I Think I Failed. Yes, I Failed.

Down the rabbit hole you go.

In my particular case I am testing a new output matching transformer design for an audio preamplifier and using one of my go to driver circuit designs. Very stable, and very reliable. Wack it together and off you go to test and measurement land without a care in the world. This particular transformer is designed to be driven with a  class A amplifier operating at 48 volts in a pro audio setting where you turn the knobs with your pinky in the air sort of thing. Extra points if you can find some sort of long out of production parts to throw in there for audiophile cred, and I want some of that.

img_2857-2Lets use some cool retro transistors! I merrily go along for hours designing away. Carefully balancing the current of the long tailed pair input. Picking just the right collector power resistor and capacitor value to drive the transformer. Calculating the negative feedback circuit for proper low frequency cutoff and high frequency stability, and into the breadboard the parts go — jumper clips, meter probes, and test leads abound — a truly joyful event.

All of the voltages check out, frequency response is what you would expect, and a slight tweak to the feedback look brought everything right into happiness. Time to fire up the trusty old HP 334A Distortion Analyzer. Those old machines require you to calibrate the input circuit and the volt meter, tune a filter to the fundamental frequency you are applying to the device under test and step down to lower and lower orders of distortion levels until the meter happily sits somewhere in the middle of a range.

Most modern circuits in even cheap products just go right down to sub .1% total harmonic distortion levels without even a thought and I expected this to be much the same. The look of horror must have been pronounced on my face when the distortion level of my precious circuit was something more akin to a clock radio! A frantic search began. Was it a bad jumper, or a dirty lead in the breadboard, or an unseated component? Was my function generator in some state of disrepair? Is the Stephen King story Maximum Overdrive coming true and my bench is going to eat me alive? All distinct possibilities in this state of panic.

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Using LTSpice to Measure Total Harmonic Distortion

Audiophiles spend a lot of time and effort worrying about audio specs like Total Harmonic Distortion (THD). Makes sense, because THD affects the quality of audio reproduction. However, THD can also affect interference from radio signals and even losses in power transfer systems. A simplified definition is the THD is the ratio of the sum of the power of all harmonic frequencies to the power of the fundamental frequency.

If a circuit produced a perfect sine wave, there would be no harmonics. There are many ways to measure THD in practice, but [Michael Jackson] has an interesting video showing how he easily visualizes THD using LTSpice. Assuming you already have the system in question in LTSpice (or you could use another simulation tool, if you prefer) it is fairly straightforward.

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Cheap guitar amp repaired by replacing the distortion circuit


It’s an understatement that [Troy] is not impressed with the distortion circuitry built into this guitar amp. He picked it up for $40 on Kijiji (basically local classified ads run by eBay) so he wasn’t afraid to get elbow deep in its inner workings to see what was going on. It only took him a few minutes to solder together the distortion circuitry that fixed it. Figuring out what needed fixing is another story.

[Troy] uses some colorful language and metaphors to illustrate his disdain for the sound of the overdrive option. He hooked it up to an oscilloscope and his trained eye immediately tells him that it’s not working as it should. After studying the PCB and working out a schematic he reworked the circuit with this pair of diodes and a resistor. It still uses a bit of filtering on the board, but does away with all of the other cruft. What remains is a cheap amp, but one that actually functions.


Bubble wrap cure for Nexus 7 speaker distortion


[Home Brand Cola] is quite happy with his Nexus 7 with the exception of the built-in speaker. It produces fairly good audio quality until he reaches about 50% volume level. Anything above that produces distortion. He figured out how to fix it using a small piece of bubble wrap.

The eureka moment came when he was using his Nexus 7 and discovered he could fix the distortion by gripping the top and bottom parts of the case strongly between his finger and thumb. This led him to realize that the speaker unit is a bit loose and the unwanted noise is produced when it vibrates against the case. The video after the break shows the fix, which places a strip of bubble wrap (looks to be about 1″ by 3″) on top of that speaker unit. When the case is snapped back together the packing material helps hold everything in place and now he can use his tablet at full volume without any problems. One of the comments on the Reddit thread asks about heat problems with the addition of this plastic. He’s been using it for a few weeks and so far no issues there.

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