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Old 15th April 2007, 08:07 AM   #1
adi81bv is offline adi81bv  Romania
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Question "classic" amp and begginners problems (pcb wiring, component values)

hi

i want to build this amp (based on esp design). My problems are:

1. The 10R resistor between SGND and GND is ok? or i do not have to put it there? if it is ok what power the resistor should be?

2. The capacitor groups (100n + 1000uF polarized, down left and down right corner), are a good choice?

3. Are the pcb traces wired correctly??? (i am not used to do pcb's and it's one of my fist tries)

4. I choose the BD139/140 + TIP35C/36C transistors, is this a good choice?

5. The output wires connect to the output pin for signal but i don't know if the output ground should be connected to the GND (power ground) or the SGND (signal ground, the star grounding point)

Please help me to get this right. I am new to amps. Thanks.
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Old 15th April 2007, 08:35 AM   #2
adi81bv is offline adi81bv  Romania
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sorry the picture of the pcb didn't load up

Click the image to open in full size.
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Old 15th April 2007, 09:22 AM   #3
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hi adi81bv,

A very good first attempt. I'm no expert but here is a few comments anyway.

1. The ground lift s OK. Some people use them, some don't, others put a diode or 2 across it.

2. I would prefer to see the filter caps closer to the output transistors. Check the diameter of the 1,000uF cap they seems to be small on the PCB. BTW: Rod reckons those caps are unecessary.

3. The accuracy of your traces really depends on your schematic. If you did the schematic right, and back and forward annotation is working then everything should be OK.

4. I haven't tried TIP35C/36C.

5. I'd put the output ground to the off board star earth.

With your power traces I'd try to have thick traces from the fuses to the filter caps then to the output transistors. Then take a thinner trace back to earlier stages.

With such a big board, the bottom right hand corner seems a little too cramped.

regards
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Old 15th April 2007, 10:09 AM   #4
adi81bv is offline adi81bv  Romania
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thanks greq

is it better now?

Click the image to open in full size.
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Old 15th April 2007, 10:21 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
the Zobel must not link to the signal ground (SGND).
Take it to the power ground (GND) or it's own separate wire to offboard audio ground.

Change the shape of the output traces.
Connect the two emitter resistors with a straight wide trace.
Tap off this wide trace with a leg to feed the output pin and from the output pin a trace to the feedback pad.
From the trace feeding the output pin take off the Zobel link and the bootstrap link.

The speaker return and the PSU 0v and the Zobel return and the disconnecting network to safety earth and the signal ground and the power ground all meet at the off board audio ground.

Add an extra pin location on the input DC blocking cap to allow a better metallised film to replace the electrolytic.
Add two inverse parallel diodes (or zeners) across the NFB blocking cap.

Keep the big decoupling caps and KEEP the fuses.
Add two emitter degenerating resistors to the LTP pair (even if you decide to link them out).
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Old 15th April 2007, 10:25 AM   #6
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hi adi81bv,

That's the general idea, but I didn't mean you had to get rid of the caps and fuses though. If you think they are worthwhile, then leave them in. You can try them for yourself and see if they benefit this design.

regards
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Old 15th April 2007, 10:28 AM   #7
adi81bv is offline adi81bv  Romania
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i will modify the pcb regarding andrewT advise as well

the 1000uF caps i decided not to use them because I will have about 12000UF on the power supply pcb and the fuses will be there also.

just trying to make it better. i am just a chemist
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Old 15th April 2007, 10:32 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
keep the onboard decoupling.

I recall a story about a commercial amp that kept blowing up because the decoupling was only 100nF and the rails oscillated as much as the output. The solution was 100uF//100nF on board.
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Old 15th April 2007, 11:07 AM   #9
adi81bv is offline adi81bv  Romania
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ok i will put also the decoupling caps on the board

observation: for the people out there who ask themselves why I do not buy esp pcb. for me to buy a esp pcb is to expensive - due to custom fees and taking in account that i have no income and the components value for this amp is around only 2,5euro - so i think that building this pcb is no harm to rod. beside his pcb i understand is different from the schematics that he has on his site. hope you understand.
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Old 16th April 2007, 03:26 PM   #10
adi81bv is offline adi81bv  Romania
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i am totally confused

1. do you have a link to some pictures how to make a star ground point?

here how i understand: i have 2 kinds of ground (GND and SGND) I collect the SGND on the PCB with a star and take a wire from this poit to a certain star point in the amp box. at this certain point i also collect the GND wires from PSU (0V or also known as reference voltage), ground (or return) from the speakers, GND from the amp pcb, GND from the preamp, box ground. Probably I mistake a lot.

-can anyone explain it to me( but in simple word, because i do not understand a lot of abbr. and terms.)

2. emitter degenerating resistors to the LTP pair - again can someone point me to a picture of an schematic - i presume that the LTP pair is Q1-Q2 transistors (BC546). From what i found these resistors are linked between GND and Q1-Q2 emitter?

3. in p3a schematic C3 100uF electrolytic cap is the NFB blocking cap. i received some info to add two inverse parallel diodes or zener across this cap. is this recommended? what value are the diodes? 1n4148 are ok? the diodes are inverse relative to them or the cap?

4. i modified the pcb again.... do you think this is better?
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