Project No. 1: Beta22 Headphone Amplifier - diyAudio
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Old 3rd April 2008, 10:12 PM   #1
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Join Date: Jan 2008
Default Project No. 1: Beta22 Headphone Amplifier

http://www.amb.org/audio/beta22/

For my first project I decided to jump in more or less head first, with a 5-channel beta22 balanced/single ended w/active ground headphone amplifier. Four of the channels are for driving 2-channel headphones in balanced configuration or two pairs of unbalanced headphones, and one is an active ground for one or two pairs of unbalanced headphones. I ordered parts from Glass Jar Audio and they came in a few weeks ago, and I got to work right away. I ordered enough boards for the 3-channel configuration to start out with, intending to order the other boards after I had completed the 3-channel amp. The kit included 3 B22 boards, one S22 board, an E12 muting delay/DC offset protection circuit board. I had ordered a 100VA transformer, but he upgraded it to a 120VA, I guess because that's what he had on hand. It saved me a ton of time to be able to order everything as a package from one place. I highly recommend it.

I started this thread because this is my first project, it's a bigger project than most people start out with, and I want to stay connected to the community throughout the process, to make sure I get it done and that I don't make any huge mistakes. I also thought that following a beginner's acquaintance with DIY through building a beta22 would make for an interesting read. Hopefully this will be the first of many projects. I began a thread at head-fi a while back, but I haven't been getting a ton of replies and this forum is more heavily trafficked by knowledgeable DIYers, so I'm posting here hoping to get more responses.

When the kit arrived and I pulled everything out and got to work. I started with the E12, since it was the smallest. I worked by finding parts on the parts list and then soldering them into their respective locations on the board. I had to google several parts to find out about their polarity. I would place several parts of the same type on the board and then flip the board over, cut their leads and solder them into place. It was going very slowly at first, but as I got used to soldering it started going faster, and I finished it that night. I didn't read its schematic until the next day. I'm pretty sure it was the first complete schematic I'd ever read. Clever, simple circuit. I noted that if one of the channels has a DC offset of more than 70mV, but |VL + VR + VG| < 70mV (which is the circuit's threshold), say, the DC offset is positive on one channel and negative on another, the headphones wouldn't be disconnected. That situation's not very probable and I guess Ti determined that the simplicity of the circuit was more important.

I put the E12 away and the next day I got started on the S22. I would've finished it before I went to bed that night, but I ran out of solder! I had ordered some solder earlier that week and it arrived the next day, and I finished the S22 all except for the mosfets, because I didn't have any heatsinc mounting kits. I still need to order those.

For the past few weeks however, I've been very busy and haven't had any time to work on the amp. I'm hoping to get some time after my Calc 3 test on Tuesday.

As of now I have no idea what I'm going to do for an enclosure. I have no tools for building one, but I can probably borrow some from someone. I have a few friends who can weld, too.

I'll post some initial pics asap. I'll be posting pics and updating this thread (and asking questions) throughout the building process.

I actually have a few initial questions:

I ended up going with Kester "44" activated rosin core solder, .031", 63/37 eutectic solder. I chose the size pretty arbitrarily (from a range), but I thoroughly researched the different flux types and alloys, and did a lot of searching to find out if there was anything to those expensive "audiophile" solders (there wasn't) or if they were just nonsense (they were). Apparently silver and gold containing solder were originally intended by legitimate companies like Kester for soldering silver and gold (or silver/gold plated) components, respectively; not for a perfectly negligible decrease in resistance or any kind of audiophile voodoo. I went with the cheaper, slighly better handling Pb/Sn eutectic solder and I'll buy some silver or gold containing solder if I ever need to solder silver or gold contacts.

I haven't been able to find any info about cleaning the joints, however. Kester claims that the flux is non-corrosive if not cleaned, but would it still be better if I cleaned it, and if so, what should I use to clean it? Ti's website says that I can clean it with alcohol, but another source said that this is far from ideal; that it would be far better for the board if I used something made specifically for cleaning circuit boards.

My second question is about what tips I should buy for my soldering iron. I have a Hakko 936, and I've been using the stock tip, but I'm afraid that it's too large for the beta22. My grandfather bought me a few tips, but I haven't seen them in a while and I'm afraid they're lost. I couldn't tell you what tips they were. I just need to know which tips I should have in my inventory. I'm also interested in tips about caring for my tips.

I also need to order RCA and 1/4" stereo jacks soon. Are there any high quality 1/4" jacks that have a switch built into them? As I said this amp will eventually be balanced, and it will also have an active ground board for driving unbalanced headphones. I want to be able to control volume of the two pairs of headphones with separate stepped attenuators and the balanced headphones with a single, 4-channel stepped attenuator. I want to design a circuit that would drive the balanced headphones in unbalanced configuration when I plug a pair of headphones into one of the 1/4" jacks and disable the XLR outputs entirely when two pairs of unbalanced headphones are plugged in, and that would be simpler if I had switches in the 1/4" jacks that would open or close when I plugged a pair of headphones in. But these are signal path switches, so they need to be high quality.

Finally, what about soldering technique? I've been having problems with the solder sticking to the tip of my iron. Is this a tip care issue or should i be doing something differently?

Thanks guys!
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