Top Shelf

As threatened last week, I gathered all the components of a top shelf for my Leslie 125 together and made a trial assembly...


You can see at top left the MTCLogic controller, a power strip providing 110V from an isolation transformer, the motor, the horn and the belt tensioning arrangement, all mounted on a piece of 19mm MDF.

Here's the single-speed Leslie motor in close-up...

I turned up some spacers (not visible above - they're hiding under the rubber mounts) from mild steel to raise the motor up to the correct height above the board to match the pulley on the horn, which you can see in the next photo...

Also visible is the stock Leslie idler pulley, which tensions the belt. The motor and the speaker are in the correct positions, copied from the layout of a Leslie 145 - but I improvised with positioning of the idler wheel.

Here'a a close-up of the controller. It is held in place for testing with my customary blobs of Blu Tack...

The system works perfectly with our new controller (he says, swelling with pride). I also tried it with some of the competition...

This is a unit from Caribbean Controls. I'm pleased to be able to say that there is no choice for me as to which speed controller I'll build into the modified 125. I'm even more pleased to be able to say that the choice is made entirely on merit!

...-.- de m0xpd

In a Spin

Aside from admiring, owning and using them, your humble servant is now active within the global economy of Leslie Speakers, having jointly developed an accessory which is commercially available. More of that sordid, mercenary semi-professionalism later. First, true to my Ham credentials, I'd like to share some details of my new Leslie interface, just for the love of it.

Readers have learned how I got my first Leslie, an 825, from Bob, just down the M56 in Chester. A couple of weeks before Christmas, I got another Leslie from another Robert, a little further west in Holywell. I'm pretty sure that the relative remoteness of Chester and (particularly) Flintshire kept the prices down to my beer-budget levels.

The new (old) Leslie is a 125 - similar to the 825 in that is has only a single speaker (but that's about to change - watch this space). The 125 is, however, critically different in at least three important respects. Firstly, it has a veneered wooden enclosure (rather than the ugly "Tolex" vinyl covering of the ProLine 825) - it LOOKS like a Leslie. Secondly, it has a valve(/tube) amplifier, with all the romance that entails. Thirdly, it uses a different interface to the outside world.

The 825 uses Leslie's "9 pin" interface, as described in this previous post. It achieves speed control by a simple, low voltage switching scheme, ready for plug 'n play connection to my homebrew half-moon switch.

The 125 uses an older interface standard, called "6W". The 6 indicates the fact that there are only 6 conductors in the cable to the speaker (and 6 pins on the special, expensive Amphenol connectors). The "W" actually stands for Wurlitzer, in order to differentiate from another contemporary six-pole Leslie interface, "6H", in which the "H" meant Hammond. The 6H interface was used for - you guessed it - Hammonds, as exemplified by the Leslie 122. The 6W interface, developed from a Wurlitzer wiring standard, is also called the "Universal" interface and is used for other applications, as in the Leslie 147. These two interface standards are dangerously INCOMPATIBLE, yet they share the same physical layer in the special "6 pin" connector cable, which is used in both standards.

Here, for my own records as well as your edification, are the pin assignments in the 6W standard...

Pin	Colour	Function
1	Black	Signal Ground
2	Yellow	Motor Relay
3	Gray	AC Power In
4	Blue	AC Power In
5	Brown	Motor Relay
6	Red	Signal Input

The colours refer to the cores in a real Leslie cable.

Note the apparently inoccuous pins 2 & 5, connected to the Motor Relay. In order to switch between the two available speeds ("chorale or tremolo"), one has to apply 240V AC between pins 2 and 5 (in the UK at least). Speed control certainly isn't a "simple, low voltage switching scheme" and it implies switching mains electricity. No voltage gives tremolo (the fast rotor speed) and 240V gives chorale (the slow speed).

