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New old-school electret mic preamp
New old-school electret mic preamp
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Old 1st February 2018, 01:01 PM   #11
JMFahey is offline JMFahey  Argentina
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Location: Buenos Aires - Argentina
3V is not enough.
1.5V up is enough for the Electret capsule alone, so is 3V but once you add a preamp, 2 large problems arise:
1) transistor biasing and voltage drop, that alone will eat up about half of your meager 3V available, leaving just 1.5V for peak to peak headroom (best case) .
So in practice you gain nothing compared to plain capsule straight fed 1.5V
2) a preamp is supposed to provide *gain* , so combining that to low (terrible) headroom it will distort easily, most of the time, unless picking a hummingbird wing fluttering 100 meters away.
3) in any case, as mentioned above, a Zoom H1 has *ample* sensitivity to be driven by a straight unamplified Electret capsule, so if not enough you are not setting it right.
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Old 1st February 2018, 01:13 PM   #12
Rufinus is offline Rufinus  Germany
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Quote:
Originally Posted by JMFahey View Post
3V is not enough.
1.5V up is enough for the Electret capsule alone, so is 3V but once you add a preamp, 2 large problems arise:
1) transistor biasing and voltage drop, that alone will eat up about half of your meager 3V available, leaving just 1.5V for peak to peak headroom (best case) .
So in practice you gain nothing compared to plain capsule straight fed 1.5V
2) a preamp is supposed to provide *gain* , so combining that to low (terrible) headroom it will distort easily, most of the time, unless picking a hummingbird wing fluttering 100 meters away.
3) in any case, as mentioned above, a Zoom H1 has *ample* sensitivity to be driven by a straight unamplified Electret capsule, so if not enough you are not setting it right.
Thanks for your answer. But in this case i do not need anywhere close to 1.5V peak. Regarding distortion i have provided simulation results.

H1 optimal gain is 37-60, after that it gets noisy. And with that gain it's a bit quiet for me at least.

Besides, there are not so many capsules with optimal voltage of 1.5, most of them are 3-5V type.

Last edited by Rufinus; 1st February 2018 at 01:15 PM.
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Old 1st February 2018, 07:21 PM   #13
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by Rufinus View Post
...about 20-25 cm (10 inch for US guys ),
Here in Canada, we have a wonderful mix of units. I buy gasoline in litres, but the air pressure in my car tyres is measured in pounds per square inch...

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Originally Posted by Rufinus View Post
THD results: Total Harmonic Distortion: 0.020929%(0.022341%)
Looks OK to me.
The schematic looks good to me. The very small signal level, and the fact that there is now some overall negative feedback, is helping to keep the distortion of the preamp low.

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Thanks a lot! The next step would be trying it on the breadboard and tweaking the R3/gain for H1!
You're welcome, that sounds like a plan!

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P.S. One point, which troubles me a bit is the removed R4 - will we not get thermal instability?
You still have DC feedback via R3. If, for instance, Q1 collector voltage is too high for some reason, that increases the voltage at Q2 emitter, therefore at the junction of R6/R7, therefore feeds more base current into Q1 via R3, which brings Q1 collector voltage back down.

In fact, if you neglect any DC drop across R3, the voltage at Q1 collector is set to 3 Vbe, which seems to be close to 1.5 V in your case.

Vbe for silicon devices changes by about 2 mV per degree C, so the collector voltage of Q1 will change by about 6 mV per degree C (it will fall as temperature goes up).

So thermal stability is not perfect, but it would take rather large changes in temperature to have any significant effect on the circuits operation. As long as you don't dip it in boiling water or bury it in the Siberian tundra, I think you'll be okay.

-Gnobuddy
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