Samson S-Phantom Low Audio Output

Help Support GroupDIY:

tristangregorysmith

New member
Joined
May 1, 2020
Messages
3
Location
WA
Hi folks - I have a Samson S-Phantom 48V mic supply on my bench right now, could use some help troubleshooting.

The phantom power section seems to be working fine, I'm able to measure 48V on pin 2 and 3 of each Mic In XLR. However, the audio output is extremely low. When I use the same mic and cables with my interface's phantom power, it works just fine.

Some details:
- measured 48V phantom on the Mic In XLR connectors
- I can hear and see phantom power turn on
- tried different mic and cables
- contact cleaner on audio jacks and the phantom switch
- I don't see any obviously blown electrolytic caps
- audio just barely registers on my interface's VU meter when I tap the mic loudly. If I turn up the gain on absolutely everything, I can hear audio (and of course a huge noise floor)

Traced the audio part of the circuit a bit, attached a little hand drawn snippet.

Additionally, I think I may have applied 48V from my audio interface to the output of the S-Phantom at some point... I assumed it would have enough protection and bipolar caps, but I may have been mistaken. This could cause the zener diodes to fail, correct? And then maybe the 47uF coupling caps?

Any ideas of what could be going wrong here?

My next step was going to be reflowing solder in the audio path. After that, maybe replacing the protection diodes and the electrolytics. Thoughts?
 

Attachments

  • s phantom.jpg
    s phantom.jpg
    150.1 KB · Views: 14

moamps

Well-known member
Joined
Sep 24, 2005
Messages
1,819
Location
Croatia, HR
First you need to check what the voltage is at pins 2 and 3 when the phantom power is loaded with a microphone (when the microphone is connected to the input), and whether it is much different from the voltage when that same microphone is plugged into the audio interface.
If they are equal, it means that the phantom power source is working ok.

After that you can pull out the zener diodes, and you can test the electrolytic capacitors so that while the device is off you connect your DMM in the "continuity check" position where you need to hear a short "beep" when you connect the probes to the capacitor leads. Reverse the probes and try again. If you don't hear a short beep then it's probably capacitor faulty.

The last check is to disconnect the zener diodes, short pins of the electrolytic capacitors and with the phantom power off on your interface, turn on the phantom power on the samson power and check if it works.
Note: this test may only be performed if the audio interface has a phantom power option (there are phantom power blocking capacitors inside the interface).
 

JohnRoberts

Well-known member
Staff member
Moderator
Joined
Nov 30, 2006
Messages
20,969
Location
Hickory, MS
perhaps measure impedance (resistance) between pins 2 and 3.

+1 confirm voltage when loaded by a mic (check pin 1 also).

JR
 

tristangregorysmith

New member
Joined
May 1, 2020
Messages
3
Location
WA
Hey, thanks for the responses! Here are some updates:

Without mic: ~49V
With mic (KSM141): ~31V

Impedance b/w pins 2 and 3: 13.3kOhm

Brought it to work and probed with the scope today, looks like one side of the mic audio has a much higher amplitude than the other side. Pin 2 has about a +/-200mV spike when i tap the mic, Pin 3 has a +/-600mV spike. That's not really what I expected to see...

15.5V drop across the 6.81k resistors with the mic attached.

The coupling caps all beep on the continuity checks.

I'll try to get to the more invasive/desoldering troubleshooting steps soon. Maybe the above can give you an idea in the meantime?
 

JohnRoberts

Well-known member
Staff member
Moderator
Joined
Nov 30, 2006
Messages
20,969
Location
Hickory, MS
The active circuitry inside phantom powered mics may be single ended or not have symmetrical outputs so that might be natural.

Perhaps look for a difference using your scope on both pins, between the normal interface and the low output interface.

Did you confirm pin one is solidly 0V?

JR
 

tristangregorysmith

New member
Joined
May 1, 2020
Messages
3
Location
WA
Sorry, Pin 1 in reference to what? Pin 1s are shared ground throughout the entire board.

Good point. This from the KSM141 manual, so not symmetrical outputs seems natural after all:
1628099423280.png

Probing the mic input pins 2 and 3 and the mic output pins 2 and 3 and they look identical. Let me see if I can track down something else here that supplies known good phantom power so I can compare...
 

Latest posts

Top