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MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« Last post by JPK on August 06, 2026, 04:47:16 PM »

Check that R46-L is connected to the 0V. Check on the resistor leads, not the PCB pad.
Reflow the solder joints of R45, R46, R47, R48.

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MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« Last post by guenter1910 on August 06, 2026, 07:08:42 AM »

what should I check next? 

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MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« Last post by guenter1910 on August 03, 2026, 06:35:16 PM »

the connections are all ok

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MP566-II Tube microphone preamplifier with HPF / Re: MP566-II Phantom Power problem - signal fades to zero

« Last 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!

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MP566-II Tube microphone preamplifier with HPF / Re: MP566-II Phantom Power problem - signal fades to zero

« Last 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.

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MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« Last post by JPK on August 03, 2026, 05:34:53 PM »

You have a broken connection.
Check that R47-Right is connected to
- R46-R
- R45-L
- U4-pin2
Check that R47-Left is connected to
- D11-L
- R48-R
- U4-pin6
If one link is missing, repair it with some wire.

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MP566-II Tube microphone preamplifier with HPF / Re: MP566-II Phantom Power problem - signal fades to zero

« Last 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.

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MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« Last post by guenter1910 on August 02, 2026, 10:26:54 AM »

it gives also 1 Meg in the circuit

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MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« Last post by JPK on August 01, 2026, 07:48:11 PM »

Check the value of R47.
The value is 1Meg (brown, black, black, yellow, brown) but when measured in circuit, it should give you about 12k.

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MP566-II Tube microphone preamplifier with HPF / Re: MP566-II Phantom Power problem - signal fades to zero

« Last 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.