Vintage AMB - part 1
The first ever audio DIY project I did back when I was 14 years old (circa 1977) was to make a VU meter box for my parents' stereo. Sadly, I don't have any pictures of that contraption and it has long since been dismantled and discarded.
But I caught the DIY bug and began some pretty ambitious builds. Like many junior high school kids, I was going for as many switches and lights as I could possibly put in. The result was the AMB alpha1 preamp and beta1 power amp. You see, that greek letter and number scheme began a long time ago. As well as the logo!
Like the VU meter box, these have been dismantled, but I do have pictures of them. Here are a couple of scans. Unfortunately, I only took exterior shots. I'd be embarrased to show the insides anyway, they are quite messy.
First up, the alpha1 preamp. It has the usual RIAA phono stage, volume control and line stage, but also has a unity gain headphone amp, mic preamp with mixing capability, a separate continuously adjustable loudness contour knob, stereo/mono/reverse mode switches, low and high pass filters with switchable corner frequencies, high-blend switch, -20dB mute switch and even an output muting delay circuit with blinking LED status indicator.
There isn't much to write about as far as the actual amplification circuitry is concerned. I used very simple two or three-transistor amplifier designs found in most early-to-mid '70s stereo receivers. Remember, those were the days when the schematic diagram came with the owner's manual.
The casing is all-wood. It was major chore cutting those rectangular holes for the paddle switches. I took advantage of my school's wood shop and used this as my project. You can see, the black panel theme has stayed with me through the years until recently, my β24 power amp broke tradition with clear anodized alunimum instead.
Next is the beta1 power amp. This was a 100W/channel beasty that could also be bridged to become a 300W mono amp (via a rear panel switch). There are a pair of input level knobs, two slanted rows of level meter LEDs (inspired by some Harman Kardon Citation amps of the time) which could be switched on or off, or into a test mode to turn all LEDs on. Two additional LEDs are used for clipping indication. There is active fan cooling inside, with thermal sensors that would switch the fan to high speed if the heatsinks' temperature exceeds a certain threshold. A switch on the front panel also allows manually setting the fan to high speed. LEDs also show power on, bridged, thermal (fan at high speed), delay/protection and speaker standby status.
The amp circuit was based on a project published in a Taiwan audio magazine. It is all-discrete and employs bipolar transistors throughout, with a complementary differential input stage, complementary VAS stage and a darlington complementary push-pull output stage with two paralleled 30A transistor pairs per channel (a total of 8 output transistors). The power supply had independent transformers, rectifiers and capacitors per channel, with a total of 40000uF. There is a DC offset protection and muting delay circuit, a fan controller circuit, and the LED level meter driver circuit, which does not use the LM3915 chip. Instead, I built it with no less than
twenty-four 741 opamps, each serving as a comparator and LED driver. The clipping indicator LEDs are additionally controlled by a special circuit that would keep the LED illuminated for 2 seconds when such an event occurs.
Tha casing again is all-wood, except the top cover is perforated metal. The underside of the perforated sheet is covered with a non-perforated piece with cutouts for the areas that need extra ventilation. The right quarter of the amp is actually a special air tunnel for the fan (entry in the back, exit at the side near the front), and the heatsink fins protrude into that tunnel. There is a separate top cover over the air tunnel.
Sorry about the overexposed pics.
This duo actually served me well for a number of years, but my philosophy in amp design and construction has developed such that I could no longer bear the thought of these monstrosities, so they got taken apart for parts and I went on to building newer and better amps.
This is part 1 of (hopefully) a series of posts about my builds from the distant past.
bijou!
