microphone circuit

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leswatts

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My latest mics use pol voltage reduction pads (at Ricardo's reccomendation). It's more about totally silent pad switching than distortion though.
I was not willing to have the noise penaltly of bleeder resistors, and even with them a pop is not completely eliminated...because real capacitors have memory...in other words they are (poor) electrets. I came to this decision after having a musician damn near blow out
his PA switching in a cap pad at a live event with one of my prototypes. Just saying "don't do that" was not a solution for me.  :)

I had very little response change with the different polarization levels, but that depends on the capsule parameters. It can be very little or a lot.

On the distortion...This has been long known...probably since Wente in the 1920s. It happens.
If one goes through the numbers it can be seen that stray capacitance induced distortion goes up rather gradually...
really the biggest change is the first 10-20% of capsule active capacitance. A large pad doesn't increase it so much more.
Perhaps a few percent at very high levels. Both even and odd harmonics are present, but it's mostly second and third harmonics.
Typically 90%+ is second harmonic. One could argue that it actually gives a desireable sound color. One could also argue that
at reasonable SPL(say < 100dB) the distortion is inaudible. But we don't switch the pad in at reasonable SPL do we?

Les
L M Watts Technology
 

bockaudio

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ricardo said:
bockaudio said:
I wonder, for those who are changing polarzation voltage is if they have accounted for the change in frequency response, which becomes more dramatic as the membrane size decreases, or it doesn't matter-whatever?
In the Soundelux U195 I did this (non-capacative pad) in 1995, and wonder if it had been done before. My argument against capacitive pads was from the music engineer's view, as I and every other engineer I knew felt the cap pads altered the sound too much, not from a pure EE point of view.
I did the pad with reduced Vp on the Calrec Soundfield Mk4 circa 1980(?).

I'm not sure either method changes the frequency response enough in the audio range that it matters.  I've never designed a mike with a cap pad so can't claim to have done extensive listening tests.  There is more distortion but again I'm not sure enough to matter unless you were interested in specs rather than sound.
Excellent, thank you Ricardo. So I wasn't the first to do something, again! (I was unaware of the existence of the Calrec Soundfield mic in 1995). But to the point of changing the sound of a capsule, at least on k67 capsules and k83 or 84 capsules, the voltage method produces measurable differences in frequency response. I've done the measurements, repeatedly. Have not done it on every capsule type, but the effect is more dramatic on the smaller membranes. Even Neumann admitted so much, in the Studio Sound review of the TLM50 mic from years ago.
I have not done any measurements on the effects of capacitive pads.
 

bockaudio

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My latest mics use pol voltage reduction pads (at Ricardo's reccomendation). It's more about totally silent pad switching than distortion though.
I was not willing to have the noise penaltly of bleeder resistors, and even with them a pop is not completely eliminated...because real capacitors have memory...in other words they are (poor) electrets. I came to this decision after having a musician damn near blow out
his PA switching in a cap pad at a live event with one of my prototypes. Just saying "don't do that" was not a solution for me.  :)
Very reasonable.

I had very little response change with the different polarization levels, but that depends on the capsule parameters. It can be very little or a lot.
I found response changes once you get over a 15v differential in the capsule types I mentioned in my last posts. I was unconcerned about anything less.
On the distortion...This has been long known...probably since Wente in the 1920s. It happens.
If one goes through the numbers it can be seen that stray capacitance induced distortion goes up rather gradually...
really the biggest change is the first 10-20% of capsule active capacitance. A large pad doesn't increase it so much more.
Perhaps a few percent at very high levels. Both even and odd harmonics are present, but it's mostly second and third harmonics.
Typically 90%+ is second harmonic.
Well that explains the "murky" sound of 87's and 67's with the pad engaged. I suppose Neumann chose that compromise over the other (change in Freq response).
One could argue that it actually gives a desireable sound color.
I don't know ANY audio engineers who would say that.
But we don't switch the pad in at reasonable SPL do we?
Some engineers  do it preemptively.

 

ricardo

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ricardo said:
I did the pad with reduced Vp on the Calrec Soundfield Mk4 circa 1980(?).

It certainly wasn't new when I did it.  The real reason I did it was the remote control box brought things up to Line Level with a stupid amount of gain and range.  It was a way to change gain quietly at very high spls.

