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Old 4th March 2005, 07:26 AM   #1
G33/33 is offline G33/33  Indonesia
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Default Lazer Amp - modified to class A

Gentleman,

I attached the circuit, bias is 20 mA each o/p Tr. Is it possible to use this circuit as class A ? perhaps 600 mA each o/p Tr.

The reason to use this circuit is i already have the pcb.

I'm just a buider, not a designer, so I need your help to modified or to improve thus circuit

Thanks in advance,

G33/33
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Old 4th March 2005, 11:54 PM   #2
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some things in the schematic are definitely wrong.
BC546s seem 'upside down', also some minor mistakes like no emitter resistors in LTPs and mirrors.... or am I misunderstanding something?

basicaly with rails -70... 0... +70 driving 8ohm you would need 4.5A through each pair to reach pure class A I think, but raising it to 600mA using a pot may give good results
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Old 5th March 2005, 06:53 AM   #3
Marc Vi is offline Marc Vi  Netherlands
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I agree that some things in the schematic are basically wrong. Did you draw the bias circuit correctly?

Marc.
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Old 7th March 2005, 07:35 AM   #4
G33/33 is offline G33/33  Indonesia
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The manual says that the bias is 17,5 - 20 ma each.
Damping Factor = 200
S/N Ratio = 122 dB unweight
Harmonic Distortion = 0.06 %

Some friends build it and work properly.
I can't tell the sound quality since they use it as PA amplifier.
But this amp is a real hard worker, played for more than 6 hours heavy metal music in open space without problem.
So, I think there must be something good about this circuit.

I miss something when i draw the circuit, here is the revision, marked with circle under input tr.

If I want 4,5A bias per pair, I think i should change the driver transistor to MJE 15032/33 and lowering the rail to 50V ?


rgds,
Gede
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Old 7th March 2005, 02:43 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi g33,
Your schematic is definately not correct.
Please check & report back.
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regards Andrew T.
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Old 7th March 2005, 10:18 PM   #6
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hey, what you've 'corrected' was not wrong!
other things are.

It is hardly possible to run bjts with such high Iqescient.
If you insist on running it in class A (a bit pointless to me) you have to use MANY output pairs to divide this huge current among them.
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Old 8th March 2005, 04:59 AM   #7
G33/33 is offline G33/33  Indonesia
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I have check the schematic and i'm sure that I draw it correctly based on it's manual.

Sure i will check with the pcb. the pcb is working properly. I build 1 channel, bias is 20 ma each as per manual says.

Maybe pcb and schematic is different, I will check both and report back.

4,5 A is for quad, not pair. My mistake.


rgds,
Gede
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Old 9th March 2005, 06:27 AM   #8
G33/33 is offline G33/33  Indonesia
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I compare the schematic and pcb layout, and yes both different.
All of BC 546 are upside down. Seems the maker wants to make sure people buy their pcb, not just copying the schematic.

I attached the revised sch.


rgds,
Gede
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Old 9th March 2005, 10:23 AM   #9
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Now it looks acceptable (at least for playing heavy metal )
I guess you are aware that running an amp with +70..0..-70 rails in a pure class A at 8 ohm means 4,5A 'idle' current. This makes 630 watts of continuous power dissipation- you need bjts of Ptot of at least 200W and active cooling. Reconsider if it is worth that.

regards
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Old 9th March 2005, 11:34 AM   #10
wkopacz is offline wkopacz  Poland
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Default Vas stage

Hi Gede !
If U don't like smoke
You have to swap pins B & E of MJE340 in VAS stage...

Regards, wk.
Ps.
Interesting output stage!
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