Monday, 31 October 2011

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

Wednesday, 26 October 2011

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...

3YellowNo Connection
4OrangeStationary Input / Aux Control (not used in 825)
5Greendc Output (+28V)
6WhiteSlow Motor Control
7VioletFast Motor Control
8Gray240V In
9Blue240V 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.

Tuesday, 18 October 2011

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