But to the point of changing the sound of a capsule, at least on k67 capsules and k83 or 84 capsules, the voltage method produces measurable differences in frequency response. I've done the measurements, repeatedly. Have not done it on every capsule type, but the effect is more dramatic on the smaller membranes. Even Neumann admitted so much, in the Studio Sound review of the TLM50 mic from years ago.
Calrec's ancestry is AKG via Clem's (my mike mentor at Calrec) friendship with Bernhard Wiengartner (who did the C12).  We sneer at the N**mann centre terminated design.  :p  I'll claim its their rotten capsules.  ;D

I see AKG under Harman, have gone over to the Dark Side and introduced centre terminated capsules too.  :'(
 

bockaudio

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It certainly wasn't new when I did it.
Who did it first? All the example I have are capacitance based.
The real reason I did it was the remote control box brought things up to Line Level with a stupid amount of gain and range.  It was a way to change gain quietly at very high spls.
Yes the line level mic makes the most sense from a pure engineering viewpoint.

Calrec's ancestry is AKG via Clem's (my mike mentor at Calrec) friendship with Bernhard Wiengartner (who did the C12).  We sneer at the N**mann centre terminated design.  :p  I'll claim its their rotten capsules.  ;D
Well they certainly are rotten, especially those pesky U47's. ;D. Agreed the edge termination is better regardless of diameter, for midband flatness. Great that you got to work with those guys. Did you ever end up dealing with Mr. Giles when he took over the company?
I can't help but wonder, if you've ever seen an xlr input labeled "mike"? (not making fun of you, just curious).
I see AKG under Harman, have gone over to the Dark Side and introduced centre terminated capsules too.  :'(
[/quote]Yes, dark like this: I saw an advert for the Perception 100 four years ago, it had a distinctive brass ck12 behind the wire mesh in the photo. I ordered one immediately! When it showed up, Chinese center terminated k67 in the mic. I saved the photo from the original packaging as a souvenir. Point is SOMEONE at AKG knew exactly what they were doing in deceiving the customer.
I have seen several new nylon ck12's with the snap in membranes where you can rotate the membrane at will. :(
 

ricardo

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I did the pad with reduced Vp on the Calrec Soundfield Mk4 circa 1980(?).
bockaudio said:
Who did it first? All the example I have are capacitance based.
I've no idea.  Maybe we were the first to do this but it seems pretty obvious with hindsight.  It's all 30 yrs ago for me.

Great that you got to work with those guys. Did you ever end up dealing with Mr. Giles when he took over the company?
I went back to my first love, speakers, in 1983(?)  AMS bought Calrec soon after.  It didn't work out and some of the original directors bought the company back but left the Soundfield with AMS.  Ken Giles bought it from AMS but much later.

I can't help but wonder, if you've ever seen an xlr input labeled "mike"? (not making fun of you, just curious).
Yus Yanks kunt spul!  Dis be propa Yorkshire spullin'  ;D
 

Gus

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I checked  to see what happened with this thread and noticed that today there are 2117 views. 
Has anyone adjusted there MXLV67, MXL2001 etc to this circuit? 
If so any feedback? 

 

Enchilada

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Hey Gus,

Are you still around? Do you know if anyone ended up having a punt at this modification? I've not used my V67G in years so I'm keen to give it a crack. Any recommendations/requests for testing methodology?

Kris
 

naive1073

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Finally got round to this and really like it, very rich and full bodied with a Russian polyprop .47uf. The rasp of the original has gone, s's are like chalk and cheese as if the sample rate had been upped 4x, everything has more detail. I've blundered somehow because there's more hiss than before but the increase in quality already overshadows that. Of course I worked out a reason for the de-emphasise move significance as soon as I engaged my brain.  Thanks Gus, I enjoyed this and learnt some things too.
 

Gus

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Thanks for the feedback. 
As I posted this uses most of the stock parts
You could try a different jfet and rearrange the jfet bias/ gain setup
change the EQ etc.
 

James-F

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Nov 16, 2019
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Why did I use a 47K in the JFET stage?
Why the bias change to the EF section and its input resistance?
Why the cap and resistor value adjustments?
Why...
Anyone try this adjustment to the circuit?

I really wish I could see you schematic and mods @Gus .

Here is my summary of useful mods, including schematic and LTSpice file.

 
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