Call me a wuss if you like, but I have a pretty well-developed fear of high voltage electricity. It all started when my childhood friend, Paul Sibley, tried to stop the smell caused by a budgerigar feather burning on the element of a small electric fan heater. Paul used a pair of scissors to remove the feather with predictable results. He flew across the kitchen at altitude of about six inches and slammed into the opposite wall. Mr Sibley was a successful builder who had built his family an impressively proportioned home, so Paul's trans-kitchen flight was a long one. He survived and I learned what has proven to be a useful life lesson.

With all the baggage of burning budgie feathers, I wasn't about to go switching mains voltages on my virtual organ console. I don't think the guitar pickup selector switch in my homebrew half-moon switch would have handled 240v AC - much less, into slightly inductive loads. So - a safer switching scheme was indicated...

Of course, I could have simply used a relay to switch the voltage to the motor relay (big fleas have little fleas upon their backs...) and used the Telecaster switch to energize the local relay coil at low voltage. That, however, wasn't entirely true to the spirit of the ideas brewing in the back of my mind for more flexible speed switching options, including 9-pin, 6-pin and maybe even 11-pin interface compatability, so I added a little electronics.

Here's the low-voltage aspects of my speed control circuit - it is based on a somewhat over-specified relay from Maplin (which required me to learn again for the umpteenth time how to create a new part for Eagle). The design includes a stabilized 12V supply (not strictly needed for the switching operation, but important for the rest of my design, as you'll see in a moment), which is at the top of the schematic below.


I've edited out all the high voltage parts of the circuit. I don't want to be held responsible for people who haven't learned the consequences of burning budgie feathers.

Here's the switching circuit in the flesh...


Still glowing with the success of my pre-amp for the 825, with its HF shelving filter, I made another identical circuit for use with the 6W interface...


All these printed circuit boards are all very well, but they only become a practically useful system when placed in an enclosure with the appropriate connectors. In sourcing a suitable box I learned that some of Maplin's enclosure range is to be discontinued - I hope this isn't yet another stage in the long, drawn-out demise of this once-useful source of components. (For overseas readers, Maplin is a high street retail chain filling the spot once occupied by Tandy - the UK version of Radio Shack. Until five or so years back, Maplin rivalled the big "trade" players in component supply with on-line and high street options. The demise of hobby electronics (indeed, of any practical pursuit) and the nonsense of trying to provide it on the high street has driven Maplin's metamorphosis into yet another consumer electronics / tech store. I doubt they will survive much longer.)

Anyway, here are some shots of my finished 6W interface, allowing the 125 to be connected to my virtual organ (i.e. a computer soundcard) or anything else which can produce close-to line level audio into high impedance.


The big red switch turns on the whole shooting match (including the Leslie) and the two controls on the front are for volume and HF boost.There are internal jumpers selecting 0, 10 or 20 dB overall gain. On the rear are mains inlet via an IEC socket, the 6-pin chassis socket (I splashed out and purchased a 32.5mm hole punch to add to the collection), audio input via 1/4 inch jack and speed control via a 3-pin DIN (as used on my other "pre-amp"). It works perfectly and makes hooking up the 125 (or any other Leslie using the 6W interface) a walk in the park.

Here is the finished 6W interface unit, sitting atop the 825 (the 125 is on the other side of the room).


Now to return to the commercial world...

I was chatting with Hammond and Leslie expert George Benton, after reading of his adventures using a Hammond Super B console as controller for a virtual organ. George had seen the games I was playing with PICs in an organ context and asked if I'd like to join in developing a speed controller for Leslie speakers. I accepted, thinking I'd learn from the experience and from friendship with George - both have turned out to be the case.

First, a little explanation.

Early Leslie speakers (the 21H, 22, 22H, 22R, 31H, 44W, 45, 46, 47and 51) were single speed (or, at least, single speed plus "stationary"). Later models evolved to the more familiar two-speed operation mentioned above. It will come as no surprise to hear that the old, single speed models are coveted and valued for their special, desirable properties, real and imagined. Prices soar accordingly. Despite this, owners of single speed units also like the two-speed feature, so opportunity exists for electronic speed controllers able to convert a single-speed unit to two-speed operation. A number of such controllers are available.

