RuudNL

Mackie 1402-VLZ phantom power buzz
« on: May 09, 2012, 04:40:37 AM »
For private use I have a Mackie 1402-VLZ. It works fine with all kinds of (phantom powered) microphones.
Recently, I plugged in a Behringer B-5 microphone.
(Yes, I know, mentioning the B*** word might sound like swearing, but their B-5 microphones are pretty good...)
To my surprise I heard a faint buzzing (100 Hz.) sound, that I never heard with other phantom powered microphones.
At the moment I switch the phantom power off and the capacitors are discharging, the microphone is still powered and the buzz is gone.
Voltage on pin 2 and 3 is 37.3 Volts during normal operation, so no excessive current consumption.
Other phantom powered microphones are completely clean. To exclude that there is something wrong with the B-5, I borrowed a C-2 from a friend and this microphone had the same problem: also a soft buzzing sound with phantom power on. So it seems it is a Behringer problem... (Could it be that the electronics do not provide a good symmetrical output, so that the common mode rejection suffers and the ripple on the phantom power voltage is amplified?)
This is the circuit diagram of the phantom power part of the 1402-VLZ:



Personally I think that the 100 uF is a bit small. Any thoughts about this problem?
There is a solution for every problem!

http://www.vansteenisaudio.nl


Harpo

Re: Mackie 1402-VLZ phantom power buzz
« Reply #1 on: May 09, 2012, 07:10:04 AM »
Max.current demand from a shorted/flooded multicore or stagebox would be 83mA or max.60mA for normal operation with all 6 phantom fed channels connected. R14||R15 10R doesn't seem right. 4.8A current limiting won't protect a thing and filter with following 47uF set at 338Hz won't help remaining ripple voltage. Value in range 100R...560R might be a better fit. Assuming transformer secondary about 46VAC and allowing mains voltage varying within +/-10% to keep it within regulation, C14 with 100uF will give about 3% (8.6V) ripple in front of Q4. 330uF will bring it down to 1%. Voltage rating of  C15 and C16 looks a little close to the edge. YMMV.
A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.  -Douglas Adams

RuudNL

Re: Mackie 1402-VLZ phantom power buzz
« Reply #2 on: May 09, 2012, 07:57:11 AM »
Personally I think R14/R15/C16 are there to reduce the noise on the phantom power line, not to serve as a short circuit protection.
(In most cases the 6K8 resistors will burn after shorting the input to ground with phantom power on!)
In theory R13/D25/C15/Q4 should work as an effective ripple filter, assuming the input voltage is high enough.
There is a solution for every problem!

http://www.vansteenisaudio.nl

JohnRoberts

Re: Mackie 1402-VLZ phantom power buzz
« Reply #3 on: May 09, 2012, 01:25:06 PM »
Look for a possible impedance unbalance (resistance from pin 2 to 1 and pin 3 to pin 1).

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

RuudNL

Re: Mackie 1402-VLZ phantom power buzz
« Reply #4 on: May 09, 2012, 01:57:14 PM »
That's what I already thought of:
Quote
Could it be that the electronics do not provide a good symmetrical output, so that the common mode rejection suffers and the ripple on the phantom power voltage is amplified?
But the problem is that the unbalance might be inside the microphone, since all other microphones are working fine.
And I don't want the risk of damaging a microphone. (I even haven't got an idea how to get it open!)
There is a solution for every problem!

http://www.vansteenisaudio.nl

JohnRoberts

Re: Mackie 1402-VLZ phantom power buzz
« Reply #5 on: May 09, 2012, 03:33:38 PM »
You don't need to open it up to measure from pin 2 to pin 1 and pin 3 to pin one with an ohmmeter.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

RuudNL

Re: Mackie 1402-VLZ phantom power buzz
« Reply #6 on: May 09, 2012, 03:42:14 PM »
That's right, but the resistance you measure can be different from the impedance.
But I'll give it a try!
There is a solution for every problem!

http://www.vansteenisaudio.nl

radardoug

Re: Mackie 1402-VLZ phantom power buzz
« Reply #7 on: May 09, 2012, 03:45:57 PM »
Your problem is not in the Mackie, its in the microphone.
It could be frame buzz from the microphone picking up TV transmission, essentially a grounding problem. The Behringer probably has some sort of active unbalanced output.  Junk is junk.

RuudNL

Re: Mackie 1402-VLZ phantom power buzz
« Reply #8 on: May 09, 2012, 05:48:38 PM »
On an other mixer the microphone is completely "silent"(=No buzz!)
There is a solution for every problem!

http://www.vansteenisaudio.nl

sodderboy

Re: Mackie 1402-VLZ phantom power buzz
« Reply #9 on: May 14, 2012, 08:26:27 AM »
You measured 37.3 VDC on the lines- did you measure VAC as well?

Mike


RuudNL

Re: Mackie 1402-VLZ phantom power buzz
« Reply #10 on: May 14, 2012, 08:38:35 AM »
230.7 Volts (=230 V for Europe.)
Phantom power voltage = 47.3 Volts without load.
All other phantom powered microphones are completely 'clean'.
I think that there is an unbalance inside the microphone, so that the ripple on the phantom power supply voltage is amplified.
There is a solution for every problem!

http://www.vansteenisaudio.nl

JohnRoberts

Re: Mackie 1402-VLZ phantom power buzz
« Reply #11 on: May 14, 2012, 11:04:23 AM »
That's right, but the resistance you measure can be different from the impedance.
But I'll give it a try!
??

To measure the AC impedance of a phantom powered mic is more complex.  You need to power it to allow linear operation of any active circuitry.

How about a different test.... Take two precision resistors  say 10k ohm 1% or better. Connect one end of the two resistors to each other, feed the other end of one resistor into pin 2 the other end of other resistor into pin 3, you probably want to cap couple the common side of the two resistors to a sine wave source or even an audio signal feed.

If the mic presents a well balanced termination, the injected audio signal should mostly cancel, if mic is unbalanced you will hear  more audio signal injected. If you keep the injected signal level modest this shouldn't hurt anything.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

RuudNL

Re: Mackie 1402-VLZ phantom power buzz
« Reply #12 on: May 14, 2012, 11:23:38 AM »
Quote
You don't need to open it up to measure from pin 2 to pin 1 and pin 3 to pin one with an ohmmeter.
I know that impedance is a complex thing. That is why I was a bit hesitant to use an ohm meter.
However: hum with phantom power active, no hum with phantom power switched off (capacitors discharging) points into the direction of ripple on the phantom power voltage. Especially because the same microphone does not produce any hum on another mixer.
So: maybe there is somtehing 'wrong' in the microphone design, but they sound very good and seem to work properly with a ripple-free phantom power source.
Until now I did not have the time to take a closer look at the Mackie, but the first thing I want to do is use a larger value for the buffer capacitor for the phantom power and measure the ripple on the phantom power output.
(That is easy to do: just use an AC millivoltmeter coupled through a capacitor to the 48 V. line.)

There is a solution for every problem!

http://www.vansteenisaudio.nl


 

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