microphone too hot - can I just insert a resistor?

[tempus fugit]

Hey all;

I've got a condenser microphone (balanced output, 48v phantom power) mounted
inside my guitar. The output however, is WAY to high, and it's distorting
the input on my mixer. Can I lower the output by simpy inserting a resistor
in series with then output, or do I need something more complex?

Thanks

 

[tempus fugit]

Hey all;

I've got a condenser microphone (balanced output, 48v phantom power) mounted
inside my guitar. The output however, is WAY to high, and it's distorting
the input on my mixer. Can I lower the output by simpy inserting a resistor
in series with then output, or do I need something more complex?

Thanks

I just thought of another thing - since the inputs are balanced, could I
just insert the resistor between in the path to ground (thus using only 1
resistor, rather than 1 for each of the + and - in the balanced path)

Thanks again

 

[tempus fugit]

You should use a pad -- a series resistor from the mic, followed by a
parallel resistor to keep the resistance "seen" by the amplifier low.

Or use a pot, so you can experiment to find the right setting.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Just to make sure I understand you Tim - the parallel resistor goes between
pins 2 and 3 (i.e. + and - signal lines)? Also, I would need 1 for each of
the + and - lines right? I'm also assuming the parallel resistor should be
fairly low (less than a couple K)?



Thanks

 

[Michael Black]

Hey all;

I've got a condenser microphone (balanced output, 48v phantom power) mounted
inside my guitar. The output however, is WAY to high, and it's distorting
the input on my mixer. Can I lower the output by simpy inserting a resistor
in series with then output, or do I need something more complex?

How do you know where the problem lies?

Yes, you may be overloading the stage it runs into.

But, you may be overdriving the microphone itself, it being too close
to the sound source.

You need to figure that out first.

Michael

 

[tempus fugit]

How do you know where the problem lies?

Yes, you may be overloading the stage it runs into.

But, you may be overdriving the microphone itself, it being too close
to the sound source.

You need to figure that out first.

That is something I've also considered, but I'm leaning towards the
distorting my inputs theory for a couple reasons:

1. Manufacturer's specs are showing 0.5% at 145 dB SPL and a sensitivity
of -34. It seems unlikely that the noise levels are hitting 145dB inside of
a classical guitar.
2. Even with the input gain virtually off, the meters on my mixer are
showing clipping without a lot of heavy strumming.
3. This mic has been successfully used for micing drums, and the 2.00
capsule I had in the guitar previously did not distort.

 

[BobG]

I'm surprised that switching in the mixer pad didnt fix it. Tried
that? Maybe theres a bunch of DC from the phantom power biasing your
preamp up near clipping. Any caps in the signal from the mic to the
mixer preamp? If you have a nice hot mic signal, you can unbalance it.
Might work into the line in then.

 

[Phil Allison]

"tempus fugit"

I've got a condenser microphone (balanced output, 48v phantom power)
mounted
inside my guitar. The output however, is WAY to high, and it's distorting
the input on my mixer.

** Hard to believe a mixer with 48V phantom cannot take the signal from your
mic.

Is there no "pad" switch ?

What mixer is it ?



....... Phil

 

[tempus fugit]

"tempus fugit"



** Hard to believe a mixer with 48V phantom cannot take the signal from your
mic.

Is there no "pad" switch ?

What mixer is it ?



...... Phil

The mixer is actually a mixer amp, and (I didn't find this out until I
actually bought it) the phantom power isn't 48v, it's only 15. I tried
recording with it, using my homebrew mic pre which actually has 48v phantom,
and still got the distortion. Unfortunately, my mic pre doesn't have a pad.
I agree though, it's kind of crazy that the output is that high, but even
with virtually no gain on my mic pre, it still clips the VUs on my recording
program. Now I did measure the resistance from pins 1-2 (on the mic) and got
3.8K, but got infinite when I measured 1-3. My other condensers show an
equal amount of resistance. I don't know if that's an issue or not.

Thanks Phil

 

[tempus fugit]

"tempus fugit"
"Phil Allison"



** Made by " Bi-Amp" by any chance.

And why is this secret ?




** That is some condenser mic.

Why it its identity a big secret?

Build it yourself maybe ?

Jeez people who REFUSE to post the most basic of details are annoying.




...... Phil

The mic is made by Karma Audio - I chose it for it's small size:
http://karmaaudio.com/kmp.html

The mixer amp is made by Carvin:

https://www.carvinguitars.com/products/single.php?product=PA620

 

[tempus fugit]

If I understand the above correctly, you are supplying 15V phantom
power to a mic that expects 48V.

I thought that might be the problem too, but, as I stated above

I triedfrom overdriving the inputs, as even with the mic pre gain almost off the
VUs on my recording program are still pegging.

 

[Phil Allison]

"tempus fugit"

The mic is made by Karma Audio - I chose it for it's small size:
http://karmaaudio.com/kmp.html

** That ( electret ) mic has extremely high output - over 40 mV at 94dB
P - when most condenser mics have only 5 or 10 mV.

But that 145dB SPL figure is complete bollocks - cos the mic would need to
deliver 15 volts rms.

