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Old 4th September 2010, 09:44 PM   #1
Katman is offline Katman  Italy
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Default Low noise transistors for electret mic

Hi all,
I am rebuilding an electret microphone in which many years ago I used a replacement capsule for Shure SM94. Now I have built a smd circuit on the solder side of a prototype board, but noise is very higher than in the old circuit. At that time I selected 2N5087 among other types a the best ones. Now I used BC860(smd). Do you think BC860(smd) could produce higher noise than 2N5087? BC560 have the same noise?

Thanks,
Gianluca from Roma, Italy

This is the schematic of my version:
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File Type: jpg SM94.jpg (52.8 KB, 616 views)

Last edited by Katman; 4th September 2010 at 10:12 PM.
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Old 4th September 2010, 10:48 PM   #2
Katman is offline Katman  Italy
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Quote:
Originally Posted by Katman View Post
This is the schematic of my version:
Complete schematic, better quality (I can't edit first post anymore...):
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Old 6th September 2010, 05:31 PM   #3
Mooly is offline Mooly  United Kingdom
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The circuit just looks a little "odd" tbh. Have you tried to draw the circuit out from your old one.
Does it actually work OK apart from the noise ?
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Old 6th September 2010, 08:29 PM   #4
Katman is offline Katman  Italy
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Quote:
Originally Posted by Mooly View Post
The circuit just looks a little "odd" tbh. Have you tried to draw the circuit out from your old one. Does it actually work OK apart from the noise ?
I have built the old one identical to the schematic I had drawn from the SM94. It had no more noise than SM94.
Today I bought 3 2N5087. I'll replace the 3 BC860 smd and i'll let you know.

Datasheets say BC560 (they should be identical to the smd version BC860) have low noise for source resistance of about 2kOhm; 2N5087 for about 20kOhm... It's very hard to evaluate only on the sheets which one among two transistors will be quieter in a specific circuit...
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Old 6th September 2010, 08:52 PM   #5
Katman is offline Katman  Italy
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LED is added by me; 47uF is a one-time voltage doubler making a flash at power on if battery is ok. No visible LEDs can be turned on directly from 1,5V and current drawn by a LED would be much higher than the current drawn by the amplifier.

Q1 is an active load for the FET. His Vce is regulated by supply voltage. Higher voltage gives more dynamic range. 15uF between b-e makes Q1 like a current generator for audio signal, because Vbe, then Ic, is independent from audio signal on the emitter. Q1 is a stable voltage source for polarization, but it has a high impedance for audio signal.

Q2 and Q3 are emitter followers to lower the signal impedance. They are in series for audio, but they have the same polarization (depending on supply voltage).

The BF245A (a current regulator field effect "diode" 1N5304 in the original circuit) with the 270 Ohm resistor is a current regulator set to 1.8mA. Note that required resistor value is different from a single FET to another, first of all because Idss can be very different from one to each other.

Last edited by Katman; 6th September 2010 at 09:01 PM.
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Old 7th September 2010, 06:28 AM   #6
Mooly is offline Mooly  United Kingdom
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Thanks for the explanation... obviously try the other transistors and see.

I'll tell you what puzzled me... and I'm not really familiar with powering mics via a phantom supply but anyway.

Q2 and Q3, it seems really odd to bias both from the same point, Q3 and the 47k tie the base to the same audio input that Q2 is receiving. That's odd

That was one thing, the other is the 1.5v battery. The fact that it's there I guess means the circuit is intended to run off just that if needed, the Ge diode giving low forward voltage drop. So with no phantom supply the bias for Q1, Q2 and Q3 and their operating point is very transistor dependant as the bias is provided by just a simple "resistor to ground" and not a network setting a defined bias point.

Anyway, there we go
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Old 7th September 2010, 07:27 AM   #7
Katman is offline Katman  Italy
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Quote:
Originally Posted by Mooly View Post
Q2 and Q3, it seems really odd to bias both from the same point, Q3 and the 47k tie the base to the same audio input that Q2 is receiving.
- Q2 and Q3 are AC-coupled, then they can be biased at the same point. Audio from Q2 via capacitor has low impedance, then it is stronger than the signal via 47k.

Quote:
That was one thing, the other is the 1.5v battery. The fact that it's there I guess means the circuit is intended to run off just that if needed, the Ge diode giving low forward voltage drop.
- Drop is 240mV

Quote:
So with no phantom supply the bias for Q1, Q2 and Q3 and their operating point is very transistor dependant as the bias is provided by just a simple "resistor to ground" and not a network setting a defined bias point.
- It works...

Quote:
Anyway, there we go
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Old 7th September 2010, 07:40 AM   #8
Mooly is offline Mooly  United Kingdom
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OK then... good luck with the other transistors, let us know how it turns out.
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Old 7th September 2010, 01:34 PM   #9
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a little odd is that pin 3 is not driven??



I'm thinking the 1.5 battery is the on/off switch... not sure how that works though.

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Old 7th September 2010, 01:56 PM   #10
Katman is offline Katman  Italy
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Originally Posted by bear View Post
a little odd is that pin 3 is not driven??
In the original there is a transformer, without the two 2k7 resistors. In my version output is only "impedance balanced", driving only the "+" output (2). "-" output (3) has the same impedance, but there is no signal.
The two 100 Ohm resistors will be differene, because on the driven side there is also about 40 Ohms output impedance.

Quote:
I'm thinking the 1.5 battery is the on/off switch... not sure how that works though.
Uh?...
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