Mic amp

[JRB]

Last Saturday I bought my first mic with the thought of amplifying it through my spare 15W guitar amp, needless to say this didn't end as planned. It hummed like crazy, so the diyer I am I decide to find a solution I found this:



So I decided to build this with the components I have on hand and was forced to sub some parts.
R1, R2 = 4k7
R3, R4 = 68k
C1, C2 = 10n
R8 = 27k
R9 = 120k
left C8 out cause I didn't have anything near it.

It fixed the hum but introduced a different problem, it made the signal about 4 times as small.
So I did some calculations and came to the conclusion it being a inverting amplifier it was decreasing the signal instead of amplifying it. What I ended up with doing is changing the inverting amplifier into a non inverting amplifier.
To my surprise it didn't do anything, to fix the volume it actually made it stop outputting any volume, the signal just disappears after it go's into the op amp.

What I am wondering now is what does the inverting amplifier do in this circuit? Could I just use the inverting amplifier and put a booster behind it to fix the volume or should my non inverting amplifier idea work?

[cmilless]

Is the mic output balanced? Sure your cable and connector were good?

[Mark Hammer]

I think its my diagram, so I should clean up my mess.  As shown, the circuit has a gain of around 225x or about 47db.  With the components you describe, the total gain is around 64x.  To increase the gain, stick another 4k7 resistor in parallel with the 4k7 units you used, to double the gain of the first stage.  Then, stick a 68k in parallel with the 27k unit in the second stage to increase that gain.

The original circuit yields a balanced out. The parts I crossed out were unnecessary to produce a simple mic preamp with balanced in and an unbalanced out to feed an amp directly...which is precisely what I did and what you want to do.

Note that, unless yu are using a mic to sample a cymbal or something with equivalent top end, you can use a feedback cap in stage two as large as 100-150pf, and still have plenty of bandwidth to spare.

[JRB]

Actually it is your diagram mark, found it in a topic from april 2011. Ill give the resistor change a go see if that changes anything.

and cmilless I am pretty sure the cable is good, made it myself and on the scope it gives out the signals you would expect.

[Seljer]

Changing C1 and C2 to 10nF moves the lowend cutoff at the input way into the treble range, if you're not phantom powering anything you can use regular electrolytics with the - towards the input.

[JRB]

So I tried what mark suggest and when I turn up everything to max volume I can actually hear myself at a decent volume. I am now just wondering how low can I go with resistors R1 and R2 so I won't have to crank everything to the max. at the moment I am down to around 1k5.

[Mark Hammer]

If you need more oomph, just make the feedback resistance in the 2nd stage higher.  Just note that you should not expect huge gains and still get a clean output.  So if you have another 68k resistor left, stick that in series with the 120k you put in there, and maybe leave it at that.  

Note that the feedback cap interacts with the feedback resistance.  If you up the feedback resistance to 188k (120 + 68), you probably don't want to make that feedback cap much over 100pf.

But at this point, unless it is difficult to get them, I suspect you would be well-advised to get some other resistors and just use single ones instead of piggybacking things.