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β24 on the move

beta24

β24 on the move

Postby cobretti » June 9th, 2015, 7:58 am

I have finally got some time to start building my B24. I will be using 4 transformers for amplifier itself and 2 supporting transformers for start up circuitry and speaker protection. I purchased 4 Plitrons for the price of one, that one Ti used in his build. Big toroids are rated 300VA and small ones are 15VA each. Big transformers can push easy 400VA, tested, and they don't hum.
I will be using my own slow start circuit (still under development) to eliminate inrush current. The case is also on its way, it should be delivered today, hopefully. I have no idea how I will fit everything inside the chassis yet, but it will have to fit. :D I need to physically see the box to have an idea about layout. More to come.
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Re: β24 on the move

Postby cobretti » June 24th, 2015, 11:25 am

I have a dilemma and thinking of removing one power MOSFET on positive and one MOSFET on negative side in σ22 power supply (for my B24 build) because I am trying to save some space. My rough guess is that each power MOSFET dissipates less than 1W of power, so removing one on each side should be no issue. Looking on the sigma22 schematic, I think I wouldn't even have to change any compensation caps value. So I will have to either yank one out or find really shorter heat sinks for them..... :?:
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Re: β24 on the move

Postby amb » June 24th, 2015, 9:21 pm

How much room do you have? Would 1" tall heatsinks fit? If so, you can use something like Aavid-Thermalloy 6396B-P2G (Mouser 532-6396B-P2G).
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Re: β24 on the move

Postby cobretti » June 25th, 2015, 11:04 am

Well, I have some room there to make everything fit I think, but just those heat sinks are sticking from the side in the way where speaker protection boards are(will be). So, I will buy those 1" sinks and go with them instead.
I was running some load tests yesterday with my fast rectifier diodes (STTH2002)
http://www.mouser.com/Search/ProductDetail.aspx?R=STTH2002Dvirtualkey51120000virtualkey511-STTH2002D
and they will need heat sinks as well, they dissipate quite a lot of heat when I push them around 5A. Definitelly not as convenient as GBPC3504 bridge that can be mounted straight on the aluminum plate.
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Re: β24 on the move

Postby cobretti » July 25th, 2015, 1:23 pm

I started putting things slowly together. Here are some photos. I am still missing speaker protection boards but those can be installed later.
Now I have to do some wiring (once again, time permited) and give it a bell ;) Hopefully, no magic smoke will escape :D
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Re: β24 on the move

Postby amb » July 25th, 2015, 1:29 pm

Nice!
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Re: β24 on the move

Postby cobretti » July 25th, 2015, 1:52 pm

I tried a new PCB manufacturer called eurocircuits. These dudes can make even one prototype PCB for reasonable price. It takes 2 weeks get it delivered. The cost was significantly less than OSH park, and you get as many boards you want. Quality is excellent. I think they are Belgium company and boards are made in Germany. However, my second small board came from Hungary, which was slightly different but also very good. Just the solder masking coat is little bit different color. I did lot of PCBs with IteadStudio before. I had no problems with them but I lways ended up with so many PCBs I didn't need. They make so many prototype PCBs per order, who the hell needs 10 PCBs if you making 1 prototype.
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Re: β24 on the move

Postby cobretti » July 31st, 2015, 5:53 pm

A little update. I have some stuff temporary wired and I was able to do a power supply test today. Everything looks good so far. Start up sequence is OK and bleeder resistors discharge storage caps on power down no problem. Plus minus 36V regulated supply was already tested, no problems there. I also did a sanity check of the main boards and didn't find anything suspicious, so everything looks promising, fingers crossed. I wish there were test points for VR4 to adjust it to middle position. As you can see from the picture, things got very heavy, so I had to mount steel reinforcement "L" profiles on the sides as well as one bar on the bottom.
I also received milled rear panel today.
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Re: β24 on the move

Postby cobretti » August 2nd, 2015, 8:53 am

First channel set up = success!
I made some progress yesterday and was able to adjust all stages on my b24.
Input stage bias reads 4.02V on all resistors R7,R8,R9 and R10. Beauty.
VAS stage balance setting and voltage adjustment of VR4 on R11 and R13 was really touchy. It moves up and down with temperature (I guess?) but it is balanced between 125mV and 132mV as it goes left and right, up and down...... So, that's all right.
Output stages, 40mV reading on all 4 resistors R45, R46, R47 and R48. Once again, there is a slight deviation plus minus 3-4mV as it drifts with temperature.
I have a nice burn mark on my leg from 22ohm current limiting resistor :mrgreen: as I accidentally touched it. Damn! they heat up to 90 deg. C. Both resistors read 8.9 and something volt.
I was pleasantly surprised how well made those Plitron transformers are. I measured precisely +45.2V and -45.2V on both halves. Perfectly symmetrical.
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Re: β24 on the move

Postby amb » August 2nd, 2015, 1:45 pm

Excellent!

Yup, those temporary resistors get hot. 8.9V / 22Ω = 405mA, which is right on target, and they dissipate 0.405A * 8.9V = 3.6W each.
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