All things Green Pre

[ENS Audio]

One basic question that I have in regards to when Im measuring voltage lets say from an IC, Transistor, e.t.c....Obviously, from what I understand (just reevaluating some very basic info incase Im wrong) I use the Positive (+) for probing and the Negative (-) goes to GND that is if Im measuing per rail or "input"?

BUT...where Im unsure is WHAT GND Reference does the Black/Negative (-) Probe connect to????  Does it go to the PWR supply GND?  IC GND (when probing Pins)  OR do I use Star GND Node as a Reference point?  I know this is such a basic question but I wasnt sure if it makes any difference when Im measuring voltage.


Thanks, and if theres a thread that answers my question..paste the link and I promise not to spam precious server space with my newbie questions :-[

 

[DaxLiniere]

Here's a quick test.

Switch the unit off. With multimeter set to reistance, touch one probe to a gnd point, then touch the other probe to another gnd point. If you have >0 ohms, there may be a problem.

Otherwise, all gnds are gnds!

 

[Hammond]

CTBC550C vs BC550C

I know they are pin compatible and are interchangeable but what's the difference between the two?
I can't find the datasheet of the CTBC550C

Thanks

 

[peterc]

Having used subs like this before (not in the Green tho'), I would say they would be fine as a swap. The original transistors were 2SD786

Peter

 

[Hammond]

Is it worth pay 15 bucks for the original 2SD786?
what ar the changes in sound?
are there any better alternatives than the 550's or the 550's are the best?

Thanks

 

[peterc]

I had some early versions with 2SD786's in them, I could not tell any difference between them & the 550's. If you do go with the 2SD parts, note the different pinout.

Peter

 

[Hammond]

Ok guys
Finished a 2ch Green pre v14 and everything works but on one channel when i turn on the hpf the sound disappear there's only little hum.
Any suggestion where to look first?
Thanks

 

[peterc]

Sounds like a break in the signal chain. Check all the components have been soldered & check all the values in the HPF circuit are correct.
Check the switch is not going open cct in that position
Peter

 

[Hammond]

OK it was a bad 5532 , now the hpf works.
One more question about star ground :
I've connected all pin 1 to the pin attached to the body of xlr which in turn go
to star ground thru the case, and i have the pre's ground go to PSU.
Do i have to connect the PSU ground to star ground? and if yes is it best where the transformer
connect to the PSU or at the output of the PSU?

Thanks

 

[Hammond]

Anyone knows where's best to put a led for the Phantom?
Thanks

 

[jcharles00]

I'm finally getting to revisit my non-operational greens after a long span of time.. neither work, but i'm tackling them one at a time. The first one had, i thought, a suspect cap, and a bad op amp, so i replaced those and the resistors that burnt as a result but i still have the same problems (despite my with and without op-amp voltages looking correct) - when i have a dynamic mic connected, some settings of the lorlin cause oscillations.. synthesizer like repeating sweeps. in a couple of positions audio from the mic actually passes but it is VERY heavily distorted, robot voice sounding.

injecting signal at various points in the circuit seems to work and sound fine.

my (probably bad) assumption is that there is an issue in the unbalancing portion of the circuit. I've tried replacing C3 with no change, so I was wondering if anyone had other suggestions before I continue replacing components one at a time.

 

[peterc]

Hammond

Straight after the phantom switch. 48v > 8k2 (1/2w) res > LED > GND

Jcharles

Start tracing from the output back to the front end. Inject a signal just before the output stage, heck both the outputs. & so forth. The heavily distorted sounds sound like overly high gain, check the resistor values around there.

Peter

 

[Hammond]

I m trying to do cmmr setting but something is going wrong, when i tunr the 1k trimmer nothing happens  ???  it's the same on both channels
Here is an image of how it is looking on the scope.
CH1 : output of pin 1 of middle 5532
CH2 : signal before the two 100ohm resistors
http://www.imagebam.com/image/e9d9e522660483
CH1:Max voltage 900 mV   Min voltage 390 mV   Pk to Pk voltage 510 mV    RMS Voltage 630 mV   Mean Voltage   650mV
CH2:Max voltage 1 V    Min voltage -1.25 V    Pk to Pk voltage 2.35V    RMS Voltage 700 mV       Mean Voltage    -70mV
Here is how i connected to test: Hot of SG to the 100ohm resistors in paralel to the + and-  input
Negative of SG to starground.
Only one question about the scope probes: positive at pin1 of middle 5532  where do i connect scope ground ?

As always thanks to everyone here ;D

 

[Hammond]

So turning the cmmr trimmer don't works at all on both channels!
Where i have to look first?

Thanks

 

[peterc]

Hamond

Not quite sure what your voltages mean, so I'll do a brief description.

The idea is to send the same signal to both the inputs, but because the inputs are out of phase with each other, they should cancel out after the summing stage. This is where you should be measuring.

http://www.mendelt.nl/greenpre_V14/_Green_trim_HPF%20CCT.gif

The signal should be very similar at the outputs of U1a & U1b (ignore the model number), but  out of phase. This gets summed by U2a, this is where to measure as I have described in the stup procedure. The scope probe should go to GND.

The idea is that the trimmer will compensate for the component tolerances in each path.

Hope this helps.

Peter

 

[Hammond]

Hi Peter
I've done exatly like you described and i have identical but out of phase signals at the output of the first 5532
but at the output of the middle 5532 (pin1) i have a signal of about 700 mV and turning the trimmer doesn't change it even of one mV.
What i'm doing wrong?

Thanks

 

[peterc]

I can only think to check all your components. The 1k trimmer should vary the balance of the + & - inputs of that opamp & you should be able to trim the signal down to a couple of mV.

Are you using the 50Hz input signal?

 

[Hammond]

Ok now it works, is 30mV Peak to Peak ok?
Just one more question when i measured with the scope i noticed that putting the ground probe
to GND or not does not changed the measurement in any way, is this normal? 

Thanks for the patience ;D

 

[Hammond]

Ok just finished calibrations and everything seems to work fine ;D
Just one question about the Peak detector DC offset (20k Multiturn trimmer) :
i've set it to -3mV DC, but when i rechecked it i noticed that the voltage changes
if i measure it after a while on one channel just a bit ( -.2.7 ) on the other some more ( -1.1 )
now i have redone it and wait for re measure.
Any suggestions?

 

[peterc]

The 2k trimmer is there to get the DC offset below the threshold of the lowest LED, so virtually anything under 0v will work.
Peter