Two Questions on Electrets
« on: February 18, 2019, 12:28:51 AM »
1. Since electrets have a fixed charge and don't take a charge to the capsule, is it correct that pretty much any electret capsule can work with any other electret microphone preamplifier, and if not, what are the parameters?

2. How long can the leads between the preamplifier and the capsule be, in practice, if shielded, without significant degradation of the signal?

2a. Is the "self-noise" spec really attributable to the capsule and not the preamplifier? I have seen cases of a capsule sold individually with a much higher noise spec stated than the commercially available mic using that capsule (e.g. TSB-2555B in the CAD e100s.).


RuudNL

Re: Two Questions on Electrets
« Reply #1 on: February 18, 2019, 03:56:11 AM »
Any electret capsule will produce a certain voltage by itself without a polarisation voltage.
So you can exchange one electret capsule for another.
As with 'real' condenser capsules, an electret capsule should be terminated with a pretty high impedance, in order not to load the capsule. This goes also for a capacitive load, so you should keep the wires between the capsule and the impedance converter/amplifier as short as possible.
This is the reason that a lot of electret capsules have the impedance converter already integrated in the capsule enclosure.

My experience is that electret capsules are usually noisier than externally polarized capsules, but there may be exceptions.
There is a solution for every problem!

http://www.vansteenisaudio.nl

Re: Two Questions on Electrets
« Reply #2 on: February 18, 2019, 04:08:02 AM »
My experience is that electret capsules are usually noisier than externally polarized capsules, but there may be exceptions.

Thanks very much.
I'm still confused about whether the noise spec really reflects the capsule or the preamp.
Obviously noise from the capsule is attributable to brownian motion of air on the diaphragm, if I understand right.
But for example if an electret capsule (a nickel measurement mic) is spec at 14db self-noise, is that the level of noise regardless of which preamplifier you use? No, right. When you see an electret like the CAD e100s and it is showing a noise spec of 3.7db, it is not the cheap capsule, right?

This is interesting:
"The mic’s input stage has been described as a “quadra-FET” design. It has four FETs, the specific selection and application of which are part of the secret of the mic’s extremely low noise floor." http://recordinghacks.com/microphones/CAD/E100S

I am curious to pair an electret measurement mic with a quiet electret preamp like that.

analogguru

Re: Two Questions on Electrets
« Reply #3 on: February 18, 2019, 06:15:34 AM »
Which "electret measurement mic" do you want to use ?
If you think about  one based on WM60A / WM61A capsules forget it, they have the impedance converter FET inside the capsule. And the description of the CAD E100S is not about a PREAMP it is about an "input stage" which is the impedance converter.

Khron

Re: Two Questions on Electrets
« Reply #4 on: February 18, 2019, 06:36:20 AM »
Well, with a bit of care and patience, one could always perform a "Linkwitz mod" on one of those capsules.
Coupled with an "Alice" circuit, that'll definitely increase the headroom-before-clipping, but i'm not sure how noise would be affected.

Which "electret measurement mic" do you want to use ?
If you think about  one based on WM60A / WM61A capsules forget it, they have the impedance converter FET inside the capsule.

Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

Re: Two Questions on Electrets
« Reply #5 on: February 18, 2019, 01:04:13 PM »
Which "electret measurement mic" do you want to use ?
If you think about  one based on WM60A / WM61A capsules forget it, they have the impedance converter FET inside the capsule. And the description of the CAD E100S is not about a PREAMP it is about an "input stage" which is the impedance converter.

No, I'm talking about a real measurement mic -- the Bruel & Kjaer. They come in 0v prepolarized (electret) versions.
These answers reflect why I asked about whether all electret capsules can be used with all electret preamplifiers.
That's interesting about the "input stage" -- at the risk of being pedantic, that is in the preamplifier (those four FETs are on the PCB there) so that's what I am trying to figure out here. 

RPC360

Re: Two Questions on Electrets
« Reply #6 on: February 18, 2019, 07:37:29 PM »
All electret capsules without internal electronics should work with all electret impedance converters/preamplifiers. Note that regarding noise there are two issues:
1) the noise of the air against the capsule; this goes down by 6dB with every doubling of capsule diameter, but other aspects of microphone behavior degrade, so this is a compromise.
2) the electrical noise of the impedance converter/preamplifier. This is a tradeoff between low voltage noise and low input capacitance.
Incidentally, shielding the cable between capsule and impedance converter is a very bad idea if the shield is grounded, as the cable becomes a capacitive attenuator. However, a "driven shield" arrangement where the shield has the same voltage as the conductor will eliminate this effect.

Re: Two Questions on Electrets
« Reply #7 on: February 19, 2019, 01:04:09 AM »
well for its membrane diameter the 4006 is quite low noise.
I dont think that the driving noise source is the impedance converter....

