Sound Skulptor forums

Build help and troubleshooting => MP566-II Tube microphone preamplifier with HPF => Topic started by: guenter1910 on July 29, 2026, 03:18:34 PM

Title: MP566-II Phantom Power problem - signal fades to zero
Post by: guenter1910 on July 29, 2026, 03:18:34 PM: Hi all, following my previous post about LD2 there is another problem that came up. When using Phantom Power, Everything works fine for a short while and than the signal fades away quite rapidly. The Voltage on IN+ and - is down to 28V when the 48V switch is engaged. What confuses me a bit is that on the kit C1 (100u, 63V) is packed but the partslist with C1 on is crossed out and on the new one C1 is no longer there. I have put C1 in initially and thought that this might cause the problem and so I desoldered it, but the behaviour is the same.
Any help highly appreciated!
Best regards,
Guenter
Title: Re: MP566-II Phantom Power problem - signal fades to zero
Post by: JPK on July 30, 2026, 08:44:57 AM: but the partslist with C1 on is crossed out
Make sure you are using the latest parts list. C1 must be populated.
Does the 48 V rail (on the finger connector, pin 15) remain stable when the voltage on the XLR drops?
What is the phantom voltage level on each side of R3?
Title: Re: MP566-II Phantom Power problem - signal fades to zero
Post by: guenter1910 on July 30, 2026, 01:54:25 PM: Phantom Power on Pin 15 is stable (49,45 V), Voltage on R3 is 49,45V on one and 47,90 V on the other side, C1 populated again.
Title: Re: MP566-II Phantom Power problem - signal fades to zero
Post by: JPK on July 30, 2026, 06:51:56 PM: Check the resistance value between:
- XLR pin2 and pin3,
- XLR pin2 and 0V,
- XLR pin3 and 0V.
Also check your mic cable.
Title: Re: MP566-II Phantom Power problem - signal fades to zero
Post by: guenter1910 on July 30, 2026, 07:37:30 PM: XLR Pin 2 and 3 on the cable 29,4 Ohm, on the 500 Rack Slot 28,94 Ohm
Pin 2 on the cable to 0V on the board 22 MOhm
Pin 3 on the cable to 0V on the board 22 MOhm
both slowly increasing

Mic Cable is ok. I checked with a different cable - same behaviour
Title: Re: MP566-II Phantom Power problem - signal fades to zero
Post by: JPK on August 01, 2026, 07:37:53 PM: and than the signal fades away quite rapidly.
Try to follow the signal on TP1, TP4, TP5, TP6.
Check what happens between the initial working state and the final faded away state.
Also cjheck if something happens with DC.
Title: Re: MP566-II Phantom Power problem - signal fades to zero
Post by: guenter1910 on August 02, 2026, 11:48:44 AM: i fear my mic died during testing. I tried another one and it seems to work. is it possible that i have destroyed the mic while testing/ measuring? What could be the cause? I just want to be certain that I do not destroy the other mic as well.
Title: Re: MP566-II Phantom Power problem - signal fades to zero
Post by: JPK on August 03, 2026, 05:43:17 PM: No, that's very unlikely. The microphone is isolated from the preamp circuitry by the input transformer, so the preamp itself cannot damage it. The only parts directly connected to the microphone are the 48 V phantom power (through R1 and R2), the pad relay, and the input transformer. Under normal conditions, none of these should damage the microphone.
Title: Re: MP566-II Phantom Power problem - signal fades to zero
Post by: guenter1910 on August 03, 2026, 06:00:31 PM: So I guess the reason was a somehow defective microphone that finally died during the testing phase. Thank you for your support and fast feedback!