[Paul[_13_]]

On 2/18/2017 1:45 PM, Don Pearce wrote:
On Sat, 18 Feb 2017 20:36:32 +0100 (CET), "Mat Nieuwenhoven"
wrote:

On Fri, 17 Feb 2017 17:24:55 -0600, John Hardy wrote:

On 2/17/17 11:58 AM, Mat Nieuwenhoven wrote:
On 17 Feb 2017 09:26:36 -0500, Scott Dorsey wrote:


The capacity of some capacitors (especially multi layer ceramic) is
dependent on the voltage across them; in some cases the value gets
halved! You don't want such in an audio path if the audio voltage is
a significant part of the blocking voltage (if any). See
http://www.eetimes.com/author.asp?se...oc_id=1330877& .

Also
http://www.intersil.com/content/dam/...n13/an1325.pdf
has a nice table of different capacitor types and their trade-offs.

Mat Nieuwenhoven


Both of those references seem to discuss the shortcomings of the
crappier ceramic capacitors, with just a passing reference to the
premium ceramic capacitors known as the "COG" or "NP0" types. The
COG/NP0 type deserves special consideration. If anyone is interested,
page 8 of my 990 data package describes some of the differences between
the three most common types of ceramic capacitors, the COG/NP0, X7R and Z5U.

http://www.johnhardyco.com/pdf/990.pdf

Very interesting document, thanks. Indeed, the COG/NP0 caps are fine
in this respect.

Are transformer-based mic amps still used? I can see that a
transformer-based gain is essentially noise-free, but aren't they
sensitive to microphone impedance?

One question about the MPC-1 mic pre-amp schematic, if I may. For the
+/- 15V the 78L15/79L15 regulators are used. I thought that these
were quite noisy? I've seen recommenations to use adjustable
regulators ones instead.

Mat Nieuwenhoven



Transformer based gain? It isn't gain - it is just impedance
transformation. And most transformers have a loss about 1dB, which
equates to a 1dB added noise figure. If you want a quiet preamp you do
away with the transformer and design the front end to present a noise
match to the mic. This will also provide the lowest distortion, which
transformers won't manage at the low end.

As for regulator noise, if your preamp has even a half-way decent
PSRR, it simply isn't a factor.


Some interesting reading he

http://www.ti.com/lit/an/slaa414/slaa414.pdf