Sound Skulptor forums

Build help and troubleshooting => MP573 Microphone preamplifier => Topic started by: edikt01 on January 14, 2023, 03:01:51 PM

Title: Intense humming in audio
Post by: edikt01 on January 14, 2023, 03:01:51 PM

Hi Forum,
 
 I recently bought the MP573 Kit and was really fun to build. Very well documented.
 Setup went well and I measured around 24,2 V without and a little less with output transformer connected. 3V could be set properly with P2 Trimpot.
 
 Unfortunately there is a really loud magnetic hum in the signal which loudness increases with the gain level. It is already markable at lower gain levels like +20dB. (MP573 installed in most left Slot)

 I moved the MP573 out of the rack as far as possible away from the power transformer with the extension cable in a test but the hum is still  hearable  at lower gain levels but it was much better. This is my first 500 module and rack, I don’t have a possibility to test in a different rack or to test my rack with a different module unfortunately…
 
 I have a brand new Fredenstein Bento 8 Pro Pure Analog rack which has a linear power supply and is described on the product page as follows: „Bento 8 Pro incorporates extensive magnetic shielding of the mains transformer to eliminate induced hum“
 Could it be that the rack is faulty?
 
 
 I did the checks described on thread „In case your MP573 doesn't work as expected“

• All DC Voltages are almost exact same as the values on the help site
• Checked for correct direction of components and resistor values several times
• Used extender cable to move the MP573 module out of the rack as far away as possible from the power supply. That reduced the hum a lot but still hearable.
• Output transformer matches the resistance
• Input transformer: Here I was not sure if the values has to be measured soldered or unsoldered so I did both:
  
  
  • Unsoldered: (measured 7-8, 8-9, 7-10 only in unsoldered state)

• [R in Ohm over Pins]	[soldered]	[unsoldered]
  2-4	24,9	24,7
  3-5	24,5	24,3
  7-9	0,2	136,3
  8-10	135,1	137,1
  7-8	x	0,2
  8-9	x	136,3
  7-10	x	137,1

The input transformer values was not completely clear for me, do think that these values are ok? 
Not sure if faulty input transformer could create such hum.
Do you have some other ideas what to check on the module side.

thanks for any help. 

kind regards P.

Title: Re: Intense humming in audio
Post by: JPK on January 16, 2023, 03:52:49 PM

Do you confirm you have a short circuit between pins 7 and 8 with the transformer disconnected?

Title: Re: Intense humming in audio
Post by: edikt01 on January 16, 2023, 08:00:38 PM

Hi JPK, 

thanks for your response. 
Yes I confirm, was wondering about that. 
I have just checked it again.

.P 

Title: Re: Intense humming in audio
Post by: JPK on January 17, 2023, 09:36:14 AM

This sounds like a defective transformer.
Please contact me here :https://www.soundskulptor.com/en/content/6-contact