rogs
Well-known member
As a relative newbie, I've been enjoying experimenting with some cheap centre terminated Chinese 34mm capsules, used in conjunction with a pretty standard Schoeps type circuit, and a simple voltage multiplier to generate the capsule bias voltage.
(Some notes here of my first attempts: http://www.jp137.com/lts/LDCX2.notes.pdf )
With the component values shown, I use a 60V bias for the capsule, which seems to work quite well. Certainly significantly more sensitive and quieter than any of the electrets I've tried previously!
I then moved on to trying a 'Chinese' edge terminated C12 style capsule, which turned out to be even more sensitive, and sounded pretty good.....
However, after a few hours of use, the sensitivity suddenly dropped dramatically, and the output sounded very odd.
A quick examination suggested that the capsule had partially collapsed (the surface was no longer completely smooth)...
So I reduced the bias voltage to 50 volts (simply a different 'tap' off the multiplier chain)...The sensitivity was obviously reduced - but only by a couple of dB or so.
The mic has performed fine since then, with no further 'collapses'.
Although I know that some Chinese 'specs' can be notoriously inaccurate, the ones I've bought list the centre tapped capsules as 0.4 micron thick, and the edge terminated as 0.3 micron..... Which perhaps helps explain what I encountered.
What I can't seem to discover is just what bias voltages are considered 'safe' limits for different capsules...
I have to say I was a little surprised that 60V caused problems, although I can see that a 0.3 micron capsule is pretty thin!
Are there any hard and fast rules for these capsule bias voltage calculations, or is it just 'suck it and see'.... ?
(Some notes here of my first attempts: http://www.jp137.com/lts/LDCX2.notes.pdf )
With the component values shown, I use a 60V bias for the capsule, which seems to work quite well. Certainly significantly more sensitive and quieter than any of the electrets I've tried previously!
I then moved on to trying a 'Chinese' edge terminated C12 style capsule, which turned out to be even more sensitive, and sounded pretty good.....
However, after a few hours of use, the sensitivity suddenly dropped dramatically, and the output sounded very odd.
A quick examination suggested that the capsule had partially collapsed (the surface was no longer completely smooth)...
So I reduced the bias voltage to 50 volts (simply a different 'tap' off the multiplier chain)...The sensitivity was obviously reduced - but only by a couple of dB or so.
The mic has performed fine since then, with no further 'collapses'.
Although I know that some Chinese 'specs' can be notoriously inaccurate, the ones I've bought list the centre tapped capsules as 0.4 micron thick, and the edge terminated as 0.3 micron..... Which perhaps helps explain what I encountered.
What I can't seem to discover is just what bias voltages are considered 'safe' limits for different capsules...
I have to say I was a little surprised that 60V caused problems, although I can see that a 0.3 micron capsule is pretty thin!
Are there any hard and fast rules for these capsule bias voltage calculations, or is it just 'suck it and see'.... ?