[Scott Dorsey]

Mat Nieuwenhoven wrote:
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?

Yes, that's where the free gain comes from. You trade admittance on
the input for it. It's not noise free, though, because of the thermal
noise of the transformer which can be substantial if you want a high
step-up.

There is a classic JAES paper from the 1980s by Marshall Leach which
is probably still on his website, which does the math for various input
topologies. Sometimes it is a win for noise and sometimes it is not.

But more importantly.. the transformer buys you ENORMOUS CMRR, and a
free RF low-pass filter. I once worked in a club where my remote truck
ground was 65V away from the stage ground... but there was no hum because
that's how good the Jensen splitter transformers were.

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.

They are very noisy, but it's all high frequency noise so a filter is not
hard. Requires careful layout, though. You can spend a lot more money for
expensive LT regulators if you are tight on space though.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[John Hardy]

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.

John Hardy
The John Hardy Co.
www.johnhardyco.com

[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

[gregz]

Scott Dorsey wrote:
In article , Paul wrote:

In contrast, you can listen anywhere in front of the Yammies, and
although they may sound bright, and have tons of "presence", they never
get HARSH like the piezos!

Okay, you saw that 4kc peak on the piezos. Now, assuming you're using a
12dB/octave filter, if you crossed over at 8kc then the peak would only be
12dB down. You'd probably want to cross over at 16kc in order to really
control the problem, or use a sharper filter. Which kind of makes it useless.

You'd think maybe you could use a Zobel network like you would with an
electrodynamic tweeter to control the peak, but really nobody has ever been
able to make that work well. It might not be minimum phase.

It MIGHT be possible to use an acoustic network in order to deal with the
problem, but it's hard to do that and not screw up the pattern.

In the 1980s some grad student built a PA speaker system using a horizontal
array of piezo tweeters and phase-shift networks, which somehow wound up in
the EE department auditorium at gatech, probably because nobody else wanted
them. They had oversized Motorola drivers with the worst of the ugliness
around 1kc, and they were crossed over around 5kc using a conventional
midrange driver. Even though the plot wasn't so horrible, there was still
severe harshness due to the nonlinearities.

Now... Jon Dhalquist with the DQ-10 actually did use a piezo tweeter and
did actually get some benefit from it. But he was crossing them over at
18 KHz or so and using them only as a supertweeter in order to add a little
more air.
--scott

Arrays tend to emphasize the lower harsh peak. Additive phase. I once cut
off half the horn length. That helped. Use a 2nd order crossover above 8
kHz.

Greg

[gregz]

Richard Kuschel wrote:
On Wednesday, February 15, 2017 at 9:13:13 AM UTC-7, Scott Dorsey wrote:
Snip

Now... Jon Dhalquist with the DQ-10 actually did use a piezo tweeter and
did actually get some benefit from it. But he was crossing them over at
18 KHz or so and using them only as a supertweeter in order to add a little
more air.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

A friend on mine had a pair of those Dahlquists and they were a good sounding system.
I knew that they had piezo tweeters, but wasn't awate that they were crossed that high.
A piezo will respond to a 40kHz signal but all that is going to do is annoy the dog.

I have a piezo in a mini box, 555 oscillator, used to test animals and
people.

Greg

[Paul[_13_]]

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....

[Don Pearce[_3_]]

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. 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

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

[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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[Don Pearce[_3_]]

On Sun, 19 Feb 2017 06:32:04 -0700, Paul wrote:

"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!

This is where simplistic cables models fall over. Four hundred feet of
cable is enough that at 20kHz a real, distributed model will give a
correct answer, but the lumped C/L/C model has failed. Four hundred
feet of typical 300 ohm mike cable connecting a 150 ohm mike to a 1500
ohm preamp will actually result in about 0.25dB RISE at 20kHz, not a
drop. The reason for this is that at high frequency the cable is
starting to act as a transformer, slightly improving the match between
150 ohms and 1500 ohms.

d

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[Scott Dorsey]

In article , Paul wrote:
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!

