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  • Spectrum Analysis in 1986

    Posted on November 17th, 2013 Iain No comments

    I have heard that when you use a tool your brain treats it like an extension of the body and, effectively, it becomes part of you. I don’t know if this applies to electronic test equipment, but certainly some of my strongest memories of learning practical electronics are the equipment I used in different periods.

    In 1986 I joined the Communications Lab at GEC Research as a student apprentice, or “intern” as people would probably say now. It turned out to be a fateful decision because I worked on early trials for the GSM cellular system and that defined the future direction of my career. The labs were proper research spaces with long wooden benches and lots of hand-made hardware which was cutting edge for the date. They also had a lot of fancy test equipment.

    We had a raft of impressive Tek mainframe scopes on trolleys, but sometimes more exotic equipment was needed. Doing radio work a spectrum analyzer comes in handy and the most common one around the site was the Marconi Instruments 2382. Marconi was part of GEC so it made sense to buy in the family, but I think Marconi had genuinely produced a product that had a price/performance ratio that lead the industry. Using a computer-driven display (controlled by the good old 8085 CPU) to replace the storage-tube traditionally used for spectrum analyzers reduced costs and provided exciting features like GP-IB based plotter output.

    Marconi2382 Spectrum Analyzer

    Marconi2382 Spectrum Analyzer

    The 2382 was a massive instrument of two brown boxes for the analyzer and the display. The combination was a foot tall (30cm) and nearly 2 feet deep. It was build like a tank. I am pretty sure that if it fell on you it would probably kill you. The controls included a vast field of clicky switches with embedded LEDs (no membrane keyboards for this monster). In fact my recollection of operating the device involves a lot of clicking. It had some kind of internal relay-driven attenuator and some operations would unleash vast sequences of clicks from the interior.

    I think the 2382 was regarded as pretty cheap for what it did. This piece from IEEE Electronics and Power in May 1985 says the prices start at £18,500. £18,500 in 1985! Probably enough for a small house.

    Learning to drive the 2382 was not just about learning to navigate its many controls. It also taught me a lot about how radio works in practice. Just looking at FM stations gave you a real visual model of ideas like noise, sidebands and carriers.

    Marconi Analyzer - IEEE Comms and Power May 1985

    Despite it’s bulk and cost the 2382 had one major problem – a maximum operating frequency of 400MHz. Not enough for us to use it on the GSM baseband operating at 900MHz and pretty limiting for a lot of comms use. In the comms lab we often had to use even more expensive and exotic HP Analyzers. HP was pioneering the use of screen-menu driven interfaces so though their stuff was the bees knees it never matched the 2382 in terms of button-load shock and awe.

     For those that want to dig deeper, try the Marconi 2382 Data Sheet, or the service manual for the RF unit.

  • Azymbol from Belgium – LushOne case builder extraordinaire

    Posted on November 16th, 2013 Iain No comments

    One of the joys of running LushProjects is seeing some of the imaginative work that people put in to building cases for my circuits. Recently Azymbol from Belgium sent me some photos of his LushOne system case which breaks new ground in design and craftsmanship.

    Working in MDF and recycled materials including placemats and tennis ball tins Azymbol has created a design that evokes 60s futurism displaying the controls to maximum advantage on a dramatically curved frame. The colour is applied in acrylic paint and creates carefully judged dramatic contrasts.

    Azymbol LushOne

    Azymbol LushOne

    Azymbol LushOne Front

    Azymbol LushOne Front

    Azymbol LushOne Side

    Azymbol LushOne Side

    Azymbol LushOne Back

    Azymbol LushOne Back

    Technically Azymbol has used the LushOne base, Contour and Inca modules to build the synth and a Velleman P8042 symmetric power supply he sourced himself.

    The design and build are magnificent. Bravo!

  • PCB Layout Tools – I am a KiCAD fan

    Posted on September 24th, 2013 Iain No comments

    From the postbag:

    Was thinking about a project building front panels for various DIY synth projects out of PCB material and mounting all the control components (pots, switches) on the PCB itself like on the Lush (eliminating most of the point to point wiring). What PCB design package do you use and how difficult was it to learn?

    Tom

     

    Dear Tom,

    As I’ve said elsewhere I hate point-to-point wiring so I made it a feature of LushOne to have all the components mounted on the board. I had previously used Eagle but the LushOne the board was too big for the low-cost edition. In any case, I like to use open source software when I can even though a lot of it is not that user-friendly. The LushOne was the 1st project I laid out in KiCAD and I’ve actually found it a really effective design tool.