George had the bare bones of such a system but needed to develop software - enter yours truly.

Our Leslie speed controller is a fairly standard motor control application, using Pulse Width Modulation techniques. The motor is placed in an H-bridge, formed of IGBTs, under the supervision of a PIC microcontroller...


Although Motor Control is a standard PIC application, we chose (for legacy reasons) to use a very simple 8-pin PIC with no hardware Capture/Compare/PWM module. This made the entire project a matter of trying to squeeze a quart into a pint pot. Not in terms of memory capacity or computational load - rather in handling the time constraints imposed by the interrupt-based software PWM modulator I developed.

The single-speed motors are built to run at "tremolo" speed when driven by 110V, 60Hz, so the controller generates this output signal when "tremolo" is requested. The motors can also be persuaded to run at slower speed, appropriate to "chorale", by lowering the frequency. However, this drop in frequency is accompanied by a drop in the inductive impedance offered by the motor, such that the motor passes more current and runs hot. This can be countered by dropping the voltage magnitude and frequency in "chorale". However, the motor speed is actually a complicated, non-linear function of both voltage magnitude and frequency, all of the consequences of which are anticipated and handled in the control algorithm.

Transitions between each speed (the controller also has a "stop" mode, giving stop, chorale and tremolo as the three available operating speeds) are handled with various boost or brake options, some of which can be selected by the installer.

What with the adaptive interrupt service routine and several other "smarts", there's quite a lot of innmovation and intellectual property in the controller. George and I are real proud of it!

Here is my development platform for the code - I implemented the IGBT H-Bridge on the white PCB for development purposes, but never connected it to a motor...


Here's the final product, which replaces the relay in a single speed Leslie...


It was rather time consuming developing real-time motor control software on one side of the Atlantic and testing it on harware on the other - but we got there! Learning the obvious lesson from that geographical separation between development system and target, I now have assembled all the bits required to add a horn, under the control of our speed controller, to my 125. I also have the single speed motor for the LF unit, so it will go "all electronic".


This will make development of the next versions of the controller a whole lot easier - plus I get to have a great organ speaker!

Over the past few days there has been some disquiet as the Bank of England engages in the Carollesque delusion called "quantitative easing". I can't see any real economic benefit in that. Instead, why don't you make a real difference by heading off to the Benton Electronics on-line store and getting yourself an MTCLogic 2-speed Leslie controller.

Every home should have one. At least, every home with a single speed Leslie.

...-.- de m0xpd

Top Totty

Isn’t it heart-warming to know that, despite the imminent threat of economic Armageddon, with all its collateral, our politicians still have time and energy to devote to matters of grave import.

I understand from her website that Kate Green MP twitted (sic) to the effect that Slaters’ branding for their blonde beer ‘demeans women’. Although her actions can hardly further demean the laughing stock that is the political class, Ms Green undoubtedly has demeaned the cause of equality.

I have the good fortune to live in Ms Green’s constituency. Evidently, I do not have the good fortune to have my views “represented” (in any sense) by the local MP.

Cheers Kate!
...-.- de m0xpd

More Matters Enigmatic

It has been so long since my last post that part of me feels like making an apology - but the bigger, better part reminds me that this is for my pleasure and recognizes the potential danger of becoming ruled by a compulsion to blog, so I'll continue as if nothing has happened...

Amongst the many nice gifts I received at Christmas was a(nother) book about Bletchley Park, to add to those already devoured (some of which are reported elsewhere in these ramblings).

This time, it is Sinclair McKay's book "The Secret Life of Bletchley Park"...

This was a pleasant-enough read, different from all the other technically focussed books in that its subject is very much the people at BP and the context in which they served. The book looks at social, domestic and human aspects, without spending much time on the cryptanalytical methods and technologies at the core of the work. Also - unusually - the book doesn't become too distracted by the "celebrities", affording equal weight to some of the humbler folk on which the activities depended.

