power amp out to balanced line- HELP ME TO HELP A FRIEND!!!

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pyjaman

Well-known member
Joined
May 31, 2007
Messages
295
Location
Poitiers,France
Hello, gentlemen.
Here the deal:
a friend of mine who works as a PA system engineer asked me to imagine and build a gizmo for him that could take audio signal at the output of a 4KW PA amp, attenuate and convert it into a balanced line level for FFT analisys purpose. Don't ask me why. ultra low noise is not a problem for PA use, but very flat frequency response, and very low disortion are important for his purpose.
He needs it for the very next days, so we're in a hurry.
It has to be simple also, because I don't have time to build and test something complicated, and despite my learning efforts, my knowledge is somewhat limited (I'm an average live tech and DIYer, not an electronic engineer).

I was thinking of a 1/200 resistive divider (as the amp can output about 300V max) in parallele to the load, made of High value resistors (to avoid any loading disturbance to the amp/speaker), followed by a good FET ultra low distortion opamp, (with offset trim?) , directly coupled to a THAT balanced line driver.
Normally, no DC is present at the amp output.

Maybe there are problems I'm not aware of, or maybe it could be much simpler, that's why I'm begging for your help. Idealy, it would be smarter if powered by the console phantome, but discrete design is far beyond my knowledge.

I'm aware that's not one of the most exciting topic of the year, but my friend is a great guy, and I owe him a lot, that's why I'd really like to be able to do it for him.

But I won't be able to do it in such a short time without any help.

So, any inputs from the good souls here?

Thanks,

Laurent.
 
quote (myself) :
"Don't ask me why".
;D
I think he wants to be able to compare different amps/speaker configurations and isolate amp response at this spot of the signal chain from the response of speaker/room .

He is in R&D stuff concerning PA systems, and didn't have the time yet to fully explain the "why", excepted he NEEDED it.

 
pyjaman said:
Hello, gentlemen.
Here the deal:
a friend of mine who works as a PA system engineer asked me to imagine and build a gizmo for him that could take audio signal at the output of a 4KW PA amp, attenuate and convert it into a balanced line level for FFT analisys purpose. Don't ask me why. ultra low noise is not a problem for PA use, but very flat frequency response, and very low disortion are important for his purpose.
He needs it for the very next days, so we're in a hurry.
It has to be simple also, because I don't have time to build and test something complicated, and despite my learning efforts, my knowledge is somewhat limited (I'm an average live tech and DIYer, not an electronic engineer).

I was thinking of a 1/200 resistive divider (as the amp can output about 300V max) in parallele to the load, made of High value resistors (to avoid any loading disturbance to the amp/speaker), followed by a good FET ultra low distortion opamp, (with offset trim?) , directly coupled to a THAT balanced line driver.
Normally, no DC is present at the amp output.

Maybe there are problems I'm not aware of, or maybe it could be much simpler, that's why I'm begging for your help. Idealy, it would be smarter if powered by the console phantome, but discrete design is far beyond my knowledge.

I'm aware that's not one of the most exciting topic of the year, but my friend is a great guy, and I owe him a lot, that's why I'd really like to be able to do it for him.

But I won't be able to do it in such a short time without any help.

So, any inputs from the good souls here?

Thanks,

Laurent.
No opamps are required, only a passive network.
I'm going to do soon a schematic.
I'm busy with the opamps but I will find 5 min. for post it.
Pier Paolo
 
thanks A LOT ppa!
I was just thinking off a well calculated +/-50db balanced pad.
Could it be as simple as that?
I was telling to my self: Why the hell would I need a line driver? We have WAY more current than needed...

I'm waiting for your drawing.

P.S. : The APPxxx OPamps look very attractive, (as their prices, too...)... very tempting.

Laurent.
 
it's a pad, regarding resistors, for 43dB of attenuation they are the following:

R1=R2= 22 kohm 5% 1W
R3= 330 ohm 1/4W


for 50db
R1=R2= 47 kohm 5% 1W
R3= 330 ohm 1/4W


zeners= 4.7V 1/2W

the earth chassis should go to PIN 1.

Attention with the high voltages! all must be well done according all electrical safety rules!

thanks for the compliments to my opamps.

Pier Paolo

PS: all resistors must be for 1kV
 

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