Around the time I’d last posted about compressors, I mentioned my next project which was going to be a stereo 1176 Revision D. It’s been a while since then and I’ve made a lot of progress, but I have hit a snag. I’ll get to that later though.
Things started easily enough, with the populating of the PCBs. This is actually one of my favorite steps of the build if only because everything is already neatly laid out and everything has a place.
As you can see, the lower left hand corners of both boards are empty, and they still are. That’s because I’m using a single external power supply to power both boards. The accepted wisdom here seems to be that it’s a good idea to keep your power supply as far from your audio as possible. That being said, my Hairball 1176 Rev A has its PSU section on the main PCB and it runs very quietly. So I guess it all comes down to the quality of the design at the end of the day.
It wasn’t long after the PCBs were stuffed that my case arrived. I get a lot of my enclosures from Collective Cases. The proprietor is an active member of the GroupDIY forums and offers some really excellent cases made specifically for the more popular DIY projects. Needless to say, the boards fit inside like a glove.
The front panel is definitely a bit jam-packed, especially with those smaller circuit boards mounted on the switches. You can also see the stereo link board (that smaller red one). I have since removed those boards in order to make trouble shooting easier since I had more than a few problems upon first test.
The first thing I did was power up the unit with the main boards disconnected from power. All I really wanted was to check that the lights for the meters and main power indicator worked. As you can see, all is well there. Unfortunately, all was not well elsewhere.
After confirming that the lights worked I double checked my voltages coming from the PSU and hooked up the two main boards. The first step of calibrating one of these units is to set the Q-bias trimmer. This trimmer basically controls how much signal flows through the FET. Without properly setting the Q-bias, the unit won’t compress the audio. To set the Q-bias, you have to send a 1kHz sine wave into the board at a level of around 1dBu, which is about .775 VAC across pins 2 and 3 of the input XLR. With this signal being sent to my two channels though, I noticed some strange behavior. First of all, upon turning up the input knob, my signal would rise steadily until a reading of about -7dB was read on the meter. At that point, the signal would stop increasing in volume and became noticeably distorted. This was consistent across both channels. I initially thought it to be a grounding issue, so I went through and re-adjusted my grounding scheme. I also removed the previously mentioned stereo link boards just in case they were causing any issues. Much to my dismay, after all that, I was still experiencing the same problems.
My next step was to check the voltages on each leg of each transistor to make sure that they were in line with the schematic. It wad during this step that I discovered I had made a colossal mistake. I had accidentally soldered a transistor in backwards. Luckily, I had recently purchased a desoldering pump, which made correcting my mistake infinitely easier. Once that was done, I went back to testing and was happy to hear a very clean signal being passed. Only this time, as I progressed to the next steps, I wasn’t getting any gain reduction.
After trying a few different things, I eventually took all the knobs and switches off of the front panel to check for bad solder joints. In the process of looking, I noticed that there was a loose wire coming from one of the switch mounted boards. After a few seconds I realized that when I had disconnected the stereo link boards, I had severed this connection and then never reconnected it back to where it belonged. I quickly fixed that issue for both channels and went back to calibrating. That must have been the last issue, because afterwards channel 2 was behaving exactly as it should, and i was able to finish calibration and then put it to some good use since I had a band coming over in about 20 minutes!
Unfortunately, despite applying the same fixes to the channel 1 board, I’m still plagued by the same issues as I was from the get go. I’ve double checked all of the components and checked the transistor voltages against the schematic, but I have yet to determine what is causing the issue with this channel. I’ve actually stepped away from this one for the last week to help clear my head, and I’ve been working on another project in the meantime, which I’ll start a post about later. I’ll be sure to post an update when I get the problem solved and the unit is up and fully functioning. I at least have one channel working until then.