rogs

Calculating capsule bias voltages..
« on: January 05, 2019, 04:49:24 PM »
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'.... ?

« Last Edit: January 05, 2019, 05:37:12 PM by rogs »


Tim Campbell

Re: Calculating capsule bias voltages..
« Reply #1 on: January 06, 2019, 05:11:33 AM »
There are no rules beyond using a voltage that doesn't collapse the membrane under almost all conditions.
For my own CT12 capsule I found that 55v sounds a little better and I have used the same capsule in designs using 48v phantom power that have also sounded terrific.
I know that some here like polarizing the capsule as high as possible. I find that higher voltages stiffen the membrane and so reduce frequency response.

rogs

Re: Calculating capsule bias voltages..
« Reply #2 on: January 06, 2019, 06:57:15 AM »
Thanks for your reply - and thanks for your tips and observations - much appreciated.
I think I shall stay with 50V  for the time being....  As I say, the drop in sensitivity form the 60V bias seems pretty insignificant, and so far it seems to be working fine.

RuudNL

Re: Calculating capsule bias voltages..
« Reply #3 on: January 06, 2019, 07:42:31 AM »
a 0.3 micron capsule is pretty thin!

I doubt that your capsule is 0.3 micron!
3 micron seems more realistic.
(3-6 micron are the more 'usual' values.)
There is a solution for every problem!

http://www.vansteenisaudio.nl

rogs

Re: Calculating capsule bias voltages..
« Reply #4 on: January 06, 2019, 09:00:09 AM »
Yes - sorry, I should have realised that the figure of '0.4microns' listed on one of the spec. sheets was a nonsense.
Thanks for the correction...

It does make you wonder just how accurate many of the other claims in  these 'Chinese' specs are?

Having said that, these capsules do allow us newbies to have a 'play'  with mic experiments, without spending out a lot of cash.

Here in the UK these 34mm capsules are quite a lot cheaper than the Transound electrets most beginners seem to start out with. Those capsules are cheaper of course, but once you've added the shipping costs from the US, they work out more expensive than the 34mm LDCs....


analogguru

Re: Calculating capsule bias voltages..
« Reply #5 on: January 06, 2019, 09:25:11 AM »
......
Here in the UK these 34mm capsules are quite a lot cheaper than the Transound electrets most beginners seem to start out with. Those capsules are cheaper of course, but once you've added the shipping costs from the US, they work out more expensive than the 34mm LDCs....
What ? Where do you get a 34mm LDC for less than € 10,-- (shipped) ?

I mean the Transound 16mm capsule can be found in a lot of chinese mics (BM-700, BM-800) which you can get for ~ € 10,-- when you have luck.  And if you want a 25mm electret capsule you can buy a broken Samson Meteor for the same money.  BTW, did you know that the Rode M2 is using nothing else than a 16mm Transound capsule ?




analogguru

Re: Calculating capsule bias voltages..
« Reply #6 on: January 06, 2019, 09:40:23 AM »
What ? Where do you get a 34mm LDC for less than € 10,-- (shipped) ?

I mean the Transound 16mm capsule can be found in a lot of chinese mics (BM-700, BM-800 style) which you can get for ~ € 10,-- when you have luck.  And if you want a 25mm electret capsule you can buy a broken Samson Meteor for the same money.  BTW, did you know that the Rode M2 is using nothing else than a 16mm Transound capsule ?

rogs

Re: Calculating capsule bias voltages..
« Reply #7 on: January 06, 2019, 02:31:13 PM »
Prices can vary - quite a lot!  The best I've done for a 34mm LDC is £1.95 (incl. shipping) from Ebay. .. (see attached copy of the invoice)
Mind you, that did include a £10 coupon from Ebay - which came as an unexpected extra.....
Even so, £11.95 shipped wouldn't have been  too bad either!

Prices here in the UK are typically around £16/£18 (incl. shipping) ... although fluctuations in the strength of the pound can make quite a difference from day to day...They are still still less than  a TSB165A from JLI in the US...
Although the capsule price is only $9, the shipping to the UK would add another $22 to that cost.

As you say, some of the BM800 style mics come with a TSB160a or similar - although some come with cheap 9mm electrets 'disguised' as 16mm capsules - and those aren't so good.

I think the TSB 2555b capsule gets some good reports, but again the shipping costs from the US leaves the the 34mm   LDCs as better value here in the UK...

It can be interesting  playing around with the EQ to  try and correct the sometimes 'bright'  character of some of the Chinese 'K67' type LDCs.... but as I'm only doing this as a hobby, that's all part of the fun!!

(Re: the Rode M2 -- according to the Rode spec. sheet the capsule is only 12mm  (0.5") so I wonder what capsule they're actually using there?.....)
« Last Edit: January 06, 2019, 02:50:57 PM by rogs »

analogguru

Re: Calculating capsule bias voltages..
« Reply #8 on: January 06, 2019, 07:42:28 PM »
Prices can vary - quite a lot!  The best I've done for a 34mm LDC is £1.95 (incl. shipping) from Ebay. .. (see attached copy of the invoice)
Mind you, that did include a £10 coupon from Ebay - which came as an unexpected extra.....
Even so, £11.95 shipped wouldn't have been  too bad either!
.....
Wow, that is really cheap.....

.....
(Re: the Rode M2 -- according to the Rode spec. sheet the capsule is only 12mm  (0.5") so I wonder what capsule they're actually using there?.....)
Never believe any sales lies..... On the box of my Rode M2 is printed " 1/2" True Condenser Capsule".  But it is neither 1/2" nor "true condenser".  In reality it is a 16mm diameter capsule with "TS" printed on the side.  "TS" normally stands for TranSound. On the back are 2 x 2 holes, so it is clearly a Transound TSB-160A-GP capsule.

(Unfortunately the upload folder is full, otherwise I would have uploaded a picture)
« Last Edit: January 06, 2019, 07:49:44 PM by analogguru »

 

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