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Another in a Continuing Line of LED clocks

The clock collection grows with this double 8×32 LED display version. I actually promised myself I wouldn’t use the LED matrices again, for aesthetic reasons, but I already had two of them extra, and it was a chance to improve my housing construction techniques. Also, it is a leap away from the AVR based hardware platform to the ST Micro STM32F103C8 processor in the form of a Maple Mini. The advantage is that the Maple is smaller, cheaper and faster than a larger Arduino board like the Mega2560, and also has multiple serial ports which is important for interfacing an internet device like the ESP-01, a Bluetooth or a GPS module. The system is made with 2 MAX7219 based matrix displays, and a proto board housing the Maple processor, an RTC module for timekeeping, and an ESP-01 for WiFi access. Functionally, the system goes to an NTP server via WiFi, to get the exact time when power is applied, and every 10 hours after that. It uses that time to set the RTC in case internet is lost. WiFi and other settings are configured from a computer through the USB port that powers it. It understands daylight savings time, and has a database of about 30 global time zones. So once it is set up, it never needs any kind of attention unless WiFi setting have to change.

The housing uses high grade 1×4 pine, which is easy to work with and finish with polyurethane. There are three main aspects that make the box fit the electronics and display effectively. There is a saw cut all around the inside of the front that forms an escutcheon (holder, if you like) for the filter. In the middle of the interior, there is a post that mounts the display on one side and the processor on the other. A sheet of thin plywood covers the back.

No doubt, someone who is experienced in woodwork would suggest improvements in my method. But I don’t have a real wood shop, just a table saw and hand drill. So my method is evolving around those limitations. First I make the shallow saw cut about 5mm from the front for the filter. Then I cut the side, top and bottom pieces from that. It helps to cut slowly, which makes a cleaner cut across the wood grain. I measure and cut the center post and back cover. This leaves gaps where the saw cut meets the edge. I fill these by gluing in wood spacers, which i trim carefully after the glue is dry. The housing pieces are assembled by laying them square on the saw table and drilling for the screws. After assembly, I apply at least two coats of polyurethane from a spray can, with light sanding in between coats. It helps to let the poly dry overnight even though they talk about 2 hour drying time. When I’m happy with the outside, I paint the inside with black craft latex, to eliminate reflections and to minimize what can be seen through the filter. A future upgrade will be to hide the top screws with plug dowels.

I have been asked whether I can develop projects using more off the shelf components, so that other people would have an easier time duplicating them. There are problems in that. I always find weaknesses in the inexpensive hardware that I get, which not everyone has the patience to fix. For example, most low cost MAX7219 displays have an excessive LED current due to a poor bias resistor choice. I think it is because the designers think that people are too dumb to turn up the brightness in software. I chose to do SMD surgery to fix that on these display boards because I loath to run components outside their specified limits. The popular library to drive these displays views each 8×8 module rotated by 90 degrees, so I had to modify the code to change that. I also have found that the ESP-01 modules can be powered down when not in use, yet most hobby circuits don’t bother to do it. So in many cases, hardware and software modifications are required to make a project really useful.

In fact, I have two other clock projects in the works, where I am trying to create something that the average experimenter could duplicate. I am trying to avoid the special mods and things and put together a code base that isn’t also a hodge podge of modified libraries like I often have to use. Mainly here, I’m showing you how to take your clock hardware, and put it in a presentable and pleasing box.

Phantom Powered Guitar Preamp

The idea of putting a preamplifier in a guitar is hardly new. There are many advantages, but a big disadvantage is the requirement for power, which usually means that a battery has to go into the guitar. That means that a space must be found for the battery, it must be replaced when it expires, and has the potential to leak corrosive liquids inside the guitar if left for too long.This circuit avoids those problems by using a common idea from microphone technology, phantom powering. Many microphones obtain power for internal circuits from an ingenious circuit that allows power to be transmitted over audio cables. This system has become a standard, and is provided by most microphone preamplifiers and mixing consoles.

Originally, the power was used to bias condenser microphones, and so the voltage is quite high, 48 volts. Originally, a transformer was used to separate the audio from the supply voltage, but in recent years transistors became available that could do that, and so the “Schoeps circuit” was developed by the engineers at Schoeps Mikrofone to replace it.

The Schoeps circuit has been adapted and modified endlessly since its introduction, and does a fine job of supplying the power provided on a phantom cable, to on board preamplifiers of various kinds. It is a differential amplifier that provides a very low impedance output to drive the output cable, combined with a balanced power extraction circuit. It works well in different configurations, some having Zener diodes for regulation and some not. The driver stage can be configured as an additional regulation stage (which I have done), however this is optional and some designers omit it. It is the kind of circuit that will almost always work, but needs some thought and understanding to achieve the best results.

In my case, I adapted a JFET input stage from some microphones to work as a guitar preamp. The JFET is configured in differential output mode, in order to easily drive the output transistors and audio cable, which are also differential. It has an extremely high input impedance which means that it presents almost no load on the guitar circuit. Thus the guitar tone and volume controls are the only electrical load that the pickup “sees”. This avoids tonal variances that normally happen when a cable is plugged into a guitar. The lower impedance and differential (balanced) signals in the XLR cable are more immune to hum and interference, and can run up to 100 meters (several hundred feet) with absolutely no loss of tone.

