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Old 20th September 2007, 03:22 AM   #1
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Default Audiolab 8000a

Hi,
I was thinking of building the amp Audiolab 8000a. But I don't know the values of R1 and R2, which aren't on the schematic. I know it's to set the bias. Maybe I could replace them for a trimpot, but don't know the value. Maybe 2k2 would be okay.
Schematic attached.

Luis Ross
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File Type: pdf audiolab_8000a.pdf (29.4 KB, 412 views)
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Old 20th September 2007, 07:51 AM   #2
Gordy is offline Gordy  United Kingdom
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My guess is that when the amp was manufactured the value of the resistor(s) would be 'select on test' to give the correct bias. So your idea to use a pot is the right approach. (However I do not know which value of pot).
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Old 20th September 2007, 12:06 PM   #3
AMV8 is offline AMV8  United Kingdom
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hi
If you do build the amp look at the 3 x 4k7 resistors that drop the rail voltages from 44v to a lower value for the input. You will probably need to use 7watt resistors for each of these parts. However I think they will still get very hot. I suggest you change these resistors and substitute a transistor set input voltage.
Don
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Old 20th September 2007, 06:59 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
replace the three resistors in the dual Vbe multiplier with a series combination of 200r +2kpot.
Look at the inverting DC servo.
It has no filters on either the input or the output. It badly needs both.
Check out Tom Gootee's thread on DC servo.

This circuit has a number of compensation components spread around the PCB.
Designing you own PCB will almost certainly require the compensation to re-configured.
This is NOT an easy Cdom type circuit, but once the stability is correctly set, should perform rather well. But, do you have the expertise to get to that successful conclusion?
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Old 20th September 2007, 08:02 PM   #5
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Many thanks everybody who answered my question.
I'm no expert. I've built some amps (P3A, for example) and they work fine (at least for me...). But you scared me here. I guess I'll will move to a simpler project.
Since you've been so kind, maybe you could help me here:
I have and old amp whose output board died from abuse. All the rest is okay. My intention is to build a new output. Nothing too complex.
The conditions are:
- Supply: +/- 49 volts
- A single pair of 2sc1943/2sc5200 trans per channel
- Class AB (100 mA quiescent current max.)

I'd choosed the Audiolab because it looked to fit into this picture. If anyone has suggestions of better (simpler) projects (with schematics), I'd appreciate.

Regards

Luis Rossi
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Old 20th September 2007, 09:45 PM   #6
Gordy is offline Gordy  United Kingdom
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Have a look in this thread...


Reference DIY amplifiers:

Reference DIY amplifiers
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Old 21st September 2007, 01:34 AM   #7
Leolabs is offline Leolabs  Malaysia
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Heard that Audiolab 8000A do have two batches with different components/sound,is it true???
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Old 21st September 2007, 06:59 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
the production run for the 8000a was near two decades.
I would expect there to be at least ten and possibly as many as twenty different versions.
But they, Audiolab, in common with many British manufacturers do not follow the far eastern practice of declaring a miracle new improved sound every time they change a component or two.
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Old 5th June 2011, 02:02 PM   #9
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I'm getting no sound from the left channel on my 8000A
Anyone have any ideas?

Thanks
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Old 6th June 2011, 02:48 PM   #10
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Quote:
Originally Posted by Craigy r View Post
I'm getting no sound from the left channel on my 8000A
Anyone have any ideas?

Thanks
Couple of common things to check are the protection relay, make sure it is switching over correctly. But on the last couple I have done with no sound on one channel it has been down to dried out capacitors in the pre-amp. I don't know what test gear you have or your technical capability but you can either follow the signal through the circuit and see where it is disappearing or use an ESR meter (something like a Peak ESR60) to check the caps.

There are two common caps which fail but I can't remember the circuit designator for them, the value is 47uF and they are located in the center of the PCB behind the record selector you might want to start with them.
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