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Solid-State Balanced Microphone Preamplifier
Solid-State Balanced Microphone Preamplifier
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Old 5th May 2019, 05:16 PM   #11
N101N is offline N101N
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Quote:
for low source impedances a transistor with low base resistance is best
...in common-base configuration.
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Old 5th May 2019, 06:38 PM   #12
Mark Tillotson is offline Mark Tillotson
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Originally Posted by H713 View Post
The input transformer is just a 1:1. I don't see any reason why a JT11P-1 (line input transformer) couldn't be used for a microphone input.
Most of the noise in that circuit comes from the resistances of the transformer windings which are 1.5k or so.

You'll note Jensen microphone transformers such as the JT-MB-CA have winding resistances of 50 ohms or below. A line transformer winding is designed to have a noise resistance an order of magnitude less than line impedances, microphone transformers have noise resistances an order of magnitude below the microphone impedances.

The classic error when first working with low-noise circuits is to add series resistance to the source - this is to be avoided as much as possible (yes, sometimes you need some for protection purposes. Its a mistake everyone makes!

In general try to keep any series resistances less than the source impedance, so 100 ohms or so max for a microphone input.

Series resistance not only adds Johnson noise of its own to the signal, but multiplies the input device's current noise to a voltage too (in fact even reactive impedance does that, even though it has no Johnson noise)
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Old 5th May 2019, 06:55 PM   #13
mchambin is offline mchambin  France
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This paper is clear about what is a ultra low noise amplifier.
Here one for measuring low noise bipolar transistors.
Ultra low noise amplifiers
Figure 1 shows clearly what matters.
Click the image to open in full size.
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Transistor junction temperature is not transistor case temperature.
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Old 5th May 2019, 08:36 PM   #14
MarcelvdG is online now MarcelvdG  Netherlands
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Quote:
Originally Posted by N101N View Post
...in common-base configuration.
Why? We're not trying to characteristically terminate a transmission line or anything like that. In fact a too low input impedance could overload the internal amplifier of a condenser microphone.
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Old 5th May 2019, 09:41 PM   #15
mchambin is offline mchambin  France
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Originally Posted by Mark Tillotson View Post
Series resistance not only adds Johnson noise of its own to the signal, but multiplies the input device's current noise to a voltage too (in fact even reactive impedance does that, even though it has no Johnson noise)
What to care for, about the caps that are in serie for phantom power isolation ?
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Old 5th May 2019, 09:45 PM   #16
H713 is offline H713  United States
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Here's the updated schematic. Interesting point about the winding DC resistances of a line transformer- not something I had really considered. That certainly could introduce noise.

I reconfigured the feedback scheme so that 1) It's actually negative, and 2) I was able to eliminate the 180 ohm resistors on the input. That should drop the noise floor a bit.

As far as the AC coupling to the output transformer is concerned, I'm going to wait until I've had a chance to prototype the circuit to see if it is necessary. I do suspect that it will be, since I won't have perfectly matched resistors and transistors throughout, but we shall see.
Attached Files
File Type: pdf H713 Microphone Preamplifier V2.pdf (48.2 KB, 43 views)
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Old 5th May 2019, 09:51 PM   #17
MarcelvdG is online now MarcelvdG  Netherlands
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Please add the antiparallel diodes, you don't want to have the noise floor ruined when someone accidentally connects a microphone while the phantom supply is on.

Quote:
Originally Posted by mchambin View Post
What to care for, about the caps that are in serie for phantom power isolation ?
If they had existed, they would have needed to have relatively large values
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Old 5th May 2019, 10:40 PM   #18
mchambin is offline mchambin  France
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With 10Ks at the collectors, you have very little headroom. I would use lower values. I would remove R18 R19. Don't forget 100nF caps as close as possible to power pins at the op-amps.

To see about DC at the output with the simulator you need non equal transistors. A 1C temperature that decreases Vbe can be simulated adding a -2mV voltage at the base. Or, to decrease Vbe you can make other BC560 BC550 models with a higher IS. To simulate for higher gain you can make transistors models with higher BF.
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Last edited by mchambin; 5th May 2019 at 10:55 PM.
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Old 5th May 2019, 11:21 PM   #19
H713 is offline H713  United States
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Quote:
Originally Posted by MarcelvdG View Post
Please add the antiparallel diodes, you don't want to have the noise floor ruined when someone accidentally connects a microphone while the phantom supply is on.
Wouldn't said diode affect the maximum gain of the first stage though?
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Old 6th May 2019, 04:18 AM   #20
mchambin is offline mchambin  France
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Simulation should show these protection diodes do not change the gain.
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