The wiring in my guitar is completely conventional, with a three way switch and tone and volume control. The output goes to the preamp, which is connected to an XLR connector that replaces the usual output jack. In fact, it is possible to keep the original jack and have both powered and unpowered outputs from the guitar, but I didn’t want to drill a new hole. In future, I would because the XLR is not a perfect fit in the existing hole (to be honest, a bit of an understatement!).

Here is the installation. The prototype board fits nicely inside with a safety wrapping of insulating Kapton tape.


I wrestled with the design for a long time. There are a lot of variations on it online but no complete explanations that would allow me to optimize the components.

I thought about Zener regulation. The entire circuit is differential except for the JFET current. So I reasoned that I could omit it and depend on a pair of filter capacitors to do the job. What is strange about the Schoeps circuit, is that the pair of driver transistors function as a first stage voltage regulator as well as signal amplifiers. The collectors are common, and go to the first stage filter capacitor. Then a dropping resistor feeds the second stage supply, which depends on the second stage filter capacitor for audio frequency suppression. In my Spice simulations and also in testing, I found that this was more than adequate. There is no noise or instability from omitting the Zener, in fact it is probably quieter because Zeners are notorious noise sources. The only disadvantage that I can find in this arrangement is that all the component values are interdependent, and so you can’t really change one value or component without changing them all. The voltages have to be checked after construction to verify that they are sane. You should measure approximately 10 volts on C6. However, this circuit has a good chance of working the first time if it is built exactly as shown.

Note: R1 in the schematic above is just a simplification for the sake of Spice testing. It really represents the entire usual volume and tone control circuit of the guitar.

I’ve been playing my Godin Redline 2 with the circuit installed and I like it. I have it plugged into an Art Accessories Phantom II Pro, which then goes into my guitar amp. I can just as easily plug it directly into any phantom powered microphone preamp or mixing board, as I mentioned before.

Undercover Pickups

For quite a long time, I have been wondering about a phenomena called “eddy current losses” in guitar pickups. Essentially, these are signal losses due to various electromagnetic aspects of the pickup’s construction, that cause the tone to become dulled to some degree. All pickups have them, and some depend on a calculated amount of losses to produce a certain tonal balance. However, because the high audio frequencies are conducive to a sensation of “clarity” or “brilliance” in the sound, it is generally good to reduce the losses as much as possible.

While analyzing pickups and examining the analyses of other testers, I began to realize that the metallic covers that contain the coils and internal parts of a pickup, are a prime source of loss. This was actually known to the early designers of the 1950’s era. They responded by finding metals that have low losses, and used those as the base material for pickup covers. These would then be electroplated to any desired appearance.

J.R. Butts, a designer for the Gretch guitar company, chose a different way. He considered the electromagnetic problem more carefully, and designed a metal cover shape that was almost completely immune to the losses – the “Filtertron”. Subsequently, the Fender guitar company adopted the design for a specialty guitar – the “Cabronita”.

After 1960, nobody thought much of the whole thing. High quality pickups always used an alloy called nickel-silver, while the Filtertrons and some other covers remained the more inexpensive brass. But when brass is used in a non-Filtertron design, the sound is usually very dull due to the eddy current losses.

I wondered, why does the J.R. Butts design work so well? The patent mentions it, but offers little explanation. So I began some experiments to determine the exact nature of the eddy currents in a guitar pickup cover. These were extremely revealing. Soon, I realized that the Butts design barely scratches the surface of the techniques that could be leveraged to improve a pickup cover.

I designed and built several prototype alternative designs made from brass to test my theories. These were wildly successful. However, as I considered what I would do with my invention, I realized that I lack the funds and resources to obtain patents, trademarks, set up inventory, place manufacturing orders and such things that are necessary to make and sell a product.

guitar pickup

Prototype Humbucker Cover

So after almost a year of development, I feel that the best course of action is to simply release the information into the public domain. I hope that if it has some small success as a product, that I can at least boast that it was my idea. After all, it probably won’t be the last one coming from me.

The technical article is long, so I should gPrototype Telecaster Neck Pickupive you a summary. The idea is that by cutting small slots in strategic locations on the cover, the tone-sucking eddy currents can be mostly eliminated. This has two applications. One is that cheap brass covers can be used where a nickel-silver one would normally be used. That is a cost advantage. Another is that when a nickel-silver cover is slotted, the losses are so small as to be both non-measurable and inaudible. This means that a protective cover can be added to pickups that have previously shunned covers for reasons of tonal purity (this habit began with the heavy metal players of the late 1970’s).

There are a few different possibilities for placement of the slots, however not all are mechanically sound or aesthetically pleasing. Here is a practical alternative for the Tele neck design, fully tested and found to eliminate losses equally as well as the version shown above.

Here is the full story: pickup_cover_geometry

Rendering Again!

nine_lives_1 These are historical batteries that I have subjected to a cylindrical scan and then rebuilt, using computer rendering techniques. The floor is the actual floor outside my apartment in Hubei, photographed and used as an surface colouring. There are few of these batteries in existence now, because they leak and corrode with age.

Introduction

I am a person of many interests, and I plan to show some of my ideas and projects on this site. I hope that others will find some interest or novelty in them. Welcome to my world.