All things Green Pre

[DaxLiniere]

Hey Flatpicker,

That's with the gain set to maximum (66dB) with an unterminated input. Output connected directly to HEDD192 A/D, measuring in RME totalmix.

This seems insanely noisy to me. I tried my friend's FF ISA428 the other day and at 66dB gain you can't hear any noise. Otherwise the Green pre kicked it's arse. (Much wider freq response for starters).
Normally I wouldn't need this much gain, but I've got an old 30ohm ribbon mic that I'd like to use. It worked well with the 428, low noise too.

The noise floor does drop slightly when the input is loaded. I checked that with a Beyer M69 (dynamic).

 

[Yoannv]

Does the voltage to the +15 & -15v supplies also go down a lot when switching on more than 1 phantom supply?

Even if I turn on all phantom power, and plug in a microphone, these voltages doesn't move : +16,13v and -16,25v .

I don't if it is a good or a bad new for my Leds, but I am ready to sacrifice them if necessary ! :wink:

yoann

 

[Flatpicker]

[quote author="DaxLiniere"]This seems insanely noisy to me. I tried my friend's FF ISA428 the other day and at 66dB gain you can't hear any noise....[/quote]
Yes, as long as you are truly comparing apples to apples... Yuck! You could try the 2SC3329 used on the Supergreen, however the pin-out is different, so you'll have to kludge them in there. You could also kludge in an LM394, although that'd be a little more expensive.

 

[DaxLiniere]

So you really think it's those transistors that are making it so noisy?

"kludge" - What a beautiful word

 

[DaxLiniere]

I'd be happy to kludge a couple 2SC3329 but I can't seem to find them at RS or Farnell (Newark).

Found the LM394 - WOW! What a cool little device. Unfortunately they're almost 200 times more expensive!
Also, I think that any benefit I'd gained from those supermatched transistors would be lost in having fly leads to join them to the board

 

[SALSA]

You can find the 2SC3329's at lobtron.de

Cheers,
Mike

 

[peterc]

I don't if it is a good or a bad new for my Leds, but I am ready to sacrifice them if necessary ! Wink

The only thing I can think of is to increase the size of the caps in the voltage doubler. Try 220uF, not sure that it will work though.

Peter[/code]

 

[Flatpicker]

[quote author="DaxLiniere"]So you really think it's those transistors that are making it so noisy?[/quote]Maybe not, but you need to isolate the problem and that's as good a place to start as any. Also, try swapping out the opamps or using different types all together (MC33078's would be cool to try and should work just as good).

 

[Yoannv]

The only thing I can think of is to increase the size of the caps in the voltage doubler. Try 220uF, not sure that it will work though.

Are these capacitors the three 47 µF caps ? (two at 63V and one at 100V)

I think it could be interesting to try this...

Just for my knowledge, what is it supposed to do ?

I think I'm gonna do it, I willl report my results.

Thank again,
Yoann

 

[peterc]

Are these capacitors the three 47 µF caps ? (two at 63V and one at 100V)

Those are the ones. They should provide a more stable charge pump.

Peter

 

[Yoannv]

Hi all,
So, I have change these capacitors and...

IT WORKS !!

Even If I turn on all phantom Power, and plug Microphones, no hum !!

Thanks a lot Peter ! :thumb:

In fact, I have notice a very low Hum, present when using 48v, but it doesn't get up when turn on other phantom power.

It is very low, I 've measured it at -55db FS, at maximum gain, and it's 60Hz hum (strange, because here the electricity is at 50hz...)

For me it's Ok, but, if someone wants to try this mod, think about it.
But, I'm not really sure this little Hum was present before changing PSU caps, but I think it was not.


Thanks again, Peter.
:guinness: :sam:

Yoann

 

[peterc]

Yoann

Can you measure the ripple (AC) voltage on the 48v line with all the Phantoms on? Also measure the ripple on the +-15v supplies as well, be interesting to see what it is. Measure before & after the regs, use a scope if poss.

What were the first 2 caps you used in your PSU on the +-15v lines?

