Shunt resistors

[raysolinski]

On my mic input module (pm1000_) that I racked there is mention of a mod to change the shunt resistors for 100K to 680K to improve the high end response..I am probably wrong but wouldn't one reduce the resistor value to increase the high frequencies? Or is this some inverse relationship I don't quite grasp...

Thanks again...

Ray

 

[Guest]

[quote author="raysolinski"]
I am probably wrong but wouldn't one reduce the resistor value to increase the high frequencies? Or is this some inverse relationship [/quote]
Relationship of load resistance to high freq response is not so
straightful.
They plays main role input wires capacitance.
Optimal values can be figured in dimensionless form, it
is some X transformer capacitance, or X series inductance + winding
resistance.
If you have short mic cables, you can enlarges loading resistor.
That may cause some improvements in middle band, and can affect (maybe negatively, hights; or may underdamp resonancy, which can pe pleasant).
If you have long cables, 100 k load can be large, and use 50 k, or
extremly 3 k.

Or, you can make resistor bank (can be outer, between two XLR connectors) and change resistor to obtain good sound. But
I still mean, that you results in somewhat like 10 k.

xvlk

 

[raysolinski]

I should clarify that these are shunt resistors following the eq section of the circuit..not input or load shunting resistors..the theory was they rolled off the high end a little to protect PA horns since these were live gig consoles...

Ray

 

[CJ]

A resistor on the secondary of a x-former will roll off the resonant peaks, perhaps this is what is going on. Hard to tell without schematic.

 

[tubejay]

Here's the schematic:

http://home.new.rr.com/lordjimbo/PM1000E_2.pdf

I thought I'd bump this, as I'm interested in possibly doing this as well.

Thanks,

Jay Lison

 

[CJ]

I would say try it and see. Shouldn't take long to change one resistor.

 

[PRR]

> change the shunt resistors for 100K to 680K to improve the high end response..

WHICH "100K"?

Those ones inside the high-pass filter switch? Why would they change the high-end? They are in shunt with 3K3. They affect the low-end... somewhere below 0.1Hz.

> Shouldn't take long to change one resistor.

Yeah, but it does not take long to clobber-up a perfectly good and irreplaceable PCB with ill-advised "tweeks".

 

[Analogique]

[quote author="CJ"]A resistor on the secondary of a x-former will roll off the resonant peaks, perhaps this is what is going on. Hard to tell without schematic.[/quote]

On the ADM780 schematic I have it shows a 620 ohm resistor across the secondary of the output transformer. I'm assuming the tranny load is a required part of the design response and should be included. I'm going straight into a DAW btw. Since my schm is hard to read I ask, is this the correct termination value ? thx!

 

[raysolinski]

OK..looking at the schem this mod makes no sense...so I was probably fooled by some placebo ear effect ..not the first time!

Ray

 

[PRR]

> On the ADM780 schematic I have it shows a 620 ohm resistor across the secondary of the output transformer. I'm assuming the tranny load is a required part of the design response and should be included. I'm going straight into a DAW btw.

Apparently this transformer gives best response with a load.

If you were driving a true 600Ω load, you would remove that resistor.

Since true 600Ω loads are nearly extinct, and your DAW is surely 10K input, you need the 600 ohm dummy load. (That comes out to effective 10000||620= 584Ω load, but transformer loading is not that critical.)

 

[Analogique]

Thanks very much PRR !!

... J