Retrotechtacular: Tube Amplifiers


It’s hard to beat this vintage reel for learning about how vacuum tube amplifiers work. It was put together by the US Army in 1963 (if we’re reading the MCMLXIII in the title slide correctly). If you have a basic understanding of electronics you’ll appreciate at least the first half of the video, but even the most learned of radio enthusiasts will find something of interest as they make their way through the 30-minute presentation.

The instruction begins with a description of how a carbon microphone works, how that is fed to a transformer, and then into the amplifier. The first stage of the tube amp is a voltage amplifier and you’ll get a very thorough demo of the input voltage swing and how that affects the output. We really like it that the reel discusses getting data from the tube manual, but also shows how to measure cut-off and saturation voltage for yourself. From there it’s off to the races with the different tube applications used to make class A, B, and C amplifiers. This quickly moves onto a discussion of the pros and cons of each amplifier type. See for yourself after the jump.

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Build a Simple Audio Amp

[Ynze] has built an audio amplifier that looks and sounds great. His amplifier uses a National Instruments (now TI) LM3886 Overture series 68 Watt power amp. The LM3886 places [Ynze’s] amp squarely in the “Gainclone” catagory. Gainclone or Chipamp are terms long used by the DIY community to describe audio amps based upon highly integrated semiconductor amplifiers. The Gainclone name stems from the original Gaincard audio amplifier sold by 47 labs. The Gaincard used less than $100 USD of parts when it was introduced in 1999. It sounded good enough to command a $3300 USD price tag on the audiophile market. The low parts count and simple construction spawned the audio DIY community to build their own versions of the Gaincard. Hundreds of variants exist now, and wading through the different versions can be a bit of a daunting task. [Ynze] found a basic design that works, and built from there.

One of the interesting things about [Ynze’s] amp, as well as many of the Gainclones, is the fact that they use no circuit board. All wiring is done point to point. resistors are soldered directly to the pins of the amplifier chip. This can be some tricky soldering for beginners, but several PCB kits are available. [Ynze] built his amp in two cases. One case holds the power supply, and the other contains the amplifier itself. [Ynze] is using a large toroid transformer to drop his local 230V mains down to +25V and -25V. The amplifier circuit itself is simple – a few discrete components surround the LM3886 and it’s heat sink. [Ynze] also did some very nice carpentry work on his wood chassis. The resulting amp looks like it’s right out of the 1960’s – but hides 1990’s electronics inside.

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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.


Measuring tiny current with high resolution


[Paul] knew that he could get an oscilloscope that would measure the microamp signals with the kind of resolution he was after, but it would cost him a bundle. But he has some idea of how that high-end equipment does things, and so he just built this circuit to feed precision data to his own bench equipment.

He’s trying to visualize what’s going on with the current draw of a microcontroller at various points in its operation. He figures 5 mA at 2.5 mV is in the ballpark of what he’s probing. Measurements this small have problems with noise. The solution is the chip on the green breakout board. It’s not exactly priced to move, costing about $20 in single quantity. But when paired with a quality power supply it gets the job done. The AD8428 is an ultra-low-noise amplifier which has way more than the accuracy he needs and outputs a bandwidth of 3.5 MHz. Now the cost seems worth it.

The oscilloscope screenshot in [Paul’s] post is really impressive. Using two 1 Ohm resistors in parallel on the microcontroller’s power line he’s able to monitor the chip in slow startup mode. It begins at 120 microamps and the graph captures the point at which the oscillator starts running and when the system clock is connected to it.

I keep my tunes in an ammo can


Calling this a boom box is at least slightly ironic. Instead of high explosives it now carries high decibels in its new life as a self-contained sound system.

Despite the conspicuous power cord a peek inside reveals a big enough battery to keep the tunes playing for hours on end. [King Rootintootin] kept the cost on the build down since he was given the used speakers and amp by his girlfriend’s dad. The amp kicks out about 25 Watts with the battery rated at 7.2 Ah. He added a charger and routed the controls to the side of the ammo box so that it can be charged without removal. The only external component is the audio jack which connects it to the music source.

One of the suggested improvements from the Reddit thread is to add baffles inside of the enclosure so that sound from the two stereo channels doesn’t interfere with each other.

Soldering Small Components for a Video DA

Video DA Board

Video distribution amplifiers are used to amplify a video signal and split it into multiple outputs so multiple displays can be driven. They are also used to correct the gain of an incoming video signal. [Andrew] was having trouble with the video signal from an interferometer, and found the issue was caused by a low output gain. His solution was to build his own video distribution amplifier.

The THS7374 appeared to be the perfect chip for this application. It’s a four channel video amplifier IC, and only requires a few passive components to run. The only problem was the package: a 14 pin TSSOP with 0.65 mm pitch. Not fun to solder by hand, especially if you don’t have a PCB.

[Andrew]’s solution was to build his own breakout out of copper-clad board. He worked under a microscope and cut out a pattern for the part, then soldered 30 AWG wire to the pins to make connections. After cleaning off any copper that could cause a short, the board was working, and the video waveform looked great on an oscilloscope.

After testing, even more gain was needed. [Andrew] ended up cascading two of the amplifiers. This method of prototyping doesn’t look easy, but could be worth it when you need a single board.

Adding AirPlay to a shelf system


AirPlay is a great system. It allows you to send whatever media is playing from one device to another. Sure, we wish it were a bit more open (Apple is certainly not known for that) but there are several option for creating your own AirPlay receivers. After coming across a project that does just that, [Matt Shirley] decided to turn his shelf system into an AirPlay receiver.

The path to his goal depends on the Raspberry Pi’s ability to receive AirPlay audio using the Shairport package (we just looked in on another player that does this last week). He uses an Edirol UA-5 USB audio interface as an amplifier for his record player. He wasn’t using the USB port for it and knew that it would be simple to connect the RPi USB as a host for the device.

Wanting to keep the look of the system as clean as possible he popped the lid off of the amp. There is just enough room to fit the small RPi board inside. He hacked (literally, look at the pictures) an opening for the USB ports into the side of the metal enclosure. A short patch cable connects from one port to the USB jack on the back of the amplifier. The white cable leaving the side of the case provides power to the Rasperry Pi. The surgery was a success and now he can listen to his tunes with a tap of his finger.