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Daniele's Beta24 thread

beta24

Daniele's Beta24 thread

Postby daniele86 » February 25th, 2017, 6:58 am

Hi Ti,
I'm going to start the bigger project you have made: the Beta24. 8-)
I think to start it slowly, completing it in the next years.
The Amb site is one of the best site for DIY in audio world because every project is very well documented without considering the assistance that you do in AMB forum.
Despite this, the Beta24 is not so well documented; is there a reason for this or do you think to do it in the next future?

To start, I have a question that is already discussed a lot of time..
In a room of < 20m^2 with medium speaker efficiency >89dB if you have to decide, which beta24 (AB or A) would you do? I don't require high spl

Thanks!
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Re: Daniele's Beta24 thread

Postby amb » February 25th, 2017, 11:30 am

The β24 was developed in open source fashion just like all my other projects, but that was on the headwize forum which had gone defunct, and all of that history is lost. Nevertheless, copies of the schematic floated around and some people (particularly in China) began making "clones" of the β24, either as a kit or a full product. Many of them are modified from the original, uses inferior parts, with different PCB layout, etc. Then they claim their products to be AMB's β24, which tarnishes the image of the real thing. Apparently a large, high-powered and sophisticated power amp draws lots of interest, including the unscrupulous. Other AMB projects have also been cloned (such as M³, Mini³, σ22, etc) but not to the same scale as the β24.

So, in a way the demise of headwize had a silver lining, it (mostly) stopped further cloning.

I spend time and effort to design these projects for the benefit of the DIY community, and I share the design openly for all to see, but I take a dim view toward those who take advantage of my work for their own profit.

Instead of a website like my other projects, I provide a full, detailed guide for the β24 in the form of a PDF document. It is sent via email to anyone who purchases the β24 board, and also available by request.

Anyway, to answer your second question, 20m square is a fairy large room. If you don't listen at high volumes then the class A is probably enough, depending on acoustics (and as you mentioned, speaker efficiency). The question is whether you'll want to live with an amplifier that runs very hot at all times and runs up your electricity usage.

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Re: Daniele's Beta24 thread

Postby daniele86 » March 3rd, 2018, 5:16 am

Hi all!
I’m at the final step in building the beta24.
All parts are on place, the enclosure is almost done and the 4 boards fixed on Dissipante so I started the last notorious section “the initial setup”.

The boards are OK at the “before powering up” section: trimmers OK, no short circuit from the listed pads.
Come on, connect 22R 10W resistors and let’s power up!

The first board went well with the leds lighting, no high temperature to any components, BUT my second board…
In the second board I immediately found that Q47 was getting hot, the only one, extremely hot (can’t touch). Turned off.

Some point for troubleshooting:
VREG+ = 34VDC
VREG- = 34VDC
V+ = 40VDC
V- = 40VDC
VR1 is in the right position: From the - pin of D1 and R5 I’ve found: 200 Ohm
VR2 is in the right position: From the base of Q17 and R25 I’ve found: 200 Ohm
VR3 is in the right position: From the base of Q19 and R26 I’ve found: 200 Ohm
VR4 is in the right position: From the - pin of D1 and source of Q1 I’ve found: 50 Ohm
I’ve compared the two boards each other and
all resistance are the same value
all diode (D) have the same voltage drop

I could imagine it’s Q47 defective, but how to be sure?
There’s a test for the CR diode?
What voltage measurements could help?

Thanks a lot.
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Re: Daniele's Beta24 thread

Postby amb » March 3rd, 2018, 12:15 pm

Check the board thoroughly to make sure all parts are correct for each position, and installed in the proper orientations. Also look for solder joint issues.
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Re: Daniele's Beta24 thread

Postby daniele86 » March 4th, 2018, 2:50 am

Hi Ti,
I checked all the parts a lot of times, but I didn't find any problem. Every parts is in correct position and orientation. I also checked the solder joints and resolder some of it.

Nothing changed. What can I check now? Could it be a good idea change Q47?

Thanks!
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Re: Daniele's Beta24 thread

Postby amb » March 4th, 2018, 2:57 am

It's rather unusual for Q47 to heat up like that. Are Q41-Q44 JFETs installed in the same orientation as Q1-Q4? They should not be according to the silkscreen.
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Re: Daniele's Beta24 thread

Postby daniele86 » March 4th, 2018, 7:42 am

The position of those components have been my concern since I started with them.
I have putted the maximum concentration and I can't see anything wrong.
But I'll post some picture to get more help.

https://www.dropbox.com/sh/zlsyb6oc3rt252e/AAA4dvjAnOoQ_JzVqCQnRfBGa?dl=0
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Re: Daniele's Beta24 thread

Postby amb » March 4th, 2018, 12:40 pm

The JFETs appear to be installed correctly. Do BJT sanity check on all the BC550C and BC560C transistors around the common mode feedback block (see schematic) to test if any of them fail. Note that the B-C junctions are shorted together intentionally for Q49 and Q50.
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Re: Daniele's Beta24 thread

Postby daniele86 » March 10th, 2018, 12:53 am

Hi Ti,
I checked all the BC550C and BC560C around common mode feedback, but seems all of them are ok.

What can I do now? Is it possible to test Q41-Q44? Or test zener diode D11/D12?

If you remember I sent you a private email where I told you that one board have a "scratch" on pcb, do you think that could cause a problem like this?



Thanks
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Re: Daniele's Beta24 thread

Postby amb » March 10th, 2018, 4:47 am

JFETs are difficult to test while in-circuit, especially when one of your transistors would heat up rapidly upon turn-on, so there is not enough time to make a lot of measurements. Same thing for the zener diodes.

Perhaps go beyond just the CMFB section and test the BC550C and BC560C transistors everywhere else?

Did you measure that "scratch" to see if it breaks a trace or causes a short? If there is no measured problem then it wouldn't be the cause. I seem to recall it was merely a blemish on an internal plane and it's not near anything else, if I remembered correctly.
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