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Old 27th August 2005, 11:51 AM   #1
jeapel is offline jeapel  Canada
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Default ccs question on mini A

hi

The upper transistor with mpsa18 is the aleph current source
but on the lower side there is another mpsa18 to control
current why not just a transistor like zen v4 i
don t understand this new ccs.

aleph newbie :-)
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Old 27th August 2005, 01:42 PM   #2
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The NPN on the neg half of the Aleph variants is a current limiter on the gain side, and it can be omitted if you like riding a motorcycle 300MPH in rain and snow.

In theory, if the source resistor hits a preset (by the voltage divider on the base) voltage drop (current draw throught fet), it shuts down the voltage to the gate, reducing the current throught the source resistor
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Old 27th August 2005, 04:45 PM   #3
jeapel is offline jeapel  Canada
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hi

ok the lower side mpsa18 is normally off line

check me if my understanding is good ??

1-for a zen v4 with just a +v supply the aleph upper
side current source limit the max. current in both
upper and lower side transistors

2-for a aleph:
the upper side:
idle current=.7/.47=1.5 amp plus maybe 30% aleph
modulation for a max.=2 amp

the lower side:
there is no current limit from the -15v and the lower
mpsa18 is for that but ??:

for .7v on mpsa18 base you need 2.25v on .47ohm
because (2.25*100)/(221+100)=.7v
I limit =2.25/.47=4.78 amp it s very high

for a 8ohm speaker maybe 6ohm there is a natural
limit = -15/6=2.5 amp it s very below 4.78amp

thanks
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Old 27th August 2005, 05:18 PM   #4
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I'm not sure about the Zen4, but you are correct for the Aleph. The current limit is set well above normal opperation. It's to make protect the amp in the case of shorted output. This is the production Aleph's only mode of protection.
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Old 27th August 2005, 06:17 PM   #5
jeapel is offline jeapel  Canada
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thanks for help :-)

i see a photo on the left below your name

is it a aleph nimi 3 or 30 and how do you find the sound
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Old 27th August 2005, 06:57 PM   #6
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It's something between Aleph 2 and Aleph 1.2
+- 60 volt rails 16 mosfets/ monoblock.
I'm still playing with the bias. but I've been running around 5amps.

Sounds to die for, and 100% efficient room heater. (too bad it's 95 deg outside.)
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Old 28th August 2005, 12:40 AM   #7
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hey brian, sorry to impose here, but what are your secondaries. My secondaries for my A2 are at 38V (1500VA), just want to be safe, as i will have 12 outputs per/ch, orig bias (AT FIRST )
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Old 28th August 2005, 02:00 AM   #8
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60 V secondarys, I think it's 1500 va, CRC 60,000 uf, Huge 2.5ohm 100w 55,000 uf per rail. I was thinking of regulating, but after buying tranny, decided it would be better to just burn the extra volts in a ceramic resistor then waste heatsink capacity with a reg

I've pushed them really hard, but my mind clipps before they do.

I think 38 comes out almost dead on the 45V mark, but don't sweat it. The circuit works fine anywhere from 20V to 75V.
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Old 28th August 2005, 12:51 PM   #9
jeapel is offline jeapel  Canada
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hi Brian

your big amp seem very interesting have you
a web site for details

my actual setup
www.loginnovation.com/jeapel

more questions ?

1-for the 3 220uF you use top cap like blackgate
or just junk cap ?

2-on the mini A schematic the output mosfet are irf644 is it
possible to use irfp140 or 240 more available without
change the sound ?
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Old 28th August 2005, 03:57 PM   #10
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I did very little documentation. I did a few posts as things progressed, but many thing changed over the last few years

At last, My Aleph 2+

I used Nichicons, not boutique, but not quite junk. I plan to bypass the one in the neg feedback, but the other 2 I don't really consider critcal.

IRFP 140,240, 244 have all been used successfully. Using 4 sets of 8 matched, and a pain in the **** to install, I didn't listen to any others than the 244's I used.

A friend and I listened for a few hours, tweaking bias, current gain, and rail voltage (with a variac), and the biggest improvement was upping the rails. It seemed to open the sound and deepen the soundstage. We couldn't really tell a differece in the sound of the bias or gain, but most people seem to prefer the sound of higher bias.

To summarize, if I did it all over, I would run the rails higher, set the bias as high as the sinks can handle without running too hot, and adjust the gain so it clipps symetric. I would advise against running hotter than 150 deg. The stress of thermal cycling makes the silicon die delaminate from the copper case, then it melts down. We weren't watching, and ran mine up to 175 deg. About 40 times on and off after, the source resistors lit up like lightbulbs.
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