Simple Single Ended Mic Pre

[VictorQ]

Hi,

Old post but new question from a newbie.  Could the last schematic just above this from Samuel Groner titled "simple 5V mic pre.pdf" be powered by the usb port with the line out going to the line in on a netbook?

And, would it work with an electret capsule like the Primo EM-172?  The Spec sheet for the Primo I think says it operates at 2.4k ohms +/- 30% at 1kHz.  (I unfortunately do not know what that means).

James

Yes and yes.
To adapt to electret replace R9 with 3.9 k to 5 V and reduce C5 to fit with the new source resistance, say around 150 pF for the same HF rolloff.

Current in the input transistor may need to be scaled down as well for good noise performance... Maybe about 0.24 mA assuming hfe of around 500. This is about an order of magnitude less than the original design, so just scale R1, R5 and R6 up ten times.

But...

Do you really want to apply 40 to 60 dB of gain? This would make the output clip at about 86 to 67 dB SPL. At what level does the input of the computer clip? What is the level of the signals you wish to record?

Overall there seems to be much simpler routes to go with this, since it will be quite 'ghetto' regardless.. A simple opamp stage could probably perform well here, and be more flexible wrt. gain range..

[jmccanna]

Hi,

Wow, I got a response! Thanks so much.  I am still learning what all the terms and concepts mean.  I do not want 40x gain but want line level signal (if those are different). I think I am looking for about 20x gain maximum?  I could put in a potentiometer I guess?  (Or, maybe not).

I hope my questions/suggestions are not too odd.  I truly am trying to educate myself on this.  As to simpler routes you mention, I would be very grateful if you could elaborate. First, let me say what I am trying to do.

I am using a Linux-based netbook when I travel, which is a lot. I have modded it to run quite well as a mini-DAW.  I use it to just capture ideas but have been frustrated with just using PIP from its onboard preamp. I have made a simple stereo microphone with a set of Primo EM172s. They require a minimum of about 3.35v to operate well and they get it from the PIP of the netbook..  They sound good but there is a constant "hiss" which I believe is the crappy preamp on the netbook. So, I am trying to go around it with a line level signal.  The input is both mic/line so I am assuming there is some sort of switch in the input that senses the input level and adjusts accordingly.

I made battery powered preamp that is better but has little "musicality" to it?  It just seemed to me that it would be cool if I could use the 5v power provided by the USB to power the mics and preamp. One challenge I have not wrapped my newbie brain around is how to ground it.  I am assuming that the ground going back to the USB power port might work?  That might mean though it conflicts with the ground going into the line in jack.

Anyway, that what prompted my question above.

I am happy to answer any questions and really appreciate the response.

[ricardo]

James, I think you said on MicBuilders that this is a Samsung NP-n135 and the input is a 'combo' mike & stereo line input.

I'll repeat my question here to everyone.

Anyone know how these 'combo inputs' are wired up?
Is it stereo?
Does the mike have PIP?

The old IBM laptops up to at least the T4x series had excellent stereo line inputs with textbook 16b performance.  You could make good recordings with just a pair of mikes and a P48V preamp.

Alas, only some Macs have good stereo line inputs today.

Anyone know different?

[jmccanna]

Hi,

You are correct it is a Samsung NP-n135 and I am running Linux ArchBang on one partition and AVLinux on the other.  It is actually a potent, though low-powered, little DAW station.  It does have PIP and puts out about 3.5v by my crude measurement with a Multimeter.  I have not been able to find a reference circuit for how it is put together.  It does recognize a stereo input.  I made a stereo microphone using a pair of electrets and it works fine with PIP but it has a hiss to it. 

Last weekend, I built the LowZ-PIP preamp designed by Scott Wurcer off of the micbuilders site and that works quite well - the hiss is gone. It is not terribly musical but that may be the builders (meaning me) problem rather than the designer!  (I also recognize that this is not intended to be studio quality).  I could not get the LowZ circuit to work with his alternate addition of J305 jfet though. Again, I am sure that was me not the designer.

I am looking at all options hence my query here.  I am also trying to figure out something with adjustable gain now.

   

[ricardo]

James, I take it the Samsung has a 3.5mm TRS stereo input for the 'combo' mike & line input.

Do you measure 3.5V on both the tip & ring connections?

I'm trying to figure out how it might sense a 'stereo' line input.  Perhaps drawing more than eg 10uA DC switches the gain to high.

Anyone have experience of this type of input?

[jmccanna]

Hi,

I measured only on one of either the tip or ring but do not recall which. It is a standard 1/8" stereo plug.

Not sure how it works. Any explanation as to how using the LowZ circuit eliminated the "hiss" ?

[VictorQ]

Not sure how it works. Any explanation as to how using the LowZ circuit eliminated the "hiss" ?

I just had a look at the LowZ PIP circuit. Very clever stuff. I can't really imagine anything simpler.

It probably reduces hiss because of the lower source impedance presented to the netbook input, eg. current noise (which runs through the source impedance) produces less input voltage.

Maybe it makes the netbook switch gain or something else entirely. It's a lot of guesswork when the input circuit is unknown.

Could you draw up a diagram of exactly how you connected the mic before and with the LowZ-PIP-circuit?

Also: what is the DC voltage at the netbook input with the circuit connected?

[jmccanna]

Hi,

This is an interesting subject to me.  I am still trying to understand the role of impedance.  I have been doing reading but it is taking a while to sink in.

As to your questions:

Could you draw up a diagram of exactly how you connected the mic before and with the LowZ-PIP-circuit?

Also: what is the DC voltage at the netbook input with the circuit connected?

I am afraid I am going to have to expose the depth of my newbieness.  Not sure how to diagram it but will try to explain.

The stereo mic was just two electret capsules wired to a standard 1/8" stereo to dual RCA adapter.  I removed the RCA plugs and used the hot lead to positive and the bare wire shielding wrapped around it as ground for each side.

This was plugged in to the Netbook and the netbook provided sufficient PIP to make it work but it had the very obvious "hiss". 

So, I built the LowZPIP preamp and doubled it, one for each side of stereo.  I wired in a 1/8" Radio Shack stereo jack for the inputs and one for the outputs. And, I powered it all with a 9v battery. 

Then, I just plugged the above stereo mic into the input and a male/male 1/8" stereo cable into the preamp output to the netbook input.

Now to my newbieness:  I do not know how to measure the DC voltage with the circuit connected.  What I did do, which may be the same thing, is I used a stripped adapter like above and put the multimeter across the ground and lead to get the measurement when it was plugged in to the netbook. Is that the same thing you are asking?

Thanks for the interest.

James