Thursday, April 21, 2016

Speaker Impedance Measurement

Comparing two speakers side by side is the best way diagnose a fault, but it's likely you will not always be able to do this. Speaker impedance measurement software is a useful tool for recording the characteristics of a woofer or speaker box for future comparison.

Different from a frequency response graph, an impedance graph indicates load on the amplifier vs frequency. 
Software produces a sine sweep (or pink noise). This signal is taken from the PC output and amplified. Current (voltage across a known resistance) and voltage of the output is measured, and fed back into the PC. Software then calculates Impedance and compares it to the source signal.

Here is a comparison of a speaker before (gray) and after repairing a fault in the crossover (black). Note the significant difference in impedance at 4kHz.

LIMP by Artalabs is the software used in this project.
It has great calibration tools, and a thorough tutorial.

My example has an amplifier built in (based on TDA7294 monolithic chip).
3.5mm TRS connectors were used for simple interface with PC.

Output of the amplifier passes through a 1R 10W sense resistor. 
Sense outputs are voltage dividers, and PC mic level inputs are buffered.

Wednesday, April 13, 2016

Octopus Tester / Component Curve Tracer

The Octopus Tester  is an accessory for an oscilloscope which permits a method of power-off, in-circuit testing (ie; it is used to test damaged equipment without powering it up, and without removing components). Touching the probes to a device will produce a voltage-vs-current characteristic diagram on the scope screen, so a component under test can be quickly compared to a known working component simply by comparison of the image on screen.
One channel of a stereo amplifier is not working. You can use the Octopus Tester to compare suspect transistors on the blown channel to the same transistors on the working channel. Each time you find a damaged device, remove it, and test all components to which it was immediately connected (Resistors should be checked with a multimeter).

The mains transformer produces a 12V 50Hz sine wave with a floating output. This signal sweeps the Device Under Test, and an image is produced on an oscilloscope in X-Y mode. The voltage across the DUT is shown on the X axis, and the current through the DUT (measured as voltage drop across a known resistance) is shown on the Y axis.

Shown here is a diode. Voltage across the diode increases until the diode turns on at 0.6V. Current through the diode is then at maximum.

When testing components in circuit, there will typically be resistive, and reactive elements to the image on screen. You can first familiarise yourself by testing components out of circuit.

After using the device for a short time, you will begin to associate the images on screen with the devices you are testing, and the surrounding circuitry. It then becomes simple to quickly troubleshoot faulty circuits, based on knowledge of the devices you are probing.

The octopus circuit is housed in a small plastic enclosure. BNC connect to scope X and Y inputs, banana sockets are for regular multimeter probes.
Due to the simplicity of the circuit, point to point wiring is the quickest method of construction. Be sure to ground your unit correctly.

Setting up the octupus tester.
Ensure that your scope is set to DC coupling, and that the circuit you are testing has no other path to ground, ie; disconnect all power / audio / data cables to the unit.

It's also necessary to discharge any large caps before testing (You cannot damage anything, but your trace will disappear from the screen).

Some scopes come with a curve tracing function build in. Set-up and use of the component test function of the Hung Chang scope is the same as most other brands I have used.
I have two scopes on my test bench, the Hung Chang pictured is used solely for this function, such is the frequency of it's use.

This is a schematic of the Hung-Chang 3502C scope front end, from which I copied the design of my device. The component testing circuit is highlighted.

Thursday, April 7, 2016

Mains Current Limiting Bulbs

When working on a mains electricity powered device, you will need to safely power up the unit by limiting mains current.
This is achieved here by placing a non linear load in series with the unit. The resistance of the incandescent globes increases as the current through them increases, so under normal conditions (where the device under test does not draw a large current) the globes will not light up.
Under a fault condition, the device under test will draw a large current, and the globes will light up, dropping voltage to the device under test, preventing further damage to the device.
4* 100W incandescent globes is enough to provide idle current for a very large amplifier.
If a lower current limit is required, simply unscrew a few globes.

Current limiting bulbs permanently installed on my test bench.

Example of fault in device under test
(here provided by a short circuit)

Please note, this method cannot be employed on some "universal" switch mode power supplies, as the supply may enter a mode designed for a different voltage mains supply (ie; a 240V supply might think it's receiving 110V). I have found this only rarely to be problematic.

Monday, February 8, 2016

Differential Scope Probes

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 has provision to split a single floating supply for dual rail operation. My example is a x100 probe using a 12V plug pack. Input voltage divider resistor values, and rail voltages can be changed to suit your needs.


Thursday, December 11, 2014

Soundcard Scope Interface

Soundcard Interface
A computer soundcard can be used as an oscilloscope for testing frequencies below 20kHz. This circuit is a simple protection and control interface, with BNC connectors for oscilloscope probes.

This device is useful for generating 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).

This circuit is designed to be used on a work site where a full size scope is impractical. It contains only passive components (therefore has a very low input impedance), and as such should be used only for rough troubleshooting.

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.

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

Visual Analyser uses your 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 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.


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.

Original Circuit

Modified Circuit

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.
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.
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.

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.

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.