This is why audio folks use low-Z balanced systems. That estimate is kind
of silly, though, as you really need to specify the source and load impedances
as well as the cable. With the Schoeps mikes that have a 50 ohm source Z,
you can run longer than 400 ft. before getting 1dB down. Which is good for
classical work where running a thousand feet of cable from suspended mikes
out to the truck is not unusual.

But... there are two cases you will encounter frequently in the studio where
this isn't necessarily the case, and both have to do with high-Z systems.
The first, obviously, is guitar amp cables. Very high impedance line, so
it becomes a huge issue not only the shunt capacitance of the cable but with
some weird cable designs even the series inductance.

But the second one isn't so obvious, and it's the ribbon microphone. Ribbons
have pretty weird source impedances which can be pretty high at some
frequencies, so a lot of people like to use preamps with very high load Z
so that source impedance is not a problem. But, when you do that, the cable
then does start to be a problem, especially cables with a lot of shunt
capacitance like star-quad types.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Phil Allison[_4_]]

Don Pearce wrote:



This is where simplistic cables models fall over. Four hundred feet of
cable is enough that at 20kHz a real, distributed model will give a
correct answer, but the lumped C/L/C model has failed. Four hundred
feet of typical 300 ohm mike cable ...




** Who sells "300 ohm mic cable" ??.

Or are you saying typical twisted pair mic cables have a characteristic impedance of 300ohms in the audio range ?

IME, 400 feet of common or garden mic cable will significantly attenuate high frequencies from a mic like the SM58 and most others - assuming there is the usual 1500 ohms load at the other end.

The rated impedance of a mic cable is not defined anywhere I can find, but IMO ought to be the value of terminating resistor that minimises or eliminate shunt capacitance in and somewhat beyond the audio range.

Users would at least then know how to get the best HF response with long runs.


..... Phil








connecting a 150 ohm mike to a 1500
ohm preamp will actually result in about 0.25dB RISE at 20kHz, not a
drop. The reason for this is that at high frequency the cable is
starting to act as a transformer, slightly improving the match between
150 ohms and 1500 ohms.

[Paul[_13_]]

On 2/19/2017 5:55 PM, Phil Allison wrote:
Don Pearce wrote:



This is where simplistic cables models fall over. Four hundred feet of
cable is enough that at 20kHz a real, distributed model will give a
correct answer, but the lumped C/L/C model has failed. Four hundred
feet of typical 300 ohm mike cable ...




** Who sells "300 ohm mic cable" ??.

Or are you saying typical twisted pair mic cables have a characteristic impedance of 300ohms in the audio range ?

IME, 400 feet of common or garden mic cable will significantly attenuate high frequencies from a mic like the SM58 and most others - assuming there is the usual 1500 ohms load at the other end.

The rated impedance of a mic cable is not defined anywhere I can find, but IMO ought to be the value of terminating resistor that minimises or eliminate shunt capacitance in and somewhat beyond the audio range.


Time Domain Reflectometry (TDR) is used in telecommunications to
find impedance discontinuities in cables.

RF engineers use the stand wave ratio (SWR) to find the least
amount of reflected power with a given termination load.

Not sure if audio frequency people use these techniques....








connecting a 150 ohm mike to a 1500
ohm preamp will actually result in about 0.25dB RISE at 20kHz, not a
drop. The reason for this is that at high frequency the cable is
starting to act as a transformer, slightly improving the match between
150 ohms and 1500 ohms.

[Scott Dorsey]

Phil Allison wrote:
Don Pearce wrote:

This is where simplistic cables models fall over. Four hundred feet of
cable is enough that at 20kHz a real, distributed model will give a
correct answer, but the lumped C/L/C model has failed. Four hundred
feet of typical 300 ohm mike cable ...

** Who sells "300 ohm mic cable" ??.

Or are you saying typical twisted pair mic cables have a characteristic impedance of 300ohms in the audio range ?

