annoying HUM/noise at my stereo PULTEC eq

[matthias]

Hi !

I have an annoying hum/noise in my stereo pultec, :sad:
it's around 80-100hz and comes up after a few seconds, when the tubes had lighten up...
so I think it's not that general ground/loog hum problem... :?:


http://www.soulset.de/forum/pultecfreq.JPG
http://www.soulset.de/forum/pultechum1.JPG
wavefile: http://www.soulset.de/pultechum.mp3 (normalized)



the pin1 of both input and output xlrs are connected to ac ground,
to the groundplanes on the board and to the chassis...
all pin1s of all xlrs input and output are connected together


both trafos are packed in grounded aluminium boxes as you can see in the picture to prevent magnetic interferences...


http://www.soulset.de/forum/pultec-hum1.jpg


I also disconnected both filterboards and bridged the in and out leads of the power boards
with a resistor to exclude that the noise is coming from the filterboards.



so do you have any idea what could be the reason for this strange hum...

maybe bad tubes (valvo e88cc nos) , bad power caps (220µ , 400V from german supplier Reichelt) ??

my main trafo (12 V, 50VA ) makes a little noise itself, when it gets power ...
maybe it is broken and occurs this f***ing hum-noise...

should I try other tubes ??


:?: :?: :?:

thank you

mat




Jakob wrote in another thread:

Check for ripple in your HT voltage. Maybe too small a resistor between the two psu caps?

Jakob E.
.


the resistor between the 2 big power caps should usually be the 3k3 2W resistor...
I changed it to a 12k to reduce the voltage to 250V , it was too high due
to the tolerances of both power transformers...
all other values are as shown in pultec.pdf
the 2nd board gets the ht voltage after the 12k resistor from the 1st board, but this should be ok??

 

[DrFrankencopter]

I wouldn't rule out a bad ground scheme. Does the noise go away when you power the unit down as well (I.e. tubes are still hot, and still have a little plate voltage from the PSU caps)? Try powering down by removing the mains cord from the wall.

I had grounding problems on my Pultec, and it was a matter of attaching the mains ground, chassis ground, and SRPP amplifier ground to one spot (and one spot only).

Hope this helps.

Cheers,

Kris

 

[rafafredd]

It could be your xlr´s pin 1.

Try disconnecting the pin one of the output XLR´s and use a little as possible wire to connect the input XLR´s pin 1s to the chassis DIRECTLY, as close as possible to the conector. I usually use the same screw that fixes the connector there.

Sometimes you have to play with your grounding schem to get it right. Go for it! It´s a little pain when you have to do it, but it´s also fun when you get it right!

Just my 2 cents!

 

[matthias]

@Kris: when I switch off: the deep hum is gone immediately and the noise stays a few seconds... as long as the tubes are "hot"...
when the tubes are cold everything is quiet as it should be...
so i have no probs with my mixer...

@rafafred : I will try it!!

should I connect the chassis to the ac ground ??

my xlr connectors have a pin thats mounted on the outer metal surface of the connector that touches directly the chassis at the drilled hole... that's the shortest way...


mat

 

[cayocosta]

Have you fully assembled the pultec yet?

If not, make sure your front panel is grounded to the rest of the chassis and that all controls are fastened to the panel.

My la2a had a bad hum while the front panel was off the chassis. Once I clipped a lead from the front panel to the chassis it disappeared.

 

[Mark Burnley]

Hi,

To start off with find a "ground free" amplifier/monitor. Have you got a small battery-powered practice amp? If so, make a lead to connect from this amp input to the output of your EQ.

If you have a jack input to the amp, connect Pin 2 XLR to the Tip of the jack, and Pin 3 XLR to the Shield of the jack. Use as short a piece of wire as is possible. (You may need to connect Pin 1 to Pin 3 to provide a screen for the amp- but you can often get away with it for a quick test)

Then, power up your EQ and switch on the amp. Make sure that you have no other external connection to the EQ- remove it from any rack and place it on a non-grounded bench (wooden table, rubber mat etc).

This way, you know that you will be listening to the output of the EQ without any external ground problems. Then, connect an input cable from a signal source- mixer, DAW etc. Monitor the output using the mini amp again and see if there is any difference in hum level.

If all is quiet with only the amp connected, then it will be a grounding problem. If it hums even with just this connection, then the hum problem lies within the circuitry itself. Ground hum isn't always 50/60Hz, if the ground path passes through a power supply rail carrying DC, the audio path can be modulated by the 100/120Hz rectified-AC frequency.

