HI-FI Electret/Dynamic microphone preamplifier

[Crontox102098]

Hi guys, just want to show you a little (UNTESTED) HI-FI preamplifier for electret (i love it  ) or dynamic microphone.

Just i've got a little question... how the level pot will behave?

Here's the schematic:

ALL RESISTOR ARE 1/4W METAL FILM

Cheers! 

[PRR]

Someone used a sticky slide-rule.

Max gain is 33dB, not "46" dB as noted.

Input impedance will go from 16K to zero, not "23.5K" as noted.

[Mark Hammer]

Anybody notice a problem with the way the Level control is wired, when it comes to producing a "proper" Vref?

[Crontox102098]

Someone used a sticky slide-rule.

Max gain is 33dB, not "46" dB as noted.

Input impedance will go from 16K to zero, not "23.5K" as noted.

How you can know it?  i justo used commons math equation.

[PRR]

> How you can know it?

Turn the Level pot so wiper is at top of pot (as drawn), shorted pins 1+2.

What is the input impedance?

Turn the other way (shorted 2+3). Now I see 47K||47K||50K, which is 16K.

The Maximum gain is nearly 50K/1.1K, which is about 45. A gain of 100 is 40dB, so gain of 45 must be less than 40dB. 2:1 change is 6dB, and 45 is about half of 100, so we'd rough-guess 34dB. Actually 45 is 10% short of 50, and 10% is about 1dB, so we'd refine-guess 33dB. I usta do this for a living and I am shocked that I forgot how to do dB on the slide-rule. I been using the TI calculator too long. It sez 33.15dB.

> a "proper" Vref?

Ah, and that also. Good catch. Fixable; but I think this herring is red.

Ditch the level pot. The only thing it does, that the Gain pot doesn't, is (try to) turn gain down below unity. You probably do not need a mike-amp gain below unity. Anyway the top-side of the Level pot *tries to* reduce gain by loading-down the source (microphone). Assuming a dynamic mike, 200 Ohms, there won't be any audible reduction until that pot is 1% from the end.

[therecordingart]

This is what I'd do.



Your gain pot takes you from unity to a gain of 45. Remove the level which gives you an input z of 23k.

EDIT: PRR beat me to it. DAMN!

[Crontox102098]

> How you can know it?

Turn the Level pot so wiper is at top of pot (as drawn), shorted pins 1+2.

What is the input impedance?

Turn the other way (shorted 2+3). Now I see 47K||47K||50K, which is 16K.

The Maximum gain is nearly 50K/1.1K, which is about 45. A gain of 100 is 40dB, so gain of 45 must be less than 40dB. 2:1 change is 6dB, and 45 is about half of 100, so we'd rough-guess 34dB. Actually 45 is 10% short of 50, and 10% is about 1dB, so we'd refine-guess 33dB. I usta do this for a living and I am shocked that I forgot how to do dB on the slide-rule. I been using the TI calculator too long. It sez 33.15dB.

> a "proper" Vref?

Ah, and that also. Good catch. Fixable; but I think this herring is red.

Ditch the level pot. The only thing it does, that the Gain pot doesn't, is (try to) turn gain down below unity. You probably do not need a mike-amp gain below unity. Anyway the top-side of the Level pot *tries to* reduce gain by loading-down the source (microphone). Assuming a dynamic mike, 200 Ohms, there won't be any audible reduction until that pot is 1% from the end.

So, that Level por is a bad idea? Remember i dont understand english as well and google translator is a little piece of... You know what.

This is what I'd do.



Your gain pot takes you from unity to a gain of 45. Remove the level which gives you an input z of 23k.

EDIT: PRR beat me to it. DAMN!

Need the Level por, cause the box i've got, hace got 2 holes in it.

[anotherjim]

I think all you need do is to fit an ordinary volume control potentiometer (10k log?) between R7 and the output jack.
If the preamp is remote from the mixer, it could be useful to have.

[Crontox102098]

I think all you need do is to fit an ordinary volume control potentiometer (10k log?) between R7 and the output jack.
If the preamp is remote from the mixer, it could be useful to have.

Isn't remote, thats the deal.

[Mark Hammer]

Just as a side-note (already conveyed to Carlos/Crontox in a PM), it is a common strategy in many low-cost solid-state amps to use a single pot to both adjust the gain of a stage, and control attenuation of the output.  One outside lug of the pot goes to a lower-value bleed resistor on the output of the stage, and the other outside lug is part of the "ground leg" of the non-inverting op-amp used for the gain stage.  Both legs of the pot connect to ground, and have complementary action.  As gain is increased by reducing the ground leg resistance, more of the stage's output is "preserved" by increasing the total bleedoff resistance.  And, as the ground-leg resistance is increased (reducing the gain), the bleedoff resistance is simultaneously decreased.

[duck_arse]

another something - with SW1 open, C1 will have 4V5 (maybe) on its (+) end. with SW1 closed, it will have +9V on its (-) end. it would need a NP electro.

[Crontox102098]

Just as a side-note (already conveyed to Carlos/Crontox in a PM), it is a common strategy in many low-cost solid-state amps to use a single pot to both adjust the gain of a stage, and control attenuation of the output.  One outside lug of the pot goes to a lower-value bleed resistor on the output of the stage, and the other outside lug is part of the "ground leg" of the non-inverting op-amp used for the gain stage.  Both legs of the pot connect to ground, and have complementary action.  As gain is increased by reducing the ground leg resistance, more of the stage's output is "preserved" by increasing the total bleedoff resistance.  And, as the ground-leg resistance is increased (reducing the gain), the bleedoff resistance is simultaneously decreased.

Just tell me Carlos  ... That mean what i can use this before my mixer? And will work as well? 

[Mark Hammer]

I think you should be able to use it.  But that will depend on whether there is a good impedance-match between the output of the (revised) circuit and the input of your mixer.

[Crontox102098]

I think you should be able to use it.  But that will depend on whether there is a good impedance-match between the output of the (revised) circuit and the input of your mixer.

My mixer have got 18K in input impedance... and the output of this preamp is 120, think will work as well, gonna try.

[Crontox102098]

Still is untested  ... is viable my circuit? I will produce it in series to sell and want to know if it will work fine. Can anyone help me please?

I'm a little bit stupid i know , but this time really i don't want to f**k it all and lose my money (as you know, hi-fi components aren't cheap at all).

[Seljer]

Thats exactly what breadboards (and perf/veroboards) are for! Test it out before committing to making proper PCBs

[anotherjim]

I still think the only simple and sensible place for a level control is on the output AFTER the amplifier.

I doubt the microphone is so hot that it needs attenuating before the amplifier, BUT, in the case of the Electret, it is the internal JFET that will cause distortion with high sound levels and an attenuator before the amp will not fix that..

Tolerance to high sound levels can be improved by running the Electret as a Source follower (read on Linkwitz mod), but that means it has no gain itself and your pre-amplifier must provide more gain.

Go here and read the paragraph "Sound Pressure Level".
http://sound.westhost.com/project93.htm

Or just use the mic at a greater distance from the sound source or put a thick foam wind shield over it...
or a sock 

[Crontox102098]

Thats exactly what breadboards (and perf/veroboards) are for! Test it out before committing to making proper PCBs

Mine is it broken, and in my electronics local store do not have in their inventory.