Actually, typical 24 ga mike cable tends to run around 110 ohms or so, which
is why that was picked as the impedance for AES/EBU, since they wanted to be
able to run AES/EBU over existing cable plant.

Older 18 ga mike cable with thicker rubber comes in around 150 ohms.

If you wanted to get up to 300 ohms you'd have to separate the conductors
more, or make them tiny.

IME, 400 feet of common or garden mic cable will significantly attenuate high frequencies from a mic like the SM58 and most others - assuming there is the usual 1500 ohms load at the other end.

If you're using an SM58, you probably want a load of around 600 ohms in order
to make it happy. If you're using a condenser microphone, the output Z is
likely far lower than the load and so if you're thinking about it as a lumped
sum system with the impedances in parallel the source impedance is dominant.

The loading issues are more of an issue than the cable issues.

The rated impedance of a mic cable is not defined anywhere I can find, but IMO ought to be the value of terminating resistor that minimises or eliminate shunt capacitance in and somewhat beyond the audio range.

Users would at least then know how to get the best HF response with long runs.

The problem is that microphone noise and microphone dynamic behaviour due to
electrical damping (in the case of dynamic mikes) are usually much more
important than any HF behaviour.

Still, the HF behaviour gets more interesting with cables like star quad that
have far higher shunt capacitance without any change in series inductance.

I'd like to see a model of the system. Shouldn't be hard to do, and I would
not be surprised to see an HF peak appear in the top octave if you get the
loading right for the cable.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Scott Dorsey]

In article , Paul wrote:
RF engineers use the stand wave ratio (SWR) to find the least
amount of reflected power with a given termination load.

Not sure if audio frequency people use these techniques....

They do, only when cable lengths are long enough for transmission line effects
to be significant. Which is to say many miles.

Back in the days of the analogue telephone plant, looking at long distance
lines with a tdr would let you see discontinuities like loading coils and
line damage. These days you seldom see even telephone loops more than a
few miles before the signal is digitized.

Most of the time audio people view cables as lumped-sum networks because
cables are far shorter than a quarter-wave.

And, as Phil notes, microphone cables aren't even specified for characteristic
impedance, and usually the diameter and dielectric thickness aren't controlled
as tightly as a cable intended to be constant-Z.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Phil Allison[_4_]]

Scott Dorsey wrote:
Phil Allison
Don Pearce wrote:

This is where simplistic cables models fall over. Four hundred feet of
cable is enough that at 20kHz a real, distributed model will give a
correct answer, but the lumped C/L/C model has failed. Four hundred
feet of typical 300 ohm mike cable ...

** Who sells "300 ohm mic cable" ??.

Or are you saying typical twisted pair mic cables have a characteristic impedance of 300ohms in the audio range ?

Actually, typical 24 ga mike cable tends to run around 110 ohms or so, which
is why that was picked as the impedance for AES/EBU, since they wanted to be
able to run AES/EBU over existing cable plant.

Older 18 ga mike cable with thicker rubber comes in around 150 ohms.

If you wanted to get up to 300 ohms you'd have to separate the conductors
more, or make them tiny.

IME, 400 feet of common or garden mic cable will significantly
attenuate high frequencies from a mic like the SM58 and most others -
assuming there is the usual 1500 ohms load at the other end.

If you're using an SM58, you probably want a load of around 600 ohms in order
to make it happy. If you're using a condenser microphone, the output Z is
likely far lower than the load


** But not very likely - plenty of condenser mics have 250ohm or so impedance.


The loading issues are more of an issue than the cable issues.


** They are inseparable.

The rated impedance of a mic cable is not defined anywhere I can find,
but IMO ought to be the value of terminating resistor that minimises
or eliminate shunt capacitance in and somewhat beyond the audio range.

Users would at least then know how to get the best HF response with
long runs.


The problem is that microphone noise and microphone dynamic behaviour due to
electrical damping (in the case of dynamic mikes) are usually much more
important than any HF behaviour.


