Categories
Hardware Tools

Universal Radio Tester

This simple device is a basic signal generator that can be used to debug audio and AM radio frequency circuits. It produces a (roughly) 1 kHz periodic wave output that has enough harmonic frequency content to go into high MHz range – this will make the signal audible even when injected into RF and IF radio circuits.

The schematics is based on a simplest possible two-transistor multivibrator that can be powered by one 1,5 battery (AA or AAA) but will also work fine with a higher voltage source, for example a 3,6V lithium coin cell. It draws around 300 mA at 1.5V. The components values can vary, also pretty much any pair of low power transistors will work. In case of PNP transistors, the battery should be flipped (+ as the ground and – as the power rail). It should start working right away and doesn’t require any tuning.

The output waveform and frequency will vary depending on the tested circuit impedance, without the load it will be similar to this:

Github source: https://github.com/badscrew/RadioTester

I simply soldered mine on a perfboard and it proved itself very useful:

Categories
Audio Hardware

3. Dead Kings bass effects pedal – EQ

This is the 3rd post in the Dead Kings preamp series.

Now to design the EQ module!

My initial idea was to create a circuit inspired from the classic tube bass amplifiers, like Ampeg Portaflex or SVT. I did some research and got somewhat carried away, so the circuit I came up with has ideas from many sources and doesn’t reproduce any of the existing modules. There were also ideas suggested by OnlyBass forum members that I’ll try to test out and maybe incorporate into the design.

So this is the first version that I came up with and ran through LTSpice simulation:

This is basically a “symmetrical Baxandal EQ in the negative feedback circuit”, with RC Bass and Treble and LC Mid sections. The LC/RC bridges are all high-impedance and require high-impedance next stage, hence the usage of the bootstrapped emitter follower (Q1) as a buffer with 250-300K input impedance. Q2 is the identical input buffer, and the Q4 buffers out the output – all of these make the EQ module completely independent and usable all by itself (perhaps as a separate pedal). As is, this EQ can be also easily transplanted into a tube circuit with minor or no modifications.

The pots are linear 100K, easy to source and more precise than logarithmic pots used in some of the vintage passive tone stack designs. For the Mid EQ inductor I’ll be using a primary winding of a small audio transformer, I have some handy but will need to test them in the circuit – it’s unclear what their inductance is, depending on the source it’s from 3 to 11 Hn.

The overall amplification is 0db (no amplification), the frequency bands give about 12 db cut and boost for the Bass and Treble and 8 db cut and boost for the Mids.

One of the things suggested on OnlyBass is to make the band frequencies modifiable, and I will be certainly testing this, to do that I just need to vary the capacitor values: C8 for the mids, C4 for the bass and C16/C21 together for the treble. In the final product I’ll maybe choose two values for each of the bands (using three switches, one per band).

Next step: to design and solder the board!

Categories
Audio Hardware

2. Dead Kings bass effects pedal – clean boost

Continuing the Dead Kings preamp series

The tests in real hardware have shown very close characteristics to what LTSpice have simulated, this means the models of the transistors I’ve been using are pretty accurate, that’s good news!

i’ll be testing some slight modifications – reducing maximum gain and maximizing undistorted output – smaller R9 and R10 and higher R8.

The first stage (Q1 and Q2) didn’t need any modifications whatsoever. Even the modifications I’ll be doing are pretty minor and may not be necessary, time to try this and see if it worked out better at all!

Categories
Audio Hardware

1. Dead Kings bass effects pedal

I’m starting this new project on Github, heavily inspided by my recent discovery of the band Royal Blood https://www.royalbloodband.com/

This is the first post in the Dead Kings preamp series.

This preamp / effects pedal isn’t supposed to emulate the sound of Mike Kerr, I’ve just got some ideas I would like to try 🙂

Just for the kicks and as a challenge, I’ll be building this using only (or at least mostly) germanium transistors. I’ve been compulsively buying germaniums for quite some time, need to put them to good use!

The design will be licensed under Open Hardware permissive license, the sources will go here: https://github.com/badscrew/DeadKings

The main idea is to have two or three mixable channels, one clean and two effects – an overdrive/distortion and an octave up distortion.

Overall block diagram so far:

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Hardware Minitel

Minitel

Hey, look what I’ve got, it’s small, cute and it works.

Well, the Minitel network is no more, but I have some interesting ideas (and three of these!)

Will speak about this more in near future!

Minitel terminal running
Minitel