Weltmeister Combo Bass

My pal Vic has given me her Weltmeister Combo Bass keytar to reinvigorate. There's not a great deal of info online so I'll collate what I know here in the hopes that it will be helpful to someone else in the future.

The Combo Bass was more commonly known as a Basset. These were in production from 1963-69: the '6510' serial number on this one makes me think it's probably from 1965. It runs two and a half octaves from F to C (ish), has a sweet spring loaded volume swell button and two weird looking outputs (two different impedances by the looks).


Each key triggers a mechanical linkage that plucks a metal tine of a given pitch. If you wanted to tune it you'd have to fractionally adjust the weight of the tines, maybe with nail polish and sandpaper? I personally like the wobble so will leave it as is.



There's a magnetic pick-up that runs the length of the keyboard and fed into an on-board 4.5V preamp. As an exercise in nerdiness I traced the circuit and discovered it is a pretty basic single-transistor preamp, besides the fact that it is positive ground. Back in the sixties transistors were pretty hit-and-miss; positive ground PNP transistor circuits were the most reliable at the time. (More info on positive ground here.) Positive ground confuses the crap out of me so I might have some polarities mixed up but as best as I can tell, the circuit looks like this:

The transformer is tricky to measure but I think it's something like a standard 1:1000 audio. The transistor is an OC1044, but I can't find out any more than that - anyone have a comprehensive transistor resource?

Circuit tracing sucks because you have to have X-ray vision to see both sides of the board at once. Unless you have photoshop. Take a photo of each side, vertical-flip the back side, and overlay it on the front. Distort and transform til the layers match up, then change the opacity so you can see both at once. See:

Front, distorted:


Back, flipped:


Both:

If you listen closely you can hear the sound of me being really proud of myself for thinking of this.

I'm going to experiment with replacing the preamp with a nice high-impedance FET buffer, then replace the weird old-school output jack with a standard 1/4" one. Maybe if Vic lets me I'll modify that volume pedal so you can switch it over to a wah control. Mmm... maybe I should just let this priceless piece of pre-synth history rest in peace.

4-Stage Gated Oscillator

The first project I'm teaching in my DIY synth course is a 4-stage gated oscillator. It takes a couple of hours to put together, costs about $30 in parts, and is a good starting point for absolute beginners. So what's going on in there?



Four square-wave oscillators are chained together. The first one is always on, in the video it can be heard driving the main rhythm. Each of the following three oscillators are switched on and off depending on the state of the previous one. The rate of each oscillator is controlled by its own knob. Adjusting the oscillation rate results in changes in rhythm, pitch, and timbre.

The circuit diagram and simulated output are shown below. Further detail is available in my lesson plans, which you will get when you sign up for you DIY Synth lessons with me! It's cheap, and choice!

Electronic Audio Hardware

I've been reading Handmade Electronic Music by Nicolas Collins and it's possibly the most informative electronics book I've ever read. It presumes absolutely no prior electronic knowledge of the reader, starting with the most basic introduction to circuit bending (think wet fingers inside open radios). He focuses on maximal fun for minimal effort and it's late in the piece before he starts explain circuits of any appreciable complexity. For the most part these circuits are incredibly clever and pack a lot of functionality into a few components. I'll be working through some of his suggestions and posting my progress here. A simple project that caught my eye was the DIY spring reverb (guitar amp -> cheap audio transformer -> piezo -> slinky -> contact mic -> amp). He also has a lot of helpful information on using CMOS logic as the basis of synthesisers and signal processors (something I've been reading a lot about lately). I'll be playing with a lot of these ideas in the next week and providing in-depth details, but for now here's a teaser for what you can expect...

The simplest CMOS oscillator, a 40106 Schmitt inverter chip with a single resistor and capacitor. The component values set the pitch. You can replace the resistor with a potentiometer for pitch control. Start with a 1uF cap & a 1M pot. You can add a voltage divider on the end to drop the (very loud) signal level. To mix more than one of these oscillators together you can either connect a 100k resistor to each output and tie all the resistor ends together, or replace the resistor with a diode for a different sound (half-rectified).


Replacing the potentiometer with a photoresistor makes a psuedo-theremin. The second photoresistor is used to alter the volume.


These circuits produce a nearly-perfect square-wave. You can also get a triangle wave (of much less volume) out of them. The 10k-1k voltage divider on the main output drops the volume of the square wave to match. Both these waves will be at about line-level.


The complexity increases slightly from this point - in the book he makes this transition very gentle. One excellent notion he shares is a very simple vibrato circuit: the oscillator blinks an LED on and off at a variable rate. The photoresistor is mounted end-to-end with the LED and varies its resistance with the blinking, dipping the volume of whatever you want to plug in.



If you replace the inverter with a 4093 NAND chip, you get a gated oscillator. This is essentially the same thing, with an aditional input that can switch the oscillation on or off. With one oscillator running into the next, the first will modulate the second. If the first oscillator is cycling at a slow rate (1-10Hz) and the second oscillator is running at audio frequency, the audio tone will be switched on and off rhythmically. If both oscillators are running at audio frequencies the result is a warbly modulation similar to a flanger.


Collins also shows how to use a 4049 inverter as a mic/guitar preamp. The ratio of RF to RI sets the circuit gain - in this case it is set to a tenfold increase, but replacing RF with a potentiometer allows you to alter this. CI and CO are decoupling capacitors - they pass the audio signal (AC) but block the DC signal from getting into your amp or mixer (audio circuits don't like DC).


Too much gain and the circuit will start to distort. This is the basis of the Big Muff Pi guitar pedal. It's a good idea to place a small capacitor (10-100pF) between the input and output of each gate to get rid of the very highest frequencies as these tend to get a bit out of control.


Feeding one of these distortion units into a 4040 divider chip makes for a really easy octave-down effect. This chip will actually produce up to 12 octaves down - far below audio range.


Some other ideas I will be looking into in the coming weeks are the sequencer and envelope follower. The sequencer is known as a Baby 10, the easiest possible sequencing circuit. Here's an example video of it driving an Atari Punk Console (similar to the simple oscillator shown at the start of this post). In the given circuit the 40106 oscillator is used as a clock and the 4017 just blinks each of the LED's on in turn at the rate set by the clock.


The envelope follower can be used for a lot of things, but to start with I'll be using mine to to make a LED's brightness change with the audio volume.


These schematics are reproduced without permission so consider purchasing Nic Collins' excellent book if you find them helpful.