter junction is reverse biased to roughly 7V the junction zeners (thats how zener diodes are made). If low noise devices are allowed to zener they get noisy. So clamp diodes like that are pretty common in low noise design.

JR

PS; Over time low noise devices that get zenered will heal somewhat (self annealing), but best practice is to never allow them to zener.
 

abbey road d enfer

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Figured as much, but do you think that it would be possible to clip the quad summing amp on peaks, as is, or is that something that would rarely happen?
I already answered that. The voltage across a VE mixer input is close to zero. Actually it's the output voltage divided by the Open Loop gain of the summing amp and multiplied by the number of stems attached to it. mere millivolts worst case. The diodes have no role in normal operation, as gyraf and JR explained they are here to protect the input transistors from turn-on/off transients.
 
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