G9 noise and hum problem

[Wolferl]

Hi everybody!

My new G9 Mic Pre is nearly finished but there is a hum and noise problem which I could not solve yet by myself:

My G9's attributes
2x ECC82 EI low noise & hum selected
2x ECC82 JJ low noise & hum selected
OEP transformers shielded
HT buffered with 2 x 470µF / 400V BC (the second one! First I had 2 x 100µF)
Everything works fine - no overheating of ICs,...
Ground concept is like many here built (mass star point at XLR in,...)

G9 operating mode:
gain and output at minimum position
microphone is connected
Output is connected via XLR to a little soundcraft mixer

Noise and Hum?
are very solid at both channels :sad: even when I change some gnd lines, put a DI-Box between the output and the mixer, lift the ground, change the tubes, short the input, is on both channels,...
The level is about -60 dB or smaller but very good audible and that gets on my nerves....

So what have I done till now?
I measured the resistors, check the capacitors visual, check some knot points (voltage, resistance to gnd, ...) but all seems to be good.

So have anyone an idea? Or maybe a similar problem and solved?

Thanks and regards!
Wolferl

 

[radiance]

[quote author="Wolferl"]
Output is connected via XLR to a little soundcraft mixer
[/quote]

Is the shield (pin 1) connected on both ends? If so, try cutting the shield on ONE end.....

 

[Wolferl]

I've already lifted the gnd - with a DI-Box. No influence. :roll:

 

[radiance]

Sorry, should have read your post more carefully...

 

[kdawg]

Are your front panel 1/4" input jacks isolated from the chassis? Since all signal routes through those, you may want to try that...

-kdawg

 

[maxime]

just a few questions:
did you test your device alone unconnected to other pieces of gear?
is your building ground consistent?
what's the type of noise ? 50Hz harmonics? no pops? no hiss? is it the same on each channel ?
are your front and back jacks of the isolated type (from the chassis, you can ohm it)?
are your regulators isolated from the chassis?

hope these help
maxime

 

[ioaudio]

did you reference heater supply?

 

[Wolferl]

yes, my front panel jacks are isolated.

I did test with and without any gear connected - result is the same.

Ground concept is metal box with star ground point connected with the chassis XLR Pin1s and AC earth line.

The noise has two components: 50Hz with harmonics and an overall broad band noise. And that with zero output gain and lowest input gain. So it ist possible that my problem is the srpp part maybe?

My regulators are isolated.

Heater supply is connected to gnd and almost ripple free (I have a oscilloscope).

 

[maxime]

hi
I'd rather consider myself a novice in electronics, but here's what I'd do:
unplug the pcb in/out connectors, see what happens (measure the outputs at that point)..
try to shortcut the instrument in/out directly at the pcb (shortcut it before it leaves the main pcb at the 5 pcb solder points level )
that point is high ohmic therefore more prone to noise pickup
did you ground the instrument jack or not, shield that wire?
unplug 1 tube (disconnected), see what happens, to track any source of noise.
did you track the AC power lines, is it wired cleanly, crossing clearly any audio cables? power transformers far from input transformers? AC line behind the power trans?

best wishes
Maxime

 

[Wolferl]

Thanks for advice!

When I unplug the input stage tube the noise and hum remains - so the srpp stage is the source of noise. I could even connect the middle pin of the poti to gnd (same as turn it to full left position) - the noise and hum remains.

My AC-Power lines are shielded and even drilled (was a special cable I found). Moving them have no influence on hum and noise.

Moving the transformers etc. have no influence. I have checked all resistors and capacitors in the srpp stage and even at the input stage - everything seems to be ok.

Next step is to exchange the TL783 + Zeners... but I expect not very much because the measured ripple voltage on HT is very low - about 4mV.

 

[R. Weiss]

I know if you aren't using shielded cable between the control boards and the main, you may experience some hum at higher gain stages, but in your case something else is most definately contributing. Just a thought in case you solve this problem and still get a bit of oscillation or hum at higher stages.

Ryan

 

[Wolferl]

Thanks Ryan. I will shield my connections to the front pcb in the next step. I have high oscillation (not audible but see it on the vu of my mixer) on the right channel in the last two steps of gain. But first I have to solve the problem of my srpp stage noise... :roll:

This kind of noise is not very loud - but disturbing at low level mikes like ribbons, because of the higher gain of the mixers input channel...

 

[Wolferl]

In the meantime I did some mesurements and some shieldings. I shielded the lines to the front instrument plugs and even the supply lines from the transformers to the main pcb!

Results:
No more Oszillation on both channels :razz:
About 3dB more S/N
BUT: The basic hum remains. I did some frequency analysis - it is 100Hz + harmonics up to 2KHz. Max Level at 100Hz is about -80dB and this is very good audible... :roll:

What I don't understand: My HT is very clean DC - max. 4-5mV ripple. I have 2x 470µF/400V C's in. Why for heaven's sake is there so much hum???

 

[gyraf]

Magnetic inteference from the power transformer to the in/out audio transformers?

Have you tried moving around the power transformers to see if that changes the hum?

Jakob E.

 

[Wolferl]

Hallo Jakob,

nice to meet you here! :grin:

This kind of basic hum is very stable even when I move the power transformers outside of the box. No influence on level - unfortunately.

It changes a bit by exchanging of tubes - I have two pairs of ECC82's each selected for low noise and hum. One pair is from EI one from JJ.

Could this caused by the heater? Or by the 470µF AC to ground C's in the SRPP-stage (maybe to high esr or something)? Thanks!

 

[Campistron]

[quote author="Wolferl"]The basic hum remains. I did some frequency analysis - it is 100Hz + harmonics up to 2KHz. Max Level at 100Hz is about -80dB and this is very good audible...
I have two pairs of ECC82's each selected for low noise and hum. One pair is from EI one from JJ. [/quote]
Very strange... I have the hum on my G9 (100Hz@-80dB) with the same tubes, JJ (input) & EI (SRPP), and instrument input is also wired with shielded cable.
I have no idea about the origin of this noise :roll:

Edit : :idea: maybe a ground problem : C14 & C15 (220µF) are wired with a common ground cable (~20cm), so I'll try (next week) to wired them with two separate ground cables. Wolferl, do you have the same wiring configuration with C14 & C15 ?

Robert.

 

[Wolferl]

Hi Robert,

yes, I have the same wiring configuration with C14/C15 and is has about 10cm lengh. But I don't think that this is the reason for this kind of hum, because in an early version of my g9 I had this capacitors very close to the pcb - about 2cm. But give it a try! I'm curious about it.

Wolfgang

 

[Wolferl]

... one thing:

To make shure that I (we?) have no tube problem I exchanged the tubes of one channel with two tubes from another gear - same hum...

One thing is really good on searching for failures: You learn more and more about tube circuits... :wink:

 

[Wolferl]

Update:

In the meantime I changed TL783 the diodes around it and did some additional measurements. I also disconnected the front pcbs and connect the signals (transformer out +/- ) with short bridges for testing if the front pcbs has any infuence - this kind of basic hum remains.

My G9 sounds very very nice but -85dB@100Hz hum without any gain is too much...

Any other ideas? Robert?

Wolfgang

 

[Campistron]

I have no time this week, I'll work on the G9 hum problem next week.
Robert.