As Rafa says, have an investigate! But don't remove any safety ground connections from the mains input to the chassis itself.#

Mark

 

[matthias]

I tried some things:

connected nothing but the power chord and my headphone to the eq...

result: the hum and noise comes from the circuit...


I cutted all connections at pin1 of the input and output xlr

and connected the groundplane of the tube boards to the chassis...

result: hum was gone ... signal passes thru...


is that ok so ??

or do i have a problem when nothing is connected to pin1 of both xlr...???



thanks
mm

 

[Flatpicker]

I would hook pin 1 of the input and output back up, one at a time, and see if it?s coming from one (and if so, which one) or both. On John Hardy?s pres, pin one of the input is permanently connected to chassis ground while pin one of the output is switchable from analog ground to no ground at all. Seems like there was a pdf file at Jensen about this, but I can't find it at the moment.

 

[rafafredd]

just use short wires to connect pin one of the INPUT XLRs ONLY directly to the chassis and you may be fine. This same thing was causing problems on my Langevin4000 mic pre/EQ project.

 

[haima]

there is a good rane note which covers this ground (pun..) i can't remember the number. basically explains the way that all modern equipment should be wired...

 

[CJ]

Looks likw pwr supply ripple from over here.
Why?
It's 120 cycles, not 60, which means B+ hum.
Experiment with grounding and ungrounginf stuff to get the lowest aggravation.
cj

 

[chriss]

cj,

since matthias runs the unit in germany the frequency will be 50 or 100 cycles. :idea: 50 Hz / 230 V.
i know this doesn't matter in the discussion, but i wanted to say anything! :shock:

Chris

 

[CJ]

ok, that makes more since, because the chart he has up looks closer to 100 hz.
You could try bridging the filter caps with a big beefy 450 volt 220 uf if you have one around.
That is how I test for inadequate filtering.

 

[ChuckD]

Hey Mat,

What was the final solution to your hum problem?
I could be experiencing the same thing.

I'm goning to try and isolate the problem's frequency range and make sure iut's not sothing outside of the box.


Thanks

Chuck

 

[CJ]

Some of the Pultecs had the pwr tranny rotateed for least hum, so give that a try if you have not already fixed it.
Torroid or regularb transformer? In or outside the box?

 

[ChuckD]

My unit photo is here :

http://www.groupdiy.com/index.php?topic=751&highlight=pultec

Mat method he cliped the ground wires from the signal path and that worked. This would mean installing a ground lift? by being able to switch the Signal XLR ground into AC ground and out.

What did you mean by rotating? I mean I torriod is a circle?
Can you explain

-Chuck

 

[CJ]

sounjds like you nailed it so forget the transformer.
I have a ground lift switch on both myPultecs and they seem to run quieter with the grounsd lifted.
cj

 

[matthias]

@ chuck:

I made a better star ground scheme, lifted pin1 at both output connectors ...

with all that I get the hum down to -66db ´

but this is not good enough for me....

I think the problem is that the power supply and the tube / transofrmers are too close on the one pcb ... and in my unit are too much unshielded cables running around...

If I have finished my other projects I will go back to the pultec, throw that tube stuff away and probably build michael kruschs pultec layout (without tubes)

mat

 

[ChuckD]

Mat,

I'm not sure I understand. You lifted the Output grounds ??
And what is this new star grounding scheme? Can you show a picture or diagram? That would be a big help.

-ChuckD

 

[nik--]

hi matthias,

I just finished two gyraf pultecs, with gustav's boards, in and out transformer and two torroidal trafos 30 VA each (220:12V)
and there is no noise -they are quiter than my ssl comp ! so please do not think that jakobs design is bad.

i measured a noisefloor of -90 db !! (but the unit was connected with long cables in my studio - probably it has lower noise than 90 db)
and this little noise is just no hum it's more like hiss (hi frequency).

what is realy important is only to connect ground to the input xlr to chasis (with massive wire),
use high quality shielded cables inside the unit (e.g. sommer cable "the source" relatevely cheap for high grade cable but very gooood)
and use only torroid transformers.

I tried unshielded wire but it was terrible hum and noise !!


I placed the transformers on the right side (in the back) and the board with the powersupply and the audio transformers so that the audiotransformers pointed away fram the 220 v trannies on the very left front side close to the filter pcb.


I hope this helps you...

nik