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Old 10th March 2003, 07:17 PM   #1
Verbal is offline Verbal  Germany
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Default Battery Powered Zen Headphone Amp

I've finished building a headphone amp based on Tortello's Design, which turned out really great.
I've modified Tortello's circuit a little and added a feedback loop around the coupling caps, ala Zen V2. The gain is set to ca. 6dB, and can be changed by a jumper to 13dB.
Power supply is by two Exide 12V/3.5Ah SLAs. In case the batteries are low, there's also a LT 1085 regulated supply on board.
I'm really excited about the sound (with Sony MDR-7506s and, unfortunately only borrowed, Grado RS-1s). Very tranparent, effortless and dynamic. Very good weight and control in the bass, not sloppy at all. The AC supply comes quite close (after I finally fixed that hum...), with an overall somewhat darker characteristic. A little less tight and detailed, but still great.

Here it is:
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top cover off:
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On the left are the amp boards, on the board on the right side is the charging circuit using two LM317s, the LT1085 and the muting circuit. In the back are the two batteries, 1.5kg each.

the amp board:
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Thank you, Marcello, for sharing this great design, and of course thank you, Nelson for everything. You guys rock.

The schematic:
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couldn't resist:
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Old 11th March 2003, 05:37 AM   #2
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Blue rocks!
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Old 11th March 2003, 05:51 AM   #3
digi01 is offline digi01  China
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Very very nice!!Celebrate! !
Is your circuit of charging how to solve ? Put forward a suggestion, signal line of the amp spend analog audio interconnect cables will kind.
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Old 11th March 2003, 01:18 PM   #4
Verbal is offline Verbal  Germany
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Quote:
Originally posted by digi01
Very very nice!!Celebrate! !
Is your circuit of charging how to solve ? Put forward a suggestion, signal line of the amp spend analog audio interconnect cables will kind.
Thanks, Digi. I saw the pics of your Zen, looks really great I must say (and a lot tidier than mine...), love that polished finish.
The battery charging circuit I've come up with (if that was your question, I must admit I'm not entirely sure) looks like this:

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The first 317 limits the current to about 500mA, the second 317 regulates the voltage to 28.8V. If the current gets too low to drop enough voltage across R5/6 to keep Q1 turned on (i.e. the batteries are full, hopefully), Q2 starts conducting and the output voltage is reduced to 27.6V.
I should add I really don't know too much about charging batteries so I'm not sure whether this is a good way to do it (would be great if somebody knowledgeable could comment on this), but so far it seems to work...
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Old 13th March 2003, 12:40 AM   #5
digi01 is offline digi01  China
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Verbal
Can you talk the adjustment course of the voltage of AMP each point?
This is my AMP for the first time runin test ( unit V):
LEFT channel:
Vr18 =1.575;
Q3,b =11,c =17,e =12;
Q2,g =4.8,d =12.3,s =0.6;
Q1,g =17,d =20,s =13.
RIGHT channel,
Vr18 =1. 625;
Q3,b =9.5,c =14.5,e =10;
Q2,g =4.6,d =10.5,s =0.6;
Q1,g =14.5,d =20,s
Bias current of about 0.27A.
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Old 13th March 2003, 09:51 AM   #6
Verbal is offline Verbal  Germany
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Looks like everything's working. But there's a difference in the DC bias point. Check R9/R14 on the left channel (and remember R9/R7 also set the gain).
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