McKay describes the experiences of those who worked at BP, drawing heavily on other published materials and (apparently) a few interviews with veterans. We are given a glimpse of working conditions, billeting arrangements, recreational and cultural activities, romances between staff members and dealing with the imperatives of secrecy - both during and after the war. The book concludes with some descriptions of how BP veterans dealt with the changing conditions after the hostilities and attempts a brief sketch of the broader legacy of the work at Bletchley Park.

It is a rewarding read, but might have been better edited, not least to avoid an irritating sense of repetition. The book tries to assess the broader legacy of BP's work and occasionally is marred by repetition. [ed: scratch that - you've said it already]

Whilst we're adjacent to the subject of "Enigma", may I commend to the attention of British readers the weekly "Enigma Code" puzzle in our beloved "Radio Times"...

I'm no puzzle junkie, tending to sneer at Sudoku addicts, but this particular game interests and entertains me.

It takes the form of a "crossword without clues", in which all the squares are numbered with integers corresponding to the letters of the alphabet. Your task, should you decide to accept it, is to associate numbers and letters!

Give it a try - I saw a fellow passenger on a flight doing the same type of quiz from a different newspaper recently. I don't know which paper it came from, but I can confirm there are more of these puzzles out there if you become hooked.

Fortunately, I'm not the addictive type!

...-.- de m0xpd

Pedalling like Fury

I was fortunate to pick up an old, worn, dirty set of Hamond C3 padals on eBay for a fiver back in August. Here they are at the start of the "resurrection" process...


After a lot of elbow grease (most of it from my wife, who generously contributed her domestic skills and cleaning products) the pedals looked much cleaner. They also worked rather better, as I had replaced any badly worn felts, some of which support each pedal laterally between vertical guide pins. Finally, I had stripped off the galvanized brackets which support the operating "tabs" which do the actual switching in the real Hammond Organ (which is why this is a "resurrection" rather than a "restoration").


From this angle, perhaps you can see why I believe the previous owner(s) preferred playing in the key of F!

I wanted to use these pedals in my virtual organ project (Blogs passim), which meant that they had to be able to operate as electrical switches rather than hinged pieces of wood. There are a few technology options here for the organ builder:

• you could arrange for each pedal to touch and operate a miniature switch as in this example
• you could make electrical contact pairs (or one contact per pedal and one or more busses) for each pedal and make a physical switch as in this example.
  
• you could use an optical method, with a "shutter" tab on each pedal and a opto-detector making the translation from optical intensity to an electrical switch function
  
• you could use a magnetic method, placing a magnet on each pedal and using a Hall Effect sensor adjacent to each pedal to convert the magnetic field to an electrical switch function
  
• you could usea magnetic method, placing a magnet on each pedal and using a reed switch adjacent to each pedal to convert the magnetic field to an electrical switch function as in this example
  

There are examples of each of these methods on the internet.

I wanted to use the Hall Effect method, but I couldn't find the sensors cheaply enough (you should know me by now), so I stuck with the trusty, cheap reed switch technology...

I put 3mm Neodymium rod magnets into a hole drilled in the end of each pedal...

You can see the vertical guide rods and the felts, mentioned previously.

I also made up a series of perspex panels (chosen for its transparency, which made alignment with the magnets during "marking up" and subsequent assembly easy), each holding five or seven reed switches. I made brackets to support these panels, mounting under the nut and lock washer that secured the guide pins - the whole process involved no "butchery" of the pedals, except the magnet holes. One of the (five switch) panels is seen, in-situ, below...

I have described previously how my single-octave pedals were wired up and converted to MIDI. The same process was used again - with one significant exception...

The pedals are scanned as a matrix of (4) columns and (7) rows, the last having only one pedal (25 notes takes 6 rows of 4 and a single "extra"). In the previous encoder system, it was known that you could not play more than one pedal in a single column together whilst playing another pedal from another column without the possibility of additional notes sounding. This wasn't seen as a problem as playing two pedals in one "column" constitutes an interval of a minor third or less, which isn't done in the bass register (it is psycho-acoustically and musically meaningless).

