Building an electret microphone Acoustic Guitar pick up. Want to ask something.

[nguitar12]

source: http://www.zen22142.zen.co.uk/Circuits/Audio/ecmmic.html

Heys guys so I am building an electret microphone acoustic guitar pick up.
I have successfully built a mic pre amp with electret microphone according to the above diagram.

When I hook the output directly to the guitar amp it will produce a decent sound level but there is no comparison to that produced by a electric guitar. I know this is all about a output impedance ( hi-Z, low-Z, etc...) Here is the discription on the web site about this preamp "the overall voltage gain of the preamplifier is about 100x or 20dB"

One thing I have to mention about is I am using BC556 instead of BC549 since I don't have them around. Will it affect the amplification rate?

I guess I need a secound stage amplification in order to make the signal as  larger as a guitar (hi-Z) output. Can anyone please tell me what I need in my secound stage? Another mic preamp? or audio preamp?

Thanks in advance.

[GGBB]

BC549 is NPN.  BC556 is PNP.  I think that's the problem.

[nguitar12]

Quite strange that my preamp is actually working. I guess reversing the pinout will make it work.....just guessing.....

[Seljer]

Quite strange that my preamp is actually working. I guess reversing the pinout will make it work.....just guessing.....

Nope. NPN and PNP aren't really compatible devices per se.

You could however modify the polarity of everything else in the circuit and make it positive ground (like the difference between an NPN fuzz pedal and a PNP fuzz pedal). That would mean turning around all the electrolytics as well as the polarity of the battery.


Its probably easier to change the transistors Many different NPNs will probably work in there, what do you have at hand?

[nguitar12]

Quite strange that my preamp is actually working. I guess reversing the pinout will make it work.....just guessing.....

Nope. NPN and PNP aren't really compatible devices per se.

You could however modify the polarity of everything else in the circuit and make it positive ground (like the difference between an NPN fuzz pedal and a PNP fuzz pedal). That would mean turning around all the electrolytics as well as the polarity of the battery.


Its probably easier to change the transistors Many different NPNs will probably work in there, what do you have at hand?

This is just weird.... I connect this mic w/ preamp to my pc via interface and it will work with correct frequence response......

This is all trasistor i have in my hand right now:

BC556
BC550
BC2240
2N3904
2N3906
2N5457
2N5088
C1815
J201

BC560,BC549,BC556 is on the way from taydaelectronics

[Mark Hammer]

Pretty certain it's one of Keen's Laws: when in doubt, use a 5088.  Of lesser gain, but an 1815 will likely work fine as well.  Do note the difference in their pinouts.

[nguitar12]

Pretty certain it's one of Keen's Laws: when in doubt, use a 5088.  Of lesser gain, but an 1815 will likely work fine as well.  Do not the difference in their pinouts.

Just tested both 1815 and 5088 is not working.
2N3906 will also work.....One thing even weird..
I can install the fet in both direction with I use BC556 and 2N3906.
This thing is actually working....What's going on here?

[GGBB]

2 PNPs work.  2 NPNs don't.  How are you powering this?  Have you reversed the polarity?

[nguitar12]

2 PNPs work.  2 NPNs don't.  How are you powering this?  Have you reversed the polarity?

I bulid everything according to the schematic except replacing the transistor.
I cannot guarantee my bulid is 100% correct and maybe for some reason the mic capsule is bypassed the amp circuit ( again...guessing...)
Someone tell me that a 100x/40db amplification sould delivered a larger signal than that generated by guitar. is it true?

[GGBB]

http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

Specifically, take voltage readings at your transistor pins and post them here.  Also, tell us which transistor you are using for the measurements and whether or not you have inverted it.

[PRR]

> tell me that a 100x/40db amplification sould delivered a larger signal than that generated by guitar.

100X is 100 times bigger, true.

However in *that* circuit the gain varies directly with battery voltage.

And this from the webpage isn't written correctly:

the overall voltage gain of the preamplifier is about 100x or 20dB

20dB is a Voltage gain of 10X. Not 100X.

FWIW, my thumb-count says the gain should be hundreds, not ten or so.

There's other things I do not like about that plan, but yeah it should make signal "larger".

It is NOT clear, from the few facts you have presented here, that your build works at ALL. We don't even know the battery voltage. While you have signal and no-signal cases, two different *wrong* connections can explain that.

> take voltage readings at your transistor pins and post them here. 
> tell us which transistor you are using for the measurements
> and whether or not you have inverted it.

+1.

[nguitar12]

There's other things I do not like about that plan, but yeah it should make signal "larger".

It is NOT clear, from the few facts you have presented here, that your build works at ALL. We don't even know the battery voltage. While you have signal and no-signal cases, two different *wrong* connections can explain that.

Opps sorry...my bad. I am powering the circuit by a 9 volt battery. But consider this is not a complicated circuit. I am going to bulid this one again once I got the bc549 from taydaelectronics. This may even take fewer work than debugging the circuit. And maybe do the debugging if my second build fail again.

btw. I have just buit a even simpler one using a single 2N3904. schematic as following:



This one even generated a larger single than the previous one I built. So based on the schematic, can someone please tell me which one is better in sound quality, theoretically?

Many thanks.

[Seljer]

The first circuit utilizes the Q2 as a buffer for lower output impedance (even without the buffer, directly off Q1 it'd probably be low for plugging into a guitar amp, lower output impedance helps if you want to plug directly into a mixer)  and the biasing method utilized gives it a lower input impedance as well.

The second circuit is just a common emitter amplifier with collector-feedback biasing. It's simple and should work
The choice of caps a bit small, you probably need at least 1uF for C1 and C2, the way it is it's probably cutting significantly below 1000Hz.


The best choice for R1 in both circuits depends mainly on the mic capsule (and supply voltage) used. You should be able to get some guidance on the datasheet for the capsule if you can find it.