Happy New Year everybody! I wish everyone lots of good things and happiness!
After many tests and modifications, I should admit that I wasn’t happy with the 3 band EQ results – it either sounded wrong or was unstable at some EQ settings. Turned out making a single Baxandal EQ circuit with three independent frequency bands is difficult to design 😂. I always wondered why classic Ampeg amps had their mid frequency control made as a separate stage – now I probably know why!
So I decided to ditch the mids and only keep two bands, bass and treble. Still thinking about making corner frequencies selectable by the switches, two frequencies per band. This could be useful for different instruments, like moving treble cut-off lower for the bass but higher for the guitar…
This is how my finalized prototype looks now with the mid frequency components out:
And frequency curves (these worked the best with my basses)
Added a soundhole pickup into this guitar! It won’t be an “acoustic” anymore, but an “acoustic electric”!
First two additional holes: one for a volume potentiometer (6 mm) and another for the endpin jack (12 mm):
The pickup is a Gretch Deltoluxe – a very nice magnetic single coil designed to fit into an acoustic guitar’s sound hole. It’s sold with all the mounting hardware and the endpin jack, but it doesn’t have a volume control, so I’ll be adding one. The pickup has a cable long enough to insert the volume pot in the upper side close to the neck joint.
Moved this blog to another hosting – from OVH “Webhosting” VPS to Amazon LightSail with a Bitnami WordPress image. The reason is too many outages on OVH (I have Jetpack availability monitoring enabled and it sends me alerts way too often for my taste).
Let’s see how this one works out!
The move vent smoothly using “All-in-One WP Migration” WordPress plugin, by the way.
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:
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).
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!
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!
Well, the pandemic wasn’t easy on me but I survived. Hopefully this blog thing will continue – I’m working on several projects in parallel right now, audio electronics, vintage computers and some weird software. Will start by documenting what I’ve done during my 2020/21 “absence”, this will fill some void 🙂
As a software engineer, I am too tempted to write a blog software to write my own blog in. “Drink your own champagne” so to speak. Using the newest and the bestest framework, augmenting my CV (hey – this is important!) and having fun while recording it all.
But no. After trying things and bits and bobs, I must confess that the old good and boring WordPress stays the best for that.
So be it – so what if it’s written in PHP (insert gagging sounds?). It just works and also installs in 3 minutes from a preinstalled “module” provided by my old web hoster.
It just works and gets out of the way and has tons of useful plugins for everything from CDN for static images (through Jetpack) to antispam for comments (Akismet). Worpress, you’re the winner again. I’ll let the blog stay out of the way and concentrate my efforts on fun and funky software and hardware that won’t break the blog itself 🙂
Welcome, hope this will be useful to someone! It will be for me 😉