300-Ohm Nominal Impendance Microphone Amplifier Questions

[expertmax]

Hi everyone !

I'm trying to design a project which I find very interesting and easy to make but I don't know where to start !

My project is that I'm trying to amplify a microphone (sennheiser hd845) with a 300-Ohm nominal impedance to a signal that I can plug my earphones and the volume should be very loud but without any clipping or distortion. The final impedance should be like around 16 or 32?.

I have no idea which amplifying transistor I should use and I can use either a 9v supply(0-9v) or a 12v(0-12v) supply. And I also need help to find the right transistors. I don't need to control the output volume.

Thank you SOOO much !!
Max.

[David_AVD]

You'll probably be best breaking this problem up into 2 sections:

1) A microphone pre-amplifier with a gain of 40dB - 60dB.

2) A headphone amplifier.

There's plenty of circuits on the 'net for each.  Pick something that uses a single IC and a single 9-12V rail.

[expertmax]

Allright, I found this circuit on the internet :



I'm replacing R1 with a 10k pot'.

So that should be it ?

[David_AVD]

That's the basic idea for a mic preamp.  Maybe leave the 1K as-is and replace the 22K with 220K to give enough gain.

Try the headphones on the output via a 100 Ohm resistor to check operation.  You'll need a second opamp circuit for the real headphone driver if you want it loud.

[expertmax]

Do you think this circuit will consume more than 1 ampere ?

[DrGeoff]

Some good info and circuits here:
http://sound.westhost.com/projects-8a.htm
Since you don't need phantom power for this mic (it's a dynamic cardioid) you can leave those bits out.

[DrGeoff]

Ideally you should be using a balanced input for this mic.
Otherwise you will lose 6dB at the outset by unbalancing it and it is likely to buzz and hum a lot with mains pickup.
The circuit you show is unbalanced and not really much good for the mic you are using.

[expertmax]

Thank you for the information !



Do you think I would be able to connect the output to my computer's line in port and be able to hear myself loud enough using that schematic ?

[DrGeoff]

Yes, although this circuit is probably good for only about 40dB gain.
You also should realise that it requires bipolar supplies (+/-12V) to operate.
Try it out. It's only a sinlge chip (5532) and easy to breadboard.
You can increase the value of the 150nF caps if you want the response to be better below 100Hz.

[David_AVD]

Ideally you should be using a balanced input for this mic.
Otherwise you will lose 6dB at the outset by unbalancing it and it is likely to buzz and hum a lot with mains pickup.

I'll have to disagree with you there.  Using a balanced mic on an unbalanced input will not lose 6dB in gain.

Now if you were unbalancing a balanced mic input I'd agree about the 6dB change.

[expertmax]

What values you would put in ? This microphone is multi-purpose so the higher the better I guess.

Is it possible to use a generic 9v power adapter and convert it for negative voltages ?

Thanks !!

[DrGeoff]

Try 0.47uF caps instead for starters.
If you can get a generic 9VAC or 12VAC power adaptor then it is easy to generate a bipolar supply.
With a DC supply you can split it with a divider and run the new 'ground' point through a single supply opamp to get a low impedance ground point. You also need a coupling capacitor at the output since it will be at half rail when no signal is applied.

[expertmax]

Do you have a way of drawing it for me or something because I am totaly lost ! Thanks !

[DrGeoff]

For an AC power adaptor you can use this to generate a bipolar unregulated supply.
Add voltage regulators to each rail to generate a regulated supply, probably best to use the LM317/LM337 pair and adjust the output voltage according to your DC input level under the anticipated load, including whatever headphone amp you decide to use.

[DrGeoff]

For a DC power adaptor, you can split it into two, creating a floating 'ground' halfway. For a 12V adaptor, this only gives you +/-6V (unregulated) which may not be sufficient. Given the poor load regulator of small power adaptors, a regulator is recommended to stabilise the power supply in each rail.