Awesome, thank you!

Interesting, thanks! The next time I have to change the 9V battery, I'll give that 100p cap a try as well and see if I can detect a further noise reduction.

I regularly use this with the microphone input on my laptop. I have to dial the gain down really low, though. But the fidelity is still really good.

Thanks! I'm not sure if the circuit would work with a condenser microphone, as it needs phantom power. The 100uF input cap will block DC.

@@user-nx6ep7jz4w That 3.3meg resistor sets the amount of negative feedback to control the gain of the output stages. It could be made smaller or larger for less or more gain.

Yep, the link is broken. I've reached out to the author, waiting for a reply. If there's a new site I will update the link. For now I've updated the comments to show it as broken.

Thanks for the tip. The PG48 definitely picks up every tiny mechanical vibration. Its actually an annoyance, I have to be very careful not to touch it while I'm speaking. Wouldn't buy one for blogging, but it was what I had so I roll with it.

Thanks for the comments! Yes, I have found this amp to have a lot more gain than I need to drive the mic input on my PC. I have the pot around 10% - 20% of full travel and that is plenty of drive. At higher drive, it has enough oomph to provide a line level signal.

The gain is set by the relation of the collector/emitter load of Q1 and Q2. 10k/0.47k*23.5k/0.566k = 883, which is pretty close to 750 (57.5dB). You'll get the ridiculously high gain (in simulation) if you forget to put the mike output impedance (600 Ohm) into the equation. Why 23k5/0.566k and not 47k/0.6k? Because there is a 47k pot and a 10k resistor in parallel with the collector/emitter load of Q1, respectively. All these values are theoretical, as they ignore the input impedance of every next stage - which is both far from infinite and varies with frequency, so the real figures might be about half that. This circuit has no global negative feedback, and therefore the actual gain of the system (and the DC bias at the Q3 emitter) will vary greatly depending on the transistors type, their temperature, and probably the wind direction and the weather on Mars too. Its PSRR is virtually non-existent, so all the noise at the supply rail will be amplified and transferred to the output. The circuit is old. It probably goes back 4-5 decades, because I can totally see the valve origins of some of the design ideas.

Yep, the link is broken. I've reached out to the author, waiting for a reply. If there's a new site I will update the link. For now I've updated the comments to show it as broken.

Thanks for subscribing! I would think the BC547 is fine, that's what Andy's original circuit used. I would suggest to try it without a mic plugged in, to see if the issue remains. If so, then there likely is something amiss with your circuit construction. Also make sure you're using a dynamic mike, I am not sure if this would work with a condenser mike.

@@levelupeelab thanks for response. I checked circuit without mic and noise is more acceptable. I don't know what to do next

Try isolating the preamp from your computer. You can take a small speaker, say a 0.25 watt 2" size speaker, and connect it to the output of the preamp. With the gain potentiometer turned up to near max, you should be able to hear your voice when you speak into the mike. If it sounds good, then the problem is in your computer, if it sounds bad, then there is something miswired in your preamp.

@@levelupeelab I did like you told and it work without noise but sound is really distorted but I think It is ok becouse of high gain. In this attempt I used cheap mic with good impedance ~600 ohm but when I plug this 40 ohm speaker like a mic and rotate potentiometer it becomes a frequency generator😆. I plug also coil with ferrite like you and it works a little better. I think it is problem with all electromagnetic stuff so I will try metal case for it and find some better wires to good electromagnetic isolate. What do you think ☺️

i had the same problem try changing the power supply many power supplies dont have a (isolated) ground connection and this can cause noise i took a computer PSU and used an isolated from the mains ground connection and had no noise problem