Monday, February 8, 2016

Differential Scope Probes

Diff probe reading 60Vpp sine wave
A differential scope probe is used when a signal cannot be analysed relative to ground. An example of this is the output of a bridged amplifier; neither of the output terminals can be connected to the scope ground, and reading each separately may not be an accurate representation of the signal.

This scope probe is DC coupled, with an input impedance over 1M, and has a relatively low noise floor. A single quad op-amp chip provides input buffers, differential, and converts a single supply voltage to dual rail. My example is a x10 probe using a 12V plug pack, and will accept an input signal up to 70Vpp before clipping. Input voltage divider resistor values, and rail voltages can be changed to suit your needs.


PCB diagram

PCB Layout


Populated PCBA in enclosure

Thursday, December 11, 2014

USB Soundcard Oscilloscope

Portable USB Scope
A USB soundcard can be used as an oscilloscope for testing frequencies in the audio range using free software. This circuit is a simple protection and control interface, with BNC connectors for oscilloscope probes.

The USB soundcard has a mono microphone input, and stereo output. This design is for a one channel scope, with an output for use as a function generator. This device is small enough to carry in your laptop bag, and can be used to test equipment from line level up to speaker level audio.

Circuit Diagram
The protection circuit ensures voltage at the soundcard input is clipped at 1.2Vp-p.
Veroboard layout
Circuit will mount nicely on the back of a 16mm potentiometer.
Probe Input Circuit
Mounted on a small piece of veroboard.
USB Sound Card
These devices are available on eBay for less than two dollars. Mono microphone input, Stereo output.
The USB-A connnector and 3.5mm sockets were removed, so that wires can be soldered directly to the board.
Sound card is wired to probe input circuit board.
Output control circuit is wired point-to-point.
USB cable is soldered directly to sound card. Heatshrink is added to prevent shorting.

Completed Unit
BNC connectors are used to connect test probes.
6.5mm socket for use with audio test leads.
Banana socket to connect ground lead.

Visual Analyser Software
Visual Analyser (Sillanumsoft) is the software I have chosen to use in this project.
It is available for free download here:

Visual Analyser uses your USB soundcard to display oscilloscope traces, as well as frequency spectrum. However, the soundcard must first be set up correctly.

Right click on your volume control icon in the task bar.
Select "Recording Devices".
Choose the USB soundcard input
Select "Properties".
Select the "Custom" tab.
Un-check AGC (automated gain control).
Select "Levels" tab.
Increase microphone to 100%.
Visual Analyser is now ready to use.

This device is useful for inputting a sine tone into audio equipment, and testing the output. The amplitude control knob of the input circuit allows testing of line level audio (1V), and up to speaker level audio (100V).


Always connect the ground clip of the device to the chassis of the equipment to be tested. Using the ground clip as a probe will short live circuits to ground through your test device and PC (This goes for any oscilloscope that is not isolated from ground).


Tuesday, October 14, 2014

Epiphone Valve Jr Hot-Rod Modification

EVJ Hot-Rod
The Epiphone Valve Jr Hot-Rod is a 5W Class A all valve guitar amplifier with built in spring reverb, and is the less common, and slightly more expensive cousin of the original EVJ.
Although modifications for the original EVJ are found in abundance across the web, the EVJ Hot-Rod seems to have been overlooked by the modding community.

The EVJ Hot-Rod has essentially the same amplifier structure as the original model, with an additional 12AX7 to drive the spring reverb unit, a multi-tap transformer output for different speaker impedance, and a "gain" pot on the front panel (which is just a voltage divider after the first preamp stage). With this in mind, modifications for the EVJ Hot-Rod can be easily copied straight from existing documentation for the original EVJ.

Safety Note
Power supply capacitors in amplifiers retain charge for weeks. Always discharge capacitors before working on any amplifier. Valve amplifiers are especially dangerous, as the working voltage of the power supply capacitors can be in excess of 300V.
A 1K 5W resistor attached to probes can be used to discharge capacitors. Double check with a multimeter before working on the circuit.

Disassembly of the EVJ Hot-Rod head

Remove screws from the back panel, and pry off (The panel may be hard to remove due to traces of glue on the vinyl).
Label the cables to the spring reverb unit for re-assembly, and disconnect.
Remove four plastic caps from the top of the amplifier case.
Remove the four screws, and slide the amplifier chassis out of the case.
Remove valve covers.
Remove valves.
Discharge the power supply capacitors by shorting R14 and R15 to ground through a 1K 5W resistor.
Label all cables for re-assembly, and disconnect.
Remove the input socket retaining screw.
Remove the circuit board support screws.