Allowing for the bass boost from proximity effect ( all cardioids have
that), a peak output of 1 volt rms is not too unlikely for one sitting
inside a guitar.




...... Phil

 

[tempus fugit]

"tempus fugit"



** That ( electret ) mic has extremely high output - over 40 mV at 94dB
P - when most condenser mics have only 5 or 10 mV.

But that 145dB SPL figure is complete bollocks - cos the mic would need to
deliver 15 volts rms.

Allowing for the bass boost from proximity effect ( all cardioids have
that), a peak output of 1 volt rms is not too unlikely for one sitting
inside a guitar.




..... Phil

Thanks for that expanation Phil. I wasn't sure what a normal condenser would
have for an output. That expains the overdrive problem then. The pad seems
to have helped, but I still have a huge amount of distortion from the low
G-C notes (in the 200Hz area). No doubt this is the proximity effect,
combined with the fact that it's already exceedingly bassy inside the
guitar. I may just have to scrap the idea of using this mic and just go back
to my old capsule.

Thanks again

 

[Sjouke Burry]

Thanks for that expanation Phil. I wasn't sure what a normal condenser would
have for an output. That expains the overdrive problem then. The pad seems
to have helped, but I still have a huge amount of distortion from the low
G-C notes (in the 200Hz area). No doubt this is the proximity effect,
combined with the fact that it's already exceedingly bassy inside the
guitar. I may just have to scrap the idea of using this mic and just go back
to my old capsule.

Thanks again

Are you really sure that at low frequencies the mic does
not bounce around in its mounting?
Cause that would produce "ditorsion" alright.

 

[BobG]

Look like that button next to the channel volume knob is a gain
button.... or a pad if you like to use industry jargon..... I usually
use a pad to kneel on when weeding the garden, or to crash in when
visiting my hippy friends in the city.

 

[Eeyore]

Hey all;

I've got a condenser microphone (balanced output, 48v phantom power) mounted
inside my guitar. The output however, is WAY to high, and it's distorting
the input on my mixer.

What mixer ?

Graham

 

[tempus fugit]

What mixer ?

Graham

The mixer is here:

https://www.carvinguitars.com/products/single.php?product=PA620

 

[tempus fugit]

If I understand the above correctly, you are supplying 15V phantom
power to a mic that expects 48V. That may be the entire problem,
since it surely will affect clipping and distortion. Knowing nothing
about your particular mic circuit, here's a couple of thoughts:

If this is an electret and not a true condenser, it runs at some
specified fixed bias voltage which is less than the phantom supply.
If the regulator for that voltage expects 48V at its input and is
getting 15V, it may be biasing the mic with the wrong value. Even if
it's still providing the right value (let's say 10V), the input stage
of the preamp will then be running with only a 5V difference between
its supply and its input. This would explain the clipping.

And if it is really a true condenser mic (which requires 200V bias or
so) then running with a 15V supply instead of 48V might mean you
have so little bias you have a ton of distortion just from that.

Best regards,


Bob Masta

DAQARTA v4.00
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
FREE Signal Generator
Science with your sound card!

I've just tried putting about 15K worth of resistance in series with the
output, and using a true 48v phantom power, and the sound is really quite
good. I suppose I could hardwire the resistors and lug around the extra 48v
supply to gigs, but I think I'm going to try going back to my original mic
element and using the 15v phantom power from the mixeramp to power it. I'm
pretty sure the mic is only rated for a 10v supply V+ though. Do you think
that it would handle the extra 5v without going up in smoke, or should I
build a voltage divider?

Thanks

 

[Eeyore]

The mixer is here:

https://www.carvinguitars.com/products/single.php?product=PA620

Inadequate data. A pro mixer (even a Behringer) probably wouldn't grumble at
the mic output level since they have variable gain input stages.

This Carvin is MI grade garbage. Sorry.

Graham

 

[Eeyore]

I've just tried putting about 15K worth of resistance in series with the
output, and using a true 48v phantom power, and the sound is really quite
good. I suppose I could hardwire the resistors and lug around the extra 48v
supply to gigs, but I think I'm going to try going back to my original mic
element and using the 15v phantom power from the mixeramp to power it. I'm
pretty sure the mic is only rated for a 10v supply V+ though. Do you think
that it would handle the extra 5v without going up in smoke, or should I
build a voltage divider?

I'd suggest paying more attention to specs and buying compatible equipment in
the first instance.

Graham

 

[tempus fugit]

o------/\/\--o-----o
|
/
in \ out
/
|
o------/\/\--o-----o

Ideally you'd figure out the impedance of the balanced line, and match
that. Less than a couple of K is probably a good starting point, but I
don't know what they do with guitar audio, so I can't say specifically
what is best.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Thanks Tim

The mic is being plugged into a mic input that is expecting to see a low
impedance into it (in the 200 - 600 ohm region I would think). So would my
parallel resistor be around 600 ohms? Also, does the value of the pad
(series) resistors affect what the value of the parallel resistor should be?
The series resistors will be in the 10K - 15K range by the looks of things.

Thanks