[christophmateka]

I've recently built an MP566 and since a few weeks, I#m getting strong hum at 60-70hz.

The hum is also there if there's no input connected.
Without the module, there is no hum coming from the single slot housing.
The gain and input knobs do not change the volume of the hum.
the output pot changes it.
it's not there during the first minutes when I power on the device.
when directly connected to my AD converter (SSL Alphalink) this hum-noisefloor is at -60dBFS.

Any help is very appreciated!
Merci beaucoup!

Christoph

[JPK]

Could the hum be induced by a power device in proximity?
The input transformer even if shielded with mu-metal can be sensitive to radiated noise.
Is your lunchbox powered by a linear PSU (with a mains transformer)?

[christophmateka]

Thank you for the fast answer.

I've built the housing also from SoundSkulptor (501) - The Power Transformer is more than a meter apart. I don't know about the specs of that one but from the reading of it it says "switching".

SO I guess trying another housing might be my first step?

[JPK]

Yes, this would allow us to find out if the noise comes from the MP566  itself or from the housing supply.

[christophmateka]

Ok I hope I'm gonna be able to do that in a few days - from now I can say, that the hum starts around 7 minutes after powering on the unit - before that it works perfectly. 
I've recorded it for a few seconds and put it as a wetransfer download link here. 

https://we.tl/t-nCujNYa6M1

[JPK]

The frequency is around 70Hz so it is probably not mains induced.
Try keeping the unit ventilated to find out if it is linked to the temperature.

[christophmateka]

So I tested at another Housing - the same effect happens. there's also some microphonic noise at the tube. 

If I were to change tube - which model should I chose?

[JPK]

If I were to change tube - which model should I chose?

ECC81 or 12AT7.


Every tube is microphonic to some degree. The ECC81 is considered not very microphonic.
Regarding the hum, do you have access to an oscilloscope? It would let you find out where the hum comes from. Is it the tube, or U6, U5, U2?

[christophmateka]

I may borrow an osci soon. Meanwhile, i looked at your picture in the assembly guide and then at the preamp and I realised I may have mounted the output transformer upside down in my build. could that be the cause of my troubles or could it have ruined any other component?

pic is https://drive.google.com/file/d/1NDLS8cf43oms7t91bm7AYNvzPSeoEEoV/view?usp=sharing

[JPK]

It is not possible to mount the transformer the other way round. It is only EDCOR who changed the aspect.

[christophmateka]

oh, ok, i feel a bit releaved.
I may borrow an oscilloscope next week, can u tell me the points that i should check? since i'm not that familiar with oscilloscopes - i'll have someone who will help here!
I also found out that the hum-frequency changes when i move the gain switch (not the knob) (lowest frequency at lowest gain) and that it also appears when the preamp is not inside the housing.

[JPK]

You can start looking to where the noise appears in the signal path:
Scope on AC, high gain. Check TP1, TP2, TP3, TP4.

[christophmateka]

Thank You.

TP1 = nothing
TP2 = strongest noise Signal (+- 0,8V AC)
TP3= +-0,01V AC
TP4= +-0,005V AC (although i cannot interprete that signal good enough on the scope)

All noise signals are sine waves with approx 80hz.

[christophmateka]

and I just discovered that the hum Signal is only on the + output on the xlr connector. not on the - pin. if I switch polarity, this is inverted.

[JPK]

The output is the floating secondary of a transformer.
So you can only do measurements differentially between the hot and cold.
The audio connection to the output of the pre must be done with a balanced cable.

[christophmateka]

thank you -  i always connect via balanced cables.

So, do i get this right, if the hum occurs the first time on TP2, according to your schematics it's very likely that the tube is causing the noise because it sits between TP1 and TP2?

[JPK]

Yes the tube stage makes the noise but not necessarily the tube itself. More probably one the switching regulators for the heater or the high voltage.
TP2 is the high voltage line and TP8 is the heater voltage.
Find out which one has the 80Hz hum.

[christophmateka]

TP2 and TP8 have both the hum with the same frequency.
The Voltage is different.

TP2: +-0,8V AC
TP8: +-0,08V AC

[JPK]

It seems to be the high voltage converter U5.
Please contact me by email for an exchange: https://www.soundskulptor.com/en/contact.php

[christophmateka]

thank you a lot!