** Sorry, that is gobbldegook.

Alluding to your superior knowledge of god knows what is a sure way to **** readers off. Being very specific is far more useful.



..... Phil

[Scott Dorsey]

Phil Allison wrote:

The problem is that microphone noise and microphone dynamic behaviour due to
electrical damping (in the case of dynamic mikes) are usually much more
important than any HF behaviour.

** Sorry, that is gobbldegook.

Okay, here we go....

A lot of dynamic microphones, most especially the SM-58, rely upon the
load to provide mechanical damping of the capsule.

If you operate them into a high-Z load, you see all kinds of ringing and
overshoot, because the diaphragm becomes a poorly damped mass-spring system.

These microphones need a fairly low impedance load in order to avoid ringing.
That being the case, the load impedance across the cable means that cable
effects are pretty heavily minimized.

Alluding to your superior knowledge of god knows what is a sure way to **** readers off. Being very specific is far more useful.

This isn't any superior knowledge, this is pretty commonly known. It is
one of the reasons why people praise transformer-input preamps without really
understanding why they are getting the sound they are, however.

On the other hand, that ringing on an unloaded or lightly loaded SM-57 is
a useful thing on snare mikes, to the point where people have removed the
step-up transformer from some SM-57s to accentuate the effect.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Phil Allison[_4_]]

Scott Dorsey wrote:

Phil Allison

The problem is that microphone noise and microphone dynamic behaviour
due to
electrical damping (in the case of dynamic mikes) are usually much more
important than any HF behaviour.

** Sorry, that is gobbldegook.

Okay, here we go....

A lot of dynamic microphones, most especially the SM-58, rely upon the
load to provide mechanical damping of the capsule.


** Resistance loading on the output *only* affects the LF resonance of the diaphragm and even then not by much.


If you operate them into a high-Z load, you see all kinds of ringing and
overshoot, because the diaphragm becomes a poorly damped mass-spring system.


** That is nonsense.

Capsule inductance combined with the leakage inductance of the internal transformer plus any cable capacitance form a network that can ring at specific frequencies - normally supersonic ones.

Adding the right value load resistance will damp this nicely. If Shure thought it valuable, they could easily add a resistor ( or RC combo) inside the mic.


These microphones need a fairly low impedance load in order to avoid ringing.
That being the case, the load impedance across the cable means that cable
effects are pretty heavily minimized.


** But we are considering *unusually long* runs where cable capacitance can be a devil.


Alluding to your superior knowledge of god knows what is a sure way to **** readers off. Being very specific is far more useful.

This isn't any superior knowledge, this is pretty commonly known.


** But instead of posting like you have now, you *alluded* to it.


..... Phil

[Don Pearce[_3_]]

On Sun, 19 Feb 2017 16:55:31 -0800 (PST), Phil Allison
wrote:

Don Pearce wrote:



This is where simplistic cables models fall over. Four hundred feet of
cable is enough that at 20kHz a real, distributed model will give a
correct answer, but the lumped C/L/C model has failed. Four hundred
feet of typical 300 ohm mike cable ...




** Who sells "300 ohm mic cable" ??.

Or are you saying typical twisted pair mic cables have a characteristic impedance of 300ohms in the audio range ?

IME, 400 feet of common or garden mic cable will significantly attenuate high frequencies from a mic like the SM58 and most others - assuming there is the usual 1500 ohms load at the other end.

The rated impedance of a mic cable is not defined anywhere I can find, but IMO ought to be the value of terminating resistor that minimises or eliminate shunt capacitance in and somewhat beyond the audio range.

Users would at least then know how to get the best HF response with long runs.


I've measured the impedance of a few cables. They came out in the
range of about 220 to s40 ohms. 300 seemed a reasonable average for
the calculation.

d

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[geoff]

On 20/02/2017 6:19 PM, Phil Allison wrote:


Adding the right value load resistance will damp this nicely. If
Shure thought it valuable, they could easily add a resistor ( or RC
combo) inside the mic.

