300-Ohm Nominal Impendance Microphone Amplifier Questions - Page 1
Forum section: electronics / projects

author: expertmax
author_rank: Regular Contributor
timestamp: July 24, 2012, 02:10:05 am
content: 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.

author: David_AVD
author_rank: Super Contributor
timestamp: July 24, 2012, 02:31:47 am
content: 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.

author: expertmax
author_rank: Regular Contributor
timestamp: July 24, 2012, 02:47:08 am
content: Allright, I found this circuit on the internet :I'm replacing R1 with a 10k pot'.So that should be it ?

author: David_AVD
author_rank: Super Contributor
timestamp: July 24, 2012, 02:52:02 am
content: 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.

author: expertmax
author_rank: Regular Contributor
timestamp: July 24, 2012, 02:54:52 am
content: Do you think this circuit will consume more than 1 ampere ?

author: DrGeoff
author_rank: Frequent Contributor
timestamp: July 24, 2012, 02:56:35 am
content: Some good info and circuits here:http://sound.westhost.com/projects-8a.htmSince you don't need phantom power for this mic (it's a dynamic cardioid) you can leave those bits out.

author: DrGeoff
author_rank: Frequent Contributor
timestamp: July 24, 2012, 02:59:05 am
content: 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.

author: expertmax
author_rank: Regular Contributor
timestamp: July 24, 2012, 03:12:53 am
content: 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 ?

author: DrGeoff
author_rank: Frequent Contributor
timestamp: July 24, 2012, 03:26:02 am
content: 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.

author: David_AVD
author_rank: Super Contributor
timestamp: July 24, 2012, 03:35:34 am
content: Quote from: DrGeoff on July 24, 2012, 02:59:05 amI'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.

author: expertmax
author_rank: Regular Contributor
timestamp: July 24, 2012, 03:38:34 am
content: 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 !!

author: DrGeoff
author_rank: Frequent Contributor
timestamp: July 24, 2012, 03:51:19 am
content: 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.

author: expertmax
author_rank: Regular Contributor
timestamp: July 24, 2012, 06:23:31 am
content: Do you have a way of drawing it for me or something because I am totaly lost ! Thanks !

author: DrGeoff
author_rank: Frequent Contributor
timestamp: July 24, 2012, 09:53:20 am
content: 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.

author: DrGeoff
author_rank: Frequent Contributor
timestamp: July 24, 2012, 10:09:04 am
content: 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.