Microphones vs. ear sensitivity

[Kryten]

Hi all.

thanks for all the comments, very educational to this mainly digital guy.

I'd assumed the input impedance should match the source for maximum power
transfer.
Seems this does not give the best quality.

Surprised to hear batteries are noisy. I had a decent decoupler.

Never let the front-end transistor amplify flat out (grounded emitter)

crap circuit

I plugged the preamp into my good-quality hi-fi, and found far less hum than
using my TV.
No surprise there. Turning up the gain, I found I was picking up radio!
Sticking it in a crude faraday cage (okay, biscuit tin) didn't help either.

Sigh, more prototyping to do and more complex circuits.


JT> (1) Shunt feedback stage with a pot as the source impedance.

Hmm.. I thought "hello, that looks a bit odd"

JT> (2) CB input stage... barf.

I had to read up on why people used a transistor for unity gain, and found
that it was useful for buffering signals that need a low input impedance
(like 75R video) and avoiding the Miller effect.

Ban > You better use a specialized IC with your state of knowledge,
Ban > then you get at least some reference circuit and a
Ban > good result.

Yep, I'd be happy to buy a chip specifically designed for dynamic mic
preamping.
I expected they would be pretty common, but haven't seen any.

Ban > Win made a proposition for a good Mic preamp last year

Thanks, I'll look for it.

Has it been tested and the performance measured?

 

[Pooh Bear]

Kryten wrote:

Hi all.

thanks for all the comments, very educational to this mainly digital guy.

I'd assumed the input impedance should match the source for maximum power
transfer.
Seems this does not give the best quality.

Indeed. Power matching was only needed when gain was expensive.

You get better results with 'voltage matching'. I.e. low source Z higher input
Z.

Graham

 

[René]

On Tue, 07 Jun 2005 14:08:44 GMT, "Kryten"
<kryten_droid_obfusticator@ntlworld.com> wrote:

"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:429BBEFD.F8C0ECFB@hotmail.com...
Kryten wrote:

Just been playing with some cheap (ECM-66B) microphone inserts
The S/N ratio is quoted at 40 dB, and sensitivity at 60 dB/microbar.

In short, it's rubbish.

Okay, it is pretty small so not got a great sensing area.
And for Ł3 I did not expect much.

I went out and bought a dynamic mic insert for Ł5, and this is labelled as
having 76 dB sensitivity. It is a lot bigger and heavier.
http://www.maplin.co.uk/Module.aspx?ModuleNo=29594 (type DM13)

I made up this preamplifier
http://www.zen22142.zen.co.uk/Circuits/Audio/dyn_mic/dyn_mic.htm
which is all-discrete so should have low noise.

However I didn't find it was much improvement.
There is noticeable hiss and buzz.

Hiss seems to be coming from the first transistor (BC109, common base) and
the buzz from the mic insert, because the buzz disappears when the signal
input is shorted to ground. The hiss and buzz disappears when there is no
signal to the other transistors.

Circuit constructed on a bit of veroboard - not ideal but fairly neat and
compact.

So, is this mic insert also crap, and if so how much would I expect to pay
for a good one?

Also, is this BC109 transistor just too noisy?

Or am I going to have to make a much more complex pre-amp like this one:

http://sound.westhost.com/project66.htm

Cleaning the power line (1K - 220 uF, that sort of thing), may help
tremendously. Batteries are noisy! Power supplies more so.

Gain distribution, not too much gain in any stage. e.g. ~20x in the
transistor, same in the opamp. Never let the frontend transistor
amplify flat out (grounded emitter)

As I had the pleasure of commenting before - a junkbox electret /
transistor / low end opamp ckt. easily matched my hearing treshold.
(when I built it -long ago- my hearing was not all that bad actually)

--
- René