A 1950s Ampex Tape Recorder Microphone Preamplifier Restoration

So often when we read of a modification on a classic piece of tube electronics we prepare to wince, as such work often results in senseless butchery of a well-preserved survivor. With [Frank Olson]’s work on a 1958 Ampex 601 tape recorder though we were pleasantly surprised, because while he makes a modification to allow its use as a stand-alone microphone preamplifier he also performs an extremely sympathetic upgrade to modern components and retains it in substantially the form it left the Ampex factory.

The video below the break is a satisfying wallow in pre-PCB-era construction for any of the generation who cut their teeth on tube, chassis, and tag strip electronics. We can almost smell the phenolic as he carefully removes time-expired capacitors and fits modern replacements complete with period features such as sheathing over their leads. The larger multiway can electrolytics are left in the chassis, with their modern miniaturised equivalents nestling underneath them out of sight. We all know that electronic components have become a lot smaller over the decades, but it’s still a bit of a shock to see just how tiny even a high voltage electrolytic has become.

The Ampex would have been a very high quality tape recorder when new, and while this one has a problem with its mechanism it’s that quality that makes it easier for him to do this work in 2020. There’s every chance that this one could be returned to service as a tape recorder if someone was of a mind to fix it, and meanwhile it will give Frank excellent service as a high quality pre-amp. This is how resto-mods should be done!

Ampex are very much still in existence making digital storage products, but back in the 1950s they were at the forefront of analogue magnetic tape technology. We’ve written in the past about how Bing Crosby had a hand in the development of high quality tape recorders, and also about Ampex’s part in the gestation of the video recorder.

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THAT Preamp

It is easy to cobble together projects these days. ICs make it simple and microcontrollers even easier. However, we always respect a project that really goes from concept to finished product and that’s what we liked about [Curt Yengst’s] “THAT” Thing microphone preamp.

In part 1 of his post about it, he talks about the basic ideas including the chips from THAT — a small but high-end audio chipmaker — he uses. The first chip is a low-noise audio preamp and the other is a balanced line driver.

In part 2, we get to see [Curt] go from breadboard testing to PCB fabrication all the way to the finished rack-mounted device with a good looking front panel. It worked, but like all designers, [Curt] was already thinking about the next version.

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Hackaday Prize Entry: A BSTRD Preamp

For this year’s Hackaday Prize, [skrodahl] is building a beautiful tube preamp. It’s a masterpiece of glass and free electrons, it already works, and it sounds great.

This circuit is a modified version of the Bastard, an amp published in the Danish magazine Ny Elektronik nearly 20 years ago. The original amp was a true bastard, with a transistor phono stage, a valve line stage, and an input selector that used relays. [skrodahl]’s version only uses the line stage, but part of the name remains as a nod to the original design.

The design of this amp uses octal 6J5 tubes, a 80 VDC, 0.1 A and 6 VDC, 1.5 A power supply. This is actually two projects in one, with the power supply comprising an another entire project.

[skrodahl]’s BSTRD is built, and it works, but the question remains: how does it sound? Unlike so, so many tube amp projects on the Interwebs, [skrodahl] actually has test and measurement gear to figure out what the frequency response and THD measurements actually are. For the frequency response, this amp is dead flat from 10 Hz to 30 kHz. THD is somewhere between 0.35-0.4%, or more than acceptable.

This is a great little project, and an awesome extension to an already popular Open Source project. It’s also a great entry for the Hackaday Prize, and we’re pleased to see it entered in this year’s contest.

Hackaday Prize Entry: High End Preamps

While compact disks are seeing an uptick in popularity thanks to a convenient format that offers a lossless high-quality 44.1 KHz sample rate with 16-bit depth, some people are still riding the vinyl bandwagon of 2010. With that comes a need for the best hardware, and that means expensive cartridges and preamps designed by someone who knows what they’re doing.

For this year’s Hackaday Prize, [skrodahl] is building a really, really good preamplifier for moving coil turntable cartridges. It’s already built, it’s already tested, and the results are good: it produces between 36 and 46dB of gain, -110dB of dynamic range, and a signal to noise ratio of 79.46 relative to a 5mV input. That puts this preamplifier into the same territory as preamps sold with serial numbers, crystal lattices, and other audiophile nonsense.

The quality of this preamp comes from the design, and like any good open hardware project, [skrodahl] has made the schematic, PCB, and layout of this preamp completely open. It’s a great preamp, and a great entry for the Hackaday Prize.

Simple Vacuum Tube Preamp Results In A Beautiful Build

We have no intention of wading into the vacuum tube versus silicon debates audiophiles seem to thrive on. But we know a quality build when we see it, and this gorgeous tube preamp certainly looks like it sounds good.

The amp is an attempt by builder [Timothy Cose] to give a little something back to the online community of  vacuum tube aficionados that guided him in his journey into the world of electrons under glass. Dubbed a “Muchedumbre” – Spanish for “crowd” or “mob”; we admit we don’t get the reference – the circuit is intended as a zero-gain preamp for matching impedance between line level sources and power amplifiers. Consisting of a single 12AU7 in a cathode-follower design and an EZ81 for rectification, where the amp really shines is in build quality. The aluminum and wood chassis looks great, and the point-to-point wiring is simple and neat. We especially appreciate the neatly bent component leads and the well-dressed connections on the terminal strips and octal sockets. There’s a nice photo gallery below with shots of the build.

As much as we appreciate the miracles that can be accomplished with silicon, there’s still magic aplenty with vacuum tubes. For more thermionic goodness, check out these minimalist homebrew vacuum tubes or these artisanal vacuum tubes.

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Beautiful Tube Amp Uses A TV Tube

tube-amp-i

Most audio tube amps we see use common tubes – usually a 12AX7 for the preamp and one of the more common power tubes such as an EL34.[Daniel], on the other hand, decided to build his own audio tube amp with a 13EM7, a tube originally used for a television’s vertical oscillator. The resulting project is a wonderful stereo amp that sounds really good, to boot.

[Daniel] picked up the idea of using a 13EM7 tube from [Fred Nachbaur]’s MiniBlok SET amplifier. This very tiny 1-watt tube amplifier uses a single tube originally designed for use in old, old televisions. The secret behind this build is the fact this tube is actually two triodes in one package; one side of the 13ME7 has tons of gain but not much power, making it perfect for a preamp. The other side has a lot of power, useful for delivering two watts of power into a speaker.

After [Daniel] etched a few boards for his amp, he milled out a piece of wood for the chassis. When everything was mounted he had an awesome looking stereo amplifier that also sounds great.

Making A Phono Preamp For A First Electronics Project

Nearly everyone’s first electronic project is something that blinks a LED. There are a million ways to go about this ‘Hello World’ project of electronic design; 555 timers, microcontrollers, or maybe even discrete components if you’re really cool. When [miceuz] was asked by a friend to help with his first electronic project he eschewed the usual blinking LED project and taught him how to build something he actually needed: a phono preamp for an old turntable.

Back in the day when vinyl was king, albums needed to be mastered to play on a record player. The mastering process cuts some of the bass and increases the treble. When the record is played, this process needs to be reversed. It’s a preamp that does this job by attenuating the high frequency sounds and boosting the thumping bass.

[miceuz] found a nice DIY RIAA preamp  project and found a nice little op amp  somewhere in his parts bin. After laying out the circuit, [miceuz] etched a few boards and taught his friend how to solder SMD components.

Of course the project didn’t work the first time around, but after poking around with a meter and checking out the old turntable, the preamp came to life with the clang of chords from an old record. If you’d like to build your own, you can get the files from [miceuz]’s git.