They provide a specified load Z , 500 Ohms. They cannot control what it
is being plugged into, so surely best to leave any extra loading to
compensate for higher input impedances, damping, etc, to the user ?

geoff

[Don Pearce[_3_]]

On Mon, 20 Feb 2017 05:46:21 GMT, (Don Pearce) wrote:

On Sun, 19 Feb 2017 16:55:31 -0800 (PST), Phil Allison
wrote:

Don Pearce wrote:



This is where simplistic cables models fall over. Four hundred feet of
cable is enough that at 20kHz a real, distributed model will give a
correct answer, but the lumped C/L/C model has failed. Four hundred
feet of typical 300 ohm mike cable ...




** Who sells "300 ohm mic cable" ??.

Or are you saying typical twisted pair mic cables have a characteristic impedance of 300ohms in the audio range ?

IME, 400 feet of common or garden mic cable will significantly attenuate high frequencies from a mic like the SM58 and most others - assuming there is the usual 1500 ohms load at the other end.

The rated impedance of a mic cable is not defined anywhere I can find, but IMO ought to be the value of terminating resistor that minimises or eliminate shunt capacitance in and somewhat beyond the audio range.

Users would at least then know how to get the best HF response with long runs.


I've measured the impedance of a few cables. They came out in the
range of about 220 to s40 ohms. 300 seemed a reasonable average for
the calculation.

d

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Here are a few calculations on the behaviour of longish cables in a
microphone scenario. It is interesting that at any frequency where the
cable is starting to have an effect, a calculation using just
capacitance is wrong. It is not only the wrong answer, but the answer
is backwards, showing a loss where there is in fact a gain. Anyways,
have a look and see what actually happens.

http://www.soundthoughts.co.uk/read/cable.html

d

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[Paul[_13_]]

On 2/19/2017 11:57 PM, Don Pearce wrote:
On Mon, 20 Feb 2017 05:46:21 GMT, (Don Pearce) wrote:

On Sun, 19 Feb 2017 16:55:31 -0800 (PST), Phil Allison
wrote:

Don Pearce wrote:



This is where simplistic cables models fall over. Four hundred feet of
cable is enough that at 20kHz a real, distributed model will give a
correct answer, but the lumped C/L/C model has failed. Four hundred
feet of typical 300 ohm mike cable ...




** Who sells "300 ohm mic cable" ??.

Or are you saying typical twisted pair mic cables have a characteristic impedance of 300ohms in the audio range ?

IME, 400 feet of common or garden mic cable will significantly attenuate high frequencies from a mic like the SM58 and most others - assuming there is the usual 1500 ohms load at the other end.

The rated impedance of a mic cable is not defined anywhere I can find, but IMO ought to be the value of terminating resistor that minimises or eliminate shunt capacitance in and somewhat beyond the audio range.

Users would at least then know how to get the best HF response with long runs.


I've measured the impedance of a few cables. They came out in the
range of about 220 to s40 ohms. 300 seemed a reasonable average for
the calculation.

d

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Here are a few calculations on the behaviour of longish cables in a
microphone scenario. It is interesting that at any frequency where the
cable is starting to have an effect, a calculation using just
capacitance is wrong. It is not only the wrong answer, but the answer
is backwards, showing a loss where there is in fact a gain. Anyways,
have a look and see what actually happens.

http://www.soundthoughts.co.uk/read/cable.html


Interesting, but where is the series resistance and series
inductance per unit length in your lumped element model?

And where is the shunt conductance G per unit length in the
admittance? You only have the shunt capacitance.

Even if you assume a lossless cable, and R=0 and G=0, you
would still have the series inductance per unit length.

And what is being used for the distributed model? Some
sort of finite element analysis, using numerical methods?

