Sequels

 I seem to have spent the last year or so doing upgrades to things that worked ok first time round but had room for improvement. 

Tuner 2

My class-D bass amp incorporated a tuner that used an ESP32 and a 1.5" monochrome display back in July 2022. This has been moved into my practise amp where it can carry on being useful. 

The revisited tuner still has three basic modes - Idle, Tuning and Active. The FFT based tuner algorithm has been largely left alone, but the visuals have all been updated and now use a 3.2" colour 320x240 LCD panel. This was made easier by using a driver to control the IL1931 chipset. The tuner view is now able to be used at the other end of a 6m cable without squinting!

Some of the visual effects took some time to get right. Most notable were the drawing-a-maze algorithm and controlling multiple bounces in breakout. The 3D cube principles were found online but fathoming out a way to avoid displaying occluded lines was left to me. 

Speaker Measurement 2

A while ago I made an Arduino based speaker measurement box. It worked ok but the DDS was too wayward to give consistent results and the ADC sample rate a bit restrictive.

Seeing as the ESP32 A1S module worked well for my Basscape effects unit it seemed like a good starting place for another sequel. The built-in stereo CODEC channels are used to supply and monitor the AC signal whilst resistance measurements are handled by a separate DC coupled I2C ADC.

Topology 

In the earlier design, AC current was measured by a 3R0 resistor in series with the speaker, but for the new version an improved virtual earth or auto-balancing bridge method is used. The better separation of voltage and current signals along with higher sample rates allowed phase measurement too. After some number-crunching, sweep results are sent to the debug port for importing into a spreadsheet.

Coils of Wire

After some successful speaker measurements, the idea of changing its impedance range offered a way of measuring guitar pickups. LCR meters are ok for ordinary coils, but guitar pickups show resonance in the audio frequency range which renders simple testing at spot frequencies very hit-and-miss. The LCD shows the basic values - dc resistance, peak frequency & impedance and Q.

Pretty Pictures

During development the values measured were sent to a logfile and later imported into a spreadsheet to make graphs. The ESP32 has WiFi capability so it was reconfigured to operate as a Simple Access Point. The measurement results were then sent directly as a web page showing the graph and important values.

...and pretty good for pickups too...

Impedance is not everything

The response of a pickup to an electrical signal (measuring its impedance) is not quite the same as its response to a magnetic flux signal. There is a good degree of agreement, but it is not the whole story. Having re-read a few pages in Electric Guitar - Sound Secrets and Technology by Helmuth Lemme he describes a similar measurement system using a stimulus coil. Modifications were made yet again to allow an external coil to be driven to try this out. 

Pickup coil switching and cable length options were tested individually and the plots combined using paint.net to show the results clearly.

Sound Advice

This unit now gives a good idea of how a speaker or pickup will respond at different frequencies but there are still many other factors that can contribute to an overall lively or dull sound. It has helped remove some of the mystery and allowed me to make improvements to speaker cabinets and guitars with more certainty than flipping a coin.

A side effect of using the drive coil I was able to determine that some of my Jazz Bass pickups tested were actually split humbuckers (like Fender Precision split pickups, but in a single housing). Well, they are black boxes after all.

Deaf Ray Overdrive

 After many software based distortion variations on Basscape I thought I would go more old-school. Most overdrive circuits are too gentle and most distortion circuits are too fierce in my very limited experience forcing me to make a decision as to which one do I want. Decisions were never my strong suit so I chanced upon an interesting schematic from an Orange Crush 15R. 

The overdrive in this schematic is shown at the top centre. By varying the drive control it doesn't just change the gain before a pair of clipping diodes that either shunt the signal to ground or use it as feedback, it blends between them. With the drive at max it is full-on hard shunt clipping, with the drive at minimum it is a barely perceptible feedback soft clipping with the ability to dial in anything in-between. Brilliant!

To give it a different feel to the original, the signal is passed through a 2-pole highpass filter that can be varied from around 200Hz to 650Hz. In addition to the frequency adjustment, the Q of the filter (the peakiness of the response) is controlled in tandem with the drive control described above. At low drives the filter is fairly relaxed  removing the low end that can otherwise dominate the overdrive tone. As the drive is increased, the filter Q rises and the tonal character can become more exciting and distinctive. 

The range of Q goes from about 0.7 (vanilla) to around 2.3 (mint choc chip). As with the high-pass frequency range this was mostly determined by trying it out until the the best range was found for the controls. The economical-on-battery-juice-and-my-pocket TL062 was used and all pots were chosen to be the same value so I only had to buy one lot. 

As I tend to play bass much more than a six-string I wanted a blend control more than an output level. The blend mixes the overdiven signal with a slightly filtered dry signal to still provide the bass guitar underpinnings even if theres a good helping of edginess going on. Which there usually is...

To avoid a complete rats nest of wires I mounted the pots on a separate piece of veroboard. The whole lot was fitted into a 1590BB diecast box and some artwork made to confuse onlookers. The Deaf Ray theme came about as I liked the idea of a laser on the front and in turn this caused the sensible controls like 'frequency' and 'drive' to be renamed to 'aim' and 'force'. The 'kill/stun' switch gives a more cutting sound with higher drive (kill) or a gentler warmer tone (stun).

The artwork was printed onto A4 coloured adhesive paper and lined up with the drilled holes before covering with a transparent adhesive film layer to protect it. Much better looking than my usual permanent marker pen scrawl. 

All the parts managed to squeeze into the box along with the two 1/4" jacks, dc supply, footswitch and controls. 

I was so pleased I made two...

Aim. Fire!