However, the geometry of the new pedalboard made it easier for me to arrange that the reed switches should be normally on, switching to open circuit when a pedal is depressed. This meant that, from the point of view of my encoder scheme, all the notes in every column were being "played" most of the time - the system didn't work!

The solution is well-known and simple - you just add diodes in series with each reed switch et voila. Of course, I had to invert the logic of my encoder program (to send a "Note On" command when a reed went open circuit and vice-versa) but that was the work of moments.

Here's the expanded version of my pedal encoder system, capable of running 25 keys (plus seven spares for "toe pistons" or whatever) and the expression pedal...


The trouble with a two octave pedal board is that you need a real organ bench to sit "over" the pedals. Fortunately, eBay came to the rescue again, with a genuine Hammond A-100 bench...


I'm very surprised how easily this went together - I was frightened that the reed switches would be very difficult to set up, but they were easy. Careful location of the magnets (using a jig to drill the pedal ends) and equally careful marking up of the perspex carriers made it work first time.

All I need to do now is get out of the habit of stopping at that formerly top - now middle C!

...-.- de m0xpd

Shelving Audio Filter

I was fortunate to pick up a Leslie 825 speaker for my virtual organ project (Blogs passim) for a song on eBay...

Part of the explanation for the attractive price was that - as the seller put it - this is "a Leslie 825 speaker with a difference".

I'll let the seller go on with his description:

"the difference is that the (previous) owner had a number of keyboards and he modified the cabinet by inserting two other speakers front and back - he drilled holes in the cabinet and placed a grill over the front - the grill is available but is not in the picture so that you can see what lurks beneath".

Well, now YOU can see the ugly array of holes in the picture above. I have fixed the grille on to make thinks look a little less unsightly and, like the seller, I haven't yet bothered to test the additional speakers. But the Leslie itself works well enough (with one trivial exception, to be described below).

For those of you who don't know, a Leslie speaker includes rotating mechanical elements (horns and/or baffles) to introduce a cyclical variation to the radiation from the loudspeaker which, in interaction with the acoustics of the performance / listening space, makes some interesting spatial audio effects involving amplitude and frequency modulation and comb filtering. It was developed by Don Leslie...

in an attempt to emulate the chorus effect produced when ranks of (inevitably somewhat de-tuned) organ pipes speak together, giving electronic organs a "pipe voice". The mechanical components are contrived to rotate at one of three different speeds (slow/stop/fast or, to respect the orthology, chorale/stop/tremolo) under the control of the iconic "half-moon" switch. Paul, g1dva, tells me he has seen rotating speakers with continuously variable speed control - but we'll dismiss these as pathological. Certainly, the classic Leslie only operates at a few discrete speeds, latterly the three described above.

Older Leslies used a horrible electrical interface, in which mains power, control signals and the audio were all applied through a long, multi-way cable terminated with special (i.e. expensive) Amphenol connectors. In the case of the 825, it is a 9-pin system, with the following connections...

Pin	Colour	Function
1	Black	Ground
2	Red	Input
3	Yellow	No Connection
4	Orange	Stationary Input / Aux Control (not used in 825)
5	Green	dc Output (+28V)
6	White	Slow Motor Control
7	Violet	Fast Motor Control
8	Gray	240V In
9	Blue	240V In

(The colours are those of the cores of the "official" Leslie Cables).

The seller of my 825 kindly gave me a 9-pin cable to connect up the device - thanks Bob!

Well, I had the cable but no socket for the organ end, so I lashed things together with a very unsafe Heath Robinson connection, involving individual pieces of "Choc Bloc" connector...

on each of the pins of the plug - DO NOT TRY THIS AT HOME, CHILDREN!

All this gave me opportunity to confirm that the speaker was working, to bask in the glorious spatial effects of a real Leslie (emulations only work up to a point) and - after the honeymoon was over - to become disappointed with the sound!

