Automatic Audio Leveling Circuit Makes Scanning More Fun

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[Alan’s] friend came to him with a problem. He loved listening to his scanner, but hated the volume differences between stations. Some transmitters would be very low volume, others would nearly blow his speakers. To solve the problem, [Alan] built up a quick automatic leveling circuit (YouTube link) from parts he had around the lab.

[Alan’s] calan-scope2ircuit isn’t new, he states right in the video that various audio limiting, compressing, and automatic gain control circuits have been passed around the internet for years. What he’s brought to the table is his usual flair for explaining the circuits’ operation, with plenty of examples using the oscilloscope. (For those that don’t know, when [Alan] isn’t building circuits for fun, he’s an RF applications engineer at Tektronix).

Alan’s circuit is essentially an attenuator. It takes speaker level audio in (exactly what you’d have in a desktop scanner) and outputs a limited signal at about 50mv peak to peak, which is enough to drive an auxiliary amplifier. The attenuator is made up of a resistor and a pair of 1N34A Germanium diodes. The more bias current applied to the diodes, the more they will attenuate the main audio signal. The diode bias current is created by a transistor-based peak detector circuit driven off the main audio signal.
But don’t just take our word for it, watch the video after the break.

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Hackaday Space: Transmission 3 Puzzles Explained

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Yesterday we did a run down of Transmission 2 as part of a series of posts covering the ARG that we ran throughout April. Today I’m going to reveal all the details in Transmission 3, how we put it together and what the answers were.

In classic Hackaday fashion we hadn’t planned any of this, so by this point all our initial ideas we already used up and we were now running out of creativity so it was a real slog to get Transmission 3 out the gate. However we somehow managed it and opened Transmission 3 by posting a series of 5 images of space telescopes:

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Create Your Own J.A.R.V.I.S. Using Jasper

Tony Stark’s J.A.R.V.I.S. needs no introduction. With [Shubhro’s] and [Charlie’s] recent release of Jasper, an always on voice-controlled development platform for the Raspberry Pi, you too can start making your own J.A.R.V.I.S..

Both [Shubhro] and [Charlie] are undergraduate students at Princeton University, and decided to make their voice-controlled project open-source (code is available on GitHub). Jasper is build on inexpensive off-the-shelf hardware, making it very simple to get started. All you really need is an internet connected Raspberry Pi with a microphone and speaker. Simply install Jasper, and get started using the built in functionality that allows you to interface with Spotify, Facebook, Gmail, knock knock jokes, and more. Be sure to check out the demo video after break!

With the easy to use developer API, you can integrate Jasper into any of your existing Raspberry Pi projects with little effort. We could see Jasper integrated with wireless microphones and speakers to enable advanced voice control from anywhere in your home. What a great project! Thanks to both [Shubhro] and [Charlie] for making this open-source.

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Fully Integrated HiFi Studio Monitor

Studio Monitor and PCB

Have you ever wanted to build a high quality audio crossover and amplifier? [Rouslan] has put a lot of thought into making his dual amplifier studio monitor both high quality and simple to build.

With a concise schematic, a meaningful block diagram, and simulation results to boot, his well-written post has everything you need to build self-powered bi-amped speakers based on the LM4766 from Texas Instruments. It is great to see simulations which verify the functionality of the circuit, this can go a long way when working with complicated analog filters and audio circuitry. For those of you who do not have access to PSPICE (an expensive professional simulation tool), [Rouslan] uses LTspice from Linear Technology. TINA-TI from Texas Instruments is another great free alternative.

Additionally, [Rouslan] goes over the typical issues one has with a bi-amplifier studio monitor, such as phase misalignment and turn-on pop, and then provides a solution. Note that his project is powered by 20VAC, which requires an external transformer to convert the 120VAC in the wall to 20VAC. Be careful with high voltages! In the future, adding a high quality voltage regulator will most likely increase the performance.

His post finishes up with a very clean circuit board, which he ordered from OSH Park. With such a complete design, there is nothing keeping you from building your own. Go out and put that old speaker sitting in your basement to good use!

If you don’t have an old speaker sitting around, check out these very cool DIY speakers.