Re: Two Questions on Electrets
« Reply #8 on: February 19, 2019, 01:47:12 AM »
well for its membrane diameter the 4006 is quite low noise.
I dont think that the driving noise source is the impedance converter....

Well, the spec is about 14db self-noise.
The capsule is 12mm.
The CAD e110s has a capsule of 21mm.
Self-noise spec 3.7db.
Even if the capsule was twice the diameter and down 6db... it's more than that. So as I said above, the CAD's four-FET preamplifier makes the difference, all other things being equal. It uses a very cheap capsule. There are FET based preamps that yield "zero" self-noise nowadays. Mics haven't developed much, but technology didn't entirely stop in the 60s...

analogguru

Re: Two Questions on Electrets
« Reply #9 on: February 19, 2019, 05:40:41 AM »
The capsule is 12mm.
Well, the spec is about 14db self-noise.
.....
"11-15 dB-A is still very good. You may be able to discern some very slight noise in critical applications. But usually, such noise is impossible to hear in the context of the whole mix."
https://www.neumann.com/homestudio/en/what-is-self-noise-or-equivalent-noise-level


The CAD e110s has a capsule of 21mm.
Self-noise spec 3.7db.
.....
3.7dB ?  If this alleged claim is true, do you believe every sales-lie what you read in the web ?

"Anything below 10 dB-A is extremely low noise. The exact figure is unimportant, as even a very quiet recording room will contribute quite a bit more ambient noise than 10 dB-A. Typically, extremely low self-noise figures are only found on modern day large diaphragm condenser mics, such as the Neumann TLM 103."
"The proper way to measure self-noise is to put the entire microphone into a soundproof container. However, some manufacturers simply measure the microphone without the capsule. The latter method gives (seemingly!) better figures."
https://www.neumann.com/homestudio/en/what-is-self-noise-or-equivalent-noise-level
« Last Edit: February 19, 2019, 08:03:08 AM by analogguru »


Re: Two Questions on Electrets
« Reply #10 on: February 19, 2019, 06:06:00 AM »
I have seen no evidence that the CAD e100s has an electret capsule. Perhaps you're confusing it with the e100 or e100^2? Both of them are electret.

mhelin

Re: Two Questions on Electrets
« Reply #11 on: February 19, 2019, 09:18:17 AM »
I have seen no evidence that the CAD e100s has an electret capsule. Perhaps you're confusing it with the e100 or e100^2? Both of them are electret.
There are rumours on the internet, and also if you zoom into the image of e100s  you'll recognize the visuals. Also TSB2555B uses nickel  diaphragm, the CAD's spec sheet just says "Operation Principle ... Condencer" but for e300s "Externally Biased Condencer".

Can't find the schematics for e100s, but it's got four FET's + two 5087's and then some other  SMD  transistors and components o n the PCB. I guess that the circuit is some kind of bootstrapped one like on LSK489 application note (http://www.cordellaudio.com/JFETs/LSK489appnote.pdf, Figure 6 c).
Mikko

rogs

Re: Two Questions on Electrets
« Reply #12 on: February 19, 2019, 09:34:50 AM »
It seems as if  the actual noise floor of the CAD e100s (as opposed to the noise floor of the published spec) can vary quite a lot between units....

One of  the You Tube reviewers  had noise problems with his, and returned it to the manufacturer... 
(see the full story  here: https://www.youtube.com/watch?v=laeTTf604LE  )

So it looks like there's an element of luck as to whether or not your CAD e100s  is actually a low noise mic?....

rogs

Re: Two Questions on Electrets
« Reply #13 on: February 19, 2019, 03:02:51 PM »
All the reviews of the mic attest to the low noise. ...
I'm afraid that just isn't so ....the You Tube review  I linked to in my post above will attest to that.

It looks like there are big noise variations between different examples of this mic - 
So while some of them may meet the published noise spec, it looks like some -  perhaps many? - don't...
« Last Edit: February 19, 2019, 03:35:00 PM by rogs »

Re: Two Questions on Electrets New
« Reply #14 on: February 19, 2019, 05:34:33 PM »
I'm afraid that just isn't so ....the You Tube review  I linked to in my post above will attest to that.

It looks like there are big noise variations between different examples of this mic - 
So while some of them may meet the published noise spec, it looks like some -  perhaps many? - don't...
Yeah I've heard many CAD e100s's that are really noisy. In fact I've only ever heard samples of it where it is noisy (relative to the proposed 3.7 dB self noise). At 3.7 dB we shouldn't be able to hear a thing.

At least the Lewitt LCT 550 has a disclaimer that it will never truly be 0 dB self noise because even air hitting the diaphragm is around 4 dB IIRC.
« Last Edit: February 19, 2019, 05:38:35 PM by Icantthinkofaname »


 

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