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Old 3rd November 2002, 10:27 AM   #1
Thomas is offline Thomas  Denmark
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Default The Mini-A lives!

Hi forum, I just finished one channel of the Mini, and I'm glad to tell that the PCB's I made realy works.

Then i exposed my own PCB's, I accidently mirrored them, so I had to turn all transistors 180 degrees. This also means the output devices are below the PCB. Not intended, but it works and looks pretty good too.

The criter runs from a 12V trafo with 66000MFD per rail, that gives about 13 volts per rail including diode losses. The idle current is 2.45A. The DC ofset is about 50mA which could be lower, but I matched or measured nothing before soldering, so I saw that one comming.

I used replacments for all transistors. Output devices are IRF640, others are IRF9630 and BC550B.
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Old 3rd November 2002, 10:32 AM   #2
Thomas is offline Thomas  Denmark
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Here's a picture of the skinned and polished 'cans' I'm using. It looks realy good, if I may say so.
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Old 4th November 2002, 09:25 PM   #3
joensd is offline joensd  Germany
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Hi Thomas,
glad to see your PCB works out.
Didnīt Grey mention in the original thread the idle current would be 1A with +-12V? And youīre having 2.45A?? How do the TO220 transistors survive that. I guess they get quite hot!?
Canīt you just take the fetīs from original aleph with TO247-case for better thermal resistance?
Whatīs the sound like ? Are you satisfied with the result ?
OK, enough questions.
And please send some more pics when itīs finished.

Jens
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Old 4th November 2002, 10:28 PM   #4
Thomas is offline Thomas  Denmark
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Hi Jens, thanks for the interest!

The idle current puzzles me. Grey did indeed mention 1A at 12V. But I don't know how it's measured. I measured mine by running one rail thrugh my amp meter.- Maby that's wrong, anyone?

I did promise a PCB version for TO-247's, but I haven't got the time yet. Might do it this week...

The sound? Pretty sweet in my ears! Exelent voice reproduction. It compares a lot to my Szekers headphone amp. I'll get back to the imageing when I have finished the second channel. I noticed some distortion with high energy program material (rock and electronic music) when turned up. R4 & R5 measured just above 5 volts, a bit on the high side. Q1-Q3 measured both 3,56, a bit on the low side, should be 4 volts as i recall. - Any ideas??

Satisfied? When I'm finished I'm satisfied. But results so far looks and sounds REAL promising.
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Old 5th November 2002, 09:55 AM   #5
joensd is offline joensd  Germany
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Hi,
I havenīt build any class a amp before so no experience with that. But the idle current really seems to be too high.
Iīve made a PCB of this scaled down aleph as well but am not so satisfied with the results and wanna definitely include the TO247.
Can send you the layout if I get reasonable results.

Jens
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Old 5th November 2002, 11:51 AM   #6
grataku is offline grataku  United States
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Thomas
The amp meter should give an accurate value is it possible that you are measuring AC amps instead of DC?
You can measure DC voltage across the source resistor and use ohms law. The tolerance of source resistor may be very large, though and 0.22 ohm is difficult to measure.
I thought Grey tested this circuit very well, I don't know, you may need to tweak the CCS.
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Old 5th November 2002, 02:21 PM   #7
Thomas is offline Thomas  Denmark
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Jens, if you can wait one or two days I'll get the site updated and put up boards for to247. I'll keep you informed via e-mail.

Grataku, new measurements confirm old ones. I measured 1.16 volts over source resistors (0.47ohms) 1.16V / 0.47R = 2.46A ! Way too high.

Must I go thrugh all component values again, or do you have any experience I can rely on?
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Old 5th November 2002, 03:18 PM   #8
grataku is offline grataku  United States
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Thomas,
something is got to give, there are not that many components in the circuit.
I haven't played with the mini-A so I cannot pinpoint the problem from here, I am an experimental kind of guy. I think you should try talking to Grey. I am sure that a comparison of a few voltages in the CCS region between what you have and what Grey has will clear the issue right up.
Meanwhile you may want to re-measure all your components. is the device temperature under control? Does it reach a steady state?
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Old 6th November 2002, 04:59 PM   #9
Thomas is offline Thomas  Denmark
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Quote:
Originally posted by grataku
Thomas,
something is got to give, there are not that many components in the circuit.
Errr, I thourght so. Just being in a lazy mood looking for an easy way out.

Meantime I checked the schematic the PCB was done after and all component values, and it all checked out. - At least what I can see. And the temperature: I can't touch the thing after 30 mins.

A few days ago I e-mailed Jason, asking if he could set me in contact with Grey, because his settings doesn't alow me to e-mail him directly through this board. I haven't got an reply yet.
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Old 7th November 2002, 04:41 PM   #10
Thomas is offline Thomas  Denmark
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grataku, I changed R11 from 47.5K to 10.0K. R11 sets the bias, which now are 1.5A. I don't know if this is the propper way, and i should change other values too, but it works for now. 1.5A is a good value, because my speakers are 6ohms. The sound seems allright with the new value.
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