If you are using 1000uF, try 2200uF to get rid of the hum.

Peter

 

[Yoannv]

Hi Peter,
As I was not shure I had perfectly understand where to make the measures, here is a pict with the voltages I picked up :

Here what is on this pict (in case it woudn't be present anymore on server in the future).

? Phantom Voltage before the reg :
- without phantom power on : 63,3 v
- with phantom power on on 1 board : 62,4 v
- " " " 2 boards : 61,6 v
- " " " 3 boards : 60,9 v
- " " " 4 boards : 60,2 v

? Phantom Power after the reg : with and without phantom power on = 47,8v

? AC Voltage on the 48v line (input of the PSU, after the transformer)
- without phantom power on : 16,61 v
- with phantom power on on 1 board : 16,58 v
- " " " 2 boards : 16,56 v
- " " " 3 boards : 16,55 v
- " " " 4 boards : 16,53 v

These voltages are unstable, the results are an average.


? AC Voltage on the +/-15v line (input of the PSU, after the transformer)
More instable, I just give 2 measures :
- without phantom power on : 16,68 v
- with phantom power on on 4 boards : 16,63 v

? +15v voltage before the reg :
- without phantom power on : 21 v
- with phantom power on on 4 boards : 20,9 v



I don't have a scope, so all the measures were done with my multimeter, I hope it is accurate enough.

What were the first 2 caps you used in your PSU on the +-15v lines?

If you are using 1000uF, try 2200uF to get rid of the hum.

All the caps I used are exactly the one which are give by the BOM you send when ordering PCBs.
The only caps I change, so, are the three in green on the pic (now, it's 220µF).

The 2 1000µF you think I should change are the 2 caps in blue on my pic ?

Give my a few days to change this, and I post the results.

Yoann

 

[peterc]

The idea is to measure the AC (ripple) voltage that is sitting on top of the DC. This gives a better idea of how well the PSU is regulating the supplies, & if your ripple is high, it could cause hum in the preamps.

So instead of using the DC scale, change your meter to AC volts & measure again. The reading before the regs will be in the volts range (approx 1-2v) & after the regs should be mV (10mV or so)

Peter

 

[Yoannv]

Ok, I understand now, I hope these results are the good one.

I don't know if my multimeter is accurate enough (the smallest range for AC voltage is 2 v !), but with or without phantom power on, I have about 3mV before and after the reg.
The measure take a few second to stabilize.

On the +15v lines, with and without phantom on, I have 2mV after the reg.

Before the reg (on the spot shown in my last pic), I have 82mV without phantom power, and 90mV with all phantom power on.

I don't know if it is what you expected , but I hope it's enough to make sense for you.

Yoann

 

[peterc]

I'll have to take some measurements with my own meter, but it sounds fine.

Peter

 

[enthalpystudios]

well, i had my green's working, passing audio, etc, just fine about a week ago. I've been finishing the front panel, and went to power them up just now, to test out the power on lamp.

Anyway, before I could do anything with the lamp, my 2nd and 4th channels started smoking, 4 10r resistors just "left" of the tl074 (looking at the V14-ALL pcb layout file). These 2 channels have no tl074, the other two do. (looking at the V14-ALL pcb layout file)

Any obvious ideas? I can't seem to find where this has happened before.

Thanks, Billy

 

[peterc]

Check the electrolytic caps associated with these resistors. It sounds like there is a short circuit. Measure the resistance between +v & gnd, & also between -v & gnd. Shorts?

Peter

 

[enthalpystudios]

i'm going to keep looking to make sure there isn't something else going wrong, but there was a short in the far front corner of the board, from the red led to the 10k resistor that stretches behing the row of leds.

Would this cause such a problem? Hopefully there isn't another one, but I'm going to go keep looking.

thanks peter,

billy

 

[enthalpystudios]

well, now i'm totally lost

with the psu removed, not hooked up to any pre's or the transformer, i'm getting resistance/shorts between all terminals of the psu, +/-/0/P

thats totally wrong isnt it?

i need to go think this one through some more.

billy