[Don Pearce[_3_]]

On Mon, 20 Feb 2017 03:25:42 -0700, Paul wrote:

On 2/19/2017 11:57 PM, Don Pearce wrote:
On Mon, 20 Feb 2017 05:46:21 GMT, (Don Pearce) wrote:

On Sun, 19 Feb 2017 16:55:31 -0800 (PST), Phil Allison
wrote:

Don Pearce wrote:



This is where simplistic cables models fall over. Four hundred feet of
cable is enough that at 20kHz a real, distributed model will give a
correct answer, but the lumped C/L/C model has failed. Four hundred
feet of typical 300 ohm mike cable ...




** Who sells "300 ohm mic cable" ??.

Or are you saying typical twisted pair mic cables have a characteristic impedance of 300ohms in the audio range ?

IME, 400 feet of common or garden mic cable will significantly attenuate high frequencies from a mic like the SM58 and most others - assuming there is the usual 1500 ohms load at the other end.

The rated impedance of a mic cable is not defined anywhere I can find, but IMO ought to be the value of terminating resistor that minimises or eliminate shunt capacitance in and somewhat beyond the audio range.

Users would at least then know how to get the best HF response with long runs.


I've measured the impedance of a few cables. They came out in the
range of about 220 to s40 ohms. 300 seemed a reasonable average for
the calculation.

d

---
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Here are a few calculations on the behaviour of longish cables in a
microphone scenario. It is interesting that at any frequency where the
cable is starting to have an effect, a calculation using just
capacitance is wrong. It is not only the wrong answer, but the answer
is backwards, showing a loss where there is in fact a gain. Anyways,
have a look and see what actually happens.

http://www.soundthoughts.co.uk/read/cable.html


Interesting, but where is the series resistance and series
inductance per unit length in your lumped element model?

And where is the shunt conductance G per unit length in the
admittance? You only have the shunt capacitance.

Even if you assume a lossless cable, and R=0 and G=0, you
would still have the series inductance per unit length.

And what is being used for the distributed model? Some
sort of finite element analysis, using numerical methods?



I used just capacitance and source resistance, because that is what
people go to when they try to calculate the high frequency limit of a
cable. I could use a C-L-C model to make it appear more accurate, but
in fact it makes things worse. What you end up with is a multiple pole
lowpass filter.

The distributed model is the standard Spice transmission line - which
almost all RF simulators use. The parameters it needs are delay and
impedance.

I currently use normal coax cable at up to 60GHz (60,000,000,00 Hz) so
it is clear that no cable has any kind of upper frequency limit -
until it starts moding. That is when the inner diameter of the cable
approximates a half wavelength.

d

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[Don Pearce[_3_]]

On Mon, 20 Feb 2017 10:52:34 GMT, (Don Pearce) wrote:

On Mon, 20 Feb 2017 03:25:42 -0700, Paul wrote:

On 2/19/2017 11:57 PM, Don Pearce wrote:
On Mon, 20 Feb 2017 05:46:21 GMT, (Don Pearce) wrote:

On Sun, 19 Feb 2017 16:55:31 -0800 (PST), Phil Allison
wrote:

Don Pearce wrote:



This is where simplistic cables models fall over. Four hundred feet of
cable is enough that at 20kHz a real, distributed model will give a
correct answer, but the lumped C/L/C model has failed. Four hundred
feet of typical 300 ohm mike cable ...




** Who sells "300 ohm mic cable" ??.

Or are you saying typical twisted pair mic cables have a characteristic impedance of 300ohms in the audio range ?

IME, 400 feet of common or garden mic cable will significantly attenuate high frequencies from a mic like the SM58 and most others - assuming there is the usual 1500 ohms load at the other end.

The rated impedance of a mic cable is not defined anywhere I can find, but IMO ought to be the value of terminating resistor that minimises or eliminate shunt capacitance in and somewhat beyond the audio range.

Users would at least then know how to get the best HF response with long runs.


