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Old 3rd April 2013, 10:26 PM   #1
borys is offline borys  Poland
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Default My first PCB - some help needed

Hello

I am trying to make my first board and need someone to check if it looks ok.
Pcb is for LazyCat's amplifier called vssa (big THANKS for him for schematic).

On my board I didnt put 2 fuses and 2 2,2mF caps (there will be on psu board).
Some small "blocking" caps can be soldered directly to "big" ones on the bottom of board.
Transistors are fitted on the bottom of board - one on top the other one.
The top layout has only 3 connections - which can be replaced with some wire in case of single pcb layout.


If someone can have a look what I need to correct please ?
THX
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File Type: png vssa v3.png (71.9 KB, 186 views)
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Old 4th April 2013, 03:32 AM   #2
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In the ultra performance amp thread active at the moment there is a pcb by Alex.
Look at how he does the power leads next to the power earth and the input far away. His work is always very tidy, you could search his other posts and learn a lot.
I had a chat with Harry Dymond and others about layout in thread "Has anyone seen this..." You could search for that and it may help.

Best wishes
David
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Old 4th April 2013, 07:49 AM   #3
borys is offline borys  Poland
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Thanks very much !! I will try to find those posts.

Regards Peter


Ps
I looked at alex work and must say he is the KING, thanks again.

Last edited by borys; 4th April 2013 at 07:56 AM.
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Old 4th April 2013, 10:36 AM   #4
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Quote:
Originally Posted by borys View Post
Thanks very much !! I will try to find those posts.
It starts here has-anyone-seen-front-end-before-80.html#post3151499
Quote:
I looked at Alex work
He has an artist's eye for an attractive layout as well as a functional one.

Best wishes
David

Last edited by Dave Zan; 4th April 2013 at 10:44 AM.
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Old 4th April 2013, 11:07 AM   #5
AndrewT is offline AndrewT  Scotland
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Does you bias controller work?
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Old 4th April 2013, 01:41 PM   #6
borys is offline borys  Poland
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Bias for vas and output devices is the same as vssa 1.4 oryginally posted by the lazycat, so it schould work corretly.
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Old 4th April 2013, 04:02 PM   #7
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Quote:
Originally Posted by borys View Post
Bias for vas and output devices is the same as vssa 1.4 oryginally posted by the lazycat, so it schould work corretly.
Hi borys

Looks you were too fast at VSSA sch redrawing, this bias controller will not work. Please check again and be careful at 1:1 inspection.
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Old 4th April 2013, 05:06 PM   #8
rsavas is offline rsavas  Canada
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First thing I noticed,
is that you are using two sides, so double up on the high current traces on both top & bottom layer where you can and make it the highest priority nets to routed first.
This reduces the track Z = lower temp rise & adds extra strength to the PTH.
The ground returns, for the o/p transistor filter caps and zobel network should be on separate leads back to the common point ground.
Design, doubling up on O/P devices is always a good idea to improve performance, hands down.
Actually when I look at the placement some more, the gate drive length is way to long for my liking. i'd put the o/p FETs side by side.
Rick
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Old 4th April 2013, 05:37 PM   #9
CBS240 is offline CBS240  United States
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Did you even glance at the datasheet for the output transistors? If you had, you would have noticed that they have Zener protection diodes built in. Lateral fets should be bias around 100mA or so, and this can be done by a simple pot between the gates.(parallel C4) Source ballast resistors are a good idea even for lateral fets, it helps reduce the % difference in Gm for large and small signals, somewhat. It appears the IPS and VAS bias is dependent on B+/B-. This might fluctuate under load and rail sag. Perhaps a CCS would be better instead of R16/R17? Looks like a job for a J-fet. If I were you I would construct the circuit on a breadboard and get it right with proper compensation before drawing the PCB. You may find bugs in the real circuit that must be exterminated. As Rick points out above, there are some design issues with the PCB.
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Last edited by CBS240; 4th April 2013 at 05:40 PM.
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Old 4th April 2013, 06:45 PM   #10
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CBS240 I agree, since this sch comes from PeeCeeBee thread and is derived from VSSA's sch which is different kind of animal, some things here should be changed, yes.

ALF mosfet used in VSSA doesn't have integrated protection zeners, so here can be omitted. Bias trimmer is not recommended here since VAS BJTs are on the same temp as an output laterals, so TL431 serves to stabilize bias voltage without tempco influence of the main heatsink. Source resistors are not necessary for ALFs, here they can be implemented. Input bias CCSs are in VSSA, here also recommended but since this sch comes from PeeCeeBee thread, there CCSs were omited. Compensation in relation with PCB layout is also another story.
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