The 825 has no horn (and associated HF unit) - only a large "full-range" speaker firing through a rotating drum. In consequence the top end is hardly what might be described as "sparkling". In fact it is so poor that I began to think there was something wrong with the "key click" function in the fantastic VB3 software and I actually contacted the programmer before realising that the problem lay right there in my new 825 - sorry Guido!

All this led to some creativity - I acquired a 9-pin socket to replace the temporary lash up (thanks George) and I decided to build a filter to boost the HF response and try to rescue some of the sound. The (sound card) output from my virtual Hammond wasn't enough to drive the speaker to satisfactory levels, so a Pre-Amp stage with some gain was indicated too.

Here's my HF shelf filter, seen as an extract from the LTSpice simulation of the entire circuit...

The series combination of R5 and R6 was actually realised with a potentiometer, configured to give me a variable HF lift. Here's the magnitude response simulated in LTSpice for an arbitrary setting...

The lift above 1kHz is the "shelf", made into more of a "bump" by the low-pass corner (deliberately) introduced by R1||C4.

I turned the idea into reality in a nice sloping front RS box, picked up at a rally somewhere, intentionally copying the format of the Leslie "Combo" Preamp. Here are the innards...

and here's a view of the back panel...

In addition to the variable high frequency shelf, there is an internal jumper to select 0, 10 or 20 dB overall gain and an external volume control. I haven't had chance to fit knobs to the controls yet!

There is (as is usual) a fly in the soup - the "d.c. output" from my new Leslie (pin 5) isn't working. I looked at the schematic and there is little to go wrong (I suspect a dead diode, D14, or an open connection), as the d.c. comes straight from the power supply to the amplifier driver stage, which most emphatically IS working! Still, at the moment, my new PreAmp is powered from a PP3 battery.

Result - FANTASTIC. The 825 is totally transformed and a pleasure to use.

I have some rather more exciting Leslie-related stories to tell - but they can wait for another day.

For now, I'm all in a spin!

...-.- de m0xpd

Update: Fly Rescued from Soup!

This evening I made up a shorter cable for the 825 - having 30 feet of cable coiled up is a recipe for trouble, not to mention hum. I so doing, I discovered that the 28Vdc issue isn't a speaker fault at all - just an intermittent connection in the long cable. The shorter one is fine (with new connectors) so now the PreAmp is powered from the Leslie as planned.

Most Secret War

Just finished reading R.V Jones' autobiographical book "Most Secret War, British Scientific Intelligence 1939 - 1945"...



being an account of his work as leader of the Air Ministry's Scientific Intelligence Section in WW2. In short, the book is a masterpiece and a must-read for anybody remotely interested in some of the technological and scientific tussles between the allies and the Third Reich.

I purchased the book during my last visit to Bletchley Park and there is obligatory mention of the activities of the Code-Breakers in providing the most reliable of data streams for the Scientific Intelligence community in the UK - an "anchor of truth", as the author describes it (p 530).

There is much else to entertain those with radio-related interests - including the detection, understanding, prediction and ultimate jamming of the radio navigation beams by which the Luftwaffe we able to achieve accuracy in their bombing activities and the similar location, interpretation and jamming of the radar systems which allowed German night-fighters to engage Bomber Command missions over Germany. However, beyond these triumphs, it was the author's involvement in the development and deployment of the countermeasure we now call "Chaff" - then inexplicably known as "Window" - which impressed me most.

This is a giant of a book, fully deserving the hyperbolic notices which adorn its paperback cover ("Among the best of all war books", "Every bit as good as a Deighton or Le Carre Yarn").

Yes - this deserves to be read. Unfortunately, in reading it, I found myself admiring Dr Jones' achievements but becoming anything but warm toward him as a person. Perhaps that's a by-product of the determination and single-mindedness that made him so successful in his great work.

What would I give for determination and single-mindedness?

Not a lot!

...-.- de m0xpd