I've measured the impedance of a few cables. They came out in the
range of about 220 to s40 ohms. 300 seemed a reasonable average for
the calculation.

d

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

Here are a few calculations on the behaviour of longish cables in a
microphone scenario. It is interesting that at any frequency where the
cable is starting to have an effect, a calculation using just
capacitance is wrong. It is not only the wrong answer, but the answer
is backwards, showing a loss where there is in fact a gain. Anyways,
have a look and see what actually happens.

http://www.soundthoughts.co.uk/read/cable.html


Interesting, but where is the series resistance and series
inductance per unit length in your lumped element model?

And where is the shunt conductance G per unit length in the
admittance? You only have the shunt capacitance.

Even if you assume a lossless cable, and R=0 and G=0, you
would still have the series inductance per unit length.

And what is being used for the distributed model? Some
sort of finite element analysis, using numerical methods?



I used just capacitance and source resistance, because that is what
people go to when they try to calculate the high frequency limit of a
cable. I could use a C-L-C model to make it appear more accurate, but
in fact it makes things worse. What you end up with is a multiple pole
lowpass filter.

The distributed model is the standard Spice transmission line - which
almost all RF simulators use. The parameters it needs are delay and
impedance.

I currently use normal coax cable at up to 60GHz (60,000,000,00 Hz) so
it is clear that no cable has any kind of upper frequency limit -
until it starts moding. That is when the inner diameter of the cable
approximates a half wavelength.

I've just made a lumped/distributed model that uses 500 L-C-L elements
in series, and it reproduces the gain and cyclic variation very nicely
up to 1MHz. Above that it turns flaky.

I'm not bothering with admittance and conductance. For a low impedance
speaker cable they become important, but for this they are irrelevant.

d


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[geoff]

On 20/02/2017 11:52 PM, Don Pearce wrote:
On Mon, 20 Feb 2017 03:25:42 -0700, Paul wrote:

I currently use normal coax cable at up to 60GHz (60,000,000,00 Hz) so
it is clear that no cable has any kind of upper frequency limit -
until it starts moding. That is when the inner diameter of the cable
approximates a half wavelength.

Jeeeze. 60gHz. My hearing isn't that good any more, but your bat must be
really ****ed off !

geoff

[Don Pearce[_3_]]

On Tue, 21 Feb 2017 08:05:07 +1300, geoff
wrote:

On 20/02/2017 11:52 PM, Don Pearce wrote:
On Mon, 20 Feb 2017 03:25:42 -0700, Paul wrote:

I currently use normal coax cable at up to 60GHz (60,000,000,00 Hz) so
it is clear that no cable has any kind of upper frequency limit -
until it starts moding. That is when the inner diameter of the cable
approximates a half wavelength.

Jeeeze. 60gHz. My hearing isn't that good any more, but your bat must be
really ****ed off !

Yeah, I can't hear that when I have a head cold.

d

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[Mat Nieuwenhoven]

On Sat, 18 Feb 2017 17:40:05 -0600, John Hardy wrote:

snip
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.

Thanks for taking the effort to explain all this. Most of it was new
to me, and it's interesting to learn about the technology behind the
audio.

Very impressive, these Jensen transformers.

Mat Nieuwenhoven

[Mat Nieuwenhoven]

On Tue, 14 Feb 2017 22:31:54 -0700, Paul wrote:

snip

Hi Paul,

I don't know if these will physically fit as a replacement, but the
"Img Stage Line MHD-152" horn is doubtless much better than the piezo
tweeters. It is 8 ohms, has a -6db range from 0,8 - 20 kHz straight
ahead, and sensitivity of 102 dB (2,83 V, 4 kHz, 1 m distance). It is
not cheap though, 110 euro in Germany. Measured under 30 degrees
horizontally it's about 10 dB less (that's a good value, and it's
constant directivity, meaning the 10 dB is fairly constant even if
the frequency gets higher). Vertically 30 degrees off, it is much
more, 20-30 dB less.

Usable from 2 kHz upwards, because of some resonances below it: you
need to put it behind an 18 dB 2 kHz high pass filter (one L, 2 C).

Mat Nieuwenhoven