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