LM 3886 stereo in Altec 1590C chassis - diyAudio
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Old 2nd April 2013, 05:25 PM   #1
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Default LM 3886 stereo in Altec 1590C chassis

Hi folks,
Yesterday I finished my LM 3886 stereo amp built into an Altec 1590C chassis. The amp was built on chipamp.com PCBs I'd bought on eBay along with two toroidal transformers, a project that someone had apparently been planning on doing but gave up on. The other transformer and extra boards I built into a Dyna 120 chassis. The Altec 1590C supplied two large double-sided heat sinks as well as most of the wire and hardware. There's a lot of extra real estate in there but I love the build quality of the Altec rackmount chassis and it looks great in my studio.

The 1590C was not working. It was also not liftable, using a giant output transformer as it does. That model was made exclusively for "sound distribution" and did not have 4, 8 or 16 ohm outputs. The smaller amps in the series, the 1593 and 1594, would also work well as chassis, if not working. I have one of each, though I wouldn't personally be able to sacrifice them, as they work well and are built too beautifully, physically. However, they are certainly not hi-fi amps. These amps seem to be available. It's too bad they don't have double-ended transformers.

I'd thought it would be looking a little neater when done. I probably should have routed the DC wires around instead of over the heat sink, but I wanted to keep the wires short. I used heavy wire, supplied by the Altec, for the DC. Honestly, no, I'm not going to redo anything. It works perfectly. This means I'm not going to open it up a lot to show my friends, saving them from certain boredom. -- The heat sink is double-sided and the back side is partly exposed to the outside where it goes over one of the giant transformer cutouts. After being on a few hours the amp generates a only a little warmth.

Both the amps I just built fired up perfectly the first time, and that's a novel experience. I owe it partly to good advice I got from you guys, particularly Daniel, and liberal use of a magnifying glass to check my solder joints. I strongly recommend this practice to everyone, no matter what your eyesight and experience. -- Sound-wise, wow! No noise. Without the pilot lights I wouldn't know if they were on. I've now heard things on recordings I've never heard before! The veil has been lifted. The "Altec" is driving old KLH Model 17 speakers. I didn't know they could sound so clear and transparent. The preamp is a Dyna PAT, and I know they are not loved. I'm more than content.

Thanks for your help,
Keith in Winters, CA
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Old 2nd April 2013, 06:51 PM   #2
JonGain is offline JonGain  United States
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looks great! I have a nice set of those KLH 17's they are nice!
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Old 4th April 2013, 05:52 PM   #3
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I wanted to add that speaker protection, against possible DC, is by two 6800 mfd caps, 63 volts, back to back. Two per channel, of course. Its sound continues to make me very happy. I owe that idea to Danielwritesback. He implements it in a more sophisticated way, and probably a better way, using 4 caps/channel, but this has been working well for me and I was able to find the caps for cheap at All Electronics.
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Old 5th April 2013, 10:38 AM   #4
AndrewT is offline AndrewT  Scotland
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Two back to back (equal value) capacitors have an effective capacitance of half the value of the 6800uF you have chosen.

Is 3400uF sufficient for the bass response you require from your amp/speaker combination?

Add a resistor across each capacitor. Maybe 1k or so to allow the caps to discharge ready for the next start up and also to help with keeping the voltage across them more equal during playback.
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Old 5th April 2013, 11:13 AM   #5
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The 1590 is a commercial work horse designed to drive a 25v, 70v, 100v, or 200v line. It is not designed to be a home amplifier. If it were me I would have repaired the Altec and sold it used that money to put into your chip amp if you still desired to build one. The chip amps are primitive and let the common tinkerer play with a simple amplifier. Caution must be exercised in the size of the power supply caps. They must be large enough to provide a voltage storage so the amp doesn't clip easily and has a low end. I built the same amp years ago using a pair of 15,000 mfd caps in the power supply for each channel and it sounds ok for what it is. Push it and make it clip and it sounds nasty. If your happy with it then fine. Just do not audition it next to a high dollar amp or you will be very dissapointed by the outcome. Speaker protection in this type of design is recommended because speakers and DC rail voltage do not mix hand and hand. Feed the speaker DC and you have a junk speaker.

The Jukebox community really pioneered the use of this type of chip amp. My Rock-ola has several large bridged amps inside it.

Last edited by Original Burnedfingers; 5th April 2013 at 11:28 AM.
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Old 8th April 2013, 04:27 PM   #6
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Default Big Green

I was about to give the Altec away but didn't relish lifting it into my truck. As far as I could tell there is no market for these, without a 4,8 or 16 ohm output and weighing as much as they do. I certainly had no desire to fix it, but as a source of heat sinks and a chassis I've always admired, five stars. -- I'm guessing there are many discussions on this forum about the relative merits of chip amps, and of the LM3886 in particular. Not something I can address. I'm a guitar and guitar amp repairman and a musician, and like a lot of musicians I know I'm not an audiophile. I really don't know why that is, probably because most of us didn't have enough dough when young to afford expensive stuff, time was spent practicing and maybe most jazz and rock musicians are more interested in the notes than the tonal nuances, if that makes any sense. So for years my stereos have been stock Dyna 70s and 1970s and '80s Japanese consumer amps, which seemed just fine. (Jeez, among my young friends I was considered the total lucky nerd, because both channels generally worked and I had pretty good speakers, thanks to lucky garage sales.) For my old ears this chip amp has been a major revelation. I heard slight distortion on a couple of tracks of a well-received cd I played on. I heard little mouth noises and breathing that I never had heard before, and I've listened to that cd a bunch. I'd say it was revelatory. I have no doubt that there are much finer amps and circuits in the world. However, most would cost more than the $75,+/-, I spent on this (I got a good deal on the transformer and boards), I enjoyed building it, and I've always loved the looks of Big Green. (It's surprising to me how many of my guitar repair customers recognize that classic Altec design if not the model.)

As far as the 3400 mfd output caps, the bass seems full and punchy down as far as my old KLH speakers want to go. If I want to go deeper it would be easy to double the capacitance. And thanks for the tip on the resistors to bleed the caps.

Best,
Keith
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Old 16th April 2013, 12:04 AM   #7
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Oh, good job on fine tuning the output impedance! Those classic speakers probably appreciate the non-zero output impedance at lowest pitches so that the solid state amp can't ride the brakes. When your output capacitance is precision sized to fit the needs of the speaker, really rockin bass extension may be available.

To "touch up" the resolution of your series pair of 6800u, you could add 10u or 4.7u Nichicon ES (little green bipolar muse) serving as bypass cap to carry the treble around the big caps. ES is already bipolar, so use just one per channel. Want more "revelatory," then try my little green friend. You might actually appreciate a smaller value bypass cap, like the 2u2 Nichicon ES, depending on needs of the amp, speaker, room and AF receiver.
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