Green pre CMRR?

I have the doc for setting the CMRR trimmer, but how do I know where to set the output level of my signal generator? Should the pre be connected to the PSU? What should the signal on the scope look like? I'm getting the 100Hz waveform on the scope and the amplitude changes as I fiddle with the output level on the signal generator. Eventually it starts clipping if I turn it up too high. It's not 1 or 2 mV anyway. Help!

Signal generator has to be MIS-connected for CMRR testing. SG ground to preamp ground, SG hot to both preamp hot terminals shorted together. With perfect CMRR this would give zero output (equal signals on both pins of a differential input cancel out). In practice, try 1V input and trim for not-many milliVolts output.

bump

how do I know where to set the output level of my signal generator?

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Like PRR said, set your signal source to a 1Vpp output. It shouldn't be *too* critical what this voltage is, as long as it's in the operating range of the pre.

Should the pre be connected to the PSU?

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Yes, the pre should be powered up and operating exactly as if you were using it to record with.

What should the signal on the scope look like?

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It shouldn't. The goal is to have zero signal coming out of the opamp during this test. That might sound strange, let me try to explain. Your mic cable has two signal wires, each of which are polarity reversed from each other. Any noise your mic cable picks up on the way to the preamp will be added identically to these two wires. After entering the preamp, one of these wire is "flipped", and added to the other one. This does two things: doubles the signal voltage, and *theoretically* cancels all noise. CMRR stands for Common Mode Rejection Ratio. Basically, it is a measurement of how well the opamp "cancels out" the noise. When you adjust the 1k trimmer, you are calibrating the opamp to reject as much noise as possible. In your case, you are "simulating" noise with the signal coming out of your generator by applying it equally to both the + and - inputs of the preamp.

the amplitude changes as I fiddle with the output level on the signal generator. Eventually it starts clipping if I turn it up too high. It's not 1 or 2 mV anyway.

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Don't fiddle with the output level on the signal generator--fiddle with the 1k trimmer. Fiddle with it until you can minimize the signal you see on your scope.


Hopefully this helps! Let us know.

The object is to feed the same signal into the non-inverting (+) & inverting (-) inputs at the same time. The trimmer should be set to cancel out any signal, so that the + input & the - input are exactly balanced.

I dont have the doc with me right now, but I think I said put the gain control in the middle? Make sure the HPF is not in circuit when doing this test.

Peter

I think I have figured out what I was doing wrong. I guess the setup doc is for a later version of the board than I am making. I am doing the V1 board from Peter's site. The "middle" opamp is the one closest to the output side of the board. I had the scope on pin 1 of the wrong opamp.

Here is the image from the scope at the 5mv setting with an input signal of 1v. It gets bigger in amplitude if I turn the trimpot
If I turn down the input the distortion/clipping goes away. Is this what I should be seeing?

Dust,

you should find a spot where it is a minimum. So it will get less, and less, and less, & then start increasing again. Trim so it is at the lowest amplitude.

Peter

Once again, an old thread, I know. But why does this need to be done with a scope? Won't a DMM do the job just fine or am I missing something?

I have a scope and have no idea how to use it yet. So I just hooked up the DMM and got the numbers down to .001 in the 2 setting on the DMM. Am I doing this all wrong?

I think this is all that is holding up the closing of the chassis for me and it's only because I don't understand how to take these measurements correctly. I'd like to learn, but as it seems all scopes are different, there isn't really any help in that regard. I've read part of the Tektronics doc that has been floating around here, but it's a little over my head.

Any Advice?

EDIT: This is the scope I have: http://img.photobucket.com/albums/v371/lanxe/scope.jpg

I only have a cable with gator clips attached to a RCA cable for probes. I don't know what all the settings do.

Matt

Depends on the DMM. Some are very accurate, some arent. Just set it to minimum with the DMM & then check it with the scope once you have sorted it out.

Peter

quick question: Where do i measure the output signal from? Pin 2-3 on the xlr output or directly on the opamp?
which pins on the opamp if that is the case?

I have to do it with my dmm, because i dont have a scope at home.



Thanks!
/J

pin 1 on the middle 5532 opamp.

check this file:
http://www.mendelt.nl/greenpre_V14/_Green Pre Setup.txt

HT

tack!
(thank you in swedish)
What mode do i set my dmm in? So i can get something up and running until i can get a scope.
/Jonas

What mode do i set my dmm in?

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?????

AC Volts

im sorry if i ask stupid questions... :green:

I have a lot of reading up on electronics to do, i know.

I only know how to solder, not much else about these sorts of things.

Thank you for the advice.

Now...where did i put that book i had that explained ohms law.. :thumb:

/J

[Swedish mode]
Fixa en ELFA-katalog, där finns det bra faktasidor.
[/Swedish mode]

HT

ok, it turns out my dad actually has a scope back from the time he built audio stuff that i can borrow..

Yay!

Next time i have a stupid question, ill ask him first so i dont have to bother you smart guys with this...:green:

/J

Hi,
I suddenly ran in to problems when adjusting the cmrr, after changing to two opa2604. the signal passes through without changing amplitude when adjusting the pot. I have followed instructions with the two resistors connected in Y. I managed to adjust it right the first time.
It seems one of the zeners is busted. I tested them (in the PCB)with the diode tester on my DMM with the power turned off. One zener tested 0 volts in the positive direction the others tested between 500-600 mV.
I have checked for broken leads on the PCB.
-what is the minimum power rating for the zeners?
-Is there a voltage schedule for the Green Pre mk1?
-What else can I look for?

Best regards

Sunyata

Well, I wanted to bump the thread, because I just had an idea for CMRR trimming the green.
Constantly it was asked how to do this without the scope...

Please correct me if you think the idea is BS, it is highly appreciated... :razz:
Wouldn't it be sufficient to make a custom cable unbalanced to XLR with the signal to both hot and cold?
(and put a big warning mark on it, as this is pretty useless in any other application :razz: )

Then feed a signal (ANYthing, from signal generator, soundcard, whatever is in the working range) to the pre while it is in a normal working setup, and then trim for minimum volume?
(no special equipment needed at all...kamikaze measurement by ear)

hmmm, just an idea....please comment.

Kind regards,
Martin

YES - that will work very well!

Adjust CMMR trimmer until minimum output is measured - or heard - on the output.

Excellent way of doing it without test gear!

Sounds like it should work OK.

I did 4 preamps just using a multimeter to measure the AC output of the preamp, & when I checked them on the scope, they were all good to go.

Peter