Help needed to Fix Inward connections Pre

[Tempestb]

Hello folks !
My first post here .
I have following problem . I've bought second hand Inward connections Discreet plus mic pre.
It sounds great - but have some strange low pitched HUM up to -70db to the line without the mic plugged in.
Noise appears when the power cable plugged in to the power supply (before the power up)
i've tried  to take out the inlet grounding . Inspected visually - and not found any burns .
checked the voltage coming to DOA (it correct) . Swapped the DOA and took them one by one out - nothing helps.
Contacted the manufacturer - he gave me the schematics .
Thanks for any help

 

[Tempestb]

Forgot to mention - it's API style dual channel mic pre - made in late 90s  ....in/out trasformers ...external PSU  and Dual Fader (for Output )
HP and LP filters
External PSU have dual transformers  - One of them gives 48v Phantom
Second supply the 24 + /-  and 15 +/-

 

[ruairioflaherty]

Noise appears when the power cable plugged in to the power supply (before the power up)

Have you tried separating your supply and mic pre physically?  If you are seeing hum on the line output of your mic pre`without powering up pre it is likely being coupled magnetically from the power supply toroidial transformer to your audio line output transformer.

 

[Tempestb]

Here is the transformers
..Yeah i'm tried  -  i measured some voltage leaking from PSU when power cord plugged
i'm resoldered all weak looking points in the PSU - but there is only transformers /fuse and switches ....also flushed the switches with the contact cleaner spray

 

[Tempestb]

Also checked the Transformers -with Data ****
The Triad one - should give 56 v  - it gives 70v
The Signal one - should give 36/18 - it gives 40/20

 

[Tempestb]

here is the back side of transformers plate
also checked they wired right

 

[ruffrecords]

Low level hum with nothing plugged in is not that unusual. If the gain is 60dB then hum at -70dbU represent an input signal of -130dBu. Does the level of hum vary as you change the gain? What happens if you plug in an XLR with pins 2 and 3 shorted. Does the hum go away?

Cheers

Ian

 

[Tempestb]

Yeah with the gain and output volume maxed - noise come to -30 db ..and it also pulsating a little bit...few db.
2 and 3 pins shorted on input ?! - not tried it yeat
...i'm not found any position in which hum goes away - but it very sensitive if i touch with the finger 100 piko
condenser near the input transformer ...it behaves like the microphone if you touch the mic head with the finger  ...on both channels same effect
...This brown condenser

 

[ruffrecords]

If it is 100pF it may be a ceramic type which are known to sometimes be microphonic. Might be worth just removing it and seeing what happens.

However, the fact that you have a sort of pulsing sound implies the pre may be motorboating. This is often due to an increase in power supply impedance. This can be caused by a  number of things but often it is due to deterioration of decouplig caps or power supply caps.##Can you put a scope on the output and show us the waveform?

Cheers

Ian

 

[pucho812]

can we approximate the frequency of the hum?

 

[Tempestb]

i've tried to short the pins 2 and 3  on the mic input  - it bring the noise half down  maybe .
here is the HUM picture
Thanks for you all 

 

[ruffrecords]

Looks to be 100Hz plus harmonics which suggests a power supply problem.

Cheers

Ian

 

[Tempestb]

You mean - power supply recap ?!

 

[ruairioflaherty]

You mean - power supply recap ?!

Do you have an oscilloscope?  If so probe the power rails and show is the waveform.

 

[Tempestb]

i don't have the oscilloscope yeat

 

[pucho812]

100hz, would be the harmonic of 50hz.  Recap the psu 

 

[ruairioflaherty]

Noise appears when the power cable plugged in to the power supply (before the power up)

This line from the very first post confuses me a little.  Are you saying that the noise appears in your DAW as soon as you plug the power cable into your mic pre supply but before you power it up?

Can you tell us the full signal path?

it should be -

1) pins 2 & 3 shorted together with 150 ohm resistor (to simulate a mic) > 2) Mic pre input > 3) Mic pre output to  A/D converter of DAW. 

Are you sure that you are using a good balanced line cable to your DAW A/D input? 
Are you sure that your A/D input is balanced / differential?

 

[Tempestb]

the path exact as you noted  1>2>3  ...i'm 100% about my cables .
i have a lot of preamps and they not behave like this in same situation and same cabling .
i'm not performed test with shorting with resistor - just with peace of wire and mic with negative gain
piece of wire seems lower a bit the noise and the mic looks like not affect the noise at all.
looks strange to me too - the noise appears when power cord plugged
(i measured some voltage leaking from PSU when power cord plugged)
the power switch on on the power supply near the IEC  - disconnect the LINE .....the Neutral stays connected  - it might be some voltage entering the transformers ? since AC it's sinusoidal pattern?!

 

[ruffrecords]

You mean - power supply recap ?!

Maybe. Something is wrong in the power supply. Bad joint, dead cap.  That is where I would look. You need to check for crap on the rails with a scope. Might just be a dud cap in an early stage. Without a schematic it is hard to suggest exactly what to look at first.

Cheers

Ian

 

[Tempestb]

schematic removed due to wish of manufacturer