Build an Audio Spectrum Analyzer the Analog Way

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[Ryan] wanted a spectrum analyzer for his audio equipment. Rather than grab a micro, he did it the analog way. [Ryan] designed  a 10 band audio spectrum analyzer. This means that he needs 10 band-pass filters. As the name implies, a band-pass filter will only allow signals with frequency of a selected band to pass. Signals with frequency above or below the filter’s passband will be attenuated. The band-pass itself is constructed from a high pass and a low pass filter. [Ryan] used simple resistor capacitor (RC) filters to implement his design.

All those discrete components would quickly attenuate [Ryan’s] input signal, so each stage uses two op-amps. The first stage is a buffer for each band. The second op-amp, located after the band-pass filters, is configured as a non-inverting amplifier. These amplifiers boost the individual band signals before they leave the board. [Ryan] even added an “energy filler” mode. In normal mode, the analyzer’s output will exactly follow the input signal. In “energy filler” (AKA peak detect) mode, the output will display the signal peaks,  with a slow decay down to the input signal. The energy filler mode is created by using an n-channel FET to store charge in an electrolytic capacitor.

Have we mentioned that for 10 bands, all this circuitry had to be built 10 times? Not to mention input buffering circuitry. With all this done, [Ryan] still has to build the output portion of the analyzer: 160 blue LEDs and their associated drive circuitry. Going “all analog” may seem crazy in this day and age of high-speed micro controllers and FFTs, but the simple fact is that these circuits work, and work well. The only thing to fear is perf board solder shorts. We think debugging those is half the fun.

Home Theater for One Shakes Souls, Removes Fillings

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Sometimes an earth-shaking home theater setup just won’t do. A speaker enclosure can only fill the average sized room with so much sound. [Kevin Bastyr] has figured out a way around this. Do away with the room, and build the home theater INSIDE the speaker enclosure! [Kevin’s] creation is called Humorously Maniacal Milwaukee Makerspace Multimedia Machine, (or HMMMMMM for short). As the name implies, HMMMMMM was created at the Milwaukee Makerspace. The HMMMMMM reminds us a bit of the sensory deprivation chambers which were so popular in the 70’s. HMMMMMM’s purpose in life however, is anything but deprivation. The user (victim?) climbs through a 27” hatch and settles into a reclining position. An LCD display is mounted a comfortable distance away from the users eyes. Then movie (or brainwashing program) begins.

The sound system is what sets the HMMMMMM apart. The HMMMMMM utilises a 5.16 surround sound system. That’s 5 speakers and 16 10″ high efficiency subwoofers. We’re not sure if it would be better to call it a sound system, or a full-out frontal assault on the senses. We’re not kidding when we say senses as well. Bass this loud can be felt as much as it is heard. The HMMMMMM is has been measured at 148.6dB at 40Hz. That’s well into the hearing damage range. To be safe, HMMMMMM users must wear double hearing protection: foam earplugs and earmuffs.

[Kevin’s] graphs aren’t all smoke and mirrors either – he’s an audio engineer by trade, and made his measurements with a laboratory grade 1/2″ Bruel and Kjaer microphone. Sound pressure level testing isn’t without its dangers. During testing the 2050 watt amplifier powering HMMMMMM encountered a fan failure. The amp’s circuit board ended up scorched black with delaminated traces. The HMMMMMM however was none the worse for wear. Future plans for the HMMMMMM include RGB LEDs that flash to the beat, and a smoke machine to create that extra atmosphere when the escape hatch is opened.

Building an audio box out of thrown away boards

The last time [Mark] was at the scrap yard, he managed to find the analogue input and output cards of an old Akai DR8 studio hard drive recorder. These cards offered great possibilities (8 ADC inputs, 12 DAC outputs) so he repaired them and made a whole audio system out of them.

The repair only involved changing a couple of low dropout regulators. Afterwards, [Mark] interfaced one of his CPLD development boards so he could produce some sine waves and digitize signals generated from a PC based audio test unit. He then made the frame shown in the picture above and switched to an Altera Cyclone IV FPGA. To complete his system, he designed a small board to attach a VGA screen,  and another to use the nRF24L01 wireless module.

Inside the FPGA, [Mark] used a NIOS II soft core processor to orchestrate the complete system and display a nice user interface. He even made another system with an USB host plug to connect MIDI enabled peripherals, allowing him to wirelessly control his creation.