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Build help and troubleshooting => MP512 Microphone preamplifier => Topic started by: hanssongt on August 17, 2026, 08:03:05 PM

Title: Clipping/distorting noise on High Z input
Post by: hanssongt on August 17, 2026, 08:03:05 PM

Hi!

Recently finished my latest build. Everything works except when connect the DI. I can hear the source (bass in this case), but it sounds like it's clipping/distorting even on low gain. Any idea what could cause this?

I should mention that when I first tried the DI I coulnd't hear anything, but that was because I had missed soldering a couple of the resistors on the DI board. After doing that I got sound but got the problem mentioned above. I hope I didn't mess anything up by testing it without all parts soldered first.

Thanks,
Tobias

Title: Re: Clipping/distorting noise on High Z input
Post by: JPK on August 18, 2026, 11:04:36 AM

Check that Q2 (2SK170) is in the correct position A.
Can you measure the DC voltage (against 0V) on both sides of R51: near connector (16V) and near C24 (15V) ?
Can you measure the DC voltage (against 0V) on both sides of R48: near C23 (8V) and near R41 (1V) ?

Title: Re: Clipping/distorting noise on High Z input
Post by: hanssongt on August 20, 2026, 10:10:41 AM

Thank you for getting back so quickly.

Q2 is in the correct position.

R51:
15.77V near C24
15.92V near connector

R48:
131mV near R41
1V near C23

So something seems off when looking at the measurements at R48?

Thanks,
Tobias

Title: Re: Clipping/distorting noise on High Z input
Post by: JPK on August 21, 2026, 09:57:31 AM

Check you didn't swap Q1 & Q2.
Check the voltage on R46 top pin (7.4V).
Check the solder joints on R45, R46, R47.
Check the direction of C23.

Title: Re: Clipping/distorting noise on High Z input
Post by: hanssongt on August 24, 2026, 10:15:24 AM

Looks like the components are inte right place and soldered correctly. Voltage on R46 top pin measured to 7.7V.

What do you think about the measurements on R48? That's whats has been off so far.

Thanks,
Tobias 

Title: Re: Clipping/distorting noise on High Z input
Post by: JPK on August 25, 2026, 06:52:18 PM

Your Q2 may be damaged, try replacing it.