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Old 13th October 2004, 05:16 PM   #1
Edlo is offline Edlo  Hong Kong
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Default Schematic for Audiolab 8000a

Hi,

My friend gave me a faulty Audiolab 8000a amp and I want to fix it. Is there anyone like to share?

Many thanks in advance


regards,


Edlo
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Old 27th May 2005, 03:53 AM   #2
dw887 is offline dw887  United Kingdom
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Hi

I too would be interested in obtaing copy of an 8000a service manual. Also does anyone know where I would be a ble to source a power on off swith for this amp.

Thanks

Dave
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Old 27th May 2005, 07:25 PM   #3
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The schematic of the Audiolab 8000A amplifier is attached below. It is a schematic that I downloaded many years ago, don't know if it is accurate.
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Old 28th May 2005, 06:40 PM   #4
Steven is offline Steven  Netherlands
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The circuit above looks OK. I have an old scan (very bad quality) of the Cambridge Audiolab 8000A (dated 1984) and it is almost the same. Difference is that my scan shows a Vbe multiplier with only one NPN transistor instead of the two transistors (NPN + PNP) in the circuit above.
Another difference is that in my scan the feedback is taken after the output relay. The relay is double throw (loudspeaker and headphone selection I assume), the 2k7//47p for feedback is used twice, once form the loudspeaker connection, once from the headphone connection. The servo is connected right at the 0.22 output resistors, just as in the circuit above.

My scan is too big and too bad quality to attach.

Steven
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The Analog Art shows no sign of yielding to the Dodo's fate. The emergence and maturation of monolithic processing finesse has perhaps lagged a bit behind the growth of the Binary Business. But whereas digital precision is forever bounded by bits, there is no limit excepting Universal Hiss to the ultimate accuracy and functional variety of simple analog circuits. - Barry Gilbert, 1973
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Old 28th May 2005, 09:16 PM   #5
dw887 is offline dw887  United Kingdom
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Hi

than you for you replies, there are slight differences from the circuit posted but on the whole it is quite accurate. All I need to source now is the power on off switch.

Once again thanks

Dave
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Old 29th May 2005, 08:22 AM   #6
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Hi Steven is that you in the photo.......nice looks ....impressive
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Old 29th May 2005, 10:09 AM   #7
AndrewT is online now AndrewT  Scotland
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Hi,
that schematic has some really nice details, no miller comp, cascodes, LTP compensation , DC servo, low voltage input stage, properly scaled Thiel network, etc.
However can someone explain the circuit from second LTP to VAS (cascoded). There appears to be voltage gain here as well as Hi freq comp for the LTP. The 3k9 is the load for the LTP. The second LTP seems to have no method of balancing collector currents.
What do the 3 diodes in the collectors of the first LTP do?
Thoughts please.
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Old 29th May 2005, 02:57 PM   #8
Steven is offline Steven  Netherlands
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Yes, second LTP has gain, emitter resistor 680 Ohm, collector resistor 3k9. Freq. comp. is also there.
Collector currents of second LTP are balanced because the gain from both collector outputs is the same. They are both loaded with 3k9 to the VAS. The gain of the VAS is determined by the 47k load resistors; the same for positive and negative half. The overall feedback takes care now of properly balancing the currents in the second LTP.
The 3 diodes in the first LTP are for clamping in case of overload. It is a Baker clamp. The BC556 in the current mirror of the first LTP cannot saturate bacause 2 diodes take care that the Vce is at least one diode drop (Vbe). Going from top to bottom along the 4 diodes: the first one is part of the mirror, the second and third are the Baker clamp against overload in one direction, the fourth one is against overload in the other direction. This avoids that the second LTP gets reverse biased.

Steven
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The Analog Art shows no sign of yielding to the Dodo's fate. The emergence and maturation of monolithic processing finesse has perhaps lagged a bit behind the growth of the Binary Business. But whereas digital precision is forever bounded by bits, there is no limit excepting Universal Hiss to the ultimate accuracy and functional variety of simple analog circuits. - Barry Gilbert, 1973
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Old 29th May 2005, 04:14 PM   #9
AndrewT is online now AndrewT  Scotland
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Thanks Steven.
I knew I was here to learn.
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Old 13th June 2005, 07:45 PM   #10
mikeks is offline mikeks  United Kingdom
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Default Audiolab's design philosophy...

...from Derek Scotland...:
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