Part 4: Amplifying mic level input signal

On our last blog, we talked about distortion and harmonics. and how this enriches 2nd harmonics in our signals.

Here’s a nice animation showing how the 2nd harmonics get stronger the more we push our preamp harder, and note how we’re getting soft, assymetrical clipping…. thanks to our vacuum tube amplifier stage.

Overdrive/Clipping Animation

All this time, we’ve been working with a strong line-level signal. For this to work as a mic preamplifier, we need to further increase our gain capability, and be able to interface with the low impedance output of typical microphones.

TA DA!!!!


It’s working. We’re passing mic-level signals. For now, I used a dynamic microphone for testing since I don’t have 48V phantom power setup on this prototype. I used a Shure SM57 mic, not the best mic for vocal testing but it works for this testing purpose.

My voice on an SM57 mic is just boring and bland, but one thing I did noticed running through this tube prototype is how richer it seems to be, especially the low end. It seems more “bassy”, more “thicker”, and has more low end the sound of my voice running through an SM57 to this preamp. It definitely has mojo.

Basically, I can adjust the TRIM knob to reduce my output signal, while increasing the front end gain higher, pushing the intermediate tube stage harder, and letting the tubes do it’s magic of imparting it’s 2nd harmonics. At the output stage, I have another make-up gain stage (just in case) so if I want to amplify further the tube output stage before hitting the balanced driver. A quick measurement on my oscilloscope shows it’s capable of hitting +20Vrms output, or +28dBu running on +/-18V supply. Not bad at all.

I’ve done some prep work, designing PCB vacuum tube adapters to be used in our board. Also designed part libraries we’ll need to layout the PCB.


Next step now is designing and drawing the schematic and designing the PCB layout! Stay tuned!

One Comment

  1. Hi,

    I noticed that you’d been playing about with Circuitlab:

    and thought you might be interested in the possibilities offered by for this sort of simulation work but also – in the context of this series of blog posts – for tube simulations.

    There are some hardy folk doing tube simulations using Circuitlab but it is a rather limited (no tube symbols that can be used with models, no support for subcircuits) – and as such quite expensive tool – for doing this sort of simulation work.

    EasyEDA is free for up to 2 private projects (there are ways to earn more) and an unlimited number of public projects and it offers a number of tube simulation models together with suitable symbols so it can support the full schematic capture/simulaton/PCB design cycle.

    If you’re looking to do schematic capture, simulation and PCB design (with entirely open and optional low cost PCB manufacture) all in one web interface then you might like to give it a try.

    Here are some of their tube examples:

    and an example including PCB design:

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