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β24 on the move

beta24

Re: β24 on the move

Postby oneplustwo » August 4th, 2015, 10:38 pm

Man, I need to eventually finish mine up. :( Three kids and other priorities will eventually have to give way, right?
Don't forget to listen to the music!
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Re: β24 on the move

Postby Biecher » August 5th, 2015, 2:55 am

very cool build. Can you tell us something about your self-designed pcbs and maybe show a close up picture?
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Re: β24 on the move

Postby cobretti » August 5th, 2015, 5:13 pm

I did my own start up control from scratch and speaker protection where I used Ti's idea from epsilon12 that's not yet finished. I am using 4 transformers for amplifier itself, and two small supporting transformers. I could not start two 300VA toroids with a large filter caps without tripping the breaker. So I have in series resistor by-passed with relay to limit transformer inrush current.
One small supporting transformer is the stand-by power that powers ATtiny and it's constantly ON. Another one is independent 2x12VDC for speaker protections. I still have to tidy up some wires inside. I have wires flapping in the breeze everywhere. The purple PCB is made by OSH Park and the green one is made by Eurocircuits in Germany, that is actually a Belgium company. They make very good quality PCBs. First time I tried them.
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Re: β24 on the move

Postby cobretti » August 12th, 2015, 4:32 pm

Just finished my speaker protection. Adjusted and pre-tested on the bench with generator, everything seems to work OK. Sensitivity to disconnect speakers is DC to around 3-7 Hz any polarity but also depends on monitoring voltage. I can still tweak its response with adjustable gain. Current consumption is 21mA and I just found out that I will have to change the voltage regulating transistors because each one will dissipate plus minus 630mW, not so good idea for transistor in TO-92 package. :oops:
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Re: β24 on the move

Postby KooKoo » August 28th, 2015, 3:10 pm

Wow, a beautiful build!
Congratulations!!!
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Re: β24 on the move

Postby cobretti » August 29th, 2015, 6:40 am

Thanks. I did some low voltage measurements into 50ohm scope termination and some measurements with higher output voltage into 22ohm resistor, that's just for basic check. I don't have any other load that I can connect to the amp at this moment. There is a slight rolloff at 100Hz at 20Vpp. And I noticed some ringing from 50kHz and up on falling edge. Set up was really crude, I run into ground loop problems so I used my isolation transformer for my signal generator. Next time I will also try to install Zobel network and see how it will respond.
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Re: β24 on the move

Postby cobretti » August 29th, 2015, 6:41 am

and more here.......
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Re: β24 on the move

Postby amb » August 29th, 2015, 1:06 pm

Most scopes have their probe grounds internally connected to AC ground. On a differential output amp you should not be measuring its output directly because the scope would be short-circuiting one of the active output terminals to ground. I'm surprised the amp isn't damaged.

The 100Hz frequency response aberration and square wave ringing at 20KHz are most likely from the isolation transformer you installed at the signal generator, not the amp. The slow 300-ish nV rise time is indicative of this.
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Re: β24 on the move

Postby cobretti » August 30th, 2015, 6:54 am

No, there is nothing to worry about because the scope is floating, there is no ground. The 100Hz roll off was my mistake, I overlooked the scope coupling, set to AC :oops:
Here are more captured signals with full rated output swing, generator set to 1.8Vrms or less, some waveforms didn't fit the screen.
Square wave is not perfectly square especially on rising edge, this is due to my wiring set up I am pretty sure. I was using few BNC cables coupled in long run with end RCA/BNC adaptor. My internal wiring needs revision as well.
Could the slower rising edge response be caused by the C1,C2 feedback 1.5pF cap? I couldn't even get close to 1MHz. That's my best guess.
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Re: β24 on the move

Postby cobretti » August 30th, 2015, 6:55 am

more pics
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