[Paul[_13_]]

On 2/19/2017 3:38 AM, Don Pearce wrote:
On Sun, 19 Feb 2017 03:17:16 -0700, Paul wrote:

On 2/18/2017 4:40 PM, John Hardy wrote:
On 2/18/17 1:36 PM, 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


The 78L15A and 79L15A are only used for the DC servo op-amp, which is
now the OP97FP. The regulators do not add any noise to that circuit. The
main regulators for the +/-24V power supplies for the 990 op-amps are
LM317 and LM337 with lots of filtering after the regulators, 1000uF per
side on the power supply card and 1000uF per side on each mic preamp card.

Deane Jensen designed the 990 to have very low noise when dealing with
low source impedances. Here is an excerpt from the JE-990 paper that
Deane wrote:

=======
Its application may be considered where some of these parameters are to
be improved:
1) Input stage for any application where the source impedance is 2500
ohms or less,
2) Line output amplifier for driving a 75 ohm load up to an rms
voltage level re 0.775 V of +25 dB, which is an rms voltage of 13.8 V
and a peak-to-peak voltage of 39 V,
3) Summing amplifier,
4) Active filters requiring a high degree of stability,
5)Laboratory preamplifier for extending the sensitivity of noise or
distortion measurements.
=======

Contact Jensen for a copy (www.jensen-transformers.com).

I am increasingly emphasizing the importance of the use of the
lowest-ratio mic input transformer with the 990, the Jensen JT-16-B (or
"A"):

http://www.jensen-transformers.com/w...8/jt-16-a1.pdf

Jensen makes several ratios of mic-input transformers, each one the best
it can be for the ratio that it has. A summary of the specs for those
transformers is here, with links to pdf files for each model:

http://www.jensen-transformers.com/t...ers/mic-input/

The laws of physics dictate that the lower the ratio, the better the
transformer will perform: lower distortion, wider bandwidth, linear
phase response over a wider bandwidth. The trade-off is, the low-ratio
transformer provides less voltage gain than a higher-ratio transformer.
I am sure that this is why Deane came up with the two-stage design (two
990 op-amps in series), known as the Jensen Twin Servo 990 Mic Preamp.
The JT-16 input transformer provides 5.7 dB of voltage gain (I'll update
my specs some day). If you need 60 dB of gain for a particular situation
(ribbon mic, etc.), a preamp with a high-ratio transformer such as the
Jensen JT-115K-E which provides 20 dB of voltage gain would require one
op-amp that provides 40 dB of gain to provide a total gain of 60 dB.
With the JT-16 you get 5.7 dB of voltage gain, so a single 990 would
have to provide 54.3 dB of gain to provide a total of 60 dB. The
two-stage design of the Jensen Twin Servo has each of the two 990
op-amps providing 27.15 dB of gain to get to the total of 60 dB of gain.

In terms of overall noise, the combination of the JT-16 mic-input
transformer and the 990 op-amp is about as quiet as you can get. The
typical voltage gain of 5.7 dB would suggest that you only lose 0.3 dB
along the way. The distortion specs are shown in the pdf for the JT-16
and they are quite low at low frequencies. In the world of mic-input
transformers, the JT-16 is as good as it gets.

Also note that when I converted the 990 to surface-mount in 2013 (except
for the output transistors, which remain in the TO-225AA through-hole
package), I changed the two 0.1 uF power supply bypass capacitors to the
COG/NP0 type. The constant current source filter capacitor was also
changed to the COG/NP0 type. Capacitor manufacturers finally introduced
0.1 uF COG/NP0 caps in the 1206 package at a very reasonable price.

Thank you.


I don't mean to open a big can of worms here but...

What is the current state of the Transformerless Vs. Transformer
mic preamp debate?

Here's what someone said: "Transformers reject RF much better than
transformerless inputs. Transformers exibibit certain non-linear
loading characteristics that some mics like which give a characteristic
sound that you just can't get otherwise (NEVE). Transformers are
generally less transparent. Transformer-less can be more sterile"

Surely the Friis Noise Figure equation applies to any signal
chain, whether RF or audio, but I assume since in actual audio usage,
long XLR cables will be used, so you don't really care too much
about losses in the front end anyways? (this is why cell phone
towers have the pre-amps in the towers, close to the antennas, for
improved signal/noise ratios)

Just curious....




At audio frequencies and impedances that are typically 150 ohms source
to 1500 ohms load, the losses in a mike cable are essentially zero.

From he


https://www.bhphotovideo.com/explora...cable-right%3F

"The issue of frequency-response degradation in cables is sometimes
brought up, and while there is a small potential for a cable to do this,
the problem is largely immaterial except when dealing with extremely
long cable lengths. It takes close to four hundred feet of cable to
produce 1 dB of attenuation at 20 kHz (nominal), which in most live
situations, is virtually inaudible."

This I did not know!

Thanks!


I
have no idea what an adjective like sterile means in an audio context
unless it is a word of praise - the sound of the performer doesn't get
messed with.

Ignoring the transformer (and CMRR these days is more than good enough
without them), the noise of a preamp is set by two factors - voltage
noise and current noise. They are produced by independent mechanisms,
but sum to a noise power at the input. The effects are represented by
two opposite slopes on a graph of noise vs impedance, and the lowest
noise is where they cross. You choose the preamp devices to put that
crossing at the impedance of the mikes you are designing the preamp
for. In other words, a low noise preamp for a high impedance mike is
totally different to one for a low impedance mike. Voltage noise
inserts more noise power into a low impedance, while current noise
puts more into a high impedance.

d

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