Remove the circuit board from the chassis.
Thoroughly wash the circuit board with isopropyl alcohol to remove flux and dust.
The circuit can now be worked on.

Clean dust from chassis.

Clean all pots and sockets with de-oxidiser fluid.

I have largely copied the EVJ "marshall mod" (schematics for which can be found elsewhere). After about six hours of changing component values, I found a sound which I was happy with.
The result was a brighter sounding amplifier, with a well defined low end, and higher gain. Adjusting the controls for low gain settings produces a smooth overdrive, and higher gains ramp up to a broad fuzz sound.
A word of caution; the quality of the EVJ circuit board is very poor. Excessive heat will easily lift circuit board pads and traces. Ensure that you use a good heat controlled soldering iron.
EVJ Hot-Rod Original Schematic
Schematic highlighting modified components
Lowering R3 will increase input level to the first triode stage. Lowering R6 increases quiescent current through the second triode stage, while lowering C8 and C9 serves to 'tighten' the bass response.
Increasing C16 improves bass reproduction of the power pentode.
The addition of a 1K 1W resistor at the pentode screen will limit current through the amplifier in the case of valve failure. This modification requires cutting a circuit trace, and drilling holes to mount the additional resistor on the top of the circuit board.
Addition of protection resistor on pentode screen
I found the reverb on the stock unit to be very thin, and so decided to modify the reverb circuit also.
Lowering R22 and R25 increases quiescent current through the triode stages, while removing C21 and C22 produces a more linear frequency response.
Increasing C18 prevents low frequency rolloff (use ceramic cap due to voltage present).
Increasing R27 reduces output of reverb to a more subtle, controllable level.

Details and schematics for the EVJ Hot-Rod can be found here.

Wednesday, October 1, 2014

Designer Focus - Lewis Waters [New Complexity]

Lewis Waters is an experimental instrument designer and luthier based in Melbourne, Australia.

His startup company 'New Complexity' centres on developing instruments with unique sound and tonality, while retaining the familiarity and playability of traditional guitar design.
All devices are available for sale, and each are crafted with acute attention to detail.

Harmonic Master #1
Harmonic Master #2
Harmonic Master Designs
The 'Harmonic Master' is a twelve string guitar which emits chiming overtones through use of an amplified third bridge. The bridge pieces can be tuned to different ratios, and the third bridge pickup can be routed through effects separate to the primary dual course strings.

Harmonic Isolator
Onboard Preamp
The 'Harmonic Isolator' takes third bridge sympathetic resonance to an extreme, with 4:3 string field ratio. The neck features a microtonal fretboard to more closely approximate a "just intonation" harmonic scale, and both the primary and complementary courses have accurate intonation adjustment.
The instrument is also available with a sustaniac - all of the droning harmonics you could ever wish for.


For the faint of heart, Lewis also produces more traditional designs.
The 'Contra' will provide you with vintage Teisco styling with the superior build quality and reliability of a modern guitar.

Tuesday, March 11, 2014

Japanese Toolbox

Not sound related, but I love the design of this toolbox. It was published on the MAKE website quite a while ago, and was recently included in a MAKE Magazine publication. My toolbox was chosen to appear in the article.
MAKE magazine article

The toolbox is simple to construct, and is very sturdy. I use this design to transport my tools all the time for work as an AV tech. The box should only take a few hours to complete, and can be made on a small budget; a single sheet of plywood will be enough for a large box; 

First iteration (800x400x400).

Second iteration (500x250x250).

Monday, December 9, 2013

FAB Pedal Multi Effects

This analogue multi effect unit can be made from Danelectro FAB pedals sourced cheaply on ebay. Other parts include press button switches, LEDs, 6.5mm mono jacks, and some hot melt glue.
Aluminium sheet was cut to size and squared, then marked and drilled for each of the existing pots on the circuit boards. Press button switches, jacks and power was also drilled.
Each circuit board was fixed to the face plate with hot melt glue, then each pedal wired in a chain (output to input of next...).
Tinned copper wire was used for the ground connections, to give some rigidity to the rear circuit boards.
Plywood box was made to suit.

Sunday, September 22, 2013

True Bypass / AB Box Circuit

Back to basics; here is a super simple true bypass pedal. It can be used to bypass a chain of effect pedals with one switch, to switch between two sources (eg, two guitars), or to switch between two outputs (eg, an amp and a tuner).
The pedal uses a DPDT foot switch, 4* 6.5mm jacks, and the project should cost ~$25.