    Like all CAD systems it has a bit of a learning curve, but generally I’ve found it easier to use than Eagle. It’s not as powerful as Eagle and lacks the scripting capabilities but it is perfectly adequate for my projects.

    The worst part of KiCAD is the Gerber viewer. I tend to use the online Gerber viewers like the one at OSHPark.

  • Building a LushOne system

    Posted on September 17th, 2013 Iain No comments
    LushOne-System

    LushOne system

    It has always been my intention that it should be possible to use the LushOne to build big systems. Now we’ve got three modules designed we’ve got the basics of a capable modular synthesizer. The building blocks available in the three modules are functionally very similar to those found in classic systems like the mini Moog and MS20. Still, it’s natural to want more of everything and it is particularly helpful to have more oscillators and more mixers in any synthesizer. Over the last few months I’ve been working on my own LushOne system.

    The physical design is very simple. It’s just a piece of plywood with holes on a grid to mount LushOne boards on spacers. The layout is set up for a 3 x 3 grid of boards. I’ve designed a special board to hold the RECOM DC to DC converter. Power is distributed by “chocolate block” terminal strips under the LushOne circuit boards. Currently there are two LushOne base modules, one LushOne Contour and two LushOne Inca modules.

    psu-pcb

    Power supply – hand made board

    The boards have been somewhat modified for my own requirements. The same MIDI input controls both LushOne Base modules. This allows four oscillators to be controlled from the keyboard. The LushOne Base modules are running the prototype version 2 software which provides additional wave shapes as well as the ability to run the OSC2 to an octave higher or lower than the main oscillator. One of the LushOne base modules has been modified so that the OSC2 output is at a signal level rather than at a control voltage level. This makes is really easy to have textured base notes with several harmonics.

    The LushOne Contour module is standard. But, one of the LushOne Inca modules has a potentiometer and photo cell fitted instead of the joystick as an alternative way to provide input.

    The additional flexibility of the system with so many oscillators and signal processors is great. When I get time I’ll write up in detail some of the modifications. At the moment I am playing with digital delays using the PT2399 chip and I really must build a second LushOne Contour to get a second ADSR.

    It may not be the prettiest synth in the world, but I think it looks cool and the bang-for-the-buck is hard to beat.

  • LushOne Base – What should we have in revision 2 firmware?

    Posted on August 11th, 2013 Iain No comments

    I’ve been prototyping some new firmware for the LushOne base revision 2. I am using my experience with the original to add features that make it easier to create an even bigger variety of sounds and to use the LushOne as a serious instrument. I also wanted to add features that would help with building the LushOne in to bigger systems.

    Here’s what I have added so far:

    • Settings memory so that selections are saved when the LushOne is powered off. LushOne will return to its previous state allowing you to pick-up from where you left off.
    • Three new waveforms for the oscillators. 30% and 15% duty-cycle square wave and combined saw/square (like the MiniMoog). New base sounds enhance the range of capabilities.
    • +/- One octave settings for OSC2 in OSC mode. Get really rich, deep tones when mixing with a LushOne Inca.
    • Selection of MIDI input channel for use in more complicated MIDI systems.
    • Ability to get MIDI velocity output as a control voltage (substitutes for the “log f” out). Allows for touch sensitivity.

    What features would you like?

  • LushOne Base – High pass filter mod

    Posted on June 20th, 2013 Iain No comments
    Changes to convert the LushOne Base filter from low-pass to high-pass

    Changes to convert the LushOne Base filter from low-pass to high-pass

    Once the LushOne had a fairly complete set of basic synthesizer functions available I always intended to built a powerful multi-oscillator and multi-filter system. You may laugh, but in the back of my mind I had the brief-case sized systems built on the boutique Mattson Mini Modular components.

    Once you start thinking about building a full system then you are going to want a high pass filter option to complement the low pass filter in the LushOne Base. Fortunately it is easy to modify the LushOne Base filter to be high pass instead of low pass. Here are the changes:

    • Omit R218 and C210 and instead link the two footprints with a wire as shown on the left.
    • Change the value of C208 to 2.2nF. This is the same value as C210 so you can do a substitution there. Obviously this capacitor is now non-polar so ignore the polarity markings on the PCB.
    • Change R221 to 4.7k
    • Change R220 to 47k

    That’s all there is to it! Sit back and enjoy some new sounds.

    Generally the low pass filter is more useful so if you only have one LushOne Base then I wouldn’t make this a permanent change. However if you want to build a system with more than one LushOne base, or if you want to take the LushOne Base schematic and build your own filter on vero-board then it’s well worth having a high pass option.

    If you are using both the filters I recommend putting the high pass first in the signal path and then the low pass. This will reduce the risk of any high-frequency noise getting though in to the output. This arrangement can produce quite natural sounding instruments from the LushOne oscillators.

    Remember that with both a high pass and a low pass filter it is rather easy to cut the signal off all together by having non-overlapping filter bands!

    Here’s a little multi-tracked sample from a dual-filter LushOne:

  • Equation for op-amp sum/difference amps

    Posted on June 2nd, 2013 Iain No comments

    Warning: this post contains maths

    I can never find on the web or in my text books the general equations for op-amps used as combined multi-input summing and difference amplifiers (ie they have several positive and negative inputs). It makes designing mixers for synthesizers annoyingly awkward as I have to rederive the equations each time. So, to save myself having to work everything out from scratch again, here are my derivations and notes on multi-input Op-Amp circuits. I will also take the opportunity to point out some interesting parts of the results.

    Main Results

    Op Amp Sum/Difference Amplifier

    So, here’s the setup:

    We have an op-amp circuit with “N” negative inputs and “M” positive inputs as shown above. All the positive and negative inputs are identical.

    For an ideal op-amp the output is:

    Or, in other words the negative gain is:

    The positive gain is:

    eq3

    Positive and negative gain

    The negative gain is nice and easy and only depends on the input and feedback resistors and not on any other variables, like the number of inputs. Why is this? Well the inverting input of the op-amp is a virtual ground and the voltage isn’t changed by the negative inputs. Therefore the current through each negative input only depends on its input voltage. You can have as many or as few negative inputs as you like and it works the same.

    The positive inputs are not in this lucky position! Voltages at the positive inputs change the voltage at both the inverting and non-inverting inputs of the op-amp. The non-inverting input voltage changes because of the voltage drop over Rg. The inverting input voltage changes due to the feedback action of the op-amp keeping the input voltages ideally identical. This means that currents flowing through all the input branches depend on the positive input voltages and hence the complicated positive gain equation.

     

    Limits on positive gain values

    Once the negative gain is set, this configuration limits the range of values of the positive gain depending on the number of positive and negative inputs. One particular example:
    If the negative gain G- > 1 and number of negative inputs N < M, the number of positive inputs then G+ < G-.

    To derive this then consider that the maximum positive gain is when the input resistors R+ = 0 (obvious from the circuit and also by inspection of the equation).

     

    Special cases and derivation

    There are several interesting special cases from these equations (including the basic op-amp single input amplifiers) and the derivation is worth reading. So I don’t fill the blog with equations you can read it all in this pdf file.

  • Third LushOne module – development report

    Posted on May 16th, 2013 Iain No comments

    The last few weeks I’ve been busy working on the third module for the LushOne synth. This module is going to be all about signal processing and noise effects.

    This is what’s planned to go in:

    • Four channel mixer/signal processor – combine CVs or audio signals within the LushOne or change signal levels for compatibility with external equipment.
    • Noise source for percussive and random effects
    • Sample and hold function for interesting effects
    • Extra square wave LFO, primarily intended to drive the Sample and Hold
    • 3.5 mm jack breakout for easy interfacing to Eurorack modular synths
    • Joystick for dynamic control of two analog control voltages

    All this packed on to the same size board as the LushOne base and the LushOne Contour.

    All the circuits are prototyped on breadboard. Just finishing the first PCB layout over the next few days.

    Suggestions for a suitable name for this module are welcome.

  • Could be the smartest Vibrati Punk Console ever

    Posted on May 16th, 2013 Iain No comments

    Lovely work from David Mead encasing his Vibrati Punk Console. It looks like a very smart and very expensive piece of Hi Fi. He says that it is his first project. Great job!

     

  • 3D Case Printing for the LushOne

    Posted on April 15th, 2013 Iain No comments

    Another nice LushOne project – Simon Reimers in Germany made this rather elegant printed case for his LushOne and LushOne contour. He calls it the “MicroMoog”, but I am really not worthy of that accolade. I do like his comment of “tiny size and mighty sound” though.

     

    Simon Reimers’ LushOne with printed case