Sound Skulptor forums

Build help and troubleshooting => MP512 Microphone preamplifier => Topic started by: mbogner on May 24, 2023, 09:59:31 AM

Title: Noisy signal without low-end
Post by: mbogner on May 24, 2023, 09:59:31 AM: Hey,

I build a MP512 one year ago and it worked fine. I didn't use it much, and when I used it now I realized, that there's something wrong.
It has a realtively high noise level nad pretty much no low-end. The signal sounds filtered.
Anyone an idea how that could have happened?

Thanks, Malte
Title: Re: Noisy signal without low-end
Post by: JPK on May 26, 2023, 04:58:18 PM: This is probably a balancing issue. I would consider 3 possibilities:
- The input transformer doesn't receive the 2 phases. Check the solder joints on the input transformer, RLY1 and RLY2.
- The output transformer doesn't send the 2 phases. Check the solder joints on the output transformer and pot P2.
- It is an external connexion issue.
Title: Re: Noisy signal without low-end
Post by: mbogner on May 27, 2023, 02:14:54 PM: Thanks for the reply!
I checked all the solder joints, but the problem remains the same.
I would exclude an external issue, since I tried different cables and slots in the 500 Box
Title: Re: Noisy signal without low-end
Post by: mbogner on May 27, 2023, 02:37:59 PM: It is especially in the high impedance setting. I'm concious that it has an emphasis on the higher frequencies, but it seems very big, especially compared to the MP573.
The noise is gone though
Title: Re: Noisy signal without low-end
Post by: JPK on May 27, 2023, 07:32:48 PM: Measure the DC resistor between input hot and cold, with the power on and gain on MID or HIGH --> 36 ohms.
Measure the DC resistor between output hot and cold, with pad at max --> 37 ohms.
Title: Re: Noisy signal without low-end
Post by: mbogner on May 29, 2023, 12:44:28 PM: Thank you. I'm not sure where to measure. Could you tell the resistor numbers?
Title: Re: Noisy signal without low-end
Post by: JPK on May 30, 2023, 04:57:10 PM: The measurement must be done on the input and output XLR between pins 2 & 3.
Title: Re: Noisy signal without low-end
Post by: mbogner on June 07, 2023, 12:07:47 PM: Those values are correct
Title: Re: Noisy signal without low-end
Post by: JPK on June 10, 2023, 04:58:08 PM: It is especially in the high impedance setting. I'm concious that it has an emphasis on the higher frequencies, but it seems very big
There is no impedance setting in the MP512, only gain setting. In the high gain setting there should be no obvious change in the frequency response.

What is the PCB revision of your unit (green or black PCB)?
Title: Re: Noisy signal without low-end
Post by: mbogner on June 13, 2023, 03:52:20 PM: It's black
Title: Re: Noisy signal without low-end
Post by: JPK on June 16, 2023, 11:40:49 AM: Do the test Try to follow the signal here (https://www.soundskulptor.com/forum/index.php?topic=15.msg15#msg15) and check the levels you get.