Our friend [Pete] was reminiscing over the golden days with his
old and broken antique Grundig Majestic console when he realized it deserved proper refurbishing. Now, any generic stereo record player might not be worth the time and effort to fix, but this was not any generic stereo record player. [Pete’s] inherited Grundig Majestic was his childhood favorite due to the distinct sound it had from the tubes that were used as the active elements. So he set out to fix both tube amps inside of the system.
[Pete] has had some experience working with audio equipment in the past. He did what we all aspire to, and got paid for doing what he loves by creating tube amps as a side gig. When he had finally had enough of the sub-par quality of bluetooth speakers that we all put up with for convenience sake, he decided to finally fix his favorite radio that had been lying around for far too long. He got to work immediately in his notebook finding what parts would be necessary for the reboot. The build ended up consisting of a HT supply regulated at 350V, an LT supply half DC-regulated at 6.3V, a 12AX7 input/bass/treble section, 6922 concertina tubes, and an EL34 ultra-linear output section. The end results yielded one amp that sounded just like it did in his youth, and one that isn’t quite there yet.
The Grundig Majestic is not done with just yet though. [Pete] plans to add a couple of additional modifications to his beauty when he’s not too busy with the kids. Firstly, perfecting the second amp is a top priority. After that, installing red LEDs that illuminate underneath the tubes would indicate low voltage presence, whereas blue illumination would indicate HT was locked and loaded. Bias monitoring to keep him informed on the status of the circuit conditions would insure smooth sailing down the road. Adding a relay connection of the speakers to the output transformer would minimize a popping sound that is currently being made in the speakers when the HT is initially turned on. These small improvements are just that — small — but that is part of what makes home projects so rewarding. The more you use something at home, the more you realize what needs to be refined further, so you are constantly learning more. It is a gratifying experience that I hope all of our readers have the chance to come across.
Tube amps are no stranger to Hackaday. Some of us have even built a few ourselves.
[B Arnold] is hearing voices and needs help from the Hackaday community. But before any of you armchair psychiatrists run off to WebMD, rest assured that [B Arnold] suffers not from schizophrenia but rather has an RF coupling problem.
The project (which isn’t posted yet) is an attempt to turn a C.H.I.P into an Amazon Echo, for which [B Arnold] needed an audio amplifier. Turning to the junk bin, he unearthed an LM386, that venerable power amp chip that first appeared in the mid-70s. Dead simple and able to run off a 9-volt battery, the LM386 that has found its way into thousands of commercial products and countless hacks.
Shortly after applying power to the amp, [B Arnold] started hearing things – faint, far-off voices, scratchy but discernible. A bit of repositioning of wires and hands improved the signal enough for a station ID – an FM talk radio station on 97.1 MHz. [B Arnold] doesn’t mention the call sign, but it might have been KFTK out of St. Louis, Missouri; in any case, it would be helpful to know the range from the transmitter to the inadvertent receiver. Two low-fidelity audio clips are included below for your listening pleasure – you’ll want your headphones on, and Sample 2 is better than Sample 1 – as are photos of the offending circuit.
What do you think is going on here? We’ve heard of RF coupling of AM radio stations before, but how would FM signals be making it into this circuit and out of the speaker? Is there anything [B Arnold] did wrong to get this result? Sound off in the comments and let us know your horror stories of RF coupling.
Continue reading “Fail Of The Week: The Accidental FM Radio” →
Hacker [Dino Segovis] is back again with the fifth installment in his “Hack a Week” series. This time around he has put together a 1/2 watt audio amplifier that would make for a great weekend project. He’s a big fan of the LM386 amplifier chip because it does so much in such a small package. Since it is so versatile, he used it as the centerpiece of his Altoids tin amplifier.
Now an audio amp inside an Altoids tin isn’t exactly a new concept, but [Dino] takes the time to discuss the circuit in detail, which is great for any beginners out there who are looking for a fun and relatively easy project. After a high-speed video of the assembly process he walks us through the completed amp, then treats us to a couple of short demos.
One thing that makes his amp different than others we have seen in the past is the addition of a 1/4” guitar jack, which allows him to use his amplifier as a combo amp/distortion effect box.
It’s another job well done, so be sure to keep reading if you’d like to watch the latest Hack a Week episode in its entirety.
Continue reading